Camera Sensor Crop Factor
and Lens Equivalent Focal Length

Crop Factor Calculators are below.

Digital camera sensors are of a certain size (the sensor area capturing the image), much like film frames were a certain size. Nikon calls their DSLR sensor sizes FX (full frame) and DX (APS frame). Canon calls them Full Frame and APS-C (EF-S). Full frame compares to the same size of a 35 mm film frame (36x24 mm), and APS is the smaller APS film frame size (about 24x16 mm).

The lens projects a circular image onto the sensor, the diameter designed to cover the corners of the sensor or film. Then the camera sensor captures a center rectangular portion. The picture above attempts to show these things:

Red Full frame

Blue Cropped sensor
Less Field of View, simply a cropped and smaller image.

But then enlarged more to be same size again

We might think "Hey, that last one looks like we zoomed in with a longer lens". And enlargement does look telephoto, except it is of course the exact same smaller image (from the same lens in this case), simply enlarged more to view it. The lens did not do this, and technically neither did the crop, but the smaller crop did require the greater viewing enlargement (back to same size), which is what enlarged it. Zooming in on any image in your photo editor shows the same effect. The resulting crop and enlargement does appear as if we did zoom in. The 1.5x more necessary enlargement has some downside, the 1.5x enlargement reduces resolution to 2/3. zoom

Here's an animated example of same cropping effect, simply zooming any existing image in a photo editor. Zooming in the editor is not permanent, but it crops the view that we see smaller (limited by the viewing frame), and then enlarges it back to the same image size, and that visual result appears the same as if we had zoomed in the camera with a longer focal length lens. This example is a bit more than 7x zoom (comparable to a cell phone, if it could use the longer lens). The sensor crop factor "magnification" works exactly the same way. The sensor does not zoom, but the smaller sensor simply does permanently crop it smaller, and then we must enlarge that image more to still view it at the same size. Permanently cropping in the editor does lose many cropped pixels, but the smaller camera sensor can be built to still provide more but smaller pixels then. That's all that crop factor is... it's just a smaller sensor that crops the view it can see... which then we have to enlarge it more, back to the same viewing size. Crop factor is not about a longer focal length, it's just about a smaller cropped image, which we then have to then enlarge more, back to the same viewing size again.

The smaller sensors are of course very popular, with smaller less-expensive cameras and lenses, and have become quite good. But a larger full frame image can have advantages sometimes (larger images, less enlargement needed, a larger field of view, and often larger pixels with less noise). However, full frame typically uses a longer lens (to still see same view size), which can cause a bit less depth of field. The smaller cropped sensor simply provides the same cropped view that a longer lens would see on an uncropped sensor.

Effective Focal Length

We often hear beginner questions like this one:

What is the Effective Focal Length of a 50 mm lens when it is used on my crop 1.5x or 1.6x camera?

It's unfortunately the wrong question, which does not understand yet. A 50 mm lens is always 50 mm, on any camera body. The Focal Length of the lens is as marked, 50 mm. The sensor crop factor cannot change the physical lens focal length. Therefore, the focal length is of course still 50 mm, including even on the cropped sensor body.

The meaning and use of Effective Focal Length is:

Any lens always does whatever its focal length does do. A lens cannot change by simply mounting it on a different sensor. The smaller sensor does cause a smaller cropped Field of View, but the lens does Not change in any way. A smaller sensor is simply smaller, so it can only capture a smaller Field of View (the blue frame above) than does a larger sensor (the red frame above).

If Smaller sensor is Crop Factor 1.5,
Equivalent Field of View if Full frame has 1.5 FL.
If Same lens, Full frame is 1.5x Field of View.

Crop Factor is Not about the lens. Crop Factor is about the cropped Field of View due to the smaller sensor size. Crop Factor is the ratio of the two sensor sizes, ratio equal to larger/smaller. Crop Factor is specifically about the sensor size, as compared to 35 mm film size as being the standard comparison. This sensor size is measured on the diagonal dimension (to account for Aspect Ratio).

A Crop Factor of 1.5 means that (if using the same lens with same focal length) the 1.5x larger sensor sees a Field of View 1.5x larger than the small sensor (orange sensor case in diagram, trying to show same lens with both sensors at same mounting distance). In this case, the smaller sensor Field of View is 1/1.5 size of the larger one (1/1.5 = 2/3 size if 1.5x, or 5/8 size if 1.6x), which is Crop Factor.

Or, if the 1.5x larger Full frame sensor instead uses another lens with focal length 1.5x longer, then it sees the same (Equivalent) Field of View as the smaller sensor using the shorter lens (the green sensor case in diagram). This is the entire concept about Equivalent Focal Length, which is about the other larger sensor.

Or, if both use the same focal length lens, if the 1.5x larger full frame sensor simply stands back 1.5x further than the small sensor, it also again sees the same Field of View.

So, the angles work out that if the 1.5x larger Full frame senor has 1.5x distance either behind the lens (which is a 1.5x longer focal length) or in front of the (same) lens further back, its Field of View is then the same as the smaller crop 1.5 sensor.

That image from the smaller cropped view of the smaller sensor, when necessarily enlarged again to be same size again, does appear to be zoomed, to be the same view as if a longer lens had been used ON THE LARGER SENSOR. But on the smaller sensor, it of course appears as if THIS original lens were used (since it was used on the smaller sensor).

Equivalent Focal Length merely compares the Field of View that the OTHER full size sensor size would see with an "Equivalent" lens calculated to compare, compared to what this sensor actually does see with this original lens. The Equivalent Focal Length is mounted on the OTHER full size sensor, Not on the cropped camera. The cropped camera uses the lens that it has.

The standard convention used is that the Equivalent Focal Length compares:
  The Field of View that this lens on the cropped sensor does see with its lens,
  compared to the Field of View size that a larger "full frame" 35 mm film size sensor would see,
  if the larger sensor were used with the computed Equivalent Focal Length lens.

That CANNOT change this original lens in any way. But a smaller sensor does capture a smaller cropped field of view.

35 mm "full frame" size is always the comparison standard, useful simply because many users have many years of 35 mm film experience, and know well what result to expect from various focal lengths on 35 mm film. This comparison then tells them what this new smaller sensor should see, in terms they already well know. This is very good information in certain situations.

But if unfamiliar with using full frame 35 mm film size, then Equivalent Focal Length likely will not have meaning to you (because that is in fact what how it applies). You will become familiar with what various lenses do on your cropped camera sensor. And then if and when you switch to a full frame camera, then this may become important to you.

The literature can confuse us about this, to the point we have to already understand crop factor before we can understand their explanation about it. Some of the best sources word it as " For example, a 70-200mm lens becomes a virtual 105-300mm lens on a 1.5x APS-C sensor." However that "becomes" simply does Not literally happen, although if we are used to thinking in terms of full frame, we might think of it that way. The lens of course remains totally unaffected, it still does what it always did (that was how they meant "virtual", not physically existing). All that happens is the 70-200mm lens on a full frame and the 105-300mm lens on a 1.5x APS-C sensor simply see the same field of view (because the smaller sensor crops its view smaller, but normally its shorter lens compensates to show the same wider view). It's like two film sizes, IF using the proper corresponding lenses, they can take the same picture (same field of view). However, the smaller film is still smaller, and requires greater enlargement to view at same size.

The standard convention is to compare smaller cropped sensor size to 35 mm film size, simply because many of us were very familiar with 35 mm film, with years or decades of experience to already well know by second nature what field of view to expect from various focal lengths on 35 mm film. That crop factor Equivalenet comparison tells us what view THIS new sensor will show now, in Equivalent terms already very familiar. But if you're not familiar with 35 mm film, then it won't have much meaning to you. You can ignore this Equivalent Field of View as not applicable, and your Field of View is what you see it is. That can work when using the lens, but when shopping for a lens, many of us were very familiar due to years of using 35 mm film, and now that there are SO MANY different sizes of digital sensors, this 35mm film comparison gives us an easy standard of comparison of expected field of view (in 35 mm film terms that many were long used to).

The reasons you may want to know full frame Equivalent Focal Length are if you have considerable 35 mm experience so that the comparison means something to you, in those terms. Or maybe you use both sizes of digital sensors. It can be helpful to know what to expect from the Field of View comparisons.

Or if you use two smaller sensors, say 1.5x and 2.7x crops, then the larger crop factor is 2.7/1.5 = 1.8x the smaller crop factor number (which is also the ratio of their diagonals). In this 1.8 example, the Field of View of the smaller sensor (larger crop factor) and its lens is the same as the larger sensor with 1.8 x focal length.

Otherwise, the Field of View of your one smaller sensor is whatever it is, and that's what you need to get familiar with. A full frame comparison is unimportant if you have no interest in full frame.

Crop Factor

Crop factor is about the corresponding Field of View captured by a smaller sensor size, as compared to larger 35 mm film results. Quite simply, a smaller sensor crops a smaller image than a larger sensor could have captured larger and more completely.

The term "Crop Factor" always means "Cropped Field of View Factor" (due to smaller sensor dimensions). Crop Factor compares to 35 mm film size, convenient because many people are very familiar with using 35 mm film.

The relative size of our digital camera sensor (THIS cropped sensor that we are using, here called THIS sensor) determines its Crop Factor, which is relative to 35 mm film size (which is called "full frame" size). Crop Factor is about Field of View size, and for that comparison, the 35 mm film size is selected to be a standard of comparison. We might hear things about the lens, but the lens is always totally unaffected. The lens cannot change, its design always remains exactly what it is, it does what it does. THIS smaller sensor simply crops the Field of View to show a smaller area of the lens image (the picture above). We see a smaller view from a smaller sensor, which is called a cropped field of view (cropped when compared to 35 mm film size).

In practice therefore, the smaller cropped sensor has to use a shorter lens (with a wider view) just to be able to see the full "normal" view size again. We might compare that view to what a camera with a larger sensor (35 mm film) sees with its longer "equivalent focal length" lens. But the smaller sensor of course uses the shorter lens to be able to see that same view. This view is what's equivalent, the two lenses are of course different, necessary because the sensor sizes are different. A camera with a crop factor of 2 sees a view of half that size, so it uses a focal length half as long, in order to still see the same view as the sensor of 35 mm film size.

Numbers of Crop Factor

Crop Factor =
Diagonal dimension of 35 mm film
Diagonal dimension of THIS camera sensor

Calculate Crop Factor

Sensor Width mm

Sensor Height mm

Crop Factor is about the sensor size, comparing it to 35 mm film size as the standard comparison. If this division computes the Crop Factor to be 1.5x, it means that the 35 mm film frame diagonal dimension is 1.5x larger (the area is about 2.2x larger, varying slightly with aspect ratios). If 3:2 aspect, a 1.5x crop frame is 1/1.5 or 2/3 the dimensions of the 35 mm film. This size is compared numerically measured on the diagonals to account for different shape sensors. The diagonal of 35 mm film is 43.26661 mm. But generally, Crop Factor is used regarding the smaller Field of View seen as cropped by the smaller sensor size.

We might not be told Crop Factor, or even the sensor dimensions in our camera specifications. However we are often told its Equivalent focal length relationship of a comparison lens for 35 mm film, which can then tell us Crop Factor and sensor size.

Calculate 35 mm film
Equivalent Focal Length

Focal Length mm on

a Crop Factor body

Equivalent Focal Length for 35 mm film =
  Focal Length of lens on THIS camera x Crop Factor

The lens Focal Length stays what it is. The lens cannot be modified by the sensor. A 50 mm lens on a full frame body is of course still the same 50 mm on any smaller cropped body. The Equivalent Focal Length applies ONLY to a different lens on a different sensor (specifically the size of 35 mm film) that gives the same field of view on that camera as THIS lens on THIS camera. The focal length of any lens on any camera is of course always whatever it is, unaffected, the lens cannot change itself by putting it on another sensor. Crop Factor does not change anything that your lens does, it still projects the same image. Crop Factor only describes the size of your sensor, relative to the size of 35 mm film. The difference is that a smaller sensor "crops" a smaller image, smaller than a larger sensor would otherwise see. The effect of this calculated lens ratio is that then both sensors see the same field of view and the same subject size, meaning the subject fills the frames of both in the same apparent way.

Compute Crop Factor from Equivalent Focal Length

Crop Factor =
Equivalent focal length on 35 mm film
Focal length of lens on THIS camera

Calculate Crop Factor from
Equivalent Focal Length

THIS Focal Length mm

35mm Equiv FL mm


For example, the specification for some compact camera's zoom lens might say:

Focal Length: 4.5 - 81.0 mm (35 mm film equivalent: 25 - 450 mm)

So this lens focal length is 4.5 to 81.0 mm.

It also says that a 35 mm film camera would see the same field of view if it used a 25-450 mm lens. If you have 35 mm film experience, this may be meaningful to you, to tell you what to expect that this camera is going to do.

THIS cameras 4.5 mm lens sees the equivalent view of a 25 mm lens on a 35mm film camera (the tiny compact sensor has to use the very short lens to fill its frame with the same field of view).

So simple division can compute crop factor to be (25 mm / 4.5 mm), or (450 mm / 81 mm), Both of which compute the same 5.556x crop factor. This is the actual size relationship of the two digital sensors (diagonals), which affects the field of view that can be captured from the lens. And since 35 mm film size is well known, then with the correct aspect ratio, crop factor can also compute sensor dimensions. However these published values (crop factor, sensor size, aspect ratio, even focal length) are often rounded, to be stated as slightly less precise values than they actually are.

Macro Lens Reproduction Ratio on Cropped Sensor

1:1 is a fixed reproduction ratio, which is 1:1 object size on any sensor size. Since any lens remains unchanged by any sensor size, its reproduction ratio is also unchanged, and the focus distance where 1:1 occurs is unchanged. A smaller sensor will necessarily crop the overall field of view, but the magnification remains the same.

What 1:1 means is if photographing a ruler with a lens at 1:1, then a sensor width of 24 mm will show a field width of 24 mm. A sensor width of 36 mm will show a field width of 36 mm. Objects within the frames will be the same 1:1 real life size in the image (same magnification), but overall field of view size will simply change with sensor size (a smaller sensor crops the image).

Reproduction ratio is (actual real object size in mm) / (object image size in mm). Half size ratio is 2:1.

Magnification is the reciprocal, (object image size in mm) / (actual real object size in mm). Half size magnification is 0.5x.

Extension Tube Magnification on Cropped Sensors

Again, the Crop Factor is about the sensor size, and simply does not affect any fixed lens reproduction ratio.

Macro lenses are much more expensive, but are vastly more easy and versatile and convenient (meaning usable) than extension tubes, which can be a bit much for a beginner. Read up well first. Macro lenses focus normally and easily, from infinity to typically 1:1, very simple to use, like any other lens (you will want to use f/22 or f/32 at 1:1, and a speedlight flash helps both light and speed).

An extension tube is an empty tube (with no glass elements) mounted between camera body and lens, to extend the lens forward to focus closer for greater magnification. Normal lenses on extension tubes (if near 1:1) really don't change focus at all, instead we focus by moving the camera back and forth a couple of inches from subject to find the one place it is in focus, at whatever one ratio the extension does. If you want a different size view, you have to change the extension tube length.

There is a basic extension tube formula: Extension used = Focal Length Used / Magnification

Or rearranged, Magnification = Focal Length / Extension.

So if the Focal Length used is 50 mm, and added extension tube is 50 mm, you get Magnification 50/50 = 1x, which is 1:1 reproduction ratio (same size on sensor as object size in real life).
If Extension is 50 mm and the focal length is 200 mm, you get Magnification 50/200 = 1/4x, or 1:4, or 25% of real life size.
Extension tubes affect shorter lenses much more than longer lenses. If you want 1:1, you must double the focal length.

Crop Factor is not a factor of Magnification, although it does affect overall Field of View size (a smaller sensor crops the image).

Teleconverters on Cropped Sensor

These add-on lens adapters contain optical glass elements which actually do modify the effective focal length of the combination. The teleconverter might be named 1.4x for example, or some of these are wide angle adapters, perhaps 0.7x for example.

The formula is:  f/stop number = focal length / aperture diameter.
If focal length becomes 1.4x longer, then f/stop numbers become 1.4x greater.
A f/stop Number 1.4x greater is one EV, 2x greater is 2 EV.
The focal length causes this, Not the crop factor.

If Smaller sensor is Crop Factor 1.5,
Equivalent Field of View if Full frame has 1.5 FL.
If Same lens, Full frame is 1.5x Field of View.

The basic fact is that cropping an image with a smaller sensor can only show a smaller Field of View, but the same lens can only project the same image onto any size sensor. Then a smaller sensor simply crops it smaller, for a smaller Field of View. Then when subsequently enlarged more to view at normal size again, it has an enlarged appearance, looking as if a longer lens was used. Or cropping smaller in a photo editor also acquires the same enlarged "telephoto" appearance, as if zooming in. See the diagram for the best explanation, that's exactly how it is in the camera. A smaller sensor shows a smaller Field of View.

Digital Crop Factor today is arbitrarily referenced to the 35 mm film diagonal, size of 35 mm diagonal / size of sensor diagonal. A sensor size with 1.5x Crop Factor means 35mm film "full frame" is 1.5x larger, and that this sensor Field of View is 1/1.5 = 2/3 the size of full frame Field of View.

 Film SizeFrame
8 mm5.5 mm7.87x
Super 86.64 mm6.52x
16 mm12.7 mm3.41x
Kodak Disc film12.81 mm3.38x
11021.4 mm2.02x
APS C28.75 mm1.51x
12639.6 mm1.09x
35 mm43.27 mm1x
82848.83 mm0.89x
127 40x40mm56.57 mm0.76x
127 60x40mm72.11 mm0.6x
120 6x4.5 cm70 mm0.62x
120 6x6 cm79.2 mm0.55x
120 6x7 cm89.25 mm0.48x
120 6x9 cm101 mm0.43x
4x5 inch153.1 mm0.28x
8x10 inch325.7 mm0.13x
Full Frame
43.27 mm1x
Digital DX
28.21 mm1.5x
Digital EF-S
26.68 mm1.6x
Digital 4/321.64 mm2x
Digital CX
One Inch
15.86 mm2.7x
Digital 1/2.6"
6.86 mm6.3x
Digital 1/3"
6 mm7.2x

This Crop Factor situation has always existed, for film too. It wasn't called Crop Factor back then, but film format size affected choice of lens focal length that was suitable in the same way, for the same reason. Large film (like 4x5 or 8x10 inch sheet film) used a rather long focal length. Middle size film used an intermediate focal length, and 35 mm was smaller, but not as small as the tiny 110 or Kodak Disc film which needed a very short focal length. A "Normal" lens (meaning with a normal field of view on the specified sensor size, comparable to what the eye remembers seeing there) was considered to be a focal length approximately the same as the diagonal dimension of the sensor (a bit arbitrary, but that was a view typically around 40 to 44 degrees horizontal width was considered a "normal" lens). These different focal lengths were needed so that all formats would show about the same expected Normal field of view.

The point is, smaller sensors require a shorter lens to see the "normal" view size. But the lens mountings were not interchangeable among those film formats, so other than in the darkroom enlarger, their worlds did not interact much back then. The difference today is the digital DSLR sizes might be able to mount and use the same physical lens, and then we notice that the different size sensors cause a different field of view size. Smaller sensors crop the field of view seen. Which seems obvious, yet it causes confusion.

The term Equivalent Focal Length has the specific meaning that THIS camera with its lens and focal length and smaller sensor shows the same Equivalent Field of View as a larger full frame camera would show if using the Equivalent Focal Length. But that Equivalent focal length does NOT affect THIS cropped camera, it is on the full frame camera. The Equivalent focal length has no meaning to the cropped sensor camera, we can just forget about that. Why would we care what some different full frame camera might do? UNLESS of course, if you have experience using, or still do also use a full frame camera, then it does allow field of view comparisons to full frame usage (which you may already be very familiar with).

Newbies sometimes misunderstand at first, and imagine THEIR lens somehow magically became longer focal length on a cropped sensor. But there is no magic, that cannot happen, nothing can actually change. Any one lens always projects the same image regardless of sensor size. With the same lens standing at the same place, the smaller sensor simply frames that image smaller (crops it smaller), and then subsequent greater enlargement back to normal viewing size makes it look zoomed. But instead, if the focal lengths are adjusted to be the Equivalent relationship (full frame sensor instead uses a longer lens with focal length multiplied by the crop factor of the smaller sensor), it is possible to make the Field of View be Equivalent (no zoom effect then).

The following discusses Examples of a 1.5x crop factor DSLR camera:

Since the field of view of THIS sensor is cropped smaller, it means that a full frame or 35 mm camera standing in SAME place must use a lens 1.5x longer to see THIS SAME smaller field of view. We call that Equivalent or Effective focal length, but it refers to the 35 mm camera, NOT to THIS camera. It means THIS camera sees a field of view equivalent to the 35 mm film camera, when the 35 mm camera uses the longer equivalent lens. The "comparison" makes it seem like a telephoto effect on this camera, only because its field of view is cropped smaller than the 35 mm camera numbers. The focal length of THIS camera is still whatever its lens actually is, nothing changes. And the focal length of the 35 mm camera is whatever it is, but the difference in sensor sizes make the fields of view be different.

Or if both cameras use the same focal length lens, it means THIS camera would have to stand back 1.5x farther than the 35 mm camera, for both to see the same field of view (the larger "full frame" sees a wider uncropped view, and standing back lets the smaller sensor see the same wider view). So for THAT comparison, what THIS camera sees compares to AS IF the 35 mm camera were using a 1.5x longer focal length (if also standing back there with us). The view makes us think of an equivalent longer telephoto lens, but again, as used by the 35 mm camera. The lens on our THIS camera is always still what it always was.

It also means that any image on THIS smaller sensor will require 1.5x greater enlargement to produce the same size print or view as the full frame camera.

Sports and wildlife photographers may prefer the DSLR 1.5x crop camera (over full frame) because its smaller cropped area makes their 300mm lens appear 1.5x longer (as compared to using that Equivalent focal length on a full frame sensor, like 35 mm film). And it does, they may not have to buy a new lens. With the same lens, the full frame camera would lose 55% of its pixels if cropping to the same smaller 1.5x "telescopic" view. The cropped sensor doesn't have to crop its image as much, so it has more pixels left. However, it does still have to use the pixels to enlarge the smaller image 1.5x more than full frame would.

300 mm lens on a full frame camera result is a 300 mm view, so to speak (in terms of 35 mm film frame size).
On a 1.5x crop camera, a 300 mm result "looks like" an equivalent 450 mm lens on a full frame.
However, it is of course still very much a 300 mm lens on a 1.5 crop camera.

18 mm lens on a full frame camera result is an 18 mm view (in terms of 35 mm film frame size).
On a 1.5x crop camera, an 18 mm result "looks like" an equivalent of 27mm lens on a full frame.
However, it is of course still very much a 18 mm lens on a 1.5 crop camera.

So cropped sensors may aid a telephoto effect (with the same lens),
but wide angle photographers (perhaps real estate agents) love the fully wide view of the full frame camera, because a cropped DX sensor would require a 12 mm lens in this example (to be Equivalent field of view to 18 mm on a full frame). Lens distortion becomes worse in very short lenses.

The smaller sensor certainly has effect causing its smaller field of view, but the literal Crop Factor and Equivalent focal length numbers do not affect THIS camera at all. THIS camera always does only exactly whatever its specific sensor and lens determines it does. Crop Factor is only a method of comparing field of view to that expected from a 35 mm film camera (which many of us spent decades learning).

Obviously, the top picture below is the view of the same scene, with the same lens, at same distance - if using the two sizes of sensors. The Subject size is obviously the same (again, same lens at same distance), but the frame size is not the same. The two drawn boxes show the size of the sensor that will capture the image, what the camera will see. This is simply how it works. When we enlarge the cropped image to be displayed at same size, it appears to be telescopic, as if with a longer lens, or as if standing closer. Any cropped image shows the same telescopic effect when enlarged to same size. Nikon uses the terms FX for full frame sensor and DX for a 1.5x crop smaller sensor. DX is a smaller sensor, and this "crop" changes the viewed area, which causes the differences between FX vs. DX.

The digital DX camera uses a smaller sensor to capture the center of the lens image, which is said to "crop" the image (edges are cut off, as shown above). A full frame lens is shown above, but DX lenses are designed smaller, to project a smaller circle, which only covers the corners of the smaller DX frame (which causes vignetting if on a FX body). Compact cameras use even a much smaller sensor, around 7x5 mm is a common size, but they include several sizes, all tiny (crop factors of 5 to 7 are common).

The difference in these sensor sizes causes different visual effects. For example, the FX sensor is 36 mm wide, and the Nikon DX sensor is 24 mm wide. The ratio of these two crop sizes is 36/24, which is 1.5 to 1, called Crop Factor (normally we compare sensor diagonals, but these two are the same 3:2 shape). The DX frame is cropped smaller (simply because its sensor is smaller). The FX view is obviously 1.5x wider than DX (more wide angle than DX).

But the smaller DX view, when enlarged to show it same size, is magnified 1.5x more than the FX, which is the same effect seen when FX uses a lens that is 1.5x longer focal length (more telephoto than FX). So, the DX camera view "looks same as" if a 1.5x focal length were used on FX. The actual lens focal length is not changed, but the 1.5x number is called the DX "equivalent" or "effective" focal length (as compared to FX). The lens is NOT changed, but DX merely crops to see a different VIEW from it. DX would give same view as FX full frame would see if FX used the 1.5x longer lens at the same distance - or if with same lens, if DX stood back 1.5x times more distant than FX. Again, the lens itself is unchanged of course, it still does whatever it always does in both cases, but the view seen by a smaller DX sensor is simply a cropped and enlarged view, different than the wider view seen by a larger FX sensor.

The "effective" or "equivalent" focal length number (due to the crop factor) is simply the comparison to FX size, which is the same size as 35 mm film, which many of us were used to for many years. Back then, we knew what a 24 mm view did before, but it becomes something different on DX. And since the FX lenses are interchangeable (used on DX too), this comparison is important to some users. The only use or importance of this "Equivalent focal length on FX" is to compare the DX VIEW to FX VIEW.

We know the 1.5x number, but sometimes we miss the significance until we actually look though the viewfinder once ourselves. (or the Field of View Calculator should make it be clear). Using the same lens, and relative to each other, FX makes a wide angle view (wider view of more area, but with necessarily smaller contents within that view), whereas DX makes an apparent zoomed in telephoto view (technically, a cropped view), which shows less scene area of course, and the subject is magnified when it is viewed enlarged to be the same size again.

Below are D300 DX and D800 FX images, using the same 105 mm lens, on the same stationary tripod, at same distance. The only change was that the bodies were swapped out. The camera viewfinder shows these same views.

This is a 105 mm lens on FX (D800). Because the FX sensor is larger, then compared to DX, FX is simply a wide angle view, 1.5x wider than the smaller DX sensor. The larger sensor extends farther out from center, so now we see pixels way out there too. Therefore, because the overall view is bigger, the subject is necessarily shown at a reduced size (scene is bigger, wider, but shown here in the same space, the objects in it are necessarily smaller than DX).

The first obvious reaction looking in a FX view finder is that (compared to DX with same lens), it shows the subject smaller - but the scene is larger, a larger area, visually appearing as if from a greater distance. These two pictures were at the same distance of course, just the view is different (DX is cropped from the FX view - see next one.)

This is the same 105 mm lens on DX (D300), on the same tripod at same distance. DX is just a smaller cropped version of the lens view, but which after enlarging 1.5x more to appear same size here, then it looks like a zoomed in view, "as if" it used a lens of 1.5x more focal length on FX. In this case, DX sees roughly an eight inch view instead of twelve inch view on FX (12/8 = 1.5). It is the same lens however. The focal length is not changed. It is only the "cropped view" that is different. After we enlarge the smaller sensor image more, then the view is zoomed "as if" it had been a longer focal length on FX.

A 10 mm lens is always 105 mm on FX or DX. The "Equivalent" term just means this is the "Equivalent" VIEW (as if seen on FX) of 105 mm x 1.5x = 160 mm focal length (if on FX). The lens is always 105 mm, but after the smaller DX image is enlarged more on the monitor, the VIEW we see on the cropped DX sensor looks magnified, "as if" FX used a longer lens, or "as if" FX were standing closer. Any cropping followed by more enlargement would simulate that. The DX sensor is smaller, so then we enlarge it more, the larger view appears as if magnified by the "crop factor".

The DX image above is actually smaller than the FX image as seen here (cropped by the smaller sensor), but it is enlarged half again more above, just to show the images as the same size.

But (at same enlargement), the DX image is actually this relative size. The subject objects in the image are the same size here as in the FX image.

The same lens of course projects the same image on the two sensors. The same image is of course the same size, but the DX sensor is cropped smaller. DX is a smaller image, but you can see that the area in this DX crop size matches the central area of the FX image above (assuming same lens at same distance).

FX is 1.5x larger, so in comparison, DX is only 2/3 of FX size. We would normally show them at same enlarged size (as above), but they are not same size. The FX sensor is half again larger than DX. DX has to be enlarged half again more, which is not an equal comparison of what we have.

This is the Same FX image, but now it is marked to indicate a DX crop that matches the DX image area. And it is shown enlarged 1.5x more here, so that the cropped DX frame comes out the same size as the FX size above. DX is simply a cropped smaller view of the full size FX frame, enlarged more, because the DX sensor is cropped smaller.

Note that the cropped DX frame is smaller (2/3 which is 1/1.5), and the FX frame is 1.5x larger than DX. I am emphasizing the fact the smaller DX frame has to be enlarged 1.5x more to view or print it at the same size as the FX image. It is simply this crop (and resulting smaller angle of view) and the following greater enlargement, that zooms DX to give the apparent telephoto effect. No other magic illusions are involved. Any crop, done anywhere, anytime, will appear to show the same telephoto effect when subsequently enlarged more back to same viewing size. The smaller image has to be enlarged more to view it at the same size. This is the entire cropped factor telephoto effect, due to the smaller sensor.

And you can also see this same "FX vs DX" view by cropping any image in your editor, by just marking a crop box that is 2/3 the dimensions of the original 3:2 image you use (1/1.5 is 0.667, or 2/3). Or use any crop factor, but this 2/3 dimension will show exactly the view a 1.5x DX camera would see, as compared to the FX view, assuming if both are at the same distance with the same lens. Or just zoom in on any image in your editor (a smaller view, enlarged to same size, appears as a telephoto effect). This crop (shown just above) is the only difference of DX and FX. DX is simply a smaller sensor, which cannot capture a view as wide. When you can believe that, then you've got it.

So DX is just a smaller sensor, which crops out a smaller central area, which necessarily has to be enlarged more, to view at the same effective size. This does change the view it sees - DX sees the same view FX would see if FX used a telephoto lens 1.5x longer (at same distance). Or, if with same lens at same distance, then DX sees a smaller view width, which is necessarily enlarged more (to be the same size print).

FWIW, a typical compact camera with a tiny 7x5 mm sensor and shooting with maybe a 9 mm lens does even more of this, with a crop factor maybe 5 to 1 compared to 35 mm film FX size. Compacts use special very short focal length lenses to compensate for the tiny sensor. The lenses are not interchangeable with DSLR, so this difference is less discussed. The camera specs will mention the equivalent 35 mm film size equivalent focal lengths however (focal length multiplied by crop factor), since many of us were familiar with 35 mm, then we also know what this lens will do.

This is nothing new. In the film days, medium size roll film, and view camera sheet film, were very different sizes than 35 mm film, which again required lenses with different focal lengths to be used (to get the same normal viewing angle). Since these lenses were not interchanged among cameras of different film sizes, it is just something we knew, but did not worry about much.

Speaking of field of view... A "normal lens" is one with a focal length more or less approximately same as the sensor diagonal, significant because this lens is often considered to give what we remember as an eye's "normal" view of a scene. So different size cameras, each with their "normal lens", will take a picture covering about the same "normal" view of the scene.

Normal lens
focal length
iPhone 54.8x3.6 mm6.0 mm7.24.2 mm
APS C, or DX24x16 mm28.8 mm1.5, 1.630-35 mm
35 mm, or FX36×24 mm43.3 mm145-50 mm
120/220, 6×656×56 mm79.2 mm0.5575-80 mm
120/220, 6×984×56 mm101.0 mm0.43105 mm
4×5 sheet film118×93 mm150.2 mm0.28150 mm
8×10 sheet film194×245 mm312.5 mm0.13300+ mm

Advantages of FX or DX

Pure and simple, in all cases of same lens at same distance, FX shows a view 1.5x wider than DX, simply because DX is cropped to a smaller view, and FX of course is not. This has perceived effects.

Of course, it does not matter when we crop it (to get same view). We could crop with the smaller camera sensor area, or we could do exactly the same thing later at home, by just cropping the FX image to be smaller DX size, and then enlarging more. You can see this same telephoto effect in your photo editor by simply zooming in. Other than the initial file size and the cropped telescopic view, the only difference and concern might be about the pixel density - the final cropped dimensions would only have about 40% of the pixels left. Starting from the D700 12 megapixels, cropping FX to DX leaves 5 megapixels. The D600 24 leaves 10 megapixels, and the D800 36 leaves 15 megapixels. But this is the same result, regardless if cropped in the camera, or done later. So if you are shooting the distant wildlife with FX, and wishing for the DX telephoto effect, either select the DX menu, or just plan to crop it later. However, there is a bigger difference. If instead using the real DX camera, it returns its normal 16 or 24 megapixels for that DX view, which is a plus.

The purpose of the effective focal length comparison (compares lens view to that using 35 mm film frame size) is simply that for anyone who was long accustomed to 35 mm film (same size frame as the FX sensor), now our lenses act that same familiar way in FX digital - a 30 mm lens means the same thing on FX (same view) - that a 30 mm lens always meant for 35 mm film. So this is another relative advantage of FX - FX is like "Old Home Week" again - the way we learned to think of it in the past. "Effective focal length" (actual focal length x sensor crop factor) is used to compare a lens view to 35 mm film format, which helps old timers "know" or predict what other sensors will show. Which may be pointless to newcomers who never used 35 mm film, but nevertheless, it is important to those many who grew up thinking that way.

Compatibility: Both FX and DX lenses will "mount" on all DSLR bodies. FX lenses are those that don't say DX. A Nikon FX lens will "work" regularly and well on the DX body (but some DX bodies will need both FX or DX lens to be AF-S to auto focus). And the DX lens will "mount" on the FX body, but the big difference is that DX lens diameter coverage is smaller, and the circle diameter only covers the Blue frame above. The FX lens are designed larger to cover the red frame above, as shown. This makes FX lenses be larger, heavier, and more expensive, to cover the larger frame. So (depending on zoom value) the DX lens on a FX body probably suffers extreme dark unfilled corners, simply not designed for FX bodies. However, most FX DSLR do have the option to create a smaller DX size cropped image, and then DX lenses can work that way, but this DX crop loses more than half of their pixels, like 10 megapixels instead of 24 megapixels.

Actual Disadvantages:

K.I.S.S. tells me not to complicate things, but who listens? Just a quick note: Changing where we stand with the camera to get the same view may give the same angular view, but it does still change a few things:

The Nikon FX models give you both choices - you can shoot DX or FX mode, for both Raw and JPG. That makes DX show different in the viewfinder (not like actual DX cameras), in that the DX frame is seen as the smaller cropped area bordered with a red box inside the full FX frame (not enlarged in viewfinder - the DX viewfinder view is very much like my last picture example above - a smaller box marked inside a larger frame). The viewfinder is optical, but Live View is digital, so DX mode in Live View can show the enlarged DX frame. The final DX image result will be necessarily enlarged. Or of course, we can always crop FX to the DX view and size anytime later.

However FWIW, be aware that the big downside of using one walk-around lens like a 24-120 mm on a FX body in DX mode is that 24 mm offers no wide angle when switched to DX mode. DX requires 16 mm to do what 24 mm does on FX, so plan on needing the 16-85 mm lens for DX mode.

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