[Leica] IMG: the episode in which Hoppy becomes a criminal

[henningw at archiphoto.com (Henning Wulff)]

At 6:37 PM -0700 4/23/08, Steve Barbour wrote:
>On Apr 23, 2008, at 5:42 PM, Tina Manley wrote:
>
>>  At 08:33 PM 4/23/2008, you wrote:
>>
>>>  so the coding has nothing to do with outcome or photo quality, just
>>>  puts the lens into the exif...is that correct?
>>>
>>>
>>>  Steve
>>
>>  I read somewhere - I can't find it now - that the coding would 
>>optimize the performance of the lens - addressing known vignetting 
>>and color balance issue, particularly with wide angle lenses.
>
>
>rings a bell for me too Tina, but is this true ?!
>
>Somehow I wonder if it turned out to be speculation.
>
>
>Steve

The coding provides vignetting correction for most lenses, but this 
is of course more important for wide angle lenses. For lenses shorter 
than 35, coding is definitely of benefit as it corrects for the 'cyan 
corners' caused by the steep incoming angles of light which causes 
'frequency shifting' on lenses equipped with the UV/IR cut filters.

The filters are interference filters, and do their job by the 
interference of reflected radiation within the coating layers of the 
objectionable wavelengths. When the angle gets steeper, the incorrect 
wavelengths get cut; in this case the deeper red and not just the IR 
wavelengths, therefore the cyan colour.

If the camera knows the lens characteristics, it can do some 
pre-processing on raw as well as jpeg files and counteract this 
undesirable effect, and give consistent colour response across the 
whole frame. The Wide Angle Tri-Elmar would be unusable with an IR/UV 
cut filter without such coding.

The EXIF data on lens and focal length is just a side benefit of 
this. Vignetting is also easy to correct in software if desired.

I wouldn't bother sending perfectly functioning lenses in to get 
coded if it weren't for this correction that the camera does. I have 
a 28 Summicron where the cyan corners are just detectable, so I will 
send in the lens at some point, but 35 and longer won't get sent in.

-- 
    *            Henning J. Wulff
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