Paul,
I think the "bromide" use of magnification is referring to the relationship
between object size and image size. The images size of a 20 mm lens and a
400 mm lens are not the same at infinity. The image of the telephoto will
be many times the size of the wide angle for the same object at the same
distance. What it means, I think, is that if you focus both each lens on an
object and adjust the distance to the object so that the images on the film
plane are the same, then you have the same depth of field at the same
f-stop. A less extreme example might be that whether you shoot a close-up
portrait with a 100mm lens from 10 feet, or with a 50mm lens from 5 feet,
the depth of field will be the same if the f-stop is the same.
Winsor
>Funny you should mention it, but I was just thinking about that subject
>today. The old bromide is "For equal magnification and equal f, DOF is the
>same regardless of focal length." But working through the formula and
>thinking about extreme cases today, I realized that that is just an
>approximation that works for some cases. For ordinary closeup-macro, it
>works almost exactly, but for other cases, the approximation breaks down,
>and one of the cases it doesn't work for is distant macro backgrounds.
>Consider this case: a 20mm lens and a 400mm at zero magnification, i.e.
>infinity focus, and the same f stop. Do they have the same depth of field?
>
>Using the formula from Brandt's _Photographic Optics_, where x is focus
>distance, tf is the distance beyond x where the circle of confusion is c, F
>is focal length, and f is f-stop.
>
>tf = x**2/(F**2/(c*f) - x)
>
>For the case of a 50mm and a 100mm set at 1/4 magnification at f/8, and a c
>of 1/30mm (a good value for 35mm full frames). tf will be about 7.5mm for
>both lenses: the rule works. But a point 30m behind the object will have a c
>of about 1.24mm for the 50mm, and 2.45mm for the 100mm. Even though they
>have the same DOF, the background will be more out of focus for the 100mm!
>One way of looking at it is that the background will have different
>magnifications for the two lenses, even though the subject doesn't.
>
>Besides that, with the 100mm, you can use the narrower angle to pick the
>part of the background you want to use, as mentioned.
>
>Paul Farrar
Winsor Crosby
Long Beach, California
mailto:wincros@xxxxxxxxxxx
"Any sufficiently advanced techology is indistinguishable from magic."
-Arthur C. Clarke
############################################################
| This message was delivered via the Olympus Mailing List
| To receive the Digest version mailto:listserv@xxxxxxxxxxxxxxx
| with "subscribe olympus-digest" in the message body.
| To unsubscribe from the current list mailto:listserv@xxxxxxxxxxxxxxx
| with "unsubscribe olympus" in the body.
| For questions mailto:owner-olympus@xxxxxxxxxxxxxxx
| Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html
############################################################
|