Proximity vs Focal length

[kieranvyas]

I'm trying to achieve a shallow depth of field with my panasonic GH3, and I have a question about proximity vs focal length.

If you have two shots, both framed the same, but:

1) is with a wide lens and at close proximity to the subject
2) is with a higher focal length, at more of a distance from the subject

Imagine the shots were framed the same with the same aperture.

Will they have the same depth of field?

 

[ItDonnedOnMe]

Technically, yes - they'll have the exact same depth of field.

In practice though, the first shot will have greater apparent depth of field, because the two lenses will have different perspective in terms of what's visible in the background. The wide angle lens will show more of the background in the shot, the longer lens will show less.

Imagine that there's a picture on the wall 10 feet behind the subject. In both shots the picture will be equally blurred due to the depth of field. However, in the wide angle shot it will be smaller, so the blur will be less apparent. On the longer lens the picture will be bigger, and the blur will be more visible, so it will appear that the shot has less depth of field in comparison to the first.

 

[kieranvyas]

Technically, yes - they'll have the exact same depth of field.

In practice though, the first shot will have greater apparent depth of field, because the two lenses will have different perspective in terms of what's visible in the background. The wide angle lens will show more of the background in the shot, the longer lens will show less.

Imagine that there's a picture on the wall 10 feet behind the subject. In both shots the picture will be equally blurred due to the depth of field. However, in the wide angle shot it will be smaller, so the blur will be less apparent. On the longer lens the picture will be bigger, and the blur will be more visible, so it will appear that the shot has less depth of field in comparison to the first.

Thank you very much, that makes a lot of sense!

 

[Zensteve]

I'm trying to achieve a shallow depth of field with my panasonic GH3, and I have a question about proximity vs focal length.

If you have two shots, both framed the same, but:

1) is with a wide lens and at close proximity to the subject
2) is with a higher focal length, at more of a distance from the subject

Imagine the shots were framed the same with the same aperture.

Will they have the same depth of field?

http://www.dofmaster.com/dofjs.html

 

[jax_rox]

In practice it would be pretty difficult to get both frames so identical that the DOF is identical, but the technical DOF would be similar.

As IDOM mentions though, the apparent DOF would seem to be less.

 

[ItDonnedOnMe]

That's the problem with DOF calculators - they'll tell you what's going on from a theoretical standpoint, but that doesn't necessarily tell you what you'll see. They're most useful in terms of figuring out if you have enough DOF to keep your subject in focus. Beyond that a lot of what you see will be determined as much by the scene, field of view and composition as anything else.