Thanks. Continuing education classes begin anew...
graywolf wrote:
OK, Keith, I apologize for misunderstanding.
No problem. I suspect we all have flaws in understanding, at times. That's what this dialog is all about.
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 follow that.
The only problem is, and this may be out of sequence, so far as where to put the question... but, what about the compression of the subject you get with increasingly longer lenses?
When I say "perspective" that naturally (to me) includes this effect.
One of our members, as I recall, took a tele shot down the length of the Santa Monica pier, and managed to get almost a dozen pedestrian gawkers in the frame. They looked as tho' they were bunched up, 2 or 3 feet apart, while they were probably 10 or 15 feet apart.
I really don't know, so I must ask...if the first shot was taken with the 100mm lens and the second with the 24, and we go thru the same enlargement exercise, to bring all images to the same size, are you telling me that both photographs would display the same compression effect as I first noticed in the long lens image?
It would seem they must, yet the answer doesn't yield to expectations—I'd think somehow the long lens image would still give you a comopression effect not present in the 24mm lens image, but can't explain why I think that.
My brain tells me that both images should be identical. My "feelings" tell me, no, that dude's an idiot. Believe ME!
These questions only come up for stuff I've never made an example of, and actually tested myself.
Obviously I haven't. I've only posed the question to myself. And now posed it here...
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.
A look at my 24, 100 and 105mm lenses reveals the following aperture sizes (roughly measured) — calc. dia. in parentheses:
With my FA*24mm f/2.0: • f/2.0 was about 7/16" (12mm)
With 100mm SMC Pentax-M f/2.8, and 105 SMC Tak f/2.8: • f/4.0 was 15/16" (25mm), and • f/8.0 was ½" (12.5mm) diameter.
Which says that the DOF is close to being the same between the 24mm @ f/2.0 and the 100mm at f/8.0.
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.
Gotcha.
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.
Yeah, if I was still doing B&W. I don't use a custom lab... but, I'm thinking of it, more and more.
Thanks for the discussion. I appreciate it.
Cobwebs mostly cleared out! <g>
keith
