Originally posted by Opahujo Please correct me if I'm wrong, but the way I understand it now, is that f8 on a P&S is the equivalent of a much higher f number on, say, the 35mm negative of a film camera.
The f-stop number system is designed specifically so *exposures* are standardized across platforms and lenses, so that frame and glass and iris dimensions needn't be considered when metering and exposing. If we have 110, APS-C, 135/FF, 645, and 6x9cm cameras with 20mm, 40mm, 60mm, 80mm, and 100mm lenses respectively, all shooting the same subject at ISO 100 and 1/100 sec and f/8, the *exposures* are all the same. The FOV and DOF of each will differ because of size and distance variances.
So f/8 gives different DOF on different formats primarily because of focal length, as well as frame size. Your Canon S31S's 1/2.5" sensor's diagonal and thus 'normal' focal length is ~7.2mm which on APS-C or FF is ultra-ultra-wide. (I'm looking at your cam's specs.) A 7mm lens on any of those frames has essentially infinite DOF, no matter whether the aperture is f/2.2 or f/22. So, comparing f/8 on 7mm vs 50mm lenses: the exposures are equivalent, while the FOVs are in different leagues.
Quote: Regarding the 18-55mm kit lense. I don't recall ever using the optical zoom on the Canon - I just don't need it. I either get closer or crop and enlarge on the PC. So, for the K-r I'm thinking about 50mm lens, either the M type I have or buying an A type.
50mm is short tele on APS-C; 30mm is 'normal', and 24mm starts to be 'wide'. Their FOV equivalents on 135/FF would be 75mm, 45mm, and 36mm. I'll bet when you power-on your Canon, it defaults to its shortest focal length. On your S31S that's 6mm, FOV equivalent to 36mm on 135/FF, or 24mm on APS-C. Putting a 50mm lens on a Kr would give MUCH narrower FOV and MUCH thinner DOF. No, the cheap DA18-55 is much more appropriate for your shoots. But I still don't know why your S31S doesn't deliver for you.
Quote: I hope you'll bear with me, because I'm sure that I'll have more questions as I think about this. I turned 80 last May and I have been taking pictures and even developing my own B&W film since back in the early 1950s.
I'm a bit younger, started developing not much later, and still have much to learn. And yeah, you should have kept your old Pentax gear! Ah well, so it goes...