Sunday, September 27, 2009

Astigmatism and Color Fringing, Sigma 50mm f/1.4

I went out to Chiricahua National Monument yesterday to try some sunset photography but got there too late and ended up taking some night images at Massai Point. The moon is at first quarter so I knew I wouldn't be able to get the sort of deep sky images that I enjoy taking but figured it would be a challenge to see what I could get with the moon up. I try to take exposures that are either short enough so the stars in the sky don't smear too much or long enough so they make long trails. Lately, I've enjoyed the short exposure (< 45 seconds) shots so I tried some of those. I wanted to get some of the Chiricahua rocks clearly exposed and found that in the light of the first quarter moon I needed to shoot at a fairly wide aperture to keep the exposure times short. Both of the images shown here have ~35 second exposures taken at an aperture of f/1.6.

I've been pleased with the results I've gotten with my Sigma 50mm f/1.4 lens on my Canon 5D mk II but I hadn't paid close attention to some of the lens distortions that show themselves when shooting star fields. This first image shows a couple of the rocks near Massai point illuminated by the moon and with stars in the background sky.

In this image, in addition to the rocks and bushes, you can see a lot of stars in the sky, a galaxy, and a couple of satellite trails. If you look closely, you will also notice some distortions in the stars in the upper left and lower right. (click on images to see larger versions.)  This is meriodional astigmatism and it gets worse the closer to the corners of the field you get. I found this effect easier to see in the following image which shows a distant view of Cochise Head from Massai Point.

Look at the top of the image, the stars are distorted so they look like little line segments oriented tangential to a circle centered on the image.

I also noticed that some of the stars looked more red than they should be and closer inspection shows that there are red/cyan fringes around the very brightest stars.  This happens all over the image, not just near the edges.  I'll show a couple of full resolution crops from the second image below.   The first full res crop shows the meriodional or tangential astigmatism and is taken from the top left of the Cochise Head image.  The second crop shows the color fringing and is taken from the center of the same image.

Apparently, night sky imaging at wide apertures requires extremely fine lenses!

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