06-01-2013, 11:35 AM
If memory serves me right, the vignetting value is for the corner, but it is for a camera default adjusted image, not the lens' own performance.
Anyway, the application sounds to me a lot like astrophotography, where pixel peeping is justified. LoCA isn't so critical unless imaging is OSC. It's effects are diminished in LRGB and negligible in narrowband. What you mostly want is a point source remaining a point regardless where it is in the frame. Accurate focusing is a pain hence the question on the degree of backlash.
I use the Canon 135L in such a fashion. It is unusable wide open for OSC as the red channel focus is miles away from green/blue. But used for narrowband you focus for a single wavelength and all is well. Almost, my sensor isn't exactly perpendicular to the optical axis giving a noticeable defocus effect (like tilting a lens). We're talking in the magnitude of 10s of microns here and I don't fancy adjusting that out!
Oh, will the focusing be automated? It will likely shift too much with temperature.
The ZF version of the lens would be quite interesting for a different reason. As it has a user controlled aperture, that would allow the ease of trading speed for reduction of size of point sources. A trick I heard of is to use a front aperture mask, but I suspect that would throw off the design of complicated photographic lenses as opposed to much simpler astrographs. Might need to check if it has rounded aperture blades though, unless you like the diffraction spike look.
Anyway, sounds like Sapkas is renting the Sony to see how it works, and that's the best way to find out as terrestrial photographers don't tend to look at things in the same way.
Anyway, the application sounds to me a lot like astrophotography, where pixel peeping is justified. LoCA isn't so critical unless imaging is OSC. It's effects are diminished in LRGB and negligible in narrowband. What you mostly want is a point source remaining a point regardless where it is in the frame. Accurate focusing is a pain hence the question on the degree of backlash.
I use the Canon 135L in such a fashion. It is unusable wide open for OSC as the red channel focus is miles away from green/blue. But used for narrowband you focus for a single wavelength and all is well. Almost, my sensor isn't exactly perpendicular to the optical axis giving a noticeable defocus effect (like tilting a lens). We're talking in the magnitude of 10s of microns here and I don't fancy adjusting that out!
Oh, will the focusing be automated? It will likely shift too much with temperature.
The ZF version of the lens would be quite interesting for a different reason. As it has a user controlled aperture, that would allow the ease of trading speed for reduction of size of point sources. A trick I heard of is to use a front aperture mask, but I suspect that would throw off the design of complicated photographic lenses as opposed to much simpler astrographs. Might need to check if it has rounded aperture blades though, unless you like the diffraction spike look.
Anyway, sounds like Sapkas is renting the Sony to see how it works, and that's the best way to find out as terrestrial photographers don't tend to look at things in the same way.
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