Want to Blur the Background?
Comparing Depth of Field of Two Lenses

There is a Depth of Field (DOF) calculator here (lower below), but which was not really done for general purpose. It does do that too, but there's already many of them online, and a very good one at DofMaster, so there was no need for another if doing the same thing. And I shirk at the work to provide sensor size for every camera model known to man. :)

So the DOF calculator below is a little different, in that it also adds a few new features.

Determining sensor size is difficult for most compact cameras. The sensor is so small that their lens is necessarily very short, which ensures a great depth of field. Setting a wide aperture may not be a choice, but choosing more dim lighting can help. The best chance to blur the background some with a compact camera is to zoom in greatly, and then stand back as necessary, but stand as close as that zoom can allow (not closer than about 5 feet for a portrait). Choose a background that is very much more distant, hopefully a few hundred feet.

Maximizing Background Blur Without Suffering f/1.8

We should realize that both focal length and subject distance are Depth of Field (DOF) factors. We can use them both for our goal. My notion of a portrait at f/1.8 is that there will of course be DOF problems, usually about the hardest possible way to make a good picture, and the last thing I want if I can prevent it. Notions may vary, but studio portraits likely work at f/8 or f/11 (again, the usual goal is that the work will sell). Studio flashes are made big to allow those apertures. We like the sharpness of depth of field.

Equivalent distance for same FOV

Focal Length A mm, Distance:

Focal Length B mm, Equivalent: 

(distances can be feet or meters)

The defaults shown in the DOF calculator below initially compares a 50 mm f/1.8 portrait at 6 feet with a 200 mm f/4 at 24 feet, both using a DX APS DSLR camera (assuming the same camera sensor). It assumes the background we want blurred is 40 feet behind the subject (try other distances too, the longer lens usually wins). Pros would more likely use maybe a 200 mm at f/4 for this purpose (of hiding the background). Or 100 mm at f/2.8 can be better too. The 200 mm is 200/50 = 4x longer than 50 mm, therefore the two subject fields of view are the same size if the 200 mm lens stands back at 6x4 = 24 feet.

And if both lens with distances adjusted to be the same field of view at the subject are also at the same aperture, then they have the Same DOF range too (at the subject) - the adage about the "same image has same DOF", etc. (at the subject, it does). But where we stand does affect the perspective, and the background is certainly NOT the same then... in the long lens, most of the wide background is zoomed out and gone missing, but what's left is even more blurred focus (assuming that is a plus here). This standing back at greater distance is little problem to do outdoors, and it possibly need not be that extreme. If both could use the same f/1.8 then, the depth of field at the subject is the Same, both 0.29 feet of DOF (which is only 3.5 inches - from nose to hair may be more than that). DOF does not describe the sharpest necessarily, instead it defines the maximum blur that we can accept. But we don't have to use the same aperture, the 200 mm lens at 24 feet can use say f/4, which has 0.66 feet of DOF (8 inches). That's still not much, but it's sure a lot better, more than twice as much DOF. But it is even more blur at the background.

And the overwhelming advantage is even much better yet: We said this Field of View (FOV) at the subject would be the same in either situation (about 2.9x1.9 feet, about right for head and shoulders). But the background field at 40 feet of the 50 mm lens is 22 feet wide. 22 feet of stuff you want blurred away. However, the field of view of the 200 mm lens is only 8 feet wide at 40 feet (behind the subject). So most of the objectionable junk you want to blur is simply missing, simply gone, removed in the best possible way. And better, you can surely move the camera slightly to one side to choose to align the best 8 feet of background decently enough, probably even if it were not blurred. But in fact, it is blurred at 200 mm. Probably blurred more (perhaps a bit less if background is near), but 1) the subject DOF is so much better, and 2) there is much less of the background even showing. And 3) usually the background that is visible is blurred even more.

So again, it's like this. Two lenses, one longer and standing back at equivalent distance to have the same field of view at the subject. If at the same f/stop, then they have the same depth of field at the subject. Assuming the background is not real close behind subject: The long lens is farther from the background, so the background is still blurred quite well. A very big deal is that long lens also has a much smaller view of that background, so we can move a slight step sideways to choose a nice small spot to show, one which does not distract. And another very big deal, then the longer lens can be stopped down a couple of stops more to improve the subject DOF considerably. This does reduce the background blurriness some, but it's still more blurred than the short lens. What's not to like? The only downside is we need to bring the longer lens, and have room to stand back.

It should be obvious that this is a really big deal to know. There are three properties offered by the longer lens: Greater DOF at subject, a much smaller area of background to select and be seen, and greater blur on the background. There are many numerical combinations where the longer lens is simply better. And a few more where a property or two is still worth consideration. If you also find f/1.8 distasteful, there is this better way.

So the calculator below computes the sensor CoC at both subject and background distances, to make this point.

A Depth of Field calculator
concerned with CoC at the Background

Identify your camera sensor size by entering either actual Sensor Size or Film Size, or Crop Factor or even a final CoC value. Any of those can calculate sensor size. Sensor size can be hard to know, but CoC also determines a sensor size, because CoC is about the standard enlargement of sensor size. You can see how to determine your Crop Factor. It's hard to beat precise actual sensor size specifications though.

Full frame 35 mm cameras often use 0.03 mm for CoC, and APS cameras often use 0.02 mm. A compact camera or smart phone might have CoC = 0.004 to 0.007 mm (much more enlargement is necessary). For 35mm film, Zeiss has recommended CoC = Sensor Diagonal / 1500 (CoC = 0.029 mm), but to get 0.03mm requires Sensor Diagonal / 1442. These are standard definitions, but are somewhat arbitrary too.

Film or Sensor Size dropdown box: I have NOT researched every possible cameras sensor size (virtually none). The film sizes are known good, but the digital sensor sizes are approximations, ballpark, because actuals can instead depend on the specific camera models chip. Especially the compact and phone sizes like 1/1.8" CCD are possibly vague (actual sensor sizes are instead described as specifications of XxY mm). The approximated sensor size used is shown in results. The film sizes are known good, but other than for film, if actual sensor size is not known, I suggest the Crop Factor option may be more accurate.

Aspect Ratio does compute image diagonal and CoC of Crop Factor, and image dimension in pixels, and also Field of View (except input of direct Sensor Size or Film Size will instead use actual sensor shape for Aspect ratio).

Megapixels is unimportant here, only used to compute image size (pixels). It does NOT affect Depth of Field or Field of View. Megapixels is only for you to see and confirm that computed sensor size is about correct, which does affect CoC and FOV. Many camera numbers are approximations, so a size difference of several pixels is probable and not a big deal, if not too far off.

Depth of Field Calculator

  Four ways to specify Sensor Size
Sensor Size WxH x mm
Film or Sensor size
Crop Factor of sensor size
Circle of Confusion mm for sensor
CoC DivisorDiagonal / Default 1442
Distance Units ConvertResult is
same units
Largest print dimension
10 inches is standard DOF
Aspect Ratio Optional, for
Field of View
Megapixels mpOptional, for
image size

Lens A

Lens B

Focal length mm mm
Subject distance
Background distance behind subject
Depth of Field
DOF total span
DOF in front
Background distance
BG CoC: X times CoC
FOV at subject
FOV at background

It will be appreciated if you would please report (Here) any problems with the calculator, or with any aspect of this or any page.

If you see results of NaN, it's an error meaning an input is Not A Number (periods are OK, but don't use commas).

DOF is Depth of Field, CoC is Circle of Confusion, FOV is Field of View, and BG is background.

The next page has photo examples of these initial default cases.

The feet/meters selection needs to know which you are using (results are in same units). When it is changed, the checked Convert checkbox will convert previous numbers to keep the same distances. Otherwise that feet/meters change will leave distance values numerically unchanged (but feet and meters are different distance values).

If Hyperfocal is new to you, you need to know more about it, see below. And to see an example of what it is, try the calculator with say a 18mm lens at f/16, and then set focus distance to the hyperfocal distance indicated.

CoC is Circle of Confusion. It is used two ways.

Blue line is the focus point at S1. Red line is the background at S2. C is the blur circle, c is the reproduced CoC size on the sensor. From Wikipedia.

BG CoC is the computed actual CoC at the BackGround distance. It is shown here as "X times CoC", meaning actual CoC is X times size of that maximum acceptable CoC limit entered. It is a relative scale of blurring there. The DOF concept implies that if the background were exactly at the computed far limit of DOF, the blur diameter there would be exactly equal to CoC (1X CoC). A larger multiple is a multiplied greater blur. Should the background be closer than the far DOF limit, then the multiplier will of course be less than 1.

The point: When wanting to hide the background, in many cases, standing back with a longer lens can provide the same field of view of subject, with much less view width of the background, but which allows stopping down a bit more to provide greater depth of field at at the subject, but still offering greater blur at the smaller background. This can make a significant difference.

CoC Divisor: CoC is usually computed as (sensor diagonal mm / 1442), which is our default unless CoC or Divisor is directly specified. It is a little arbitrary, and if you decide you want it different, you can change it, and get different results. But 1442 is a standard value for a standard 8x10 inch print.

Largest Print Dimension is about the relative enlargement of your viewed image. An example is the "largest dimension" of 8x10 is 10. When we enlarge the viewed image, we enlarge the CoC too, so it's easier to see the blur then, which becomes no longer a suitable indictor. The DOF concept is all about the CoC we can perceive, when enlarged from the sensor size we use. If we are going to enlarge our view, then we need to start with a smaller CoC. Standard DOF calculations assume viewing a standard 8x10 print size from 10 inches, which is the 10 inch default here (254 mm). This feature is to describe a different image size that you may view, to account for the effect of your enlargement on the Depth of Field calculation.

The CoC used is shown in bold, if and when modified by the Largest Print not being the standard 8x10 inches (254 mm largest). Because, CoC is the largest allowable blur, to still not be perceptible by our eye. If we're going to view an image enlarged bigger, then maximum allowable CoC at the sensor has to be smaller (to not exceed what our eye can perceive). Or vice versa.

However, note that you can specify any CoC limit directly. Yes, it will then compute and show a sensor diagonal size based on (CoC x Divisor), but that sensor size is just for reference, and is used for FOV, but is not further used for DOF. It does Not affect DOF now, since CoC has already been specified directly. The DOF formula computes with only CoC, focal length, f/stop, and focus distance. Sensor size is not in the DOF formula, except that it should have of course defined CoC. So, bottom line, you need to know what you're doing if you specify CoC directly. Just because you saw someplace use 0.03 mm CoC, this does NOT mean that is a proper number for your camera and its sensor size.

If comparing results with other calculators numbers, make sure the CoC and Sensor Size used are the same value. The DOF numbers here agree perfectly with DofMaster, Canon Europe, Bob Atkins, PhotoPills when we use CoC Divisor 1442 and 10 inch print to match their fixed 0.03 and 0.02 mm CoC for FX and DX. However one major site does show different results. I don't know why it doesn't agree with anyone, it doesn't say what they're doing, but something seems off there.

My notion is that the CoC 1442 divisor is the standard value. Some want to round it to 1500, as a very slightly tighter limit in sharpness (Wikipedia and others think that now). I used 1500 here at first, however, 1442 still seems clearly the norm on the internet, so I went with the flow. CoC is a little arbitrary, and you can use either above. There's not much difference, being 0.03 or 0.029 mm CoC for full frame 35mm, perhaps a 4% change in DOF calculations. Other factors like focal length and distance seem a larger issue.

Rounding: Note that numerically, real world APS sensors are slightly smaller than 24x16 mm, and their crop factors are actually slightly larger than 1.5 or 1.6. Just for example, the Nikon D5300 DSLR camera manual provides specifications:

Nikon D5300
6000x4000 pixels
24.2 million pixels
1.5 crop factor
23.5 x 15.6 sensor
Photo results of course show the 6000x4000 pixels is correct, which is 6000x4000 = 24.0 megapixels (there are extra pixels involved in calibrations, but not part of the image area). If the sensor is 23.5 mm wide, then the crop factor is 36/23.5 = 1.532 (normally we compare diagonals, but these are same 3:2 aspect ratio). Canon would be a similar situation. If CoC or Crop Factor or megapixels are typical rounded values, they can slightly skew calculated results in a small way, but the concept should still be very clear.

Depth of Field calculators are not accurate for macro situations. Macro calculations are inaccurate because we don't know extended focal length, and maybe not f/stop reduction, and probably not the location of the front nodal point of the lens to know distance. At the close focus point, these are large factors. Accuracy depends on knowing the numbers. Macro instead computes DOF from measured magnification. Macro 1:1 means the object image is the same size on the sensor as the object in real life, true regardless of sensor size. Distances of at least a few feet will be most accurate in any lens calculation.

What is Depth of Field and Circle of Confusion?

Circle of Confusion (CoC) is theoretically zero diameter (a point) at the focus point. But this blur circle grows larger when not in focus, and the DOF range is calculated to not exceed the standard limit (sensor diagonal/1442) of acceptable CoC. CoC (and therefore Depth of Field) definitely also depends on the current enlarged viewing size, which is magnification of the DOF blur. We should know that standard CoC is considered to view as acceptable sharpness in the standard 8x10 inch enlarged print viewed at 10 inches.

Depth of Field (DOF) is certainly not ONLY about aperture. DOF is an extremely important basic of photography, however IMO, exact DOF calculators may not be great practical use, other than to get a rough idea. But we certainly do need to know the concept. We need to know this, it should be second nature to you.

DOF is increased by
DOF is decreased by

** The three lens properties above cause the CoC (blurred diameter) in the sensor image. Then the DOF that we perceive relates to how large we enlarge that CoC to view it. In practice, we do think of cameras with smaller sensors giving greater DOF, which might appear to be the opposite of just said above. And they certainly do that, a little cell phone may not even adjust focus, yet it is in adequate focus about everywhere. But that is only because the field of view of a tiny sensor is drastically cropped (compared to a larger sensor). Therefore it must use a very short lens to achieve the same normal wider view (Crop Factor). That shorter lens certainly does increase DOF drastically. But even if we could use the Same lens (and ignore the crop), then the smaller sensor image still must be enlarged more (to view at same size), which reduces DOF. In the math, a larger sensor computes a larger acceptable CoC limit, which increases DOF.

FWIW, old-timers may remember small Minox or Kodak disk film, or 110 film size, which had quality issues being so small. Compact and phone digital camera sensors have no film grain, but they are half the dimension of Minox or Kodak Disk film, and 1/4 the dimension of 110 film size. DSLR cameras and lenses are larger because they prefer a larger sensor.

Statistical tests have said the average resolution of our eye is to perceive 6 mm of detail at 6 meters distance, called 6/6 vision in Europe, or 20/20 vision in the US ( x 3.28). This scales to other similar ratios, like 0.025 mm at 25 cm is familiar. For DOF in our photos, that was judged as perceiving 0.025 mm of detail on an 8x10 inch print when viewed at 25 cm (ten inches). This size print represents substantial enlargement of the small sensor image, so CoC limits at the sensor must be divided by the enlargement factor (from sensor size). Eyes do vary, but someone established this ballpark number, used for DOF as the limit of acceptable CoC diameter (that blurriness limit that we still call sharp). So this 8x10 print viewed at 10 inches is our standard for calculating the DOF blur that will be created. Today, this judgment is contained in the CoC = Sensor diagonal mm / divisor definition. History has used /1730 and then /1000 and /1442 and /1500, and likely others. The number is hard to verify results. The DOF formula details the geometry involved in the lens, and one factor is the CoC value which is determined by sensor size, for the purpose to scale it to what our eye can perceive at this standard 8x10 enlargement. The CoC number is defined as sensor diagonal / divisor (often 1442), but which is the CoC diameter as perceived in the standard 8x10 enlargement. It will vary in other enlargement scales.

The Depth of Field is the computed distance zone around the focus point, the span where the CoC remains less than our arbitrary limit for the size of CoC, considered to be in focus. The image is only focused at one distance, and gradually degrades away from that point. Focus just outside the DOF calculation will be hardly different than the focus just inside the DOF calculation. For example, maybe the DOF limit computes 20 feet. But then you probably cannot detect much difference a couple of feet either side of 20 feet, but the exact focus point will be better. DOF is NOT at all magic numbers, it's just where the math precisely computes the CoC size crossed an arbitrary threshold size boundary. The boundary is very vague to our eyes. Sharpest focus is at the one distance where we actually focus. Depth of Field is a vague concept.

If we can see these blurred blobs in the results, that's normally considered bad, when that distance is not in focus well enough. If only slightly out of focus, it may not enough for us to even notice it, much less object about it. Which is good, and while standards vary, DOF is a way to judge it. The exact calculated numbers are rarely important (just a simple guide). But the zone of DOF we perceive is certainly really important, and the big thing to know is what the controls are, and to know how to adjust it. With a little experience, we know what to expect, and this works pretty well.

The name Circle of Confusion is from another era, and Wikipedia quotes work in 1829 and 1832 calculating Circle of Confusion. They had microscope and telescope and binocular lenses then, but this was before cameras or film. Still same concept, but maybe if invented today, we might pick a simpler name for CoC (it is the diameter of the blurred circle of an out of focus point source). Camera sensor size is a factor of enlargement. Older work used CoC = (sensor diagonal / 1730), or 0.025 mm for 35 mm film. Today, we often use the computation (sensor diagonal mm / 1442) for acceptable maximum CoC in the final standard print size. These are often rounded numbers, or CoC = 0.03 mm for full frame 35 mm sensors, and CoC = 0.02 mm for smaller APS sensors (because the smaller sensor requires half again greater enlargement).

CoC is arbitrary, and professional level might prefer it smaller, with larger safety factor. Our CoC number choice does not affect the image in any way, it only affects how we might judge it, or plan it. It is an arbitrary notion about when out of focus is judged to become too noticeable. And DOF very definitely also depends on how large you enlarge the image to view it.

What makes DOF even more arbitrary is that the larger we enlarge and view the image, the more noticeable becomes the blur blob of CoC. View it too small, and we may not even notice it. The standard of viewing DOF is considered to be an 8x10 inch print viewed from 10 inches. That's about a 9x enlargement of 35 mm film (CoC 0.03 mm), and so the CoC we see then is the 0.03 mm x 9 = 0.27 mm in this print. We enlarge our smaller digital sensors even more to see 8x10, so allowable CoC has to be smaller. Every sensor size has a different CoC (from sensor diagonal mm / divisor) - because we assume to enlarge each to the standard 8x10 inch print to judge it. DOF is a different number after enlargement, BUT the standard maximum CoC value was chosen to be acceptable when viewing this standard print size. Today, we view first on the computer screen, or even a cell phone. But we view different sizes, and this also affects the acceptable CoC goal. There is NOT just one number for DOF of a situation.

But, you can use the Largest Print Size parameter above to describe a print size (specify the largest dimension, like 20 for a 16x20 print), and it will calculate DOF based on that instead of the standard 8x10 inch print.

Hyperfocal Distance

The Hyperfocal distance is a special idea of DOF. It is sometimes used for landscape photography with wide angle short lenses, when we want an extreme DOF range, extending to infinity, and also back to a rather near foreground subject. For example, an APS-C sensor (1.5x crop factor) with 24 mm lens at f/16 computes Hyperfocal distance as 6 feet. What that means is this:

The crude distance marking today on our lenses make it hard to set a specific focus distance, but we can approximate it, normally close enough.

These are basic ideas which have been known for maybe 150 years. The alternative of simply focusing on the near side of the subject zone typically wastes much of the depth of field range in the empty space out in front of the focus point, where there may be nothing of interest. Focusing more into the depth centers and maximizes the DOF range, generally more useful. We hear it said about moderate distance scenes (not including infinity) that focusing at a point 1/3 of the way into the depth range works for this, which is often near true, maybe a little crude, better than knowing nothing, but some situations do vary from that 1/3 depth (below). Close and macro focus situations are closer to the middle at 1/2 way in, and don't include infinity.

A good Depth of Field calculator will show hyperfocal focus distance. It does include infinity for various situations (focal length, aperture, sensor size).

Many prime lenses have a DOF calculator built into them. Speaking of prime lenses (i.e. those lenses that are not zoom lenses) which normally have marks at the distance scale showing the depth of field range at the critical aperture f/stops. However, this tremendous feature is becoming a lost art today. Zoom lenses cannot mark this for their many focal lengths. Also todays faster AF-S focusing rates can put the marks pretty close together (the 85 mm is shown from an ad, but it still gives a DOF clue). (the "dots" are the focus mark correction for infrared).

For example of hyperfocal distance, at right is a 50 mm FX lens, with focus adjusted to place the f/22 DOF mark at the middle of the infinity mark, which then actually focuses at about 12 feet, and the other f/22 DOF mark predicts depth of field from about six feet to infinity (assuming we do stop down to f/22). This places the focus at about 12 feet. The DOF calculator says this example (FX, 50 mm, f/22, 12.3 feet) DOF range is 6.1 feet to infinity.

Or another case, one not including infinity. If we instead focus this lens at 7 feet, then the FX f/11 marks suggest DOF from about 5.5 to 10 feet (at f/11, which is about 1/3 back in this case). The idea of the markings (which appear on prime lenses, zooms are too complex to mark) is to indicate the extents of the DOF range. And done because it can be very helpful. Sometimes f/22 is the best idea, sometimes it is not. f/22 causes a little more diffraction, but it can also cause a lot more depth of field. And of course, the lens markings apply to the expected sensor size for that lens.

Those DOF end point extremes will of course Not be as sharply focused as the actual focus point, but they will still satisfy the standard CoC specified. Do realize that DOF just means barely tolerable limits, where the CoC has grown to the maximum limit specified. Focus is always of course sharpest at the exact focused distance. Focus is not necessarily perfect if inside DOF, instead it is assumed unacceptable if outside DOF, but there is no sharp dividing line. If you want really sharp images, include ample safety factor for DOF; pay attention to enlargement size, stopping down at least one more f/stop, and if really important, focus on the important spot that needs to be sharp.

Your DOF calculations may not exactly be realized particularly close in practice, due to your own degree of enlargement, and your viewing distance, and your own eyes, or an inaccurately specified sensor size, and how accurately you guess the actual distances. It is just a large ballpark. You'll have to decide for yourself if your images are as sharp as you want, and then you need to know the factors to increase DOF.

A couple of tricks are to plan on having sufficient DOF with ample safety factor, and then learn to center that DOF around your subject depth. If DOF is limited, don't focus on the nose if you want the ears sharp too. Repeat this to yourself: Focusing on the closest point wastes the half of the DOF range in front of that point (where there is nothing). Instead, you can plan to better center the DOF zone around your subject.

To do that centering, we hear about the simple (rough) guide of focusing 1/3 of the way into the scene depth (1/3 of scene in front of focus point, and 2/3 behind). If we think that 1/3 of the DOF range is in front of subject, then it makes sense to focus 1/3 into the scene, instead of at front point, and instead of half way back. There is no good argument for the front point, and half way is only true if up pretty close, near minimum focus distance. So 1/3 may not be exact, but often better. That focus point may not be where the subject is, and of course that subject will always be sharpest if you actually focus on it (so there are trade offs). But 1/3 can sometimes be true enough, often not greatly wrong, and can be a better rough guide than knowing nothing.

Regardless, hyperfocal becomes interesting:

Situations will vary, and the DOF in front of focus might be from 0% to 50% (at extremes). Otherwise, 1/3 is not the worst guess (we are not actually measuring distances anyway). Generally, short lenses have closer hyperfocal, and stopping down any lens brings hyperfocal back closer to us (and brings a short lens back very near). That's a lot to know. Frankly, in practice, we never know what hyperfocal number is, so we just soon learn the general idea of what we need to do when DOF is important. Stopping down some, and focusing somewhat into the scene depth can usually help considerably. Just standing closer with a shorter lens can help DOF, and as discussed here, standing back with a longer lens can reduce DOF range (specifically, will be same DOF at the subject with same f/stop, but greatly different at the background).

For portraits at around 8 or 10 feet, I think a good tip is to focus on the near eye, after ensuring ample DOF, like f/8. IMO, f/1.8 is never the best try, and this article is about an alternative. For full frame portraits, I like about 120 mm at around 10 feet. For DX or APS crop cameras, that would be about 80 mm around 10 feet. Ten feet is very good portrait perspective, and at f/8, that's about a 2x3 foot FOV with around a one foot zone of DOF (again, of course speaking about the standard 8x10 inch print viewed at 10 inches).

Qualifications about Viewing Depth of Field

Depth of Field is NOT an exact number. Depth of Field is computed based on the Circle of Confusion (CoC), which is the arbitrary criteria defining the maximum acceptable blur circle (to be small, not quite perceptible) due to being out of focus. CoC is the diameter of the smallest possible theoretical point after it is defocused to be seen as a larger blur circle (because it focuses in front of, or behind, the sensor plane - then causing a larger out of focus circle on that plane). CoC is the maximum permissible diameter of this blur circle, arbitrarily still judged to be imperceptible in our vision (also assuming a standard viewing enlargement). If the blurred circle is too small for us to perceive it, then we imagine it's not blurred.

Carl Friedrich Gauss 1777-1855 was a most brilliant mathematician (in a class with Newton) who did many amazing great things, one of which was to formulate optical theory (1840) that is still used today. Gauss thought the eye's criteria of visibility of focus blur ought to be a CoC of (frame diagonal divided by 1730, in mm), which computes 0.025 mm today for 35mm film size. But today, CoC of diagonal divided by 1442 is a common universal value (0.03 mm for 35mm film). The sensor diagonal is involved because it is a factor of the final print enlargement required, where we see and judge the perception of the enlarged CoC. Enlargement is a big factor of perception. But it's still an arbitrary guess about blur, about what our eyes see after enlargement. Blur diameter cannot be precisely defined... kinda depends. And so a CoC limit is somewhat arbitrary, there's been a few choices. CoC is just a rough guess attempting to measure focus blur, which makes DOF numbers be a vague thing.

The DOF Standard of Viewing is in an enlargement of an 8x10 inch print (near A4 size) when viewed at a distance of 10 inches (25 cm). You should know that DOF calculators use a CoC which assumes this standard enlargement, regardless if you assume it or not. If you view it up close on a large HDTV screen, DOF will appear much less than you calculated. If you view it on a smaller wallet or 4x5 inch photo, DOF should appear better than you calculated.

Viewing the enlargement size is an important factor in what we see, and in CoC and the Depth of Field calculations. This viewing enlargement factor makes small sensor diagonal be an important factor of DOF. It's the reason smaller sensors have a smaller CoC, and larger sensors have a larger CoC (sensor size requires enlargement of CoC). But standard DOF calculations assume a standard 8x10 inch print is viewed. So this affects viewing a smaller print or a larger print:

Computing on the diagonal attempts to equalize for different sensor or print shapes, but many vague assumptions are included. You should include a safety factor, especially for large prints, one extra f/stop for safety.

Depth of Field is an angular size concept, and the math is very precise, EXCEPT for the main factor of CoC, which is rather vague and arbitrary. So there are no hard answers about Depth of Field. And since Depth of Field GRADUALLY changes with distance, there is of course no sharp line at the computed limit. There will be virtually no difference seen slightly either side of the computed limit. Numerical Depth of Field is at very best, an extremely rough guide.

Depth of Field is a fundamentally important principle of photography. However using it is MUCH LESS ABOUT any computed numbers, and VERY MUCH MORE ABOUT understanding how to use the factors that increase or decrease it (above, f/stop, distance, focal length, and sensor size). Normal situations are not much concern, but sometimes we're aware we want a lot of depth of field, or don't want much of it, and we should know how to control that, to do what we can.

Continued - Part Two, Examples

Copyright © 2016-2017 by Wayne Fulton - All rights are reserved.

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