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

["Christina Z. Anderson" <tracez@xxxxxxx>]

Ditto, Steve; thanx!  I will share this with my classes I'm in, your post
and Clive's.  You guys explain it better than the textbooks.
Chris

> > 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


*
****
*******
******************************************************
*  To remove yourself from this list, send:          *
*         UNSUBSCRIBE INFRARED                       *
*       to                                           *
*         MAJORDOMO@xxxxx                            *
*----------------------------------------------------*
*   For the IR-FAQ, IR-Gallery and heaps of links:   *
*  http://www.a1.nl/phomepag/markerink/mainpage.htm  *
******************************************************