Now that starts to make more sense. Not what you said originally though. Why not be honest and just admit you got it wrong first time round?
Antonio On 8/8/04 5:20 am, "graywolf" <[EMAIL PROTECTED]> wrote: > OK, Keith, I apologize for misunderstanding. > > What both Bill Robb and I were saying is that if you take a photo from the > same > position with both the 24mm and the 100mm it will have the same perspective. > Then if you blow up the photo from the 24mm so the subject is the same size in > it as in the one from the 100mm and crop it so both are say 4x6 inch prints > the > images will be the same except the grain in the 24mm shot will be far more > obvious. (Obviously that is not cropping in camera) > > I also said for the DOF to be exactly the same in those 4x6's you need to use > the same aperture (f-stop is focal-length/aperture, so focal-length/f-stop is > aperture). For instance f/2.0 with the 24mm is approximately a 1/2 inch > aperture, so is f/8.0 with the 100mm. > > ONLY that is WRONG (muddy thinking on my part), because of the blowup of the > 24mm shot, you have to factor the extra magnification into the equation. In > this > case it is 4x (100/24). Because of that magnification factor you would need > the > same f-stop. (DOF is determined by aperture and magnification) > > To recap, if you take the photos from the same position, with the same f-stop, > and enlarge, and crop the photo taken with the shorter lens. The photos will > be > identical except for the problems caused by the higher magnification > enlargement. > > However, if you took the photo with the 24mm from a distance where the subject > appeared the same size as in the 100mm shot , say 5 feet and 20 feet (no extra > magnification, or cropping) the apertures, not f-stops, would need to be the > same for the same DOF. But then they would have different perspectives (as you > said). > > Obviously, in this case you probably would not want to use a 24 in place of > the > 100. The quality cost would most likely be too high. But you might use your 50 > as the loss would only be a 1/2x. If you, like I do, carry 24, 50, and 100mm > lenses then the extra blowup of the enlargements can easily take the place of > 35, 85, and 135mm lenses. Giving you the equivalent of 6 lenses with the > weight > and cost of only 3, as you still get to use about an APS size portion of the > negative.
