# dof 16/35mm same same but different?

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### #1 Alexander pueringer

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Posted 22 July 2008 - 07:28 AM

Maybe it was already discussed and I couldn?t find it, so I post this question anyways.

If you frame a certain picture, lets say a close up with a 50mm lens you get a certain dof. Now if you frame the exact same picture with a 100mm lens form further away, you should get the same dof, right?

Now if you shoot on 16mm instead of 35 and you want to get the very same frame as mentioned above, shooting from the same distance, you would have to use a 25mm lens -> so less dof, right?
So if you now use a 5omm lens on the 16mm camera and go further away, you stay with the same dof again, as with the 25mm (on 16mm).

In my understanding there?s no way to create the 35mm (meaning the format) dof, with a 16mm camera!? (Clearly all the setups are meant to have the same f-stop)

Please just tell me if I?m right or not, because this topic kept me discussing with dops, directors and other assistants for hours.

Thx al

p.s. I?m aware the focal length doesn?t only affect the dof, but also the feel for distances in the picture and so on, but that?s not the point in my discussions...
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### #2 David Mullen ASC

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Posted 22 July 2008 - 09:42 AM

You'd have to change the f-stop by two stops to compensate for the DOF differences between 16mm and 35mm. Though stylistically, using longer lenses and wider apertures and tighter framing will help give the overall impression of less depth of field since you'll see more out of focus backgrounds.

But mathematically you are right more or less, changing focal length alone won't help, you need to change the aperture. Look at a depth of field chart to get the exact figures.
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### #3 David Auner aac

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Posted 22 July 2008 - 11:07 AM

Please just tell me if I?m right or not, because this topic kept me discussing with dops, directors and other assistants for hours.

Hi Alex,

good luck on your crusade. And as David pointed out, you're right, of course!

Cheers, Dave
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### #4 Alexander pueringer

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Posted 22 July 2008 - 07:46 PM

Thank u guys very much! I have to confess, I was pretty shure I’m right!

You'd have to change the f-stop by two stops to compensate for the DOF differences between 16mm and 35mm

Thanks for that tip especially, never heard that before!

greets al

Edited by Alexander Püringer, 22 July 2008 - 07:46 PM.

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### #5 Chris Keth

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Posted 22 July 2008 - 08:51 PM

The simplest way to state depth of field I have found is this:

Two things and only two things affect depth of field. (1.) Magnification, which is the product of focal length and subject distance. and (2.) f/stop of the lens.

There are even rules for the change in depth of field when one of these variables changes:

If you double focal length, the depth of field will be cut in half.

If you halve the subject distance, depth of field is halved.

For each f/stop that you open up, depth of field is divided by the square root of 2.

Edited by Chris Keth, 22 July 2008 - 08:54 PM.

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### #6 Tim O'Connor

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Posted 22 July 2008 - 09:15 PM

You'd have to change the f-stop by two stops to compensate for the DOF differences between 16mm and 35mm. Though stylistically, using longer lenses and wider apertures and tighter framing will help give the overall impression of less depth of field since you'll see more out of focus backgrounds.

But mathematically you are right more or less, changing focal length alone won't help, you need to change the aperture. Look at a depth of field chart to get the exact figures.

Can you say why two stops? Also, if this is for R16, would you open up proportionately differently for S16 to do the same thing?

Thanks!
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Posted 22 July 2008 - 09:33 PM

AFAIK it has to do with 16mm is 1/2 the size of 35mm
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### #8 Tim O'Connor

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Posted 23 July 2008 - 12:10 AM

AFAIK it has to do with 16mm is 1/2 the size of 35mm

I got that far but when Chris mentioned the part about the square root of two, I wasn't sure how it is that it works out to opening
up two stops as opposed to say one.

I am sure that David is right. I just want to be sure that I understand the how and why of it.
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### #9 David Auner aac

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Posted 23 July 2008 - 12:35 AM

Two things and only two things affect depth of field. (1.) Magnification, which is the product of focal length and subject distance. and (2.) f/stop of the lens.

Hehe, I knew that either you or Stephen (Williams) would come up with that definition. And if neither of you, I would have. Image reproduction ratio, or magnification as you call it, is influenced by the format as well. So it's a product of subject distance, focal length and film gauge.

Cheers, Dave
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Posted 23 July 2008 - 09:24 AM

Tim, I'm doing quite well. Too hot in Philadelphia for me right now though.
for the square root thing, I can only image this has to do with the F scale being squares? Opening up one stop doubles the light going in, where as closing down 1/2s it? I would think -- though I am no uber genius at math -- this would account sqrt-2 mentioned above.
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### #11 Chris Keth

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Posted 27 July 2008 - 08:09 PM

Hehe, I knew that either you or Stephen (Williams) would come up with that definition. And if neither of you, I would have. Image reproduction ratio, or magnification as you call it, is influenced by the format as well. So it's a product of subject distance, focal length and film gauge.

Cheers, Dave

I know I'm splitting hairs but format doesn't affect depth of field. You can place any format film or sensor behind an imaging system and the depth of field will not change. Format will affect your choices of lens and circle of confusion, both of which which will affect DoF, however.

In a practical sense, you are correct. I just don't want someone to come along and think that as soon as you put different film behind a lens that its depth of field properties change. I kind of take it for granted anyway that if someone is thinking about depth of field at all, that their format and circle of confusion for that format is already chosen and will be a constant.

And Adrian, about the square root of 2 thing, it's something like what you said. I do have the whole answer but it's pretty long and involved. In short, depth of field is not a linear progression. It increases at the rate of "1.4x" per stop closed down or at the rate of "/1.4" when opened up. Graphed it looks like half of a parabola approaching but never reaching zero nor infinity.

Edited by Chris Keth, 27 July 2008 - 08:11 PM.

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### #12 David Auner aac

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Posted 28 July 2008 - 02:09 AM

I know I'm splitting hairs but format doesn't affect depth of field. You can place any format film or sensor behind an imaging system and the depth of field will not change. Format will affect your choices of lens and circle of confusion, both of which which will affect DoF, however.

In a practical sense, you are correct. I just don't want someone to come along and think that as soon as you put different film behind a lens that its depth of field properties change. I kind of take it for granted anyway that if someone is thinking about depth of field at all, that their format and circle of confusion for that format is already chosen and will be a constant.

Hi Chris,

I love splitting hairs, especially optical ones! Well, Ok, so let me rephrase. CoC will affect my DoF and will itself be affected by the size of my sensor/film. I'm coming, as you probably know, from a photography background and have always been working in formats from APS-C up to 7" by 5". In photography CoC is rarely talked about BTW. It's not something you choose prior to shooting, as you probably would in 35mm cine.

Cheers, Dave
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### #13 Chris Keth

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Posted 28 July 2008 - 04:41 PM

Hi Chris,

I love splitting hairs, especially optical ones! Well, Ok, so let me rephrase. CoC will affect my DoF and will itself be affected by the size of my sensor/film. I'm coming, as you probably know, from a photography background and have always been working in formats from APS-C up to 7" by 5". In photography CoC is rarely talked about BTW. It's not something you choose prior to shooting, as you probably would in 35mm cine.

Cheers, Dave

That I agree with 100%.

I hear you about rarely worrying about CoC in stills. I have only calculated my DoF in shooting stills a couple of times. Through some combination of movements, nice efficient strobes, and the possibility of longer exposures it's not often a concern. I can just look at it and decide right there.

Edited by Chris Keth, 28 July 2008 - 04:42 PM.

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