sharpness and CA on 85mm f/1.2

[neuroanatomist]

I wasn't aware that axial CA was minimal at infinity. Are you sure about that?

I don't think it's minimal at infinity, but rather minimal within the depth of field, and stars are in focus at infinity. Put another way, the effects of CA are magnified by defocusing. They will still be there, and might be strong enough to affect the image in a demanding application like astrophotography. Also, I really don't want to contribute in any way to KR being right - I always prefer empirical data, so I hope you get a chance to test it! Technically, calling it all CA is a misnomer, since aberrations formally affect only the focused part of the image. But I think it's fair to count CA of OOF light as an aberration since the OOF parts of the field also contribute to the final image (in photography, at least - in microscopy we use tricks like confocal pinholes to optically remove OOF light, or deconvolution to algorithmically remove it).

I don't know anything

Liar!! []

[Kayaker72]

I don't think it's minimal at infinity, but rather minimal within the depth of field, and stars are in focus at infinity. Put another way, the effects of CA are magnified by defocusing. They will still be there, and might be strong enough to affect the image in a demanding application like astrophotography.

That is really awesome.....just wanted to say that.......I've been enjoying this thread............

[wickerprints]

The problem with fast-aperture lenses for astrophotography is generally not axial chromatic aberration, but rather, coma and astigmatism. Both of these are reduced upon stopping down, but coma in particular can be very severe when the lens is shot wide open.


The EF 85/1.2L design is optimized for low-contrast portraiture, not astrophotography.

[neuroanatomist]

The EF 85/1.2L design is optimized for low-contrast portraiture, not astrophotography.

Are there any Canon lenses that are optimized for astrophotography?

[Jon Ruyle]

I took some pictures with the 85 f/1.2 last night, even though I was sleepy.


Here is a picture of the Lyra area. I did not process this image as I usually would (in particular, I didn't subtract background skyglow) because my goal was to test the lens, and processing would have messed with the colors.


[img]/resized-image.ashx/__size/1024x0/__key/CommunityServer-Discussions-Components-Files/8/4377.IMG_5F00_2122.JPG[/img]


85mm f/1.2 @ f/1.2 30 second exposure iso400 5DII EM2 mount





I would say that though there is far less CA than I expected (especially around Vega- I wonder what's up with that?), it's still too much. For a wide angle pic with this lens, I'd stop down to f/2.8 at least.


Here is a 1-1 crop from close to the center of the frame:


[img]/resized-image.ashx/__size/550x0/__key/CommunityServer-Discussions-Components-Files/8/4786.m57-crop.JPG[/img]


The sharpness pretty much perfect- impressive IMO given the fast aperture. (This crop includes the ring nebula (m57)- can you spot it?)


Corner sharpness is a different matter:


[img]/resized-image.ashx/__size/550x0/__key/CommunityServer-Discussions-Components-Files/8/4505.edge-crop-85-12.JPG[/img]


So:

The problem with fast-aperture lenses for
astrophotography is generally not axial chromatic aberration, but
rather, coma and astigmatism.

I'm inclined to only partly agree in this case. Yes, aberrations other than axial CA are severe off axis. This isn't coma (which really means off axis parabolic aberration, but this isn't even "coma-like"). It might be astigmatism, but it actually looks more to me like the result of pinched optics (though I'm not sure- wouldn't this appear on axis as well? Anyway, don't trust me: I have very little experience interpreting star tests).


Still- I would say axial CA is at least as big a problem as the off axis aberrations.

Are there any Canon lenses that are optimized for astrophotography?

I doubt any are optimized for photography, but some of the longer ones perform well. I once saw a detailed article about a head to head test between the (legendary and thoroughly lust-worthy) 155mm Astrophysics Starfire and a Canon 600mm f/4, and the conclusion was that they were very close, which was pretty surprising to me. Daniel has stated that he thinks telephoto lenses outperform telescopes at twice (or maybe he said thrice) the price, though I don't remember if he meant for astrophotography or for general use.


Even with the corner problems, camera lenses are good for wide angle photography, and I know of no other inexpensive option. Even at f/1.2, I don't think the off axis problems would be that bad in an 8x10 print.


And just for fun, I took this shot of Jupiter. Notice the moons:


[img]/resized-image.ashx/__size/550x0/__key/CommunityServer-Discussions-Components-Files/8/4530.jupiter-85-12.JPG[/img]


In case anyone is interested, for the tracking I used a little takahashi EM2 mount- it's not a photographic mount but was fine for these wide angle short exposures. (The mach 1 is still packed following CalStar three weeks ago).


It was a fun evening. After taking the pics I took out my little refractor and observed the eerily beautiful galaxy NGC7331, then spent some time looking at Jupiter. (Perhaps this belongs in the "what you do when not behind the lens" thread- though like John, when not behind the lens I'm behind a different lens[] )