Re: telecentric question

Dom Jaeger

On Wed, Feb 27, 2019 at 08:03 PM, Art Adams wrote:

The cover-glass-as-optical-element is a big deal, regardless.

At Panavision we have upgraded to SP 70 lens test projectors that utilise a removeable 4mm thick glass plate in the light path, to mimic the effect of the OLPF/coverglass stack (4mm is pretty close to the thickness of most digital cinema camera filter stacks). It's interesting to compare the projection of a film-era lens with and without the glass plate. As one would expect, the least telecentric designs are affected most, especially those with faster apertures. As longer focal lengths are naturally more telecentric, this tends to limit issues to focal lengths under about 50mm. The old T2.1 32mm Arri Macro for example, which has its exit pupil only about half an inch under the small rear element, is among the worst affected, showing increased spherical aberration and astigmatism when projected through the glass. The wider Zeiss Standard Speeds and Cooke Speed Panchros are other examples. Stopping down helps of course. Interestingly, sometimes lenses actually improve in some ways - the 18mm Zeiss Super Speed for example has slightly less spherical aberration wide open with glass in the light path, which I can only assume is because the original design was over-corrected for it. 

The more telecentric lenses tend to be very minimally affected.  

It's a little like the issue reflex Bolexes had with the viewfinder prism in the light path, and the reason Bolex commissioned a whole range of RX lenses especially for those cameras, which were corrected for the aberrations introduced. That prism was a 9mm thick piece of glass, so the effect was more drastic, but even so they only made RX lenses up to 50mm since it was decided that lenses longer than that were more or less unaffected. 

Dom Jaeger
Lens Technician 
Panavision Melbourne

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