How to compare data collected with two different settings?

How to compare data collected with two different settings?

Hi there. So my friend was taking photos of Andromeda with 2.5 second exposures at f/4 and ISO 10k. I was taking photos of Andromeda with 60 second exposures at f/5.6 and ISO 1600. Both were with cameras and lenses.

I'm wondering what the proper way to compare the amount of light collected is. For our exposures, mine were 4.5 stops greater than his were. His f/stop was 1 times better than mine, and his ISO was 2.5 times better than mine. So is the proper way to do something like 24.5 (exposure) - 21 (f/stop) - 22.5 (ISO) to get that I collected 15 times the amount of light? Or would the proper way be to say that I collected 24 times the amount of light (60s/2.5s) and then the difference in f/stop means that I collected 12 times the amount of light (if we had equal f/stops), then say that I collected 1/22.5 times the amount of light (due to ISO) to get that in total I was able to collect 2.12 times the amount of light?

Thanks

Submitted October 13, 2016 at 02:40PM by Kanel0728 from reddit http://ift.tt/2dWuBfK via /r/astrophotography http://ift.tt/eA8V8J