Depth of Field, Depth of Focus, and all Circles of Confusion

erich6 · May 8, 2005

I've seen this topic discussed in different venues and I have yet to find a source that can clearly show how these things are related without being inconsistent. I see a major issue with how books, articles, and professional photographers refer to these terms almost interchangeably. I think this is part of the reason this is such a confusing (no pun intended) topic.

Now, I will be the first to admit that, being an amateur photgrapher, I lack the experience and intuitive feel for photography that many of the experts discussing this topic have. However, I do have a strong technical background in the area and I think I may have broken the code on this issue.

I'd like to share with you what I understand of this problem in the hope that many of you will provide additional insight and maybe we can come up with terminology that can consistently and clearly articulate this concept.

The best article I've seen on this subject is by Mr. Michael Reichmann (see http://www.luminous-landscape.com/tutorials/understanding-series/dof.shtml). But, even in this article I believe Mr. Reichmann makes the error that causes all the problems as I see it. Depth-of-field is NOT depth-of-focus (I believe Mr. Reichmann knows this and has actually demonstrated it in his examples but the terminology is still too vague). I've seen others state this as well but they don't take it far enough. Depth-of-focus is NOT the focal length at which light comes to focus. Depth-of-focus is the range that the image plane can move in and out of best focus and still be *in* focus. Again, this is not depth-of-field and I will explain why I think it is not. What qualifies as "in-focus"? This is the "circle-of-confusion" which is how blurry a spot can be before we say "aha! it isn't a spot anymore!".

When we talk about depth-of-focus, we are talking about an effect that occurs around the image plane. There are only three factors that affect depth-of-focus (assuming a perfect lens without any aberrations so that the only thing we're dealing with is blur from defocus): 1) aperture stop; 2) focal length; and 3) the wavelength of light. Depth-of-focus increase with a large f-number (small aperture, long focal length) and long wavelength. Indeed, it is proportional to the product of the f-number and the wavelength.

So why do long-lenses give poor "depth-of-field"? If I have a wider range of distances that I can be in focus then I should, by definition, have a wider range of depth....right? Well, the problem is with the word "field". All of the sudden, the angle-of-view of the lens becomes an important parameter. The wider the lens, the wider the perspective and the more of the background gets collected by the lens system and imaged. What happens now is that the various elements making up the background get squeezed into the film format or digital sensor. In other words, subjects that are actually larger than something closer to the camer will actually look smaller. Since they are smaller, they *appear* to be in focus. This is the same effect you get from looking at a thumbnail of an out-of-focus image that looks OK until you see it on the computer screen or on a larger print and you go "ugh!". As a side note, it is interesting to me to note that a wide-angle lens is by necessity a short focal length lens. Again, it seems counterintuitive (again please bear with my techie vs. photographic background) that a short focal length lens has more depth-of-field since it actually has less depth-of-focus.

The reason wider lenses give better depth-of-field is because of the perspective effect I just mentioned. Objects in the foreground and background really stand out from each other because of the relative sizes and how the geometry projects unto the image plane. Coversely, a long focal-length (telephoto) lens will only image objects that are relatively close to each other in depth. Therefore the shallow depth-of-field. In some books I've seen this described as a "compression" of the field. It is true, but only in the direction of the camera to the subject. In the lateral dimension the field is compressed by a wide-angle lens!

Now I see this topic further complicated by the difference in formats between 35 mm film and digital SLR's. We tend to hear about "effective" focal length due to the format factors. It is true that you will get a different field-of-view from a small digital sensor format than a 35 mm film format. [EDIT: This changes the crop for sure and may "force" a different perspective depending on the focal length of the lens]. But it does NOT affect depth-of-focus nor the actual focal length of the lens. Some have said that because of this it doesn't affect depth-of-field. That's wrong. Since depth-of-field depends on perspective, as I have explained above, then digital cameras with a format factor smaller than 35 mm will also have [EDIT: more] depth-of-field.

I'm sorry if I'm rambling on here or if I'm beating a dead horse. I'm really thinking this stuff out as I write. I hope several of you can shed more insight into this or maybe tell me I've got it all wrong and I should stick to book-worm material instead.... Otherwise, I'd like to know how this topic got so convoluted in the photographic literature.

Thanks,

Erich :scratch

erich6 · May 19, 2005

Was I way off here? I'm wondering how come nobody has replied.... Please let me know if I made a mistake.

Erich

wxwax · May 19, 2005

Erich, I suspect that you haven't had many responses because it's a long post and very technical. We have a few wizards here who are technically inclined who might eventually see this thread and respond.

4labs · May 19, 2005

I am far from an expert but there is a book "Photography"
by Barbara London and John Upton that could explain things better than I ever could..You can get it on Amazon.

Mitch · May 19, 2005

Ok I am going to shoot from the hip, but as I remember....

Depth of Field - The area beyond and in front of the object/subject focused on that is in [font=&quot]acceptable focus.

Depth of Focus - Inside the camera. The amount that the recording medium can be off plain and still yeild acceptable focus.

[/font]Circle-of-Confusion - Your's is as good an answer as any. "which is how blurry a spot can be before we say "aha! it isn't a spot anymore."

I beleive this is correct.:D

Mitch

DoctorIt · May 19, 2005

Now I see this topic further complicated by the difference in formats between 35 mm film and digital SLR's. We tend to hear about "effective" focal length due to the format factors. It is true that you will get a different field-of-view from a small digital sensor format than a 35 mm film format. This changes the perspective for sure. But it does NOT affect depth-of-focus nor the actual focal length of the lens. Some have said that because of this it doesn't affect depth-of-field. That's wrong. Since depth-of-field depends on perspective, as I have explained above, then digital cameras with a format factor smaller than 35 mm will also have less depth-of-field.

I think you are right on target, and this is a very good point that I hadn't thought of.

I'm guessing you are still going after this depth of field vs. focus argument. I think circle of confusion is pretty well defined and used correctly, but you are totally right in that these two are interchanged very often.

Lets get down and dirty then, we need some variables and equations. I'm not an optics expert, but I am an engineer. And I'm going to get my optics book right now and see what I can find...

this'll be fun

bkriete · May 19, 2005

Don't smaller sensors have greater depth of field? P&S cameras tend to have everything in focus to a greater extent than SLRs...or is this because they usually have smaller max apertures?

DoctorIt · May 19, 2005

It seems Reichmann directly contradicts your view on digital (you said digital has less depth-of-field):

What About Digital?

A common complaint about digital cameras is that when using one it's not possible to get nice out-of-focus backgrounds. Why therefore do digital cameras have greater Depth of Field? The reason for this is that the imaging chips on most consumer digitals is very small, around the size of ones smallest finger nail. This means that a normal lens for a format that small is as short as 15mm. A 15mm lens at f/5.6 has Depth Of Field from about 2.5 feet to infinity. Not too much opportunity for selective focus, is there?

This is one of the unspoken drawbacks of low-end digital cameras. Only expensive SLRs like the Nikon D1x, Canon 1D and their ilk have chips close to the size of a 35mm frame, and therefore offer enough DOF to allow creative control over out-of-focus backgrounds.

erich6 · May 21, 2005

DoctorIt wrote:

It seems Reichmann directly contradicts your view on digital (you said digital has less depth-of-field):

What About Digital?
A common complaint about digital cameras is that when using one it's not possible to get nice out-of-focus backgrounds. Why therefore do digital cameras have greater Depth of Field? The reason for this is that the imaging chips on most consumer digitals is very small, around the size of ones smallest finger nail. This means that a normal lens for a format that small is as short as 15mm. A 15mm lens at f/5.6 has Depth Of Field from about 2.5 feet to infinity. Not too much opportunity for selective focus, is there?

This is one of the unspoken drawbacks of low-end digital cameras. Only expensive SLRs like the Nikon D1x, Canon 1D and their ilk have chips close to the size of a 35mm frame, and therefore offer enough DOF to allow creative control over out-of-focus backgrounds.

He actually hasn't contradicted my statements. Mr. Reichmann says *low-end* digital cameras have greater DOF but not because they're digital but because the lens is so [EDIT: short]:

DoctorIt wrote:

The reason for this is that the imaging chips on most consumer digitals is very small, around the size of ones smallest finger nail. This means that a normal lens for a format that small is as short as 15mm. A 15mm lens at f/5.6 has Depth Of Field from about 2.5 feet to infinity.

If those small-format cameras had a long lens like those used in a 35 mm SLR they would end up with less DOF.

Erich

wxwax · May 21, 2005

Pssst, I think he said it's because the chip is so small.

Antonio Correia · October 31, 2005

CoC / DoF / etc
Lattely I have been reading about these subjects and here I send you an excellent link on this matter.
http://www.dofmaster.com/
I am "building" a Depth of Field Calculator for my two lenses. IN PAPER !!!...

I will show it to you all, as soon as it is finished. I think I'll remember to do so.

Meanwhile here is a site which gives differents formulas from the one above:
http://www.nikonians.org/html/resources/guides/dof/hyperfocal2.html

I think here they are wrong because I tried the formulas in Excel. With dofmaster Things worked fine.

Shay Stephens · October 31, 2005

I know this is an old thread but...
I studied the topic of circles of confusion and related info but have found very little practical application for this knowledge and see it as a needless complication in the photographic process. The fact that you can go your whole life without having to know this info and yet succeed in photography tells me all I need to know ;-)

Some of the calculator types might need this for camera design and such, but the average Joe doesn't. It can all be boiled down to a few rules of thumb that are easy to remember and implement.

Shay (putting my slide rule away now) Stephens

pathfinder · October 31, 2005

I think the Single lens Reflex design, as well as view camera view finders generally make this kind of information unnecessary complications for most SLR shooters. The old range finder cameras could use this information more.

Just LOOK through the viewfinder. You can also evaluate DOF after you press the DOF preview button to stop your lens down if needed. But I bet the pros rarely use this feature, just like Shay said.

mercphoto · October 31, 2005

pathfinder wrote:

But I bet the pros rarely use this feature, just like Shay said.

I was at a class held by one of the local wedding/portrait guys in town (great guy, unreal photography) and he showed an indoor portrait he did that had, in my opinion, just the right amount of background blur. I asked him if he, after all this time, simply knew what aperture setting to use to get the desired background blur in that situation and he said yes. He's done shots like that so often on that equipment that he doesn't need preview or bracketing to get the effect he wants.

David_S85 · October 31, 2005

DOF Calculator
http://www.dofmaster.com/dofjs.html

Also has software available from the menu on the left side of that page that will print out DOF charts for your own camera and lenses you use.

Good tutorial on hyperfocal distance calculations, charts, etc.

Antonio Correia · November 1, 2005

DoF / CoC / etc

Shay Stephens wrote:

I studied the topic of circles of confusion and related info but have found very little practical application for this knowledge ... The fact that you can go your whole life without having to know this info and yet succeed in photography tells me all I need to know ;-)

Shay.
I agree with you but if you study a little of this it is no harm at all... It is knowledge.

:
Finally, it gives you another way to look at the picture. It gives, shall I say culture ? and that is good.
Let me join a little example.

The photo you will see at http://www.antoniocorreia.com/album/album50/Brasil_Iguasu_2005_01?full=1, was shot in Brasil Iguasu, a place to visit.
The lens - no question - is a supperb L USM 16 35 mm f/2.8 Canon.The body is a 20D what does not matter here.
Shutter Speed 1/320; Normal Program f/9 ISO 100 Focal lenght 16 mm.

It was focused to the water. Because the depth of field was not enought my wife and I are a little bit out of focus.
This can only be seen when you blow up the photo.
We are not so sharp as we shoul be if I had rotated a little bit the focus ring after having switched off the auto-focus.

This picture made me to search for information I knew the existence of.
I'm sure you have seen on some old - say 30 years - zoom lens, the graphics on the cylinder. In blue for the f/5.6 as a can remember.
This was quite usefull (for the connoisseur).
Thank you for your point of view and forgive ye bad english for I have studied it only for 3 years.

:

Sam · November 1, 2005

Depth of field: This refers to how deep the field is. You can grow different crops based on this.

Depth of focus: This has nothing to do with depth of field. This has to do with ones ability to see past the surface and focus more deeply to find the burried meaning. If say you drink a glass of fine wine, and all you can think of is "a couple more of these and I can get a good buzz on" you have a very limited depth of focus....Or a very bad first date.

Circle of confusion: Liberals trying to discuss facts.

Sam

TOF guy · November 2, 2005

Shay Stephens wrote:

I studied the topic of circles of confusion and related info but have found very little practical application for this knowledge and see it as a needless complication in the photographic process. The fact that you can go your whole life without having to know this info and yet succeed in photography tells me all I need to know ;-)

It may not be very useful to know what a circle of confusion is, but personally I like the fact of knowing the principle behind the concept of depth of field

.

Shay Stephens wrote:

It can all be boiled down to a few rules of thumb that are easy to remember and implement.

Please share these rules with us

!

Thierry

Shay Stephens · November 2, 2005

All you need to know about DOF

TOF guy wrote:

It may not be very useful to know what a circle of confusion is...

Exactly

Please share these rules with us !

You already know them:

Wide aperture = small DOF...............Small aperture = wide DOF
Telephoto = small DOF.....................Wide angle = wide DOF
Close focus on subject = small DOF....Far away focus = wide DOF

Do you want really wide DOF? Use a small aperture (e.g. f/32), wide angle lens (e.g. 28mm), and focus far away from the lens (e.g. try infinity)

Want crazy small DOF instead? Use wide aperture (e.g. f/2.8), telephoto lens (e.g. 200mm), and focus up close to the lens (e.g. closest focus distance).

Want something in between? Modify one or more of the above variables to be somewhere between the extremes. You can add salt and pepper to taste, bake for 20 minutes, and enjoy

Antonio Correia · November 3, 2005

Shay Stephens wrote:

Exactly

You already know them:
Wide aperture = small DOF...............Small aperture = wide DOF

Telephoto = small DOF.....................Wide angle = wide DOF

Close focus on subject = small DOF....Far away focus = wide DOF

Do you want really wide DOF? Use a small aperture (e.g. f/32), wide angle lens (e.g. 28mm), and focus far away from the lens (e.g. try infinity)

Want crazy small DOF instead? Use wide aperture (e.g. f/2.8), telephoto lens (e.g. 200mm), and focus up close to the lens (e.g. closest focus distance).

Want something in between? Modify one or more of the above variables to be somewhere between the extremes. You can add salt and pepper to taste, bake for 20 minutes, and enjoy

Plain and simple.
Good point Shay !

Antonio Correia · November 3, 2005

TOF guy wrote:

It may not be very useful to know what a circle of confusion is, but personally I like the fact of knowing the principle behind the concept of depth of field .
Thierry

I like things here.
People are interesting !
Good point Thiery

TOF guy · November 3, 2005

Antonio Correia wrote:

link for a DOF calculator.
http://www.dofmaster.com/

Meanwhile here is a site which gives differents formulas from the one above:
http://www.nikonians.org/html/resources/guides/dof/hyperfocal2.html

I think here they are wrong because I tried the formulas in Excel.

These two sites use the same formula but ... different definitions for the circle of confusion. Confusing, isn't it

?. Nikonians.org also lets you enter the value of the CoC, and if you enter the same number as the one found on the other web site for a specific media you should get the same results.

But who is right

?

The latter (nikonians.org, shared for us by J. Ramón Palacios - who I don't know, but he deserves to be acknowledged for his efforts) uses a value that is close to Carl Zeiss's recommendation (1/1730th of the length of the "negative" diagonal ). The former is based on a more "generous" value that is the one used by Nikon for the markings on its lenses - when there used to be markings :cry (funny, you'd think it would be the other way, with the Nikon CoC definition used by nikonians.org

rather than the Zeiss value. Kudos, Ramón

!). Nikon was long criticized for this: all photographers who wants a very precise DOF for a specific picture will tell you that the value proposed by Zeiss is the one to start from (sometimes a smaller value is used, adjusted for a specific enlargement).

Thierry

TOF guy · November 3, 2005

Shay Stephens wrote:

You already know them:

But few people would have explained these rules as well as you did. Thanks, Shay

!

Thierry

Antonio Correia · November 4, 2005

TOF guy wrote:

These two sites use the same formula but ... different definitions for the circle of confusion. Confusing, isn't it ?. Nikonians.org also lets ... ...Thierry

Good work Thierry.
Thank you.

erich6 · November 8, 2005

Hi everyone. Glad to see a revival of this topic.

Shay, I agree with you on the practical application of these concepts. That's why I added the caveat that I was trying to reconcile my techie perspective with a practical one.

Since I posted this I've learned a lot from just shooting. I find myself worrying less and less on the specific settings I'm applying and more and more on naturally using the rules of thumb Shay laid out.

That said, there is a venue for working through these technical details. I can see it in terms of comparing lenses in a quantitative way for the purposes of design evaluation, etc. Useful for the lens makers, maybe not so much for the shooter. On the other hand, having insight into both should give you a, shall we say, "more complete picture"?

One item I didn't mention in my original post was the effect of distance to subject. What happens here is that if you are close to the subject, it doesnt take much distance in depth for the image to be out of focus on the focal plane. On the other hand, if you are far way, the same depth doesn't span as large a range at the image plane and therefore is in focus (within the circle of confusion).

Erich

chet79 · August 14, 2006

I have a question for the DOF-masters out there. I am quite new to the technicalities of photography, and don't have a dslr to practise on. I'm just keen to learn as much as possible before I do get one.

Anyway, my question is this:

I understand how to get a shallow DOF, by focusing in close to the end of the lens. But what if I want a shallow DOF, but in the middle of my shot? i.e. I want a very narrow band, in the middle of my shot, to be in focus (and everything before and after it, out of focus).

like this (OOF = out of focus):

devbobo · August 14, 2006

chet79 wrote:

I have a question for the DOF-masters out there. I am quite new to the technicalities of photography, and don't have a dslr to practise on. I'm just keen to learn as much as possible before I do get one.

Anyway, my question is this:

I understand how to get a shallow DOF, by focusing in close to the end of the lens. But what if I want a shallow DOF, but in the middle of my shot? i.e. I want a very narrow band, in the middle of my shot, to be in focus (and everything before and after it, out of focus).

like this (OOF = out of focus):

Andrew,

The problem here is, the technique that you would need to adopt to do this in a point and shoot camera is unneccessary with a dSLR.

With a P&S, say you want a certain shot with a narrow DOF at about 1-2m from a subject. You would need to adjust the zoom and move backwards away from the subject until you achieved the desired framing and narrow enough DOF. This is due to the small sensor size, that even at large apertures everything is in focus.

Hope this helps,

David

chet79 · August 14, 2006

David, I had assumed it would be near-impossible with a p&s actually. I guess with experimentation, I could pull it off.

I was actually wondering how it would be done with a dslr

devbobo · August 14, 2006

chet79 wrote:

I was actually wondering how it would be done with a dslr

set a large aperture (eg f2) then starting with the closest focal point slowly adjust the focus and the field of focus will slowly away from you... like in your diagram.

Antonio Correia · August 17, 2006

I found this.
Hope to be usefull.
http://www.night-ray.com/photores/canon16_dof_10-50mm.png
the previous:
http://www.night-ray.com/photography.php?id=6

erich6 · August 22, 2006

chet79 wrote:

David, I had assumed it would be near-impossible with a p&s actually. I guess with experimentation, I could pull it off.

I was actually wondering how it would be done with a dslr

The lens focus ring allows you to adjust what object distance gets to be at "best focus". Some of the better lenses will actually show you what distance you are focused at. If you use a large aperture you will limit the range that is in focus. This affects both the objects before and after the location you focused at.

The longer the focal length the shallower this in-focus region will be. So, if you want to have a very specific and limited range in-focus you should be using a telephoto lens. Shorter focal length lenses will give your more DOF which means more will be in-focus and it's harder to limit the range where objects look sharp.

Erich

Depth of Field, Depth of Focus, and all Circles of Confusion

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