Equivalent Depth of Field

[Pieter12]

I don't give a rat's ass about the details of the theory and physics involved, just a general grasp of what's going on. The site in post #3 is really all I need. I want to make pictures.

Let me clarify. I respect the physics involved and those who fully understand and implement it. It's just a bit more than I need to make the pictures I want.

 

[Sirius Glass]

Like the other reply, I care not about the equation behind the result, I care about the RESULT.

• I know conventional non-macro shot is based upon the size of the subject in the frame: same size, same DOF regardless of FL
• I know in macro shooting, even with same size subject in the frame, the shooting FL does not really matters, and effectively f/stop gives more control

...that is all that matters. I don't carry Excel on tablet or smartphone to compute an otherwise difficult to solve equation. In in macro, the DOF is so darn shallow that

Any photograph, whether or not macro, when the depth of field is critical, I always focus and stop down to see exactly what I will get. I do not bother with charts, tables or interactive tools because I need to see what I will get.

 

[Pieter12]

Any photograph, whether or not macro, when the depth of field is critical, I always focus and stop down to see exactly what I will get. I do not bother with charts, tables or interactive tools because I need to see what I will get.

I usually do, too. But when using strobes, the modeling lights are usually not strong enough to provide sufficient illumination to verify focus especially in shadow areas and at the edges of the frame at a small aperture, say f/22 or 32. I usually have a flashlight handy to light up those areas, but the whole procedure can sometimes be awkward. I was looking for a way to compare the focus in the FF digital test shot to what I could expect on MF film.

 

[Sirius Glass]

I usually do, too. But when using strobes, the modeling lights are usually not strong enough to provide sufficient illumination to verify focus especially in shadow areas and at the edges of the frame at a small aperture, say f/22 or 32. I usually have a flashlight handy to light up those areas, but the whole procedure can sometimes be awkward. I was looking for a way to compare the focus in the FF digital test shot to what I could expect on MF film.

I would use a bright light to focus. It does not need to be large.

 

[Ian C]

I think that the title of post #1 should have been, Same Depth of Field—Two Different Cameras and Lenses. The following illustrates how I deal with the question. It can easily be done for two different cameras and lenses. First do it with you digital camera. Then redo it with the second camera and lens combination.

I’ve long done macro photography. Tabletop is just another version in which we shoot with the lens angled downward on objects arrayed on a flat surface at varying distances from the lens. There are indeed formulas that give us the information to get the results we want, provided that they're possible. These formulas require taking some measurements and doing calculations.

I use a programmable scientific calculator to record formulas in a practical form that minimizes the labor in using them. I use a now-obsolete Hewlett-Packard 48GX calculator from 1996. Much cheaper calculators can be bought that do all that’s needed.

I recorded a program that I’ve labeled sN to compute subject distance s and required aperture number N. In functional notation I write

sN(a, b, c, f) to remind me that I have to enter the variables in the parentheses in this order from left to right.

a = the near limit of DOF (measured forward from the first nodal point of the lens).

b = the far limit of DOF

c = the circle of confusion diameter for the format in use

f = focal length. All values must be expressed in millimeters.

Here is an example. Suppose that I’ve composed and approximately focused a tabletop setup with the closest point that I require within the DOF as a = 600 mm, b = 700 mm (measured from the approximate position of the first nodal point of the lens). The circle of confusion for the 35 mm format and for a full-frame digital camera are both about c = 0.029 mm. With a 50 mm lens, f = 50 mm.

I calculate sN(600, 700, 0.029, 50) by entering these numbers in the given order and pressing the key that I’ve labeled “sN”. The screen now displays 646.15 on the top line and 11.12 on the line below it.

This means that I should place an object about 646 mm forward of the first nodal point of the lens and focus upon it, and then remove the focusing target from the setup. Then I set the aperture to f/11 and shoot. Note that this requires you to have an approximate idea of the location of the first nodal point of the lens in order to make the necessary measurements.

Now suppose that I want to repeat this with a 6 x7 cm camera equipped with a 90 mm lens. In this case I’ll take c = 0.059 mm. To maintain the perspective, I’ll position the camera so the first nodal point of the lens is at the same distances: a, s, and b as before. Then

sN(600, 700, 0.059, 90) = 646.15 and 19

meaning that the subject distance is indeed the same as before and that the required aperture is f/19. This is obtained by setting the aperture to f16 and then closing it halfway between f16 and f/22. Or I can simply set the aperture to f/22 and obtain a marginally-deeper DOF (such a small difference is unnoticeable).

I find this a fast and practical way do this.

The required formulas are:

s = 2ab/(a + b)

N = [(f^2)/c](a - b)/[b(a – f) + a(b – f)]

Once you’ve programmed the calculator, there’s no math necessary. The calculator does all the work.

The same formulas apply to general photography in which a finite DOF is required. For example, for an 4” x 5” camera and a desired acceptably-sharp field from a = 6 m (6,000 mm) to b = 40 m (40,000 mm), and a 210 mm lens (for 4” x 5” format, c = 0.1 mm)

sN(6,000, 40,000, 0.1, 210) = 10,434 and 31.88

The ideal subject distance is s = 10.4 meters and the required aperture is f/32.