In article
>In article <55hbcq$382@hyperion.nitco.com>, Tom Saylor
>
>>Hi- I've been playing around shooting aerials w/35mm. Nikon body and high
>>end f/2.8 fixed apo zoom. At 800 feet altitude and 1/4000th second and
>>lens wide open can't seem to get critical sharpness. Is it a
>>depth-of-field problem or what. Lens is racked down on infinity setting.
>One thing I found out recently when testing L39 rangefinder lenses- and
>thinking they weren't correctly rangefinder-coupled when I looked at a
>subject at "infinity"- was that focusing on something a few hundred feet
>away is *not* the same as focusing on infinity. At an aperture of f/2.8,
>you may well need to focus more accurately, and by eye, not by scale.
>Bear in mind also that at the infinity setting your lens may be focusing
>beyond infinity anyway.
You are quite right. Even with a target about three miles away, in three
or four focus attempts with lenses 200mm and over, noticeable (subtle!) variation in sharpness is visible in negatives with a 10X magnifier.
EXACT focus is necessary to get the best out of sharp lenses at wide
apertures, something that is hard to achieve while bouncing around in
a light plane or helicopter (but, then, motion generally takes the
edge off the finest lenses, also, when doing aerials...). Also, most
zoom distance scales are not particularly accurate, especially as the
lens is zoomed (many zooms are slightly vari-focal), with some slop
allowed in the markings near infinity to make sure that accurate
infinity focus can be achieved with the vari-focal lens set at any FL.
BTW, I find that it is difficult to get sharp images when doing aerials
with FL's longer than about 135mm (200 ASA, f4) - good prime 85's and
50's are MUCH easier to handle than f2.8 tele zooms, and easier to
focus accurately under difficult conditions.
Hope This Helps