The Very Basics of Focus

Doubtless most TOP readers already know all this, and more besides. But it might be of use to newcomers to photography, whom we of course wish to encourage! With the great avalanche of new photographers joining the throng by way of those clever little smartphones, I wonder how many of them know these two very basic things about focus. This short explanation is simplified somewhat for clarity.

Thing One
I was taught geometry by the memorable Mrs. Marianne Otto at the University School of Milwaukee, and received the highest grade she ever gave to any student over her long career—how I got it is a good story, but has to be left for another day. (I also should have flunked Algebra II, but Mrs. Otto let me squeak by purely out of the kindness of her heart and also some concern for my college prospects.) If you recall your own high school geometry class, you know a plane is "a flat surface on which a straight line joining any two points on it would wholly lie." The first important thing about focus is that the way a fixed-lensmount camera focuses is that the plane of best focus is a plane parallel to the plane of the sensor*. As the lens focuses, this plane of best focus is effectively moved closer to and farther away from the sensor or film. So you don't focus "on" an object or an area; you push the plane of focus back and forth as the lens changes focus, and that plane moves around as you move the camera around.

The zone of acceptable sharpness in front of and behind that plane of best focus is called the depth of field, sometimes written with hyphens even when it's not an adjective, and conventionally abbreviated d-o-f. Depth of field is influenced (not controlled) by the aperture you select on the lens; I used to have my students memorize the phrase "the higher the number / the smaller the hole / the greater the depth of field" (say it in singsong fashion).

Depth of field will always be greater behind the plane of best focus (i.e., farther away from the camera) than in front of it (closer to the camera). Because imaged objects nearer camera position are more prominent to the eye in photographs, it's more important to get those sharp if you want them to be sharp or if you don't want them to be distracting with their unsharpness; the ancient and honorable rule of thumb therefore is to focus "one-third of the way into the scene," i.e., into the zone of what you want to look sharp.

Thing Two
The second basic thing everybody should know and doubtless a lot of people now don't know is that the zone of the d-o-f is proportionate to the focus distance.

Let me 'splain. Let's say the d-o-f extends one-third of the focus distance in front of the plane of focus and two-thirds in back of it. So if you focus on an object ten feet from the camera, you'll get everything from about 6.7 feet (10 minus 3.3 feet or a third, rounded) to 16.6 feet (10 plus 6.6 feet or two-thirds, rounded) to be acceptably sharp. BUT, and yes, that's the Big But: If, without changing any other setting, you focus on something 100 feet away, then everything from 67 feet to 166 feet will be in the d-o-f and will appear sharp. And so on. So you've changed the zone of apparent sharpness from a depth of 9.9 feet total to a depth of 99 feet total...just by changing focus. It's the same aperture, but its effect is proportionate to how far away you're focusing.

That's why, when you focus close up, so little appears to be in focus, and if you focus far away, so much does.

In this example (you can see an enlargeable version on Flickr), ƒ/11 with this lens has gotten the mailbox on the right sharp enough, and the houses on the opposite shore sharp enough. That's called "pan focus"—no relation to "panning," which is a different thing altogether. Pan focus is defined  as follows:

Pan Focus refers to where everything in an image, from the foreground to the background, is in focus. As a technique, it refers to how a photographer would set a sufficiently wide depth of field to make sure foreground as well as background objects are in focus. (Canon Australia)

For the seminal 1930s American West Coast art photography movement that counted Edward Weston and Ansel Adams among its membership, Group ƒ/64, pan focus was one of it aesthetic principles. That was in opposition to Pictorialism, which favored a dreamy softness throughout the image. An aperture of ƒ/64 is one that on an 8x10-inch view camera gives good depth of field.

There's much more to focus than this, and much more that could be said. But these two things are a good place to start.

Mike

*A fixed-lens camera is one that has a single lens permanently attached to it. A fixed-lensmount camera can also include cameras with interchangeable lenses but where the mount is rigidly fixed in relation to the plane of the film or sensor. And, the plane of focus is a plane only ideally. In actuality, it doesn't always achieve flatness. But it comes close enough for it to be useful to conceive of it that way.

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(To see all the comments, click on the "Comments" link below.)
Featured Comments from:

DavidB: "About pan focus...the magazine Arizona Highways prints photos shot with great depth of field. EXIF data is often included with the photos and many photographers routinely shoot ƒ/16 and ƒ/22 to get these images. Our fear of diffraction has limited our photography."

Comments

[Grant]

Remember when all this DOF vs f stop stuff was right on the lens? Ah, the good old days.

DavidB said that dirty word, "diffraction." I never heard much about it in the film days, but once digital came along, it was on everyone's tongue. Perhaps you could share your wisdom on that subject?

[Jeff]

The Leica S and SL systems have a top display that shows the near and far (and intermediate) focus range (in feet or meters) for a given shot, so that one can manually dial in focus until one achieves the closest possible focus while still maintaining acceptable sharpness at infinity, i.e., the hyperfocal distance. This works well for wider lenses when shooting landscapes using a tripod. I don’t use it that way, but David Farkas (Leica Miami/Red Dot Forum) is a keen advocate. Of course, it’s better if one understands the underlying principles that you explained, but this provides a quick approach.

[xfmj]

You are making me nostalgic for the one B&W film class I got a chance to take. Definitely good to learn all this. If I might add: Everything but of course the fingers or camera strap that (I didn’t realize) were often immediately in front of my rangefinder camera’s lens!

The “locate an object to place the focal plane, set aperture so that you get good focus in front and behind” is definitely something you force yourself to understand when your camera can’t show you what exactly’s going to be captured. Lens distance marks nicely helped with your sense of that.

[Bill Tyler]

A couple of comments. "The zone of acceptable sharpness" is not a fixed quantity. It depends entirely on what you, the photographer find to be acceptably sharp in the final image. This is a matter of taste, and depends on the size of final image, viewing distance, subject matter, etc. etc. One other minor clarification: "behind the plane of best focus" means farther from the camera. This might be obvious to all your other readers, but I always have to think about what is meant by terms like 'behind' in this sort of context.

[Bill Tyler]

One other comment - tilt-shift lenses are very helpful in achieving great d-o-f. In ancient times, a tilting back was also useful, though the effect was not identical. Tilting the front simply extends d-o-f. Tilting the back does that and also changes the proportions in the image, a potentially useful effect.

[Rick in CO]

The basis for the Hyperfocal Distance at selected aperture. I read, a while ago, that this is how one should use large format wide angle lenses for landscape photography.

[Moose]

"The zone of acceptable sharpness in front of and behind that plane of best focus is called the depth of field"

A bit of trivia: The original d-o-f tables were created by showing prints of standard size, at standard distance from eyes to many people. They were asked to rate what was in and out of focus.

The adoption of mathematical models made d-o-f calculations easier. They are based on Circles of Confusion, how much a point source of light was spread out by the lens. But criteria as to what is or isn't sharp to humans were based on the original research into what people perceive as sharp.
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". . . the mailbox on the right sharp enough, and the houses on the opposite shore sharp enough."

Here, Mike has made his own judgement of what it and isn't sharp. And his, and your, judgement in this regard will be dependent on magnification, how large the image is to your eyes.

In the Flickr version, two clicks enlarge it to the point that details of the farm buildings above the lake are less clear than those of the houses on the lake shore.

If he handed you an 8x10 print, it would appear sharp all over. A 24x30 print, looked at closely, would reveal differences.
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". . . many photographers routinely shoot ƒ/16 and ƒ/22 to get these images. Our fear of diffraction has limited our photography."

To my mind, this is a failure of gear and/or imagination. The technique of shooting a series of otherwise identical frames, with focal plane moved between each, then combining the in-focus parts into a deep focus composite originated with micro photography, where, as Mike points out, d-o-f is razor thin.

The technique, sans specialized, automated precision focus moving devices, is a big pain. I regularly lost track of whether I'd taken the shot at the current focus or not.

Recent developments in camera design will do all that for you. Of those I'm familiar with, Panasonic and Olympus µ4/3 bodies will do all the focus bracketing auto-magically for you. The Olys will even do the compositing for you in camera, although with more limitations than creating a series to be blended later in post.

[ronin]

"...As the lens focuses, this plane of best focus is effectively moved closer to and farther away from the sensor or film"

To add to the fun, now we need to talk about Depth of Focus vs Depth of Field!

[Peter Conway]

This is may be a little tangential, but there was a period recently when it seemed to be trendy in photojournalism to have a prominent object or person in the foreground, very much out of focus, and an object or person in the background in sharp focus. A couple of examples are in this article from the New York Times, and this article in USA Today (scroll down to the photo of the "Dons" - Trump and McGahn).

I find these types of images visually jarring, with the out of focus areas distracting from the photo as a whole (unless, of course, the purpose of the image is simply to be jarring). I'd be interested in your thoughts on this approach, especially in photojournalism. I tend to be more to the "Group f/64" end of the continuum, rather than the limited d-o-f end (I did an exposure yesterday at f/5.6 and started mentally hyperventilating).

[Mike Plews]

I shoot film landscapes from time to time on either a Rollei 2.8f or a Hasselblad 500c with a 50mm Distagon.
My age predisposes me to the f64 approach. The sophisticated DOF scales on these cameras are really fun to use for deep focus photography.
I also have a light meter app on my phone that has a DOF function that is slick as can be.

[Viktor]

I can really recommend "The Ins and Out of Focus" by Harold Merklinger. When starting out in photography I found it extremely helpful in explaining the depth of field connected to aperture and focus distance.

It is also worth nothing that if you intended to use the same reproduktion size of the print, F64 on an 8x10 would correspond to roughly F8 on 35 mm. Based on lenses giving a similar field of view. But I suppose that with a large format camera you have additional possibilities like being able to tilt the plane of focus making this comparison less relevant.

[Kodachromeguy]

My 1949 5cm Summitar lens has a very curved plane of focus, almost a ball of focus. It can be handy, but sometimes I take another ltm lens with me, one with a much flatter plane.

[One advantage of owning a lens like that is it helps you learn to recognize that effect in pictures! --Mike]

[Mike Chisholm]

As far as d-o-f is concerned, the one thing that virtually no digital camera has, and which every digital camera could really use, is a [HYPERFOCAL MODE]. It would be so easy, wouldn't it? Assuming the camera's software knows the focal length and aperture currently in use, why on earth can't it do a moronic little hyperfocal calculation, and set the focus accordingly?

I keep re-reading camera manuals to see whether I've missed it, or whether they've called it something cute like [FOCOMAT], or buried it in the "scene modes" somewhere, but no. I believe some Ricoh cameras do have this feature. Even the crude three settings for "zone focussing" that used to figure on ancient film cameras (flower! / people!! / mountain!!!) could be handy.

Perhaps there's a technical obstacle? My suspicion is that enabling the lens to be focussed at an actual pre-determined distance -- rather than reactively to whatever is detected by the camera's fancy focussing matrix -- may be too great an engineering problem to be worth tackling. If so, what a shame.

Mike

[There was at least one Canon film SLR that had it. The A2, maybe? I'm getting alphanumeric-aphasia in my young old age.

The problem with a mode is that you still have to set the front and back points, because only you know what you want those to be. --Mike]

[Sal Santamaura]

Victor wrote "...F64 on an 8x10 would correspond to roughly F8 on 35 mm..."

Which naturally leads to the 8x10 shooter's maxim "f/64 and be there." :-)

[In spirit, but not exactly. The expression dates from the era of press cameras, when most newsmen used Speed Graphics and the like. So that would be ƒ/8 on 4x5. --Mike]

[Dan Khong]

Mike

Thanks for the revision on focus and sharpness. Although I've used the technique before, now I learn it's also call Pan Focus.

Sadly, many modern lenses don't have d-o-f scales anymore, Even if they do, with AF, hardly anybody uses them.

Some old manual focus lenses have two markings on the distance scale. At f8, using one marking, everything is acceptably sharp from 8-18ft. At the other further marking, everything is acceptable sharp from 18ft to infinity. And it's faster to shoot than AF.

Dan K.

[Mark Rouleau]

The first digital camera (that I can remember) that encouraged zone-style focusing was the Sigma DP series. It had a little dial on the back of the camera that let you set the manual focus distance. And boy, did you need that dial in anything other than broad daylight. Unfortunately, the dial wasn't lit, and didn't have DOF markings.

The Sigma Merrill (if I recall correctly) gave you a digital readout in manual mode that showed not only distance, but depth of field based on aperture. Pretty slick, I thought.

But of the cameras I've used, the undisputed master, is the Ricoh GRIIIx. Not only is "snap focus" (pre-focusing to a specific distance) a thing, but it also, as of the latest firmware, has a dedicated "Snap Distance Priority" mode. Here's how it works:

Enable the mode (I have it set to the Fn key for convenience.) Use the front dial to set the distance (.3/1/3/5/infinity meters), use the rear dial to set the depth of field (DOF1, DOF2, DOF3: basically narrow, medium, or maximum DOF). The camera automatically sets the ideal aperture based on the two settings (really one setting, I've never gone out of DOF3 except by accident).
So, infinity focus with maximum depth of field on a 26mm APS-C lens is an aperture of 7.1

It was a little odd at first, not having to half-press the shutter, not waiting for the AF beep, and not turning a focus dial while trying to remember which button magnifies and whether or not you remembered to turn on focus peaking ... but the final result is incredibly, "oh my god I can't believe photography can be this fast," fast! It's very freeing, almost doesn't feel like real photography.

[Herman Krieger]

You can't beat a pinhole camera for depth of field.

[Bob Johnston]

The decision on where to focus is, or should be, part of the artistic process. It seems to be assumed in all those YouTube videos that every landscape should have a near infinite depth of field. This just isn't so. It depends on the relative importance of the elements in the picture. If you have the most important subject in the foreground or middle ground it can sometimes look better if the background is a little soft. Therefore, sometimes the thing to do is to focus on the object that is the main subject and not on the hyperfocal distance.

[Christer Almqvist]

All Batis lenses have a display that can show d-o-f. It was of no relevance to me when I bought my Batises.

The d-o-f shown on the Batis lenses is much narrower that the d-o-f shown on my old Leica M lenses or indeed in the Leitz printed d-o-f tables from the film days. These were based on circle-of-confusion measurements and calculations.

In the film days the minimum shutter speed rule followed by many photographers was to shoot at a shutter speed of 1/n where n was the focal length in mm of the lens used. This is not really practicable today (unless you have IBIS) as sensors are much more unforgiving for camera shake than film was.

The same (sensor unforgiveness) is even more true for d-o-f where IBIS does not lend a helping hand.

Does this mean that my Batis lens display should take into account the number of pixels on the sensor? And of course, the distance at which the print in viewed.

Please advise.

[Thomas Mc Cann]

Thhe trouble I found with digital micro2/3 cameras was the difficulty in achieving focus differentials as they inherently have great d.o.f.
Of course could be wrong:-(

[David Bennett]

"Let's say the d-o-f extends one-third of the focus distance in front of the plane of focus and two-thirds in back of it."

I hear it all the time and I say it myself.

Is that a handy convention or is it the result of knowing many cameras and how they behave?

[It's a rule of thumb, and accurate in an approximate, general way. For specific behaviors with specific lenses you'd have to consults an optics expert. --Mike]

[Bear.]

My first rangefinder was a Ricoh 500G, it had a little mark on the focus ring which was the hyperfocal distance for the lens. Point and shoot, and hope.

[Merle]

I agree with Mike Chisholm. There should be a focus setting for Hyperfocal. It's calculated by the fStop you've chosen to give you the focus distance for which "infinity" (or its close approximation) is in focus.

For example, if I use the DOFMaster - https://dofmaster.com/m/ (or any hyperfocal calculator, I assume) - set it for my camera and pick an fStop, clicking the HD button in the lower left will always give me the same answer, the same focus distance. If I didn't want that, I'd choose another focus setting.