OK,it's been repeated twice in this thread, someone explain it to me. If I have the same magnification ratio, 1:1 example, why does the depth of field change with the focal length? Using Bob Wheeler's Vade Mecum (which I always use when shooting and have these questions) I get the exact same depth of field at 1:1 with either the 150mm or 240mm focal length. Now is Mr Wheeler's program in error or are the posters in this forum just guessing?
Dave Schneider asked "are the posters in this forum just guessing?"
A rhetorical question, without doubt.
According to Cox, "The hyperfocal distance has this property: if a lens is focused on the hyperfocal distance, then the depth of field extends from infinity down to half the hyperfocal distance." In my copy of Photographic Optics, printed 1971, the text quoted is on p. 70.
Further on, Cox added "The hyperfocal distance is not something definite for a lens alone. It is definite only for a lens at a particular aperture (in italics for emphasis), and changes as the lens is stopped down." He goes on to present formulas for depth of field which contain, among other things, the diameter of the circle of confusion.
Bulletin boards attract bullshitters like shit attracts flies and, not to overlook their important role in nature, dung beetles.
Now will you all please shut up and go back to taking pictures?
Cheers,
Dan
Oh, crap! Posted the right reply in the wrong thread.
Dave, for all practical purposes aperture and magnification control depth of field. Same aperture, same magnifiction, same d.o.f. regardless of lens' focal length. I've tested this idea informally on 35 mm with a 55 and 105 MicroNikkors and a Questar 700, all at f/8 and 1:2. The slides (KM, flash illumination, one stop more for the Questar) were indistinguishable.
There is one situation where this rule can be beat a little. At normal working distances, e.g., inside a large room, suppose one uses a short lens rather than a long one from the same position and then prints the two negatives to the same size, cropping the shot with the short lens so it covers the same field as the negative shot with the long lens. In this case, the short lens can give a print with more d.o.f than the long one.
Focal length and magnification control film-to-subject distance. At the same magnification, the film is closer to the subject with a short lens than with a long lens.
What confuses the hell out of many people is the situation in out-and-about photography where one changes focal length without changing film-to-subject distance. In this case magnification is lower, and d.o.f. greater, with the shorter lens. So shorter lenses give more d.o.f. QED. Yeah, but because of working at lower magnification.
Cheers,
Dan
Thanks Dan. That's what I thought. So the repeated posts here that when doing doing macro work you get more DOF with the shorter focal length are incorrect. You will have the same DOF with either lens, provided that you are maintaing the same magnification or image size. I thought so.
One thing to think about, assuming you're not copying flat art, is perspective. The 240 is 'long' for 4x5, the 150 is 'normal'. The 240 will tend to flatten the image out, the 150 will make the subject appear 'rounder' or more 3-dimensional. And you can use your 90mm as a macro lens, if you don't mind the subject being 1/2 inch from the front element...
Mark Sampson suggested "And you can use your 90mm as a macro lens, if you don't mind the subject being 1/2 inch from the front element..."
Mark, forgive me for being a literal minded imbecile. Were you joking? The minimum front node to subject distance is 2f. My 100/6.3 Neupolar's front node can't be more than .25 f behind the front of the front element, and I suspect other LF macro lenses are much the same. 1/2 inch, indeed!
Cheers,
Dan
Make that a sloppy literal minded imbecile. Minimum front node to subject distance to subject is f, not 2f as I typed earlier today. The argument stands, though.
Sorry,
Dan
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