The Depth of Field calculator also concerned with blurring the background
The Hyperfocal chart and calculator for your sensor
Blurring Background Without Suffering f/1.8
Standing back with a longer lens can give better background results than f/1.8
Perspective is Not about the lens, but is Only about where the camera stands
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Lenses are focused at only the ONE distance (determined by the current focus ring rotation), but which we perceive as a zone of 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 what we might see assuming a standard viewing enlargement size of an 8x10 inch print viewed at 10 inches, which you should realize might not always be your own situation. 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.
The concept of 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, 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, in preparation for what the eye can see in future enlargement to 8x10 inches). Depth of Field simply computes the distance at which the blur of a out-of-focus point matches this CoC limit, conceptually planned be where any blur becomes just perceivable by eye (in a standard 8x10 inch enlargement). See the CoC diagram (and the next page continues too). Misfocus increases blurred size very gradually, so there is NO precise border line on sharpness. But still, the DOF formula computes the distance very precisely, 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 quite that good just before reaching the limit, and not quite that bad just after. Still, DOF is a good guide of what to expect in terms of focus sharpness. We are usually just guessing distances anyway.
In any given camera, Depth of Field is determined by the combination of three lens factors, and IF with all else the same, then:
Depth of field Span is total 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.
Additional features in this DOF calculator (seen below) that you may not see elsewhere are:
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.
The lens image resolution is Not affected by sensor size (other than how we might enlarge it for viewing), but the DOF 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 DOF is all about. Smaller sensors must be enlarged more, so their CoC is corresponding smaller, and then DOF is computed for a standard 8x10 inch print viewing size.
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 these can calculate sensor size. Much more arbitrary and less precise, but even CoC can determine sensor size, because CoC relates to the standard enlargement of sensor size. You can see ways to determine your Crop Factor (perhaps even from known Equivalent Focal Length). It's hard to beat precise actual sensor size specifications though.
But in the DOF calculations, always specify the actual real focal length, 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 adversely affecting accuracy, NOT included here.
Abbreviations: DOF is Depth of Field, CoC is Circle of Confusion, and FoV is Field of View.
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 calculator will "blink" once when showing the changed result. The bright Yellow box shows the final sensor size result seen.
Example: In the chart, if with the DOF calculators initial default 23.5x15.6 mm sensor choice (Nikon DX, APS-C), 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.
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 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. The 50 mm lens at f/1.8 has almost no DOF span, noticeable vignetting, and noticeable optical defects unless maybe you spend $3000 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 hiding the background in an outdoor portrait).
This calculator always uses CoC divisor = 1442, and standard viewing size of 8x10 inches.
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 the 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 still 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 generally still choose to stop down a bit more for 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 a bit more to increase subject sharpness, but still blurring the background as much, or usually more, 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 a few strong advantages normally offered by standing back with the longer lens.
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, with 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.
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 a strong portrait consideration. A common perspective problem for portraits is from standing too close to the subject. It's also a problem with selfies held at arms length. But normally portrait perspective is no problem at camera distances of at least maybe 6 or 7 feet, and I like a longe lens at 8 or 10 feet to be sure. You've seen examples of the same portrait taken with various focal lengths showing the perspective differences. Their point seems to be that short lenses cause bad perspective effects (enlarged noses, etc), and seemingly show perspective is improved greatly by using longer lenses. Which is true enough, but you should realize that what they do NOT say is that the distance was changed dramatically in each view to show the same size, but necessarily a different perspective. Perspective only depends on the view the camera sees from the distance and angle from where the camera is. Saying, the lens simply shows what it sees from each distance. Each of those multiple pictures necessarily were taken at the different distance specifically chosen so that the subject Field of View stays the same in all (equivalent distances as described here). The longer lens is better because the subject framing requires that it must stand back at a more proper portrait distance. A strong rule for portraits with any lens, about 7 feet is necessary, and 10 or more feet is fail safe (then use whatever lens shows what you want). 4 or 5 feet is insufficient. You can get that picture, but it likely will not be flattering.
A lens 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 must choose to stand to use it.
The focal length does affect magnification, and thus framing/cropping, but those "same portrait with different lenses" examples always fail to mention that the perspective result is only because the distances were adjusted to keep the same Field of View for the different focal lengths. The distance is the one important factor of perspective. Meaning, back up a bit, don't stand too close. Simply use whatever focal length that shows the view you want to see from standing back a little, where you should choose to stand. 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 help 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. 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. No one specific focal length, sensor size affects it too, but just whatever your proper distance requires for the view you want. 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. f/1.8 is about extremely limited depth of field (or is for low light levels, but today, improved high ISO does that better). f/1.8 can blur backgrounds, but it's extreme, and we're describing an obviously better method. 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 (not 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). 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. Backing up a little more and then zooming in as desired is always a good plan.
The third page has photo examples of the calculators two initial default cases (in A and B).