</span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (tschmid @ Dec 13 2002, 07:30 AM)</td></tr><tr><td id='QUOTE'>Ed,
although your suggestion is based on common experience, you will find that more than a few enlarger lenses will show a significant focus shift when stopped down. Although enlarger lenses are designed for flat field projection, the practical performance is always a compromise within the variety of possible magnification factors and apertures.
Apart from that, the performance of an enlarger lens can be bad wide open making it difficult to find any sharp grain. In my experience, it is always better to use the grain magnifier at working apertures. If the grain is sharp, your picture will be sharp, too, no matter how many degrees you can turn your focus knob around this. However, if you check the edges, too, you will see that there is usually not that much tolerance.
I have heard this idea of focus "shifting" at different apertures many times, but from my training in physical optics it is difficult to explain. It is interesting to note that few claim that that phenomenon also applies to camera lenses, and the last time I looked the same optical principles applied.
My experience does not cover every enlarger lens ever made; it is largely restricted to Rodenstocks and Schnieder (on the turret of my Omega now) and in times past, Elgeet , Federal (Rodenstock??) and a few others ... I have never observed that "focus shift" in any of them, although I will be the first to admit to the possiblity of having "lucked out" to a major degree.
Also, in a few (actually more than a "few") hours of lens testing on the Optical Bench (dating myself here! I've found various optical flaws and manufacturing errors - but I cnnot recall a "shifting of focus" as one of them - and come to think of it - I cannot ever remember a criteria of "Stability" (?) of Focus at Various Apertures.
I am familar - very - with the difficulties of lens design, and just why it is that large-aperature capacity lenses are optimized for certain apertures (read: best compromize) but the parameters existing for enlarger lenses are far more restricted.
Personally, if I ever did find an enlarger lens with anything aproaching a "focus shift", I might conduct an experiment: find out how many times I could make it "skip" across the water of my local pond.