Depth of Field

[wolfnowl]

Hi Folks:

Was asked this question today and I didn't have answer so I thought I'd ask the experts.  Get your charts and graphs ready!

Let's say you have a 28-70mm f/2.8 lens and you make an image wide open at 70mm.  Now, you take a 70-200mm f/2.8 lens and you make an image wide open at 70mm.  Is the depth of field going to be the same?  It seemed to me that it should be but I couldn't provide a definitive answer.

Thanks,
Mike.

[pegelli]

In case the two settings of 70 mm focal length are indeed correct (manufacturers are sometimes liberal in "rounding" these numbers) both the field of view and the depth of field are identical. These are only a function of focal length and aperture, and not depending on other aspects of the optical design.

[Christoph C. Feldhaim]

Doesn't this also depend on the focal length on the camera side?
If I'm not totally mistaken lenses for reflex cameras have a retrofocus design which, if I got it right results in different focal lengths on both sides of the lens.
Shouldn't this impact DOF ?
I always wanted to know this, since I wanted to do calculations of DOF for myself to compare lenses
of various film/sensor formats, especially with respect to the different f-stops available in the respective form factor.

[Bart_van_der_Wolf]

Hi Folks:

Was asked this question today and I didn't have answer so I thought I'd ask the experts.  Get your charts and graphs ready!

Let's say you have a 28-70mm f/2.8 lens and you make an image wide open at 70mm.  Now, you take a 70-200mm f/2.8 lens and you make an image wide open at 70mm.  Is the depth of field going to be the same?  It seemed to me that it should be but I couldn't provide a definitive answer.

Hi Mike,

The answer is; possibly close, but likely to be a bit different ...

Chances are that the DOF is pretty similar, but due to differences in the optical design (factored in as the "pupil magnification" factor), there can be differences. Focus distance also matters for exposing possible differences in DOF.

The most useful source for answers to this question can be found at Paul van Walree's site:
http://toothwalker.org/optics/dofderivation.html#eq1

Cheers,
Bart

[pegelli]

Great, I learned something new, I never realized that different lens designs could lead to slightly different dof's based on exit pupil/retrofocus design issues. Thanks for the link. It even means that two similar focal lenth primes of different design can have different dof.
Whether you will notice this in practice is another matter of course, especially since in dof visual perception of actuance as well as viewing size/distance relationship play probably a bigger role in what we perceive as sharp vs. oof in a given picture.

[wolfnowl]

Thanks, folks!  This site is the best!

Mike.

[Eric Myrvaagnes]

Thanks, folks!  This site is the best!

Mike.

+10!

[RFPhotography]

+10!

Absolutely.  This site is the best photography measurebator site there is.   

Light meters work for a reason.  Meters work because f 2.8 is f2.8 is f2.8 and ISO 100 is ISO 100 is ISO 100 and 1/125 is 1/125 is 1/125. 

Yes, there may be very small differences in lenses and cameras owing to manufacturing tolerances, design, etc.  The bottom line; however, is that these differences are infinitesimally small and will make no difference for practical purposes.  Perhaps in highly specialised, scientific uses the differences will be important but I don't consider that to be a generally practical use.  If the differences between lenses and cameras were that significant, meters wouldn't work.

For practical purposes then, DOF is a function of aperture and magnification.  Ergo, same aperture, same magnification, same DOF.

[KevinA]

Great, I learned something new, I never realized that different lens designs could lead to slightly different dof's based on exit pupil/retrofocus design issues. Thanks for the link. It even means that two similar focal lenth primes of different design can have different dof.
Whether you will notice this in practice is another matter of course, especially since in dof visual perception of actuance as well as viewing size/distance relationship play probably a bigger role in what we perceive as sharp vs. oof in a given picture.

Would that not impact depth of focus more than depth of field if anything?

Kevin.

[Bart_van_der_Wolf]

Would that not impact depth of focus more than depth of field if anything?

Hi Kevin,

Depth of Field is on the object (scene) side of the optics, Depth of Focus is on the image (projection) side.

Cheers,
Bart