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

[Clive Warren <Clive.Warren@xxxxxxxxxxxxx>]

At 4:05 pm -0700 21/3/00, 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.  You are saying that
>diffraction is worse at smaller apertures/higher F numbers?  And with
>infrared film this problem is accentuated?  Thus, what aperture would be
>best for shooting infrared?  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.  At some point there must be a
>tradeoff/crossover between the two--depth of field vs diffraction, no?
>Chris Anderson>
snip

Chris,

Generally you are correct - although some lens designs break the rule and
are sharpest wide open - the 203mm Kodak Ektar for example.  Usually,
stopping a lens down by two stops is good practice if you want the sharpest
possible image across the negative.

Using large format lenses (eg 4x5), some of these lenses need to be stopped
down in order for the image circle to cover the negative.

Generally, diffraction is worse at smaller apertures, the trade off is
around two stops down from wide open. Any smaller and diffraction effects
decrease image sharpness.

The best aperture for shooting infrared for image sharpness is again
generally two stops down from wide open.  However with infrared film the
focussing point is different from that using normal film. Stopping down
further helps to cover any focussing errors with the associated increase in
depth of field.

If you want a rule then stop down three stops from wide open when using
Infrared Film. However use of the aperture is part of creative photography
and although you should avoid very small apertures, feel free to experiment
and explore in the same way as you would with visible light photography.
Throw the rules out of the window and delight in the freedom :-)


All the best,

             Clive   http://www.cocam.co.uk
                     Photographic Services, Filters and Equipment, Infrared FAQ
                            http://clive.bel-epa.com
                            Adventures in Cyberspace - Digital Art and Photos


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