Lenses are focused at only the ONE distance (determined by the current lens focus ring rotation), but which we perceive as a zone around the focus distance in which we don't notice any blur yet. A zone of "Good Enough", so to speak. But enlargement for viewing is a factor making it easier to see that blur. Users may not always realize it, but Depth of Field is computed based on assuming a conventional viewing enlargement size of an 8x10 inch print viewed at 10 inches, which you should realize might not always be your own situation. A small image will always look sharper than if enlarged more. The DoF that we can see depends on the enlargement of the sensor image, because greater enlargement magnifies the blur that we can perceive (and perceiving it is what DoF is about). The next Page 2 here covers that.

CoC (Circle Of Confusion) is the blurred area of a tiny out-of-focus area. The CoC limit considered acceptable is the sensor diagonal divided by a constant. There are different CoC divisors seen, like 1442, 1500, or even the old obsolete 1720 value. Here you can input whatever divisor you want to use, but otherwise the default here is 1442. As film resolution improved, Gauss updated long ago to use 1500. However the popular 0.3 and 0.2 mm CoC values are seen with Japanese cameras (Nikon, Canon, Sony, Olympus, etc, etc). That possibly is just rounding, but if you want to compute 0.03, then you must use 1442. 1500 will compute 0.29 mm CoC (for 35 mm film size). The vague area of Blur is very difficult to determine its measurement, and this divisor choice was just someone's opinion of what seems best, and is not a scientific fact. There is no precisely correct value. And of course, your opinion depends on your own eye too.

There are two concepts of CoC, the amount of blur that exists, but the one we hear about is the maximum allowable limit that we try to achieve.

Depth of Field does NOT try to measure the blur extent. It only calculates to a Limit of blur thought to then be detectable and objectionable in enlargements viewed by the human eye. So what Depth of Field can calculate is the diameter of an allowable blurred point source (a point of diameter zero) that does not exceed what a typical human eye can see when enlarged for viewing (the Depth of Field convention is in an 8x10 inch print). CoC (Circle of Confusion) is the calculated size of the out-of-focus fuzzy area of a calculated hypothetical zero diameter point, but it is Not a size of any object or any area you can measure. It is NOT the size of the blur, but is instead only calculated up to the CoC limit where you can see it (and it gets worse past there). What you can see is outside the CoC Limit. The numerical CoC diameter is the sensor diagonal divided by a constant, like 1442 or whichever. There is a CoC diagram above that explains what is calculated. DoF is computed so that any size sensor will produce the same CoC limit in the conventional enlargement to a standard 8x10 print. For example, the conventional CoC Limit in a 35 mm film frame image is 0.03 mm. That limit is enlarged in an 8x10 inch print to be 0.25 mm (8.3x enlargement). Smaller sensors have smaller CoC (and greater enlargement) and larger sensors have a larger CoC (and less enlargement). Smaller viewing size has larger CoC (better DoF) and larger has smaller. You could say it is technically about magnification of the enlargement to a standard viewing size.

So the point is that all sensor sizes produce the same CoC limit at viewing size. That's something, but the CoC divisor is not technically known. CoC is NOT a measurement of the blur. The distinction is intended to only be what appears acceptably sharp, and what does not. The exact focused distance is all that appears to be quite sharp. Any other distance is not at the focused point. The CoC limit is where that should appear unacceptable.

The blue object represents the camera lens, and the dark black vertical line at far right is the camera sensor where the scene is focused.

A “point” is the theoretical zero size of a hypothetical point when perfectly focused.
The Blue line is the actual focus point at S1, which is zero diameter at S1.
The Red line is the out-of-focus distance of a “point” at S2.
C is the larger blur circle of the zero size point at S2.
c is the reproduced CoC size on the sensor.
CoC is NOT the size of any scene object, it is about every computed out-of-focus hypothetical spot of original zero size.
DoF computes the near and far distance limits where c does not exceed the maximum permissible CoC specified for the sensor size, which is judged to be Not perceptible by the eye after a standard 8×10 inch enlargement at the standard viewing distance of 25 cm (10 inches). More detail at Wikipedia.

The concept of CoC  and computed Depth of Field: The computed blurred diameter of a "point" that is out-of-focus in the camera lens is called CoC (Circle of Confusion). A "point" is the theoretical size of a hypothetical speck of size zero when perfectly focused, which math can compute, but any spot computes larger when blurred. However the CoC number that is entered into DoF calculations is chosen as a specific Maximum Allowable Limit of the diameter of a blurred point (computed from sensor size enlarged to a conventional 8x10 inch print), in preparation for what the eye can see in future enlargement. Depth of Field simply computes the distance at which the blur of a out-of-focus point matches this CoC limit, conceptually planned to be where any blur becomes just perceivable by eye (in a standard 8x10 inch enlargement). The blur is gradual with distance, but the math says the tiniest bit inside the limit is sharp, and the tiniest bit outside is out-of-focus. The change is much more vague though. See the CoC diagram here (and the next page continues too). Misfocus increases blurred size very gradually, so there is NO precise or visible border line of sharpness. But still, the DoF formula computes the distance as if it were precise, ignoring our input data (sensor size and focal length are rounded, and subject distance is usually a crude guess, and even if our eye can't detect any difference just barely on either side of that limit.

Just saying, do realize that in the math of Depth of Field calculations, the blur of a "point" at the end of the computed DoF range has reached the full size of this Maximum Allowable CoC limit, but is still reported as being in DoF range and acceptably sharp. And then, just the slightest greater distance is reported as exceeding the CoC limit to be unacceptable, because it is computed to have passed the threshold of perception by the human eye (in the standard 8x10 inch print enlargement). DoF simply reports the distance where the degree of misfocus crosses that CoC limit line, even if such line of difference is pretty vague to our eye. Meaning, sharpness wasn't that good just before reaching the limit, and not that bad just after. Near that line, the two sides are really the same. Still, DoF is a good guide of what to expect in terms of focus zone sharpness. We are usually just guessing distances anyway.

The point of the computed CoC is that regardless if using a large or small sensor, the resulting enlargement to the standard 8x10 inch print will always have the same enlarged CoC limit comparable to what the human eye can detect. We rarely use a DoF chart, but the concept is very important and useful. Our experience should realize when we need more Depth of Field range, or maybe are trying for background blurring. Short lenses have more Depth of Field, and stopping the aperture down more always helps.

In any given camera, Depth of Field is determined by the combination of three lens factors, and IF with all else the same, then:  

• Greater f/stop number is more depth of field span (f/1.8 has very little DoF)
• Greater focus distance is more depth of field span (macro distance has nearly no DoF)
• Greater focal length is less depth of field span (18 mm has much more than 100 mm)

Depth of field Span is total DoF range, the sum of DoF range in front of focus, and DoF range behind focus.

These three lens properties are in the lens image projected onto the sensor, and we adjust those in our camera to control depth of field. However, there are four factors computing Depth of Field, perhaps not always understood.

** Small sensor size will appear to do the opposite. If using the same lens focal length, a larger sensor does increase DoF (due to the viewing size needing less enlargement), however a smaller sensor must use a shorter focal length lens (to capture the customary size Field of View on a smaller sensor) which then does dramatically increase DoF more.  So small sensors then do see greater DoF, only because of the very short wide angle focal length necessary (focal length is squared in the formulas, much more significant). Cell phones are so tiny and must use a wide angle lens so short that their camera doesn't even need to provide focusing.

Additional features in this DoF calculator (seen below) that you may not see elsewhere are:

• In addition to regular Depth of Field, it also computes CoC at the Background distance behind the subject (or in front of subject), to indicate the amount of blurring there, a concern when wanting to blur and hide the background. CoC is a numerical way to compare and judge the relative degree of blurring at the background, which describes the diameter of a hypothetical mathematical “point” of zero size, but out-of-focus so its blur is computed larger. CoC is Not the size of any actual blurred object, which would be larger.
• In addition, the calculator also shows Field of View dimensions, both at subject and at background (or a distance in front of subject), indicating how much of that background your focal length choice includes. An important way to hide the background is to show less of it, accomplished by standing back with a longer lens. Step slightly sideways to choose the part you want to show.

  Depth of Field uses the sensor diagonal dimension (which normally is the lens image diameter). Depth of Field does not depend on frame shape, the image size is the diagonal.

  Field of View uses the same diagonal, however it computes the shape of rectangular frames that fit on that diagonal. That causes different frame dimensions to describe Field of View with different aspect ratios. So changing dimensions for your output situation is provided, to match the printed paper aspect, or for the HDTV screen format, or for the background size needed for portrait setups. Also for cameras offering different crop factors, however that depends on sensor design. The 1x and 1.5/1.5 crop sensors surly will have sensor dimensions in the manual, so use that. If those are not concerns, just specify to use the actual Sensor Aspect Ratio (the default).

• The calculator compares numerical situations comparing Depth of Field of two lenses. Specifically, instead of reaching for a 50 mm f/1.8 lens to blur the background, this article suggests a better way with better results, by standing back with a longer lens (providing greater Depth of Field at subject, but also greater blurring at a distant background). Not only can the outdoor background be blurred better, but a sideways step or two with the longer lens only shows the best small selected part of the background that is seen. And proper portrait perspective is also assured by the longer lens.

  This "standing back with a longer lens technique" is nothing new, it has always been well known to pros. Using f/1.8 on photo work they hope to sell seems not their best choice if there is any better way (and it's very easy to imagine there is). For issues with exposure in insufficient light, higher ISO can be a good choice in digital today, when necessary (and light can easily be added with flash). Consideration of a good method for blurring the background in an even better photo is the point of this article. This calculator offers a comparison of two lens choices, allowing choices about how to blur the background.

• A chart of Hyperfocal distances is also included, for the various focal lengths and f/stops and sensor sizes. Hyperfocal is a very handy part of depth of field computation, specifically about Depth of Field reaching to infinity, and also reaching back to very near objects. The one page chart for your camera’s sensor size could be handy in your camera bag, for such situations.
• Depth of Field calculators are generally all alike, only offering Depth of Field calculations for the one native sensor size format. That was enough in film days, but digital cameras often offer multiple formats, like 16:9 for video. If your camera offers other formats like 16:9 video, or other common alternate formats, this one can compute Field of View for that too.
• Precise sensor size can be hard to determine, but the calculator offers a few ways to easily specify or compute your sensor size, which determines Circle of Confusion, which is the basis of Depth of Field calculations. Entering actual precise sensor size is the best plan if known, but otherwise, entering accurate Crop Factor can work well too.

  The lens image resolution is Not affected by sensor size (other than how we might enlarge it for viewing), but the Depth of Field formula is computed from CoC, which is computed from sensor size. The enlargement of that CoC to standard viewing size is what our eye perceives, and is what calculated Depth of Field is all about. Smaller sensors must be enlarged more, so their CoC is corresponding smaller, and then Depth of Field is computed for a standard 8x10 inch print viewing size.

• The standard convention for CoC in a Depth of Field calculator is to compute for a standard viewing size of an 8x10 inch enlargement (203x254 mm). Here too, but this calculator also has a viewing enlargement factor, called Viewing Size Dimension, to relate Depth of Field to the actual viewing size of your different size image.
• Allows changing CoC Divisor if desired to compute Depth of Field ranges more or less critical than standard CoC.
• Calculators should use the exact precise f/stop values instead of the approximated nominal marked numbers (e.g., use f/11.3137 instead of f/11). This one does, and hopefully most do.

A Depth of Field calculator

also concerned with Blurring the Background

Identify your camera sensor size by entering either actual Sensor Size or Film Size, or Crop Factor, or even Equivalent Focal Length specs. Sensor size can be hard to know, but the subject is about sensor size. More arbitrary and less precise, but even Crop values can determine sensor size. You can see ways to determine your Crop Factor (perhaps even from known Equivalent Focal Length). But it's hard to beat precise actual accurate sensor size specifications though.
But in the calculations, always specify the actual real focal length, that you actually use, and Never any Equivalent Focal Length.

Film or Sensor Size dropdown box in Option 5: The film sizes are known good, but the "1/xx inches digital sensor size" system for compact and cell phone cameras is at best an approximation, because actual size instead depends on the specific camera models chip. Especially the compact and phone sizes like 1/1.8" CCD are vague (actual sensor sizes are instead described as specifications of width and height in mm, and those usual sizes are substituted here). If actual sensor size is not known, I suggest the Crop Factor option may be more accurately known. Crop Factor also needs Aspect Ratio to compute sensor dimensions. Those are rounded values, but still reasonably precise for DoF. The computed sensor size is shown in results.

Fisheye lenses or macro distances are special cases greatly affecting accuracy, and are NOT included here.

Abbreviations: DoF is Depth of Field, CoC is Circle of Confusion, and FoV is Field of View.

Subject and Background distance can be your choice of feet or meters, and results are in the same units.

Entering changes:   Most buttons will redraw results automatically. But after changing a text field, it is necessary to either click the Compute button or hit Enter in the text field. The Yellow box shows the final sensor size result seen.

NOTE: The initial default 50 mm f/2.8 at 6 feet and the 200 mm f/2.8 at 24 feet both correctly have the same result.  X times longer focal length at X times distance IS THE SAME Field of View. And IF also the same aperture, then also the SAME Depth of Field too (within the precision of the input numbers).   More details toward page bottom.

Example: In the chart, if with the DoF calculators initial default 23.5x15.6 mm sensor choice (Nikon DX, APS-C, 1.5x crop factor), an 18 mm lens set to f/22 and focused at this hyperfocal at 2.46 feet, will have a Depth of Field span from half at 1.23 feet to infinity (sharpest focus is at the focused point). That's an extremely large span of DoF, and the hyperfocal chart is how you can achieve such results. Including an interesting near object at only a few feet can create a dramatic landscape. Yes, the diffraction at f/22 is probably a slight degrade, but in comparison, the DoF increase can be overwhelmingly awesome. You'll have to try it to see this, so you can decide which is important (DoF normally always easily wins).

Caution: As impressive as that may sound, and while hyperfocal is a strong and often very useful concept, it may not always be the best choice that it might seem. Hyperfocal calculates the maximum Depth of Field limits (normal DoF spans), determined by the Maximum Acceptable CoC, or the maximum blur at both ends of the DoF span. The sharpest point is always the actual focus distance.

So with this same 18 mm lens example at f/22 (on your crop 1.53x APS-C sensor), hyperfocal comes out as 2.46 feet. Then focusing at 2.46 feet will reach back to 1.23 feet, but which is not the same as focusing at 1.23 feet nor at infinity. Still perfect if that's your goal, but those extremes may only be fair results. DoF extremes are not maximally sharp (that’s where the blur reaches the maximally acceptable CoC limits), but the minimal blur there is still considered acceptably sharp, usually, if not too critical. Which distance is most important to your picture?

So if in this case, if you don't really need as close as 1.23 feet, then for example, maybe focusing this landscape at f/22 at 100 feet instead of 2.46 feet still reaches back fairly far. The DoF calculator then shows the DoF span of this lens to be 2.35 feet to infinity then, only a foot less but not great difference up very close, but which can improve the results at infinity. Computing background at 99999 feet (which is 19 miles), the blur at infinity is only 0.024x CoC (1/40th of the acceptable 1x CoC blur limit at infinity if focused at 2.46 feet), and is improved at 100 feet too. If you do focus at any point beyond the hyperfocal distance, the DoF span will always reach infinity easier. So use your head a little, as there are choices, and cautions, but a page of hyperfocal chart for your sensor size can be very useful.

A Better Way to Blur the Background
Maximizing Background Blur Without Suffering f/1.8

Two of the Depth of Field (DoF) factors are focal length and subject distance. We can use them both for the goal (of bypassing 50 mm f/1.8 issues). My notion of a portrait at f/1.8 is that it will have extremely limited DoF, and also optical aberrations are especially bad in the sensor corners at f/1.8. IMO, f/1.8 is usually about the worst choice to make the best picture, and is the last thing I want if I can prevent it. (Do notice that most 50mm f/1.8 lenses are inexpensive, around $100 to $200, but there are good versions for about 10X more). The 50 mm lens at f/1.8 has almost no DoF span, noticeable vignetting, and noticeable optical defects unless you spend much more on it. This is a well known subject, and if you might be unaware, here's a good look at this subject of wide lens aberrations. Such wide apertures are simply not the optical best. Formal portrait studios choose to work at maybe f/8 or more (because their goal is that the picture will sell well). We do like the sharpness of depth of field, and we can choose to work a better way. Pros know the advantages of a longer lens for this purpose (including minimizing the background size to hide it in an outdoor portrait).

Summary Chart of Numeric Examples

For the same Field of View use the same sensor size and Equivalent distance for various focal lengths. Then also for the same Depth of Field too, use the same aperture.

If the goal is to blur the background, this chart should clarify the concepts and show the evidence of a better way. The chart selects equivalent distances for the focal lengths (increasing as above concept) to create the same portrait 2x3 foot Field of View (FoV) at each subject, rotated to vertical. Each crop here is compared as 3:2 aspect ratio. So to retain the same 2x3 foot view, the 2x crop which is normally 4:3 is also shown as 3:2 (with same diagonal). The focal lengths for the two smallest sensors (largest crop factors) are divided by 2 as being more suitable for their small size. If interested in infinity, entering 99999 feet is about 19 miles. The main DoF calculator above can show all these same values for any two focal lengths, for any sensor or other subject distances or in meter units.

This calculator uses CoC divisor of 1442 and standard viewing size of 8x10 inches as the defaults unless you change them.

The term 32x CoC means that the blur diameter of an “infinitesimal point” at the background is 32 times larger than the maximum limit CoC diameter that is used to determine the maximum acceptable extents of the DoF range (where blur is 1x CoC). This is Not the size of a blurred object, but the blur of a tiniest point on it. The DoF calculator above will also show this diameter in pixels.

If the background is closer than about 15 feet from subject (speaking of DSLR size sensors), the 50 mm f/1.8 lens may blur the background as well as the longer lenses, but the longer lenses will have superior depth of field at the subject. That's a Big Deal. Farther than about 15 feet, and the longer lens wins in every way (including even a smaller Field of View of the more blurred background, which removes most of it). It is certainly something to think about.

If the bottom checkbox is Checked, all lenses will use the same aperture for comparison. Then when the same f/stop, then DoF at all focal lengths each at equivalent distances will be (very closely) the same DoF span at the subject, but background blur still increases with longer focal length (if the background is not too close). The longer lens can also stop down more for more DoF improvement at the subject.

We often tend to routinely focus on the nearest point on the front side of a subject, but then only about half of the DoF span is Behind the point of focus, so the other half is mostly wasted for portraits (in front where often there isn't anything but air). It's something to think about. Focusing on the far eye is not a bad plan for portraits, hopefully to slightly improve centering the DoF span. But the obvious point is, when another couple of inches of DoF is so critical, the longer lens standing back is a very advantageous better method, also allowing stopping down more to increase subject sharpness, but still blurring the background about as much, if background is sufficiently distant. Eliminating the f/1.8 problem is a big plus in several ways.

A 50 mm lens is too short for proper perspective on a close portrait anyway, certainly if on full frame (1x crop factor). You could better choose 100 mm f/2.8, which still offers all of the several advantages over 50 mm f/1.8. And 200 mm can work great too (speaking of a DSLR size sensor). Regardless of sensor size, we should always stand back a bit for better portrait perspective. It should be obvious that this is a really big deal to know. For portraits, there are advantages offered by standing back with the longer lens.

• Standing back a little certainly helps portrait perspective, always better than "standing too close". Zoom in all you want for the view you want, but just stand back a bit (best if not closer than 2 meters or 7 feet for a portrait is good advice as a Minimum with any lens). And it can certainly be farther, and you can zoom back in all you want. Zooming in does not affect perspective, but standing back a bit certainly helps it.
• Blurring objectionable background outdoors needs it to be fairly far behind subject, like maybe at least about 20 feet behind (or more) instead of only a few feet.

  Limits on situation details will vary (background can be too close behind, not separable), but usually, standing back with the longer lens gives better results, when at the same Field of View at subject if both lens use the same f/stop, But most longer lenses don't offer f/1.8. And the longer lens standing back more and using its larger maximum f/stop number provides even greater Depth of Field for a sharper subject, with still greater background blurring than the f/1.8, which can be a big plus over f/1.8.

  Yes, the longer lens with its larger maximum f/stop number does also sharpen both the subject and the background DoF too, but it still retains more blur on that background (at least as much, and very likely significantly more) than the shorter lens can do (typical lenses, both wide open, assuming sufficient background distance, more than 20 feet behind subject). Both calculators here will show this.

• Possibly most obvious of all, standing back with longer lens includes a vastly smaller area of background, so if objectionable, then simply moving a step or two sideways can select only the best small part to be seen. Unless it's a scenic landscape that you wouldn't consider blurring, that can be a plus if desired to hide distractions (but don't shoot the scenic landscape wide open). This reduction of background area can be an extremely dramatic improvement (see Part 3) for a sample of a quick try).
• It will be a better picture. The 50 mm f/1.8 is about the worst try for portraits. Experiment by trying this once, to see the result.

In this Summary chart, we said this Field of View (FoV) at the subject would be the same in any of the situations (arbitrarily chosen to be 3x2 feet, which oriented vertical would be just about right for head and shoulders). We simply ignored that 50 mm on full frame would be too close when at 3x2 feet (but you certainly should not ignore it). But the background field at 40 feet of the 50 mm lens is over 21x32 feet size. 21 feet of stuff you want blurred away. However, the Field of View of the 200 mm lens is only 6.8 feet wide at 40 feet (behind the subject). So most of the objectionable stuff you want to blur is simply missing, simply gone, removed in the best possible way. And better, you can surely simply move the camera a slight step or two to one side to choose to align the best (least objectionable) 6.8 feet of background decently enough, probably even if it were not blurred. But in fact, it is also more blurred at 200 mm. The 200 mm f/4 is probably blurred more than 50 mm f/1.8 (depending on adequate background distance), so the 200 f/4 subject DoF span is more than twice larger, and there is much less of the background even showing. And depending on distance, usually the smaller background that is visible is blurred even more. If this is the goal, then consider using the best tool. Also don't forget about proper portrait perspective.

What's not to like? 100 mm can do most of this too, but speaking of 200 mm (and DSLR class sensors), more than twice as much DoF range at the subject (than 50 mm), yet with greater blur on the background, and only about 1/3 of that background width even showing, which all seem like a big pluses. The only downside is we need the longer lens, and to have room to stand back. Flash power of tiny internal flashes would be an issue at extremes.

There are many numerical combinations where the longer lens is simply better in a few ways. And even with a close background, there's still a property or two worth consideration. If you also find f/1.8 distasteful, there is this better way.

Perspective is NOT at all about the Lens.
It's Only about Where you Stand to Use it

Perspective is Only about the View that the camera Sees from Where It Stands. Zooming in does not affect perspective, but the camera standing nearer certainly does. Perspective is NOT about depth of field or sharpness, but is only about relative Size and relative Position of objects in the view, due to their camera distance.

Perspective is a very strong portrait consideration. A common perspective problem for beginner portraits is from standing too close to the subject. It's certainly a problem with selfies held at arms length. The camera being too close can ruin a portrait.

The Best Perspective Rule for Portraits

The camera should be back at least about 6.5 feet or 2 meters (and a bit more is certainly better insurance), and then use any lens focal length that gives the view you want, by zooming in as desired. This is true for tight head shots, head & shoulders, waist length, full length standing, or groups. It's about the proper perspective for human faces.

For a full frame 1x crop camera at 10 feet, that's about 120 mm zoom for a 2x3 foot view, and flash at f/8 gives near a 12 inch Depth of Field depth, front to back (which is a second reason for the distance).
For a 1.5x or 1.6x crop camera, that's about a 75mm zoom, and is a bit more DoF.
The 3x2 foot field is maybe 58 mm zoom for a 2x crop Four/Thirds camera.
Or 44mm zoom for a 2.73x crop One Inch camera, and 16 inches DoF range.

If you've been in a commercial portrait studio, you probably remember the camera placed well back from the subject, and perspective is the reason. The portrait camera should stand well back and zoom in as desired.

From the same location, changing the lens focal length cannot change the perspective. Any lens can only show whatever perspective that can be seen from standing where it is. However, yes, the lens focal length certainly does influence where you would choose to stand to use it, and then location distance does affect perspective.

The focal length does affect magnification, and thus framing/cropping, but those "same portrait with different lenses" examples seen always fail to mention that the perspective result is only because the camera distances were adjusted to keep the same Field of View for the different focal lengths. The camera distance is the important factor of perspective (and the camera view angle is of course becomes important too.) Meaning, back up a bit, don't stand too close. Many longer lens work fine, simply use whatever zoom focal length that can show the view you want to see from standing where you should choose to use it. Do choose distance wisely, and stand back a bit. Zoom in all you want, which does not affect perspective, but do stand back a bit, which does improve portrait perspective.

Perspective: In photography, perspective is the depth and spatial relationship of objects, i.e., the perceived size and spacing appearance of near vs. far objects. Simply the way it looks from where you are standing. Perspective of both subject and background objects depends Only on the distance where you stand, because any lens can only see whatever view is seen when standing there. The lens might zoom and enlarge the image, but it cannot otherwise change the actual view that you see when standing there, with lens or not. The longer lens has advantages (crops an enlarged view), desirable for portraits, to force us to always stand back a bit for proper perspective, a Minimum distance of at least 6 or 7 feet (2 meters) for better perspective in portraits. And longer and a bit farther can be even greater advantage. This same Minimum distance is valid for any lens and any portrait you choose, from a tight head shot to full length standing, or even a group shot. Stand back a bit, same Minimum.

Standing back a bit is a primary rule of portraits, for the purpose to improve the perspective (to not enlarge the nose, etc). But there is more, an overwhelming advantage is even better yet: Using the longer lens, the background is also zoomed into, and only a much smaller area of it is even still visible, which can be a tremendous advantage if wanting to eliminate the background distraction. And what little is left of it is even more blurred focus (assuming that is to be a plus here). A simple sideways step or two with the camera can choose the best part of it. This standing back at greater distance is little problem to do outdoors, and focal length 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, only 3.57 inches DoF span in a APS size DSLR... so is f/1.8 really what you want to use?) DoF does not exactly describe the sharpest zone, instead it defines the limits where maximum blur becomes unacceptable. But equivalent distances don't have to use the same aperture, a 150 mm lens at 18 feet can stop down a bit, say to f/3.5, which is same picture with twice the DoF span of 50 mm f/1.8 at 6 feet. That's still not much, but it's sure a lot better, where it counts.

Maybe I'm a purist, but IMO, a "portrait lens" certainly does not mean f/1.8. Portrait lens means a longer lens to force standing back for proper portrait perspective. Nikon considered their 105mm lens to be a portrait lens (for 35 mm film), simply because it forced standing back sufficiently. But no one specific focal length, sensor size affects it too, but just whatever focal length your proper distance requires for the view you want from a proper distance. Newbies may get other notions, but a f/1.8 lens would be a laughable thing in a portrait studio. f/1.8 is certainly Not about the best capture of the face. To me, f/1.8 is about low light levels, but today, improved high ISO does that better. f/1.8 can blur backgrounds, but it's extreme, and a little brutal, and we're describing an obviously better method in the section above. A portrait studio (with the goal hoping to sell the photo) prefers depth of field, and will be using around f/8, or maybe more, and will provide the proper sufficient light this needs (easy with flash). A "portrait lens" for "head and shoulders" means 65 to 90 mm for 1.6x or 1.5x crop APS, or 105 to 135 mm for full size 35 mm frame. That longer length forces us to stand back for better perspective, to NOT enlarge noses, etc. The 50 mm lens standing back properly might do full length well, but is simply too short and close (far from the best try) for tighter portraits. My own choice is 110 to 120 mm (full frame) at 9 or 10 feet, typically at f/8 (nearly 12 inches of Depth of Field span). That would be 75mm or 80mm for 1.5 or 1.6 crops. Focusing on the Near Eye is good practice.

A cardinal rule of "Portrait" includes standing back for proper portrait perspective, a Minimum of at least 6 or 7 feet (a couple of meters), or better 8 or 10 feet. Which is very important. We guys are often too dumb to notice or realize it, but the wives will tell us they don't like their too-close portraits. And your job is to take a flattering portrait. Backing up a little more and then zooming in as desired is the good plan.

Much more about DoF continued on next page.

The third page has photo examples of the calculators two initial default cases (in A and B).