I Need A Headache

Snapshot · May 7, 2014

Thanks everyone for your input. BTW, it worked!

Jim Jones · May 7, 2014

When images of identical sizes are compared, DOF is determined ONLY by the entrance pupil of the lenses used. Simple enough?

Thomas Bertilsson · May 7, 2014

Easy.

It has to do with the size of the aperture.

The lens has aperture markings, like f/2.8 of f/8. f stands for 'focal length' and when you divide the focal length 'f' of your lens with the aperture marking you get the actual size, in millimeters, of your aperture.

So, a 25mm aperture on a 35mm lens, a 50mm lens, and an 80mm lens will give exactly the same depth of field.

On a 35mm lens that would be f/1.4, because 35 / 1.4 = 25.
On a 50mm lens that would be f/2, because 50 / 2 = 25.
On an 80mm lens that would be f/3.2, because 80 / 3.2 = 25.

The film format has nothing to do with it.

Snapshot · May 7, 2014

Thomas Bertilsson said:
Easy.

It has to do with the size of the aperture.

The lens has aperture markings, like f/2.8 of f/8. f stands for 'focal length' and when you divide the focal length 'f' of your lens with the aperture marking you get the actual size, in millimeters, of your aperture.

So, a 25mm aperture on a 35mm lens, a 50mm lens, and an 80mm lens will give exactly the same depth of field.

On a 35mm lens that would be f/1.4, because 35 / 1.4 = 25.
On a 50mm lens that would be f/2, because 50 / 2 = 25.
On an 80mm lens that would be f/3.2, because 80 / 3.2 = 25.

The film format has nothing to do with it.

Whoah! I bow to your mastery. :w00t:

dorff · May 8, 2014

Thomas Bertilsson said:
Easy.

It has to do with the size of the aperture.

The lens has aperture markings, like f/2.8 of f/8. f stands for 'focal length' and when you divide the focal length 'f' of your lens with the aperture marking you get the actual size, in millimeters, of your aperture.

So, a 25mm aperture on a 35mm lens, a 50mm lens, and an 80mm lens will give exactly the same depth of field.

On a 35mm lens that would be f/1.4, because 35 / 1.4 = 25.
On a 50mm lens that would be f/2, because 50 / 2 = 25.
On an 80mm lens that would be f/3.2, because 80 / 3.2 = 25.

The film format has nothing to do with it.

If you were to increase the format size linearly with the focal length of the lens you use, in your example above, then for the output, i.e. the printed image (considering you reduce the enlargement factor linearly as well), what you say is correct. Interestingly, if I were to frame an object the same size, using two different focal lengths on the same format size, then depth of field depends only on the f-stop and not the effective diameter. If one then enlarges the background of the wider lens, it will contain the same depth of field for out of focus items as the longer lens. The wider lens appears to have more depth of field, because it includes a wider background. What it means is that if I photograph a bird, for instance, with a 100 mm lens at f/4, and a 500 mm lens at f/4, and take care to make sure the bird fills the same fraction of the frame, the depth of field on the bird itself will be the same. However, the background will look different because of the difference in angle of view, meaning the 500 isolates a smaller part of the background than the 100.

It is the output that is important. Hence, the depth of field as observed on the output print, not the image the lens casts, leads one in defining the circle of confusion.

Because lenses are complex in design, it is not so helpful to talk about the diameter of either the lens or the aperture. The effective diameter is what counts, and that is calculated as focal length/f-stop. At least the manufacturers know what real diaphragm opening corresponds to what f-stop. I tend to take their word for it.

For a more comprehensive explanation:
http://toothwalker.org/optics/dof.html

Jaf-Photo · May 8, 2014

Snapshot said:
Thanks everyone for your input. BTW, it worked!

Yeah, my headache hit me on the second page already.

Great stuff, how can I get my enemies to read it?

Thomas Bertilsson · May 8, 2014

dorff said:
If you were to increase the format size linearly with the focal length of the lens you use, in your example above, then for the output, i.e. the printed image (considering you reduce the enlargement factor linearly as well), what you say is correct. Interestingly, if I were to frame an object the same size, using two different focal lengths on the same format size, then depth of field depends only on the f-stop and not the effective diameter. If one then enlarges the background of the wider lens, it will contain the same depth of field for out of focus items as the longer lens. The wider lens appears to have more depth of field, because it includes a wider background. What it means is that if I photograph a bird, for instance, with a 100 mm lens at f/4, and a 500 mm lens at f/4, and take care to make sure the bird fills the same fraction of the frame, the depth of field on the bird itself will be the same. However, the background will look different because of the difference in angle of view, meaning the 500 isolates a smaller part of the background than the 100.

It is the output that is important. Hence, the depth of field as observed on the output print, not the image the lens casts, leads one in defining the circle of confusion.

Because lenses are complex in design, it is not so helpful to talk about the diameter of either the lens or the aperture. The effective diameter is what counts, and that is calculated as focal length/f-stop. At least the manufacturers know what real diaphragm opening corresponds to what f-stop. I tend to take their word for it.

For a more comprehensive explanation:
http://toothwalker.org/optics/dof.html

That's too bloody complicated to think about. Congratulations to you.

I have used my method for many years and it works well for real world decisions.

dorff · May 8, 2014

Thomas Bertilsson said:
That's too bloody complicated to think about.

Okay, let's try and make it simpler :

If you want to get the same depth of field on a single format using different lenses, then use the same f-stop and make sure magnification of the subject is the same.

If you want the same depth of field using different film formats with "equivalent" focal lengths, make sure your perspective is the same, and use the same effective lens diameter.

Thomas Bertilsson · May 8, 2014

dorff said:
Okay, let's try and make it simpler :

If you want to get the same depth of field on a single format using different lenses, then use the same f-stop and make sure magnification of the subject is the same.

If you want the same depth of field using different film formats with "equivalent" focal lengths, make sure your perspective is the same, and use the same effective lens diameter.

No, it can be understood. I just don't want to, because it doesn't actually help me in practical terms. It gets in the way of making photographs. My own easy calculation works very well in practical life, and that's what I want.

dorff · May 8, 2014

Thomas Bertilsson said:
No, it can be understood. I just don't want to, because it doesn't actually help me in practical terms. It gets in the way of making photographs. My own easy calculation works very well in practical life, and that's what I want.

No problem, Thomas. I tried to simplify for the sake of others (if they cared). Yours is a useful guide, but you have to qualify it as aiming for the same output with different formats, something which was not clear from your post.

I can't recall the number of fallacious arguments around depth of field I've been witness to. It is no wonder that so many people are bewildered by the subject. And yet it is not that complicated. The related topic is how far one can stop down before diffraction takes away what you gain in field depth, and there is usually someone who touts the wisdom that changing format is the answer. Well, in that one sense at least, all formats are equal.

Jaf-Photo · May 8, 2014

dorff said:
No problem, Thomas. I tried to simplify for the sake of others (if they cared). Yours is a useful guide, but you have to qualify it as aiming for the same output with different formats, something which was not clear from your post.

I can't recall the number of fallacious arguments around depth of field I've been witness to. It is no wonder that so many people are bewildered by the subject. And yet it is not that complicated. The related topic is how far one can stop down before diffraction takes away what you gain in field depth, and there is usually someone who touts the wisdom that changing format is the answer. Well, in that one sense at least, all formats are equal.

I can't really 'think' depth of field. But I can develop a feel for how different combinations of lenses produce it. So I can shoot dof on instict. For me that's faster than thinking about it.

Thomas Bertilsson · May 8, 2014

Jaf-Photo said:
I can't really 'think' depth of field. But I can develop a feel for how different combinations of lenses produce it. So I can shoot dof on instict. For me that's faster than thinking about it.

That's how my brain works too. But the more I study something and read about it, the more it gets baked into my conscience, and then it becomes instinct instead of conscious thought. But obviously, the more complex the subject matter is, the more time it takes until it becomes natural. Practice will take care of that eventually, and I feel that is truly how we grow, by incorporating more things into our stream of subconscious things, by accumulating knowledge and then making it our own.

I usually summarize the process of making a photograph by making an analogy to that thought that is so often explained by those who nearly lost their life: 'life passed before my eyes in an instant'. With photography I know what my photo paper is capable of and its paper developer. I know what my film and film developer must be exposed and processed like in order to fit those qualities. Since I practiced so much with the same combination of materials, all those steps kind of flash by in my memory as a sort of chain reaction when I take the picture. It isn't a conscious thought, but more like a feel you get for what's in front of the camera lens, and how it will likely print some day.
Depth of field is one of those concepts that flash by, as is focus, feel, mood, composition, gesture, and so on.

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