Color Filters, Chromatic Aberration, and Focus Shift

[fiftyonepointsix]

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Had some spare time due to a cold. There have been a number of discussions with regard to using Red and Orange filters and focus shift. I tested two lenses, a Pentax 50/1.4 Super-Takumar and a Pentax 85/4.5 Ultra-Achromat with UV, Yellow, and Red filters. The 50/1.4 demonstrated a clear shift in focus, the Ultra-Achromat did show anything measurable to my eyes.

 

Pentax85_45_UV by fiftyonepointsix, on Flickr

 

Pentax85_45_red by fiftyonepointsix, on Flickr

 

I mounted the M Monochrom on a heavy tripod, unscrewed and screwed in each filter: did not want to just hold them in place as that would throw things off.

 

If the RED filter acted as a Diopter in front of the lens, focus on both lenses would have shifted.

 

Pentax 50/1.4 (RF Coupled) with Yellow filter:

 

Pentax50F14_Yellow by fiftyonepointsix, on Flickr

 

and Red filter.

 

Pentax50F14_Red by fiftyonepointsix, on Flickr

 

Shift between UV and Yellow was very small, but registration of the Yellow and Red images from the 50/1.4 were better, due to the UV filter being very tight.

 

No conclusions as this is not done on an optics bench with everything clamped into place, and I'm not setting MY Monochrom up in clamps...

 

BUT- it seems that the "ultra-achromat" (corrected for 4 crossing wavelengths) lens, corrected from UV through to IR- exhibits much less focus shift with the red filter. The Pentax 50/1.4, an achromat (corrected for 2 crossings) shows a larger shift.

 

Full res JPEGS uploaded to Flickr.

 

Use the IR index of the lens as an indicator of how much a particular Achromat will shift with use of Orange and Red filters. Lenses such as the Collapsible Summicron have an IR index almost at the F2 DOF Marks. Expect very little shift. The Pentax 50/1.4: the IR index at the F5.6 DOF mark. Expect a much larger shift when the IR Index is far from the main focus index. 

 

I'm going to guess that this may be the only Pentax 85/4.5 Ultra-Achromat to be used on an M Monochrom. Corrected for inflation, it cost as much as the 50/2 APO-Summicron.

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[Joakim]

I assume there is no IR mark on modern Leica M lenses? My ZM 50 Planar has it and my old Canon 50/1.4 but I can't seem to find anything on my Leica lenses.

[jaapv]

Use the DOF mark one or two stops down.

 

Any aid will be inaccurate anyway, as the focus shift depends on

a. the chromatic correction of the lens

b. the wavelength of the IR light.

 

For that reason focus bracketing is essential.

[Joakim]

Thanks, I have an IR filter on the way so I'll just go out and practice and see how the lens behaves.

[dante]

Thanks, I have an IR filter on the way so I'll just go out and practice and see how the lens behaves.

 

Advice: get a tripod. The M246, at least, doesn't see IR at all. Or so little that you'd have very. very long exposures. See the thread about the spectral sensitivity.

 

Dante

[fiftyonepointsix]

The M8 is the Digital M that gives good infrared images.

 

I1016067 by fiftyonepointsix, on Flickr

[jaapv]

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Possibly a 092 filter might work. It is not quite IR, but deep red with a cutoff at 650 nm.

The Monochrom1 works even (albeit just)  with a 093 filter.