# CoC and depth of field

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• 09-10-2011, 01:38 PM
David A. Goldfarb
Thanks. The mod functions in this interface can be hit or miss sometimes.
• 09-10-2011, 06:37 PM
Michael R 1974
Wow, I started a sticky thread! :)
• 09-10-2011, 06:49 PM
Michael R 1974
[QUOTE=Monito;1234252]
You can't just go with a given size for the CoC.

QUOTE]

But I would argue you can just go with CoC - as long as it is based on the variables you mention - ie film size, enlargment factor, and viewing distance (although viewing distance is kind of hard to specify). I'd even through in subject matter as a potential variable. There are likely others since perception is a complex thing. The point was once you have your preferred CoC that is what you need to balance against diffraction. Perhaps diffraction also first needs to be considered using the same variables. So I guess in the end I'm trying to figure out how far we need to stop down for sufficient depth of field, without stopping down too far, at which point any marginal depth of field (CoC) gains are more than offset by the effects of diffraction.
• 09-10-2011, 07:58 PM
JamesDean
I think this is covered well by Ken Rockwell. http://www.kenrockwell.com/tech/focus.htm

He argues that for any given scene/camera arrangement there will be an optimum aperture when the effects of depth of field blur and diffraction blur are equal. Deviating from this aperture will mean increasing one effect while decreasing the other. Since the sharpness of the image will be limited by whichever effect is greater, you can't do better than when they are equal.

Ken explains it much better than me. More importantly he goes on to show how to determine it. His formula is fairly theoretical, but it seems to give me good real world results.
• 09-10-2011, 10:58 PM
Hikari
This thread ended on the first response and was confirmed on the one following that.

BTW, I would not trust Ken Rockwell's subjective definition of permissible circle of confusion. There have been better minds than his working on this for over a century.
• 09-11-2011, 09:02 AM
JamesDean
Quote:

Originally Posted by Hikari
This thread ended on the first response and was confirmed on the one following that.

BTW, I would not trust Ken Rockwell's subjective definition of permissible circle of confusion. There have been better minds than his working on this for over a century.

If you think this thread is over then stop reading and go somewhere else. Grown-ups are talking now.
• 09-11-2011, 10:22 AM
keithwms
Rockwell's writeup on this particular issue actually isn't half bad. I landed on it many years ago when I got my mamiya 6, and found it useful. As I recall, his writeup is quite similar to a Popular Photography article that you can look up. So... try not to dismiss Rockwell's writeup on this just because he comes across as a complete ass on some other things; he probably can't help it :)

Quote:

Originally Posted by Michael R 1974
What I'm never sure about is whether to favour a smaller CoC with less depth of field or a larger CoC with more depth of field. Which will result in the higher perceived sharpness (all other factors remaining equal)?

The answer will depend on many things- not only pertaining to shooting but also to printing. E.g. the CoC becomes especially important at large print enlargement factors.

To me, it's important to remember something really fundamental about perception: we seldom miss what isn't there. If you have minor losses of fine detail here and there in a composition, in favour of more consistent focus throughout, I think you will find that the result is perceived favourably. But if you have regions of sharp detail near regions of blur, you will really notice that very quickly. So OOF transitions are an important thing. Think about a superfast lens in 35mm format- you can get disturbing bokeh lines where you least want them, if you're not careful.

But what you want of course depends on your composition and your intentions.

What I just wrote may sound vague so I will give a concrete example. Suppose you are taking a headshot of your dear uncle Bertie. He's got a pleasant, friendly face but (God bless 'im) he also has quite a lot of nose hair. Anyway, he won't be around much longer, so you want to take a nicely detailed shot for future generations to enjoy. You considered some petzvalish work with eyes in sharp focus and everything else creamy, but let's suppose you really want good detail throughout. Do you (a) give more value to critical sharpness and go for say f/8; or do you (b) go for consistent level of detail and go for f/11?

The answer, of course, is (c)... you do both, and you take multiples, because this is a valuable shot and you want to get it just right :D

The point is that you can debate this and come to many different but reasonable conclusions. And you can easily make a mistake (I think) of putting that plane of superduper sharpness right on the nose hair, and then what? At some almost subliminal level, the viewer will see that. Wouldn't you rather have consistent focus throughout?

I can't answer for you, you have to decide for yourself, maybe the nose hair is such a special thing about uncle Bertie... :)

So I'd say the bottom line is not to forget that the familiar definition of CoC is based in perception. If you really want to get the largest net amount of detail, the answer won't necessarily be appealing.... it depends on your artistic intentions. And the fact that we photographers concern ourselves with perceptual issues is what makes us artists. If we just followed some formula for maximum detail then, well...
• 09-11-2011, 11:16 AM
MaximusM3
Quote:

Originally Posted by keithwms
Rockwell's writeup on this particular issue actually isn't half bad. I landed on it many years ago when I got my mamiya 6, and found it useful. As I recall, his writeup is quite similar to a Popular Photography article that you can look up. So... try not to dismiss Rockwell's writeup on this just because he comes across as a complete ass on some other things; he probably can't help it :)

Yep..Ken actually is A LOT smarter than everyone thinks he is. Not only that, but ample credit must be given to him for actually talking a language that most photographers can actually understand and put into practice. That's an art in itself, especially today, when we need less complications to keep film alive.
• 01-30-2012, 06:55 AM
RalphLambrecht
Quote:

Originally Posted by Michael R 1974
A question about balancing depth of field and resolution. Bear with me this might be confusing if I don't write it out clearly enough. Assume 35mm negatives to be enlarged to around 8x10".

In my photographs I want everything sharp (read no selective depth of field). At the same time, whenever possible I prefer not to use apertures smaller than f11 as f16 seems to be where diffraction effects become readily noticeable. So there is always a balance that must be struck. I've started using my brother's nifty golf rangefinder to measure the distances in a scene before focusing. So I'll measure distances to the far away objects, the near objects, and/or the most important objects.

Now, most depth of field tables, including the scales on most lenses (which are not necessarily even accurate) assume a CoC of 0.03 or 0.032 as far as I can tell. But for film that needs to be enlarged a CoC this large is really pushing the limits of what I would consider acceptable sharpness. So I usually try to use a smaller CoC, arbitrarily something like 0.02 or lower if possible (I use the DoFMaster website to calculate depth of field for different CoC sizes using my distance measurements).

What I'm never sure about is whether to favour a smaller CoC with less depth of field or a larger CoC with more depth of field. Which will result in the higher perceived sharpness (all other factors remaining equal)?

Here's an example. Suppose for a scene I've determined f11 gives sufficient depth of field for a CoC of 0.025. Would it be better or not, to stop down to f16 (ie more diffraction) and theoretically get a CoC of 0.015 (ie more depth of field)?

And how much smaller does a CoC have to get from the standard 0.03 for there to be a visible improvement in resolution?

Is 0.02 significantly better than 0.03?

Is 0.01 significantly better than 0.03?

What is the lower cutoff point beyond which we can't really see an improvement (assuming a constant optimum aperture)?

Obviously the enlargement factor plays a major role...

Michael