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Controlling Depth of Field

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Probably the better title of this article would be Depth of Field for the mathematically challenged. If you’re a moderately experienced digital photographer, you know verbally what the depth of field is: The range of distance from the camera in which objects appear acceptably sharp in a photograph. As photographers its of critical importance: When I shoot landscapes I often want everything from a blade of grass 3 feet in front of the camera to the mountaintop 3 miles away in focus (and often achieve neither). When I shoot an outdoor portrait I often want my subject in focus but everything else deliciously blurred and suggestive (and in trying to do so have more than once gotten the zit on her nose in sharp focus and her pretty eyes blurred and muddled). I know the simple way to manipulate depth of field by changing the aperture, but this has limitations. Shooting at f/22 gives a great depth of field, but the diffraction it causes blurs the entire picture. Shooting at f/1.2 gives a narrow depth of field—but only if my subject is perfectly in a plane with my camera sensor. If not I only get part of the subject in focus.

What I’m NOT going to do is go through the optical physics and formulas about what depth of field is scientifically. If you want to read about circles of confusion, resolving power of vision in radians, lens front cardinal points, and formulas involving trigonometric functions—well, I’m sure there’s a forum for people like you someplace. Actually there are dozens, just Google depth of field and go there now. You’ll save us both a ton of frustration. What I am going to do is discuss the major effects of that mathematics in plain English. It will lose a bit of accuracy in the translation and gain a ton of practicality. So here we go!

Basic Rules of Depth of Field:

1. The smaller the aperture (higher f-stop) the greater the depth of field (yeah, you already know this one).

2. The smaller the camera sensor, the greater the depth of field (at a given focal length and aperture). A point and shoot sensor has a huge depth of field. Full frame sensors have a narrow depth of field. Crop sensors slightly larger, and 4/3rds sensors greater still.

3. Wide focal lengths have a greater depth of field than narrow focal lengths. A 35mm lens at f/1.4 has a larger depth of field than an 85mm lens at f/1.4. (Physics guy: I told you to go away. Spit your coffee, didn’t you? For completeness sake I will add that if the subject appears the same size to the sensor—that is you move closer or further to compensate for the change of focal length, depth of field won’t really change. I can’t imagine why any photographer would do this, but the mathematicians will argue about this for pages and pages. If you can give me an example of why a photographer would zoom from 35mm to 85mm and then step back 30 feet to make the subject appear the same size in the sensor I’ll reopen the discussion though.)

4. Depth of field increases as subject distance from the camera increases, given the same focal length and aperture. The depth of field of an 85mm lens at f/1.4 is greater when the subject is 30 feet away than when the subject is 3 feet away.

5. There is a hyperfocal distance for every focal length/aperture combination. The hyperfocal distance basically is the focusing distance for a given focal length and aperture that provides the greatest depth of field (the hyperfocal distance can be calculated mathematically). If you focus the lens at its hyperfocal distance, then everything from 1/2 that distance to infinity will be in focus. More on this later.

There are many depth of field and hyperfocal distance calculators available online.

Since we made all the math guys leave, those of you remaining will probably want a depth of field calculator (trust me on this, its worthwhile). Depth of Field Master provides a calculator for both of you who use Windows systems, ExpoAperture sells an inexpensive Field Guide on a pocketable disc, DOFMaster is a free online calculating tool, as is Focus Pocus .

For those of you with an iPhone or iPod Touch, my favorite App is DoF Plus (available from the iTunes app store for a dollar) which can calculate depth of field, hyperfocal distance, and field of view for your specific camera and even tells you things like what focal length you need to shoot a bird at 50 meters or a face at 10 meters. If you don’t have an iPhone or iPod Touch, then my suggestion is buy one—explaining to your spouse that its cheaper than a new lens—and then get DoF Plus and download the marvelous free Podcasts from This Week in Photography. Now you have a depth of field calculator with you all the time, plenty of great photography tips to listen to during the commute, and you can store all your photos to show to friends and clients (you could also play about 300 games, but we’re far too mature for that).

Using Aperture to Control Depth of Field

We all use aperture to control our depth of field to some degree. Many of you (like myself) shoot primarily in Aperture Control mode for just this reason. There are some simple guidelines to follow though, that help maximize what we can obtain with this technique.

First and foremost is to obtain one of the depth of field calculators mentioned above. Even if you just go to a free website and type in some combinations you commonly use, you’ll get a lot more accurate knowledge of what you want to do. Quick—what’s the depth of field of a 35mm lens at f/5.6 on a crop frame camera? Correct, it depends on how far away the subject is. So 30 feet away? The answer is 133 feet, everything from 17 feet to 150 feet will be in focus. What if the lens is an 85mm at f/5.6 on the same subject? The depth of field is now 8 feet. Pretty amazing difference, huh? And if the light dimmed while I was changing lenses and now I need f/2.8 the depth of field of the 85mm lens is now 4 feet. But if I change focus to a subject 10 feet away, the depth of field is 5 inches. Not enough to get their whole head in focus.

Sooooo, if you didn’t know all that, at least roughly, getting a depth of field calculator will really, really help your photography. And after you’ve used it a bit, you’ll quickly remember generalities like “you don’t need to stop that 35 mm down when the subject is far away”. Or one I wish I’d known: When shooting a pretty model 10 feet away with an 85 f/1.2, don’t let the camera use her shiny necklace for autofocus. Unless you want to try explaining why her cleavage is always in focus and her face completely blurred. (“It was the camera, I swear, it just autofocused there, and the depth of field was so narrow that your face was out of focus when you leaned forward.” Yeah, she believed me. Sure she did.)

Remember that diffraction effects make images shot at narrow apertures soft. There are a lot of factors that influence when diffraction effects begin depending on pixel density, the lens involved, etc. As a general rule they are ALWAYS present at f/16 and probably are present to some degree at f/11. Again, if you use a depth of field calculator you may find that stopping down further isn’t necessary. For example a 35mm lens with the subject at 30 feet has an infinite depth of field by f/8, stopping down further isn’t much help. Lets look at a narrow case – that 85mm shot at 10 feet, for example. At f/8 the depth of field is 1.25 feet. f/16 doubles it to 2.5 feet. Stopping down to f/22 only increases it to 3.5 feet. In this case changing lenses and subject distance is what we need to do, not soften the entire image by stopping down to f/22.

Be careful not to get too narrow a depth of field at wide apertures, especially at close distances. We see this all the time with Canon shooters renting the wonderful 85 f/1.2 for the first time. “I’m so disappointed my pictures are all soft”. When I look, the pictures are very sharp—in the 2.5 inch wide depth of field when shot at 8 feet wide open. The left eye is beautifully in focus, the right eye is barely recognizable as some bluish orblike structure its so out of focus. The same thing happens even more dramatically with Macro lenses. If I shoot a 100mm Macro at f/2.8 at an object a foot away, the depth of field is about 1/16th of an inch. You’re not going to get the whole flower in focus. At f/8 its still only 1/4 inch. If I back the lens up to 3 feet at f/8 I get a depth of field of 1/2 inch, so I might decide to give up a bit of magnification by moving back and getting the entire subject in the depth of field.

Use subject distance, sensor size, and focal length to control depth of field.

This can get really complex, but in general focusing further away increases depth of field. Let say I’m taking a shot of a street festival carrying my 50mm lens on a full frame camera. The light is bad so I have to shoot at f/2.8. The area 30 feet away will have a depth of field of 20 feet. Not going to capture the entire scene sharply (which might be fine if I want to emphasize one area). If I focus on a different part of the festival 60 feet away though, the depth of field becomes 120 feet, plenty to get a sharp image of a larger area of the festival. Or if I change lenses (or zoom out) to 35mm, that area 30 feet away will have a depth of field of 63 feet (of course it will be a smaller part of a larger image at 35mm).

While you may not have different camera bodies handy, it can be worthwhile realizing the difference sensor size can make in depth of field. A 75mm lens at f/4 with a subject 20 feet away has a depth of field of 5 feet. If I shot the same subject, same distance on a crop frame camera I’d want a 50mm lens to frame the image the same way. Calculating depth of field for this combination yields 8 feet. The pictures would appear similar (subject size in the image) but the depth of field on a crop sensor would be 33% deeper. Full disclosure: Physics guy will argue about this forever using mathematics that go on for pages. Just go try it. The difference is subtle but real.

Shoot with Your Lens at the Hyperfocal Distance

You can make a simple chart of hyperfocal distances for various focal lengths and apertures for your camera at Cambridge in Colour (scroll to the bottom of the page, print it out after you’ve filled in your camera data). Most lenses have distance scales so you can just look at your chart, set the lens to manual focus, and turn the focus ring until the distance scale is at the hyperfocal distance for your camera (remember for zoom lenses the hyperfocal distance changes as the focal length changes, you’ll have to reset it when you zoom in or out). If the lens doesn’t have a distance scale, pick something about that distance away, let the camera autofocus on it, then switch the lens to manual focus and it will stay there.

Most landscape photographers already know this: a shot focused at the hyperfocal distance will have everything from the point halfway between camera and subject through infinity in focus. So if I’m shooting a 16mm lens at f8 on a full frame camera, I’ll put it in manual focus at 1 meter. Everything from 0.5 meters to infinity is theoretically in focus. In truth, though, since I’m usually most interested in infinity, and this technique puts infinity at the edge of the depth of field, I might actually set the lens at 1.25 meters, sacrificing a bit of near field to be certain that infinity is in sharp focus. Its a great technique and also probably the best way to shoot those wonderful manual focus wide-angle lenses (Zeiss, Voigtlander, etc.) on new SLRs.

The hyperfocal technique has other uses, too. The famous street photographers of yesteryear set their wide angle lenses to the hyperfocal distance so that they could snap away without manually focusing their rangefinders. Its useful today when you’re trying to shoot rapidly changing street scenes and live action without hoping the camera autofocuses on the right place accurately, or if autofocus is too slow to keep up. Snapping away with a wide angle lens set to the hyperfocal distance means most of the scene will be in focus with every shot. Want to ‘stealth shoot’ (snapping pictures without putting the camera up to your eye?) Set it at hyperfocal distance and snap from waist level, or turn on live view in manual focus mode and frame on the LCD just snapping away.

Manipulate the Plane of Focus

When you’re working with a narrow depth of field, visualize the depth of field as a flat sheet parallel to the camera’s sensor where the subject is. If you change the angle of the camera in relation to the subject slightly, the depth of field across the scene changes, too. Picture a photo of a huge group of people, maybe hundreds, standing in a field. You’ve seen such a photo – if not look at those cellular phone adds of the entire staff of the company standing behind the phone user. Did you notice the framing? You’re looking down on the crowd. Always. Partly this is to be able to see the people in the back row, but also by shooting down toward the scene the angle of the depth of field is altered. The plane of focus is slightly more parallel to the group front-to-back. The same thing can be done to great effect in landscape work. getting to a high place and shooting down on the scenery will change the angle of the plane of focus, which brings more of the foreground into the focal plane.

The ultimate way to manipulate the plane of focus is with a tilt-shift lens. Tilting the lens tilts the plane of focus. You can do it to move the plane of focus along the image so everything appears sharp (have you seen that photo of the near road in focus, as well as the mountains a mile away?). You can also do just the opposite, tilting the lens to make the plane of focus further away from the subject so that depth of field appears much narrower than it actually is.

One thing to remember with tilting the plane of the camera is that most lenses are softer at the edges so we’d still like to keep subjects of interest away from the edge of the picture. Also many lenses (especially wide aperture primes) have a slightly curved plane of focus (meaning if the center of the lens is focused 10 feet away, the edges might actually be focusing 8 feet away). If you tilt the plane of focus, the curvature tilts too. Instead of a flat sheet the plane of focus is shaped like a parachute (well, not that much, but you get the idea). As long as you keep the subject in the center 2/3 of your frame, it shouldn’t be an issue.

Change Your Camera

OK, this is the least practical solution, but one to consider at times. Want a landscape with everything in focus? Its easier to do with a 4/3rds sensor than with a full frame sensor. Want a narrow depth of field to make the model jump out from the background? A full frame is the answer for you. For those, like working professionals, who must have more than one camera body, this may be a reason to have both a full frame and a crop frame camera.

Who else? Well, I’ll use this article to repeat my most repeated statement: “When going on safari, or out of the country to photograph, the most important piece of equipment is a second camera body.” Last year 4 more renters ignored this advice to their detriment, traveling to Africa or Alaska with a handful of rented lenses and one body which failed on the first or second day of the safari. (Can you imagine??? “We spent $5,000 to get to Kenya and go on a safari. Wish we had some pictures.” Yep, there’s no Ritz Camera there. Never mind, there’s no Ritz Camera here anymore, either. Want to guess how far you have to drive from Yellowstone to buy a camera body? Its far. I’ve been told by yet another renter.) Consider making that second body a different sensor size than your other body, both to take advantage of the ‘crop’ factor and also to manipulate the depth of field.

Roger Cicala
4/5/2009
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4 Responses to “Controlling Depth of Field”

Michael THornton said:

Hi Roger,
Regarding your comments on Hyperfocal distance charts
Since buying a Nikon D800 I find that the old standard COC of 30 no longer works.
I did read that the COC should be twice the size of the sensor pitch, in the case of the D800 2 x 4.7 = COC of 10
Readjusting the charts to use this COC of 10 results in much sharper images, the downside being that the DOF is now really restricted for landscape use. I suppose thats the penalty of using hi res sensors?

Regards
Michael

Feng Chun said:

Interesting enough, regarding the basic rule 3 of DOP, Canon has a different opinion:
"You will usually choose the focal length to suit the subject rather than to suit the depth-of-field. However, the accepted rule is that you get more depth-of-field with wideangle lenses than with telephoto lenses. In fact, this rule is misleading. What actually happens is that a wideangle lens magnifies the subject less than a telephoto lens, which means that more of the image appears sharper. A simple test will show that if you take two photographs of the same subject from the same position with a wideangle and a telephoto focal length, then enlarge the centre of the wideangle image to match the view of the telephoto image, the depth-of-field will be identical. "

Quoted from: http://cpn.canon-europe.com/content/education/infobank/depth_of_field/depth_of_field.do

Phil Thornton said:

Q: Why would a photographer would zoom from 35mm to 85mm and then step back 30 feet to make the subject appear the same size in the sensor?

A: To include less of the background in the picture.

No? Just saying - I'm no physicist.

Graham Stretch said:

Hi Roger.
Thanks for not only this but all the other great articles you have written. I only found them last month and have been reading them with great interest, from the history articles which are so well written, wish you had been teaching me history at school, I might have learned something, to these technical items which I hope to be able to put to good use improving my pictures.
I decided to post mainly to second your comment about a second body, I am an amateur photographer from England and when I went to Arizona I packed my camera bag several times to get what I wanted within the carry on limit. I had at the time a 40D with about 4000 actuations on it, I also had my 300D much older and higher mileage for backup. I took the 300D out, put it back, took it out and finally put it back in the bag.
I'm so pleased I took the second body as my 40D shutter failed at the Out Of Africa Experience near Phoenix, I just ran back to the rental car and grabbed the backup and although I was annoyed at the FPS and AF for shooting action I at least have some keepers from there and the rest of the holiday.

Even amateurs need 2 bodies!

Cheers. Graham.

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