Depth of field

[Helen B]

Does anyone actually use this stuff for anything? It's all very fun to muck around with the arithmetic, but there are so many variables for so little result I have never bothered.

I crank the aperture down enough until there's enough DoF for what I am doing, and then they press the button.

How do you decide how far to stop down?

I think that DoF theory is quite widely used to decide on the aperture that is required to achieve the required DoF or lack of it. DoF calculators have been around for a very long time - the best, like the Samuelson, having a user selectable MACC (maximum acceptable circle of confusion) because that is where the knowledge and understanding comes in. Most people know that it isn't accurate science, but it is useful in practice and there's no harm in understanding it. You have nothing to lose. On the other hand many people can get away without it or an understanding of it.

Nowadays I have a good feel for DoF, but I still use a DoF calculator now and then for critical for-print work (where the MACC can be derived from the print quality) and 40 years ago I certainly used a DoF calculator quite frequently. Using a calculator or lens markings is just passing the maths on to someone else. Plugging numbers into a calculator without understanding what the calculator is doing is not my preferred way of working, but that doesn't hold for everyone, especially those who have difficulty understanding what the calculator is doing - which is OK of course. Everyone's different.

Do you think that DoF comparisons between formats are useful? (Gavjenks simplistic and not-entirely-correct rule won't be any help there)

 

[Helen B]

In fact, let's go ahead and DO what I was talking about. Here is a point of light from a real life photograph:

View attachment 47605

Is this point of light within the DOF?

To answer this question, you do not need to know anything else about the photo. You don't need to know how tight of a crop this is. You don't need to know whether this was the sharpest point in the photo or not. You don't need to know anything about it.

All you need to know is "does this appear to me as a single one dimensional point? Or does it look like a circle with actual area to it?" If you perceive it as a point, then yes, this point is within the DOF of the overall image for you as a viewer. If a circle, then no, it is not within the DOF of the overall image for you as a viewer.



Note that your answer might change if you viewed this image from 35 feet away versus 1 foot away on your monitor. At some point, it will appear to be a dot, and at that point, it will actually be within the DOF of the image! Aperture didn't change, nor did focal length or anything like that. Only the viewing conditions changed, and yet DOF changed.

Similarly, if this were 35 feet away and you saw it as a point while you had your glasses on, it would be in the DOF, but if without your glasses, you might not be able to see it as a point, in which case it is not in the DOF anymore.





The circle of confusion in all of these situations would be exactly the same, 0.2765mm or whatever it is. But the DOF would not be the same.

DoF theory uses the maximum acceptable circle of confusion as the criterion. In you examples the MACC is not the same at 35 ft and 1 ft. You don't seem to understand this subject either.

 

[Gavjenks]

DoF theory uses the maximum acceptable circle of confusion as the criterion. In you examples the MACC is not the same at 35 ft and 1 ft. You don't seem to understand this subject either.

???

I know, MACC is not the same at 35 ft and at 1 ft. And I stated that DOF would also probably not be the same at 35 feet and at 1 foot.

How is this inconsistent with what you just stated?



My point is simply that what is "Acceptable" depends on conditions of viewing, including viewer's eyesight, how far they are from the print, etc. etc., making it impossible to calculate at the time of taking the photo, thus requiring you to eventually rely on fuzzy rules of thumb when in the field, not on hard math alone.

 

[Helen B]

It's not impossible to come up with a practically useful estimate of MACC when you know the likely viewing conditions - which is quite common.

You do seem to be disproving your own simplistic rule with this example. Aperture and magnification at the sensor/film stay the same (your definition of magnification), but DoF varies. You do seem to suggest now that it is more complicated than you first stated.

 

[unpopular]

In fact, let's go ahead and DO what I was talking about. Here is a point of light from a real life photograph:

View attachment 47605

Is this point of light within the DOF?

the only reason why this would seem to work is because the COF within DOF is small enough that it renders in focus as a point of light, and not as a bokeh circle. Truely, I do not kow if this is a low resolution telephoto lens from the early 1970's properly focused at infinity, or a modern lens properly focused at a half meter but recorded at resolution so high that the COF become apparently visible.

If I assume that the subject were taken with a modern lens at ideal distance using an ordinary recording medium, then I would surmise that it is outside of range of acceptable sharpness.

But because focus itself is a theoretical concept, not knowing how the image was recorded and using what kind of lens it was captured with, I cannot say with certainty if it is within DOF.

Now I know that tis seems like I am saying that DOF cannot be measured, but this is not really the case. What I am saying is that it can be measured, but only not in the faulty terms which we think of DOF and focus since focus itself cannot be measured in terms of depth.

But yet

For instance, (100 yards - 300 yards) is a range. It is not relative at all. It references very specific absolute values. (400 yards - 600 yards) covers the same relative distance, but is NOT the same range as (100 yards - 300 yards).

300 yards is -200 yards relative to 100 yards, likewise, 100 yards is 200 yards relative to 300 yards. A range represents the relative value of respective upper and lower limits.


I'm kind of done spinning circles going over rudimentary concepts.

 

[o hey tyler]

This thread makes me want to club a baby seal. And don't get me wrong, I think baby seals are awesome. This thread is just talks in more circles than a connect-4 set.

 

[amolitor]

Does anyone actually use this stuff for anything? It's all very fun to muck around with the arithmetic, but there are so many variables for so little result I have never bothered.

I crank the aperture down enough until there's enough DoF for what I am doing, and then they press the button.

How do you decide how far to stop down?

I stare at it and hope for the best! I am not doing the kind of critical work that pays your bills, to be sure, and I am generally NOT in a position to know my MACC anyways. Since it's a hobby and not work, things like print size come later for me than for you. AND I work in a way that is, well let's say that it's "more open to serendipity" rather than the uglier "I shoot a buncha crap and sift through the results to see if there's something there".

So, to be perfectly fair, I don't actually have strong criteria for DoF going in, except a pretty vague notion of "enough to get THAT thing pretty sharp"