Tuesday, July 5, 2016

Lens aberrations and pixel scale with my Rokinon 16mm f/2.0 lens

Last January I took my new Rokinon 16mm f/2.0 lens out for a spin to characterize the lens aberrations. The animation below shows that coma clears up pretty well along the edges by f/4, but it appears my copy has some lens tilt because the right hand side exhibits a bit more coma than the opposite side.
Modified Canon 550D, 20 second exposures, Rokinon 16mm f/2.0 lens. 
The lens tilt is pretty obvious in contour plots of star full width half maximum (FWHM) and eccentricity, shown below. The largest FWHM is on the right hand side which is consistent with my previous observation. Things improve by f/4.0 where FWHM is only 60% larger in the lower right corner compared to top-center. It is 100% larger at f/2.0. I’m not sure what to make of the median eccentricity increasing with higher f ratio.
FWHM and eccentricity at f/2 and f/4 from the animation above, plotted via PixInsight’s AberrationInspector script. Note that the shading scale is different between the images.
Though the first few images with the lens look promising with my Canon 550D, I am becoming concerned that it is not the best fit for my Canon 1000D due to the different pixel scale (56”/px vs 70”/px, respectively). Recently I captured the Summer Triangle with my 1000D and Rokinon lens and found the bijillion tiny stars were heinously undersampled in the image. This is not a surprise given the pixel scale, but it looks like debayering struggled, causing pure colors along the star edges. I hope that dithering, which I have not yet attempted with this lens, will help with this issue. I did not notice this level of undersampling in my image of the Big Dipper taken with my 550D, but in all fairness, the Milky Way region is far more smattered in tiny stars so it is not a fair comparison. In the future I would like a head-to-head match between the 1000D and 500D with the same field of view and same lens. It could just be that undersampling to this degree is just the way things are. Looks like more testing to do and possibly a conversation with the folks at Rokinon about the suspected lens tilt.

Undersampled stars, cropped image near Milky Way. Canon 1000D, single 240 sec exposure, ISO800, Rokinon 16mm f/2.0 lens @ f/4.0, Losmandy G-11 mount.

No comments:

Post a Comment