No need, how in the world can DOF change AFTER the image is already captured? It CANT. DOF is the relative sharpness of the foreground and background compared to the plane of focus. Enlarging or reducing an already captured image never changes that! JCO
-----Original Message----- From: graywolf [mailto:[EMAIL PROTECTED] Sent: Sunday, August 08, 2004 6:06 PM To: [EMAIL PROTECTED] Subject: Re: first question Well, one of us is wrong. I suggest people try it out to find out which. -- J. C. O'Connell wrote: > DOF is solely dependent on CAPTURE magnification ratio and f-stop. If > you shoot with a 24mm and a 100mm of the same object at the same > distance and same f-stop, the 24mm image will have greater DOF than > the 100mm image, EVEN IF you later "blow it up" to match the 100mm > image size in a print. Enlarging the captured image after the fact has > no effect on the DOF in an image! > > JCO > > -----Original Message----- > From: graywolf [mailto:[EMAIL PROTECTED] > Sent: Saturday, August 07, 2004 11:20 PM > To: [EMAIL PROTECTED] > Subject: Re: first question > > > 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. > -- graywolf http://graywolfphoto.com/graywolf.html
