I went outside last night and made some... pictures.
The details about my setup:
- Sony alpha 6000
- Varexon 2.8/135mm lens (would be 202.5mm on 36mm film)
- ISO 25600
- 3 seconds (a bit too long, given the Rule of 500)
- stacking of about 10 images with
Given that I was able to record 10.20 mag stars with the same lens and a single 2 s and ISO AUTO (= 5000 in that case) exposure - these faint stars appear as pale blue specks, I expected much better results with a higher ISO value. But no. They seem to be washed out.
I've seen tips to use the highest ISO when doing astrophotography, but it seems detrimental. What are some useful hints to find the right ISO value?