Everything You Always Wanted to Know About Focus* (*But Were Afraid to Ask)

And if you want to go deep on focus, here's how.

My old colleague Harold Merklinger's book The Ins and Outs of Focus is available as shareware. You can read it online or download it and look it over, and he asks for a voluntary donation of $5 if you like it and find it useful. It's a short manual, not quite 100 pages.

Harold was Chief Scientist of the Canadian Defence Research Establishment Atlantic and retired as that agency's Director-General. He lives in Dartmouth, Nova Scotia.

By the way, Harold's opinion is that manufacturers should just give us small, high-quality ƒ/4 lenses and we'd all be better off as far as recording sharpness and detail are concerned. (The reason they don't is that those don't sell.)

Mike
(Thanks to Viktor)

P.S. The title of the post is a reference to this 1969 book by California psychiatrist David Reuben, M.D., the first sex manual to enter mainstream culture in the USA and "among the top 20 all-time best sellers of the 20th century in the United States" (Wikipedia). Focusing isn't sexy, though.

P.S. I thought I might answer a few questions from the Comments to the previous post here:

Christer Almqvist: "All [Zeiss] 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."

Mike replies: The allowable circle of confusion depends only on the photographer's judgement. It depends how sharp you want things to look and how much blur you can tolerate before you consider something to be unacceptably unsharp. However, it's true that the higher the resolution of the plane of best focus, the smaller the circle of confusion needs to be to come close to matching it. A mental exercise: consider a very grainy film of low resolution. In this case, quite a lot of the d-o-f might have the same resolution as the plane of best focus. If imaged objects 7, 10, and 15 feet from camera position are all recorded with the same resolution, such that you can't even distinguish where the plane of best focus was placed, then all of that is "in focus." The d-o-f appears large. With a very high-resolution camera system on the other hand, used with good technique, lowered resolution quite close to the plane of best focus can be detected. So the d-o-f is effectively less. It isn't imaginary. It's a real effect. So it makes perfect sense that your Batis lenses, made for digital cameras with high-resolution sensors, would advise a narrower zone of d-o-f for any given aperture than an old Leica rangefinder lens made for film.

Degree of enlargement also changes the d-o-f. We can see this easily on our own computers with our own files. Just take a large, high-resolution file with a lot of front-to-back information. At large sizes ("pixel peeping," in Michael Reichmann's memorable phrase), its easy to see exactly where the plane of best focus is and that objects near, but not on, that plane of focus are slightly unsharp. Now reduce that same image to small web sizes. So much information has to be "thrown away" that areas that were clearly unsharp at the larger magnification are now just as sharp as the plane of best focus. The viewer can't tell the difference. And I think everybody old enough to have worked in 35mm film can remember instances where he or she got excited about the prospects of an image on the contact sheet only to discover that too much

Printing might also change the d-o-f. The reason is that the printer resolution has entered the imaging system, and might become a limiting factor.

This is also why it's not necessarily bad to use very small apertures that lower overall resolution. Not only is the d-o-f increased because of the small aperture, it's also increased because of the lowered resolution of the plane of best focus. And some pictures work better if everything appears to be of roughly equivalent sharpness, rather than having some parts sharper and others noticeably out of the d-o-f. (There was a great illustration of this in an old Nikon manual years ago—I think it was Nikon—and I might even still have that somewhere out in the barn.)

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."

Mike replies: Very true. The ultimate arbiter is what the photographer thinks looks best. An extreme example of what you're talking about might be some of Ray K. Metzker's pictures from his Aperture book Landscapes, many of which purposefully use radical out-of-focus to create visual elements in the compositions.

Book o' the Week

Fred Lyon, San Francisco Noir. "The version [of San Francisco] that Fred Lyon celebrates in his new book is a classic San Francisco full of smoky jazz clubs, neon lights in the fog and sharply dressed men and women stepping on and off of trolley cars. Made mostly during the 1950s and '60s, Lyon's images are big on atmosphere and style, and hit many parts of the city that visitors love." (PDN)

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B&H Photo

Original contents copyright 2020 by Michael C. Johnston and/or the bylined author. All Rights Reserved. Links in this post may be to our affiliates; sales through affiliate links may benefit this site. As an Amazon Associate I earn from qualifying purchases.

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

Steve Rosenblum: "Thanks for the links to Harold Merklinger's books. I haven't read his focus book in years, so it is great to have it again. I've always been impressed and amazed by the number of photographers with technical knowledge who have self-published helpful high-quality books like this through the years."

Comments

[Stan B.]

"Circle of Confusion?" That's been a major consideration throughout my life. Is it a photographic term as well?

[Benjamin Marks]

Just a subjective comment out of left field about the emotional (not technical) importance of focus.

I recently found some 1979 negatives of the woman who had been my first grade teacher and who taught me how to read. About five or six years after I took the photograph she died of breast cancer. When I scanned the negatives, I found that I had missed critical focus on her face. But the picture is now precious to me, because with her gone, there will be no new photographs of her made. Of course, this is not an artistic concept more of an emotional response. But aren't our artistic choices about inducing an emotional response in the viewers of our pictures? In this case, it turns out that critical focus was not an important part of that emotional response

[Andreas Weber]

Hey, I love my Voigtländer APO-Lanthar 90mm f/3.5 SL (admittedly half a stop faster than f/4 ...) for being compact, sharp and close focussing!
Merklinger's explanation of the hinge line in lens tilt helped me tremendously using Nikon's 24 and 85 mm PC-E lenses.

[Tom Duffy]

The rule of thumb I learned with manual focus lenses was to assume that the depth of field scales were about two stops optimistic, e.g., you should set the aperture at f8 and use the f4 dof marks.
I’m finding the dof scales on autofocus lenses to be fairly useless. Often the only mark on the lens is ‘f32’. Determining a hyperfocal distance involves a lot of trial and error.
On my Canon R5, if you set the lens on manual focus, the display in the viewfinder shows a scale with the focus distance highlighted. How hard would it be to show the depth of field for your chosen f-stop with shading on the distance scale? It would sure make things a lot easier.

[John Camp]

For those interested in a lot of technical stuff about photography, all in one place, check out the website "Cambridge in Color." Includes, among millions of other things, a DOF calculator.

[Bob Johnston]

I recently saw a wonderful exhibition of woodland pictures by Joe Cornish and Simon Baxter. Some of the prints were several feet wide. You could go in close and the detail was amazing. I wonder how many owners of their 50+ mp cameras make prints large enough to use that resolution and worry about dof to match. Not many I bet.

[Luke]

OK, you explained the title of this post. Are you going to explain "Thing One and Thing Two"?
I remember! And I'm 68.

[I honestly didn't even remember consciously. But...The Cat in the Hat. Makes sense. I'm of your generation. --Mike]

[Luke]

I have an app called DoF which shows all the relevant parameters of DoF as sliders. It hasn't really been all that helpful, but it shows in an obvious visual way the effect of changing any parameter, e.g., change the aperture and watch the field of focus change.

[Luke]

The German term Schärfentiefe, (sharpness depth), seems to be interchangeable with Tiefenschärfe, which gives some idea of how the parameters are interconnected. Well, to me, anyway.

[Robert Roaldi]

I remember a feature of the original Canon Elan, whereby you auto-focussed (if that's a verb) on a near object followed by auto-focussing on a far object, then the camera would choose the appropriate focussing distance and aperture to take the final shot. This was years ago and I only used it once or twice so my description of the process is probably not accurate. With cameras loaded with computing power these days, you'd think they'd revive this, given that they have so many other dubious features.

Another feature that would be nice would be for the camera to "set" focussing distance permanently (until you turned it off). It's so easy to set a hyperfocal distance with a focus-by-wire lens in manual focus mode, only to inadvertently knock it off focus by brushing against the focus ring. Since it's focus by wire anywhere, it should be trivial to "freeze" focus distance.

And another thing. Why can't I set focus distance to infinity? I think some models of Olympus cameras do something like this for astrophotography, but it should be commonplace.

Cameras are loaded with computing power these days, let's use it for something useful and convenient.

[Christer Almqvist]

Luke: Schärfentiefe and Tiefenschärfe are not interchangeable in German. No way. However, many people do confuse the two.

You got Schärfentiefe correctly translated. Tiefenschärfe is an "Unwort" and can not really be translated. Unless you think ice cream and cream ice is the same thing.

[Robert Roaldi]

Small world department. Canada's DND had a few research sites around the country and I worked one summer at the one near Valcartier QC, north of Quebec City, in 1974. A friend here in town had a summer job at the one near Dartmouth about 10 years later and knew Harold there. I sent him this link. I met this fellow socially through the local Ottawa car rally scene and it turns out that he also knew Harold in the car rally world in Nova Scotia. My friend used to do some hobby photography and may own that book; he's looking for it.

[Luke]

Mr. Almqvist: Thanks for that.
But Zeiss use Tiefenschärfe on their website:

https://www.zeiss.de/messtechnik/ueber-uns/presse/2021/zeiss-visioner-1.html

Maybe they mean something different. My German is such that I drove along the Mosel River for a couple of days before I figured out that Einbahnstrasse was not the name of a street, but a warning of One Way Street.

[Bill Tyler]

About circle of confusion. My friend, David Jacobson, analyzed lots of published DOF data from many manufacturers. Every one except Zeiss had about the same diameter circle on film. Zeiss used a slightly smaller one. If I remember correctly, these were 0.03 mm for most, with Zeiss using 0.025 mm. Of course, these are arbitrarily chosen numbers for all the reasons Mike enumerates about print size, photographer's intention, etc. etc.

[Christer Almqvist]

Luke: Yes, you are correct. Zeiss use the term Tiefenschärfe on their website for scientific instruments. If they are correct is another matter. ;-)

On their consumer web site for Batis lenses https://www.zeiss.com/consumer-products/int/photography/batis/batis-2818.html, selecting the German version and using the search function, they say: "1 Ergebnisse für tiefenschärfe". They do, however show: "22 Ergebnisse für schärfentiefe".

The German language often uses long words made up of two or more simple words. The positioning of the parts in relation to oneanother is of significance for the meaning of the long word and there are rules governing this. Neither German nor English is my mother tongue so I better do not get into this subject.

You may find this interesting http://timmermann.tv/technik/schaerfentiefe.php

[robert e]

As the source of the comment that was apparently the proximate excuse for the welcome refresher on focus and DOF, I wanted to say: Thank You, Mike!

It wasn't that long ago that I was in the habit of walking city streets with a film rangefinder, hyperfocusing by feel. How quickly changes in subjects, styles and equipment, not to mention less shooting overall, can dull skills and affect one's mind-set (hopefully to be replaced by more relevant ones).

Re the side discussion on lens diffraction: I stumbled onto an article that is refreshingly focused (pun unintended) on goals and practicalities:

https://photographylife.com/what-is-diffraction-in-photography

I'm now inspired to look for that deliberate TOP series on the basics of photography from a few years ago, and in particular on framing and perspective.

Again, Thank you, Mike (again)!

[robert e]

My search for "perspective" in the TOP archives actually turned up two articles by Ctein on diffraction, covering what it is and why we shouldn't worry about it, respectively.

https://theonlinephotographer.typepad.com/the_online_photographer/2008/11/diffractionquan.html

https://theonlinephotographer.typepad.com/the_online_photographer/2008/11/diffraction-in.html

[Rick_D]

Will nominate as a "small, high-quality f/4 lens" the lovely Olympus M.Zuiko 12-45/4. With today's excellent IBIS it allows handholding in light that would once have me grabbing for an f/1.4 somethingorother. No optical gotchas and nicely priced.

Wish to also note the not-small M.Zuiko 12-100/4, working proof a gaudy 8X zoom range isn't a necessarily a parlor trick, but can be a viable one-lens option for all kinds of pursuits.

The f/4 prime I have for m4/3 is, frankly, large.

[robert e]

Re the Ctein articles on diffraction:

As is often the case on TOP, the post is just the tip of the iceberg; there's arguably more value and entertainment in the ensuing conversation, so don't miss!

In essence, one of those finely written articles tells us that we needn't pay any attention to the other finely written article, which also strikes me as very TOP. ;)