Re: Lens optimization question

[Mark Cassino]

You are generally right, and I personally have yet to see a lens that is not 
better when stopped down somewhat. A few factors that come into play - the 
speed of the original lens is one (stop down an f5.6 les 3 stops and you are 
at f16, a point where diffraction might start to impede performance.  A fast 
f1.4 lens at f 5.6 though (3 stops down) may well be at it's sweet spot.) 
The are also some lenses that are good wide open, but better stopped down a 
bit, while others are terrible wide open but quickly get to be very good as 
they are stopped down.

IMHO, optimum performance can be anywhere between 1 to 3 from the wide open.
Pentax says that the MTF program line "matches the FA lenses performance." 
So, if you set a Pentax camera to MTF mode and then program mode, you can 
get a good feel for what apertures Pentax considers optimum aperture with FA 
lenses (I'm not sure if MTF mode if fully supported with F and A lenses, I 
suppose it is supported with FAJ and DA lenses tough I have no experience 
with those.)  I just snapped a few lenses onto my *ist-D and tried them out, 
general results:

FA 50 f1.7 --  f 2.8 - 4
FA 20-35 f4  -- f 4.5 - 5.6
FA 28-70 f4 -- F5.6 - 6.7
FA 28-105 f4/56 -- f5.6 - f8 (generally 1 stop down across the range)
FA 80-320 f4/56 -- f5.6 - f8 (generally 1 stop down across the range)
Of course, the logic in the program line is a little more complicated than 
just setting it to the optimum aperture, so it will open up at very slow 
shutters and stop down at very high shutter speeds. But you can still get a 
good idea of what is programmed as the optimum aperture in the camera by 
pointing it at light sources that will give you a reasonable shutter speed. 
And in some cases - like the FA 28-105 or FA 80-320 - the lens will not open 
up no matter how slow the shutter gets. But the MTF mode still gives you 
some idea of what is programmed in as the optimum.

- MCC
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Mark Cassino Photography
Kalamazoo, MI
www.markcassino.com
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----- Original Message ----- 
From: "Boris Liberman" <[EMAIL PROTECTED]>
To: <pentax-discuss@pdml.net>
Sent: Sunday, April 03, 2005 5:30 AM
Subject: Lens optimization question


Hi!
Recent talk about 28/3.5 and 16-45 zoom performance got me confused.
My (probably very much Boris-the-average) belief was that most lenses
are optimized at 3 or so stops from their widest aperture. So, for
most f/2.8 lenses it would be f/8 or thereabouts.
Is it right thing to assume? Or there is something quite basic that 
escapes me?

Unconfuse me, will you please? :)
--
Boris