On 3 Apr 2006 at 12:40, John Francis wrote: > Really? Both my A* 300/f2.8 and my FA* 250-600/f5.6 came with a > set of drop-in filters (albeit different sizes for each lens) that > included what appears to be a plain glass filter. I'm pretty sure > it's recommended practice to have a piece of glass in the optical > path there - why else would there be a filter blank included?
I bought my A*300/2.8 used and it didn't come with a filter so I couldn't say. > Sure, the lenses will focus just fine without anything there. But > presumably the CA corrections, etc., assume a filter is present. Well it's easy enough to test so I did (using a Hoya HMC UV(0) filter)and the lens wide open close up and at distance. It's pretty apparent that the plain of focus changes with and without a drop-in filter, I wasn't aware of this as I'd never added a filter. The focus scale is closer to the actual measured subject distance with the filter in however optically I think it is a little better with the filter out, though I'm straining to discern any real differences at 400%, there is certainly no variation in CA. I suspect that the glass is there mostly to ensure that the focus scale remains accurate. I won't be leaving the filter in my lens though. Rob Studdert HURSTVILLE AUSTRALIA Tel +61-2-9554-4110 UTC(GMT) +10 Hours [EMAIL PROTECTED] http://members.ozemail.com.au/~distudio/publications/ Pentax user since 1986, PDMLer since 1998
