Infra red focusing with Apochromatic lenses

[Augenblick]

I am about to shoot some Ilford SFX 200, using a variety of lenses. Older lenses, eg my LTM Canon 1.8 Serenar, and LTM Canon thoughtfully have an IR re-focusing mark on the focussing scale. However, my modern Voightlander Apolanthar 90mm LTM lens does not. Since the lens is apochromatic, does the focus still need adjusting with infrared film?

 

[Micky D]

I've never used SFX 200, but as far as I know it is not a true infrared film, merely over sensitive in the red wavelengths. You may not even need to adjust the focus point.

There was an interesting thread on this awhile ago. Try searching under "infrared".

 

[bmattock]

I am about to shoot some Ilford SFX 200, using a variety of lenses. Older lenses, eg my LTM Canon 1.8 Serenar, and LTM Canon thoughtfully have an IR re-focusing mark on the focussing scale. However, my modern Voightlander Apolanthar 90mm LTM lens does not. Since the lens is apochromatic, does the focus still need adjusting with infrared film?

For near-infrared, the APO is fine to focus as normal. For actual IR, it is not.

Google Books entry on Apochromatic lens vis-a-vis IR

 

[Sonnar Brian]

You should be fine in Near-Infrared as Bill stated. Silicon Sensors drop off at 1.1uM, (11,000Angstroms)

A Calcium Fluorite Ultra-Achromat lens such as the Pentax 85/4.5 in M42 mount will reach into the 7uM Longwave Infrared region.

 

[Augenblick]

Thanks for your help everyone