I never said it was always a "problem", I said its harder to manually the wider you go. Secondly, it doesn't matter whether its prime or zoom, the difference is the same because its DOF related and the DOF is the same for for primes and zooms at the same focal lengths. Lastly I use the zoom example because its the easiest and fastest way to make the comparison, changing primes makes for a slow change and the comparison is harder to make and more error prone due to the time difference between the comparisons. Its similar to audio testing, its better if you can compare back and forth instantly rather than relying on memory well after the first sampling. Its also a good example because I believe everybody knows its true. No one has posted otherwise on the entire list. If there was no difference between focussing difficulties of a wide and a tele at the same aperture, how can you account for the fact that zooms are easier to focus at the long end?
jco -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of William Robb Sent: Monday, November 13, 2006 10:37 PM To: Pentax-Discuss Mail List Subject: Re: RE: RE: Using a Super Tak w/ istDS- A challange to the list? ----- Original Message ----- From: "J. C. O'Connell" Subject: [Norton AntiSpam] RE: RE: Using a Super Tak w/ istDS- A challange to the list? > Dear Sir ( is that gentlemanly enough) , > You forgot to mention fstop used on the > 105 lens and I don't even think there is > such a thing as a K105/2.5 lens if I am > not mistaken. Pardon the typos. You've made a few yourself over time, I'm sure you understand. That would be an SMC Pentax 105mm f/2.8 lens at f/8. > > Secondly, have YOU ever used a constant > aperture zoom of ANY range that wasn't easier > to focus manually on the longer end than on the shorter > end? I haven't and this pretty much squelches > any argument to the contrary over this. Its just > simple differences in the DOF. The more DOF > you have, the harder it is to find the true > focal point because the DOF is masking it > to some extent and you don't pop in an out > of focus as quickly as you do with a longer > lens which has shallower DOF at the same > fstop and distance. I don't have much use for zooms, the only constant aperture one I own is a 70-210 f/2.8. Shel originally mentioned that he didn't see much difference in ease of focus between a couple of primes anyway, so why bring zooms into the discussion? There was a comment made that he (and I) didn't have more trouble focusing a medium wide angle lens than a short telephoto when stopped down to f/8, using an istDs (shel) and an istD (me). I am aware of the optical theory you are using for your assertion, I suspect that the difference in DOF between the two focal lengths isn't great enough to cause either Shel or myself a problem. I don't care if you disagree with what my eyes tell me or not, since what my eyes tell me is true. William Robb -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
