Re: Konica IR ...!st time, hyperfocal focusing, and Stigmata

[Steve Hodges <shodges@xxxxxxxxxxxxxx>]

"Christina Z. Anderson" wrote:
> 
> OK OK, I'm too lazy to go lug out my textbook so I'll just ask 
> this question:  

:-)

> I have heard the sharpest aperture on a lens is 2 stops down from
> smallest, i.e. an F32 lens is sharpest at F16.  

There are a number of things that contribute to sharpness.

With a perfect lens, sharpness would be at its greatest when wide open,
with sharpness decreasing as you stop down.  The sharpness of such a
lens would be limited by diffraction, which gets worse as you stop down.

In real life, lenses are not perfect.  There are aberrations (deviations
from perfection) that degrade the image.  Many of these are reduced in
effect as you stop down.

The effect of this is that most lenses get sharper as you stop down
until they reach their sharpest, then get less sharp as you continue to
stop down.

The actual aperture where a lens performs best differs for each lens
(and sometimes it is even different for two lenses of the same type!). 
Generally, the better the lens, the closet to wide open the optimum is.

A fairly good rule of thumb is that you should stop down 2 stops from
the Mx aperture for the best image quality.  This is a rule of thumb,
not some magic formula.  So an f/1.4 lens is probably near its best at
f/2.8, and f/2.8 lens at f/5.6.

> You are saying that
> diffraction is worse at smaller apertures/higher F numbers? 

That's correct.

> And with
> infrared film this problem is accentuated?  

Not the film, the light.  IR, which has a longer wavelength than visible
light, defracts more at a given aperture, and thus the image loses
sharpness faster as you stop down.

> Thus, what aperture would be
> best for shooting infrared?  

Well, that's a complex question.  The sharpest aperture is likely to be
a little wider than for visible light.  But since there's some focus
shift, it is quite common to stop down a little more so that we can
compensate for a little mis-focussing (especially where the lens has no
IR mark).

> I always assumed that the smaller the aperture
> the greater the depth of field (which, as you say, is exactly what I was
> doing with the hyperfocal focussing) which would in turn give me a greater
> chance of having a sharper image throughout.

Well, that's true.  But the maximum sharpness is reduced.  And you'll
notice this as you enlarge the image more and more.

>  At some point there must be a
> tradeoff/crossover between the two--depth of field vs diffraction, no?

Well, not so much a crossover as a tradeoff of sharpness with depth of
field.  The crossover is the point at which the lens is the sharpest, it
just gets worse either side.

Now, if you want more DOF (or less) you've just got to deal with that.

Of course, your problem could also have been due to camera shake. 
Especially if you were using a longish shutter speed.

Steve
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