Depth of field question

[Michael R 1974]

Hi. For a given film format (35mm), and a given plane of focus at a given aperture, will all lenses of a given focal length have the same depth a field? In other words, is depth of field at a given aperture and plane of focus determined by the focal length, or is it variable depending on the specific optical formula/design of the lens?

The reason I'm asking, I have two 85mm Nikkor lenses. One of them, a standard AF Nikkor has a decent depth of field scale, the other, an 85mm tilt/shift Nikkor has a small and pretty useless depth of field scale. For a particular photograph I need the shift functionality. So I'm wondering, if I use the AF Nikkor to figure out the depth of field for a plane of focus of say 15ft at f:11, can I assume the other 85mm lens will have the same depth of field if also focused at 15ft at f:11? Or will the two lenses have totally different depth of field characteristics due to their different optical designs?

Thanks
Michael

[abuelo]

I think at a same aperture both will have the same deph of field

abuelo

[Lee L]

The DoF scale is likely smaller on the tilt/shift lens because of a smaller throw in the focus adjustment. (i.e. the focus ring turns less for the same focus change compared to the AF lens you have.)

DoF should be the same between the two 85mm lenses until you apply some tilt to one lens, at which point the Scheimpflug principal takes over when the central lens axis is no longer perpendicular to the film.

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

Lee

[Christopher Walrath]

The Scheimpflug rule states that the film plane, the subject plane (plane of sharp focus), and lens plane (the plane through the optical center and perpendicular to the lens axis) must converge along a single line. If the lens and/or the film plane is changed then the depth of focus will also change in regards to the degree of change. In tilting your change your plane of focus. The focus on farther objects due to tilt or other movements will have greater ODF while you will notice shallower DOF on objects that are in the closer portion of your focal plane. Using the DOF scale on the fixed lens will not help you except for determining the DOF in the portion of your planar area that is unaffected by movements.

[dnk512]

For the same format and same distance, your 85mm lenses will render the same 'depth of field'.

The first 3 hits on google for 'depth of field' are good ones. You may find the last one of great use:
http://en.wikipedia.org/wiki/Depth_of_field
http://www.cambridgeincolour.com/tut...h-of-field.htm
http://www.dofmaster.com/dofjs.html

[Christopher Walrath]

The Scheimpflug rule states that the film plane, the subject plane (plane of sharp focus), and lens plane (the plane through the optical center and perpendicular to the lens axis) must converge along a single line. If the lens and/or the film plane is changed then the depth of focus will also change in regards to the degree of change. In tilting your change your plane of focus. The focus on farther objects due to tilt or other movements will have greater ODF while you will notice shallower DOF on objects that are in the closer portion of your focal plane. Using the DOF scale on the fixed lens will not help you except for determining the DOF in the portion of your planar area that is unaffected by movements.

There needs to be an addition here. By tilting you are effectively changing the focal length of the lens to certain portions of the subject area. And therein lies the difference between DOF locally in the subject area. I think I got that right.

[RalphLambrecht]

...So I'm wondering, if I use the AF Nikkor to figure out the depth of field for a plane of focus of say 15ft at f:11, can I assume the other 85mm lens will have the same depth of field if also focused at 15ft at f:11?...

Yes!

[Alex1994]

You can always use http://www.dofmaster.com/dofjs.html to work it out.

[Q.G.]

There needs to be an addition here. By tilting you are effectively changing the focal length of the lens to certain portions of the subject area. And therein lies the difference between DOF locally in the subject area. I think I got that right.

You're not changing the focal length really.
What you do is tilt the lens and film planes so that the conjugate distances in (ideally) all parts of the image are what the simple lens maker's formula says they should be to achieve focus for a given, fixed focal length.

DoF doesn't really change, except as you described: less for the close-up parts, more for the far away parts of the subject.
It still depends on f-stop and magnification only, just as it does with a fixed, untilted lens.

[Q.G.]

You can always use http://www.dofmaster.com/dofjs.html to work it out.

This calculator (like many) derives DoF from hyperfocal distance. That will get you close, but it's not quite correct.