Last Friday I tried my hand at some Landscape Astrophotography at the ancient stone circle at Avebury. I went with a friend from Tetbury Camera Club – safety in numbers in the land of Druids! Friday was a new moon and being July the Milky Way was high in the sky. Both of us had recced the scene in the daylight and had some specific POVs in mind. But neither of us could be sure how dark it would be.
Avebury is on the edge of the Salisbury Plain dark area, but quite close to Swindon and Calne besides having its own small group of houses and a pub in the village. As it turned out the Milky Way was visible but I would say not stunning. Good enough for an atmospheric shot of the stars and the old stones. The sodium light from Swindon and Calne dominate the horizon, and do tend to wash everything with a sodium light cast even where to the human eye it looks dark – the camera sees it all.
I was using my Sony NEX7 with a Samyang 12mm manual focus lens. I had already checked to see where the point of infinity focus was on the lens focus scale and had checked that the lens resolved point light sources as such, so I was reasonably confident that I had tha basic kit. The high ISO performance of the NEX7 is pretty good and I do use ISO1600 for interior work and it cleans up OK. I had even purchased some chemical hand-warmers which I had ready in a sock to attach to the lens if condensation looked like being a problem, but there was no need for that.
Using the so called rule of 500 I would be able to shoot at 28 secs while maintaining the appearance of reasonable point sources for the stars. Anything over that, the theory says, will produce visible short lines instead of points. The rule of 500 works like this: you divide 500 by the full frame focal length of the lens and that’s your max shooting time. So for my Samyang 12mm, that is the full frame equivalent of an 18mm lens: 500/18 gives approximately 28secs. I have read that 400 is a more practical number which would give 22 secs. I opted for 25secs at f2.0 and ISO1600, which has produced a reasonable result, perhaps slightly on the dark side. You can see though, that the shorter the focal length, the better. A 50mm lens on a full frame camera would give only 10 secs using 500.
What I was not prepared for was the difficulty of composing the image in very dark conditions. The NEX 7 has an EVF and it could see nothing! Even Matt with his D800 and its OVF said he was struggling to compose his images (they looked pretty good to me!). It was a matter of shooting blind, reviewing the result, adjusting the camera on the tripod, repeating the process until an acceptable composition was achieved. Getting everything just right was still almost impossible but the advantage of a wide angle lens and 25 megapixels is that there is plenty of scope for cropping! To speed the composition process up I used a much higher ISO and shorter shutter time.
Taking the image is just the beginning. It takes a lot of careful processing to extract the detail from the image, and to get rid of the inevitable high ISO noise. I used Topaz Denoise, which does a very respectable job. Even with the camera’s “in camera noise reduction” turned on, you are going to get noise, though presumably the camera has done away with any hot pixels.
The finished image above, was the result of processing in Lightroom (mainly) with Topaz Noise reduction and the Topaz Detail (preserve highlights preset). I am still experimenting with the processing and trying to find the ideal compromise between low noise and sharpness of image – not there yet!
I am looking forward to some more experimentation with the camera too. I need to perfect the art before my holiday next year to the USA where we will be visiting some truly dark places! On my list to try out is the concept of taking multiple images and using the Photoshop median filter to remove noise. I have seen this technique demonstrated on the internet and it looks good.