Going to chime in here and try to expand on what Milan was saying.
If you're purely talking about exposure ie what you're seeing being output onto a screen then yes for example f/2.8 at 1 minute and ISO 400 is the same as f/2.8 at 30 second and ISO 800 in that they'll "look the same".
But the problem here is the amount of actual signal the sensor is picking up is purely dependent on the aperture and exposure time where ISO is only an amplification of the signal so in the previous example f/2.8/1 min/ISO 400 has collected twice the amount of light as f/2.8/30 sec/ISO 400. The longer exposure at the same aperture has collected more light irrespective of the ISO. This is important on a number of fronts because one the longer exposure will look better and show more nebulosity for the fact that it has more actual signal than just an amplification by increasing ISO (which also adds noise etc); secondly using lower ISO means you're not blowing out the highlights as much and it allows for tighter and better colour retention in the bright stars. This is why even though I own fast UW lenses like the Sigma 14mm f/1.8 if opportunity allows I'd still mount it on a tracker for my Milky Way landscape to collect as much real signal as I can.
Jul 30, 2021 at 12:17 AM
Previous versions of bballfreak6's message #15665021 « Astrophotography? »
