Getting DOF straight

[CollinB]

I have one lens that is used on 4x5 and 8x10.
Caltar II-N (aka Sironar N) 240mm.

To focus on subject X would require the same bellows draw from either camera.
http://www.bobatkins.com/photography/technical/dofcalc.html
Yet DOF calculators take into regard the film format. Should they?
If the lens is set for f32 then the in-focus area remains the same, does it not?
What am I missing?

 

[AnselAdamsX]

I've been wondering about that myself. They also use a different circle of confusion between 4x5 and 8x10. I assume this is because your not expected to enlarge as much.

 

[terrywoodenpic]

The Circle of confusion used in each calculation differs for each format size. because the "expected" enlargement factor used for viewing the images differ. It is solely the COF factor that changes the DOF calculation.
Of course the physical focus properties do not change.

 

[MattKing]

Walk the 4x5 camera back until the field of view is the same as with the 8x10 camera. When you re-focus, your depth of field will have changed, because the camera to subject distance has changed.

 

[Ian C]

DOF is a function of:

Aperture number = N

Circle of confusion = c (usually taken as 1/1500 the diagonal of the image rectangle on film)

Magnification = m

Focal length = f

For a lens that covers multiple formats, DOF must be calculated for each format. The DOF formulas must consider the format (determines the value c).

This is a reasonable explanation of the circle of confusion and why it varies by format.

http://en.wikipedia.org/wiki/Circle_of_confusion

 

[eclarke]

Don't over think this stuff, I use view cameras from 6x9 cm to 11x14". Tilt until it's fairly sharp, focus on the middle, stop down enough and you'll be OK. It's a visual art not mathematical...EC

 

[John Koehrer]

+1 on the above.

walking the camera back isn't going to change the DOF if the bellows draw is the same.
When you talk theory or practicality the math may change but the DOF as viewed should be the same for a given aperture.
As an example look at the same scene, with the two different cameras, same lens. DOF should be the same.

 

[Leigh B]

Yet DOF calculators take into regard the film format. Should they?

As others have stated, that's because they use different Circle of Confusion (CoC) sizes for different film formats, based on the assumption that larger formats will undergo less enlargement, so a larger CoC on the film is acceptable.

I personally think that's nonsense, but nobody asked me.

- Leigh