ISO value to exposure time: Until yesterday i had a technical limitation with my CANON EOS M100(a). I was not able to expose longer than 180s per frame. This was not really a dealbreaker, since i had enough to do with increasing my guiding and get dithering working. Over the past few months i used the 180s exposures and lowered the ISO value from 1600 to 400.
Finally, I am now able to expose longer than 600 seconds per frame. But at ISO 400 i would over expose most of the deep-sky-objects, or not?
The longer the better?
In the past I have read that several times, at least when it came to increasing signal to noise ratio. It is no clever idea to go with 600s exposures as long as I am not the master of guiding. But i think it is really ok – at least i could try.
The test ISO value to exposure time
I did a really basic testing, since i sat in my room in the roof and tried to extend the exposure time beyond the 180s wall for my EOS in EKOS.
After i found the solution, i had to test more. Since I did a focus test earlier that day with an electric pylon in the distance, the test target was picked quickly.
At the beginning it was still to bright to do longer exposure times. Therefore i darkened the room, blinded the window and did my exposures. I could still see the pylon, not sharp but good enough to do some noise testing as you can see on the picture.
This picture was taken later that evening with realistic exposure times:
Meanwhile my room had a temperature of about 26°C and the camera chip was working the whole day. This explains all the color-dots on the single frames. Since i wanted an answer about the ISO to exposure time question, i focused on the noise and not on the dots and did a simple test:
- ISO 100 – 600s
- ISO 200 – 300s
- ISO 400 – 150s
- ISO 800 – 75s
The exposure time of 150s with ISO 400 is more or less the combination i used until now for my photos. In the future the combination will be a different one.
The results and how it will change my astrophotography
I can see very clearly, that the signal to noise ratio gets better longer exposure times. This result will have an effect on my astrophotography. It is not possible to compensate a shorter exposure time with a higher ISO value. I think i ll go with 360s or 600s and plan with ISO 400 or 800, depending on the object.
I hope, that i got my guiding as much under control as i think. 🙂