[sbogar]

So you want to use this lens in a manual focus fashion only. Maybe the Zeiss 135mm f2 ZE/ZF is a better idea? It won't show the MF ring play when changing direction, it most probably will not rotate any lenses, due to the way MF lenses usually focus, it is optically superior (very low LoCA in comparison to the Sony/Zeiss lens). And you only "lose" 1/3rd of a stop.

 

Indeed, I will use this in MF fashion only. Thanks for the suggestion on the 135mm f/2 ZE/ZF lens. I will check it out.

 

Regarding the ring play, now that I have such a lens, I confirm that it is indeed about 1mm.

The same backlash is present when driving the focus through the slotted screw on the flange.

 

I also looked inside the lens while focusing, and identified the group of lenses (or single lens) moving, and

it seems to me, from the barrel that is holding this moving component, that it is not rotating, but simply sliding.

 

Regarding LoCa, I don't know how serious it is for the Zeiss 135/1.8 lens. I fancy this lens because of the large aperture,

and even the difference f/1.8 to f/2.0 matters (f/1.8 takes in 23% more light when compared to f/2).

 

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.

 


I will measure it.

 

> 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.

 

What is OSC?

 

 

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.

 

Yes, I plan to have automated focusing through the focuser ring, or perhaps using the slotted screw.

 

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.

 

I can't stop down the aperture to f/2. I need the light.

 

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.

 

Will send feedback later, when I had a chance to try it. Thanks for the wise words.

 

Sapkas