Dark Sky/ISO dilemma

[Mark1489]

Hi all,

My local imaging site is SQM 21.2 Bortle 4, with my gear I can get 60 sec exposures reliably, I use ISO 1600 to get my histogram just clear of the left side.

My dilemma is that I want to travel to a nearby site which is SQM 21.8 Bortle 2 but with my gear I think I’ll need to use ISO 3200 to get separation from the left. 

How much is the reduction in dynamic range going to effect my images? Does lower dynamic range reduce the amount of faint detail I’m going to be able to capture? I use a Nikon D5300. 

 

[davew]

At ISO 1600 the Photographic Dynamic Range is near 7 stops. At ISO 3200 the PDR is near 6 stops and as the change is quite linear you can soon work out the rest.

The read noise change from ISO 1600 and up is fairly flat so no real advantage to increase.

One stop of PDR drop sounds significant but I doubt you'll see much difference in real life if you stack your subs. I've tested this on other cameras and the difference doesn't jump out at you.

However. If you're using the screen's histogram then remember it is showing the Jpeg conversion histogram and not the Raw. Try a few settings, especially down with the Sony chip, and see what happens in your processing app.

Dave.

[Mark1489]

1 hour ago, davew said:

At ISO 1600 the Photographic Dynamic Range is near 7 stops. At ISO 3200 the PDR is near 6 stops and as the change is quite linear you can soon work out the rest.

The read noise change from ISO 1600 and up is fairly flat so no real advantage to increase.

One stop of PDR drop sounds significant but I doubt you'll see much difference in real life if you stack your subs. I've tested this on other cameras and the difference doesn't jump out at you.

However. If you're using the screen's histogram then remember it is showing the Jpeg conversion histogram and not the Raw. Try a few settings, especially down with the Sony chip, and see what happens in your processing app.

Dave.

Hi Dave

Thanks, makes sense! I use the camera histogram yes, are you saying that I could possibly use a lower iso without clipping data? Would there be any way of getting a RAW histogram showing somehow out in the field? I’ve read the sweet spot for the D5300 is ISO 200 or 400 which is obviously a fair bit lower than what I’m using. 

Mark 

 

[davew]

Hi Mark,

I don't know of a way to get the raw histogram in camera and as far as I'm aware most cameras are the same.

The only way I can suggest you check for clipping is to run your camera at various ISOs and then check. 200 and 400 have been advised in the past and a few in the USA are using the 5300 at low ISO values ( 400 I seem to remember ) But whether it's in a very dark sky or not, I don't remember. 

I know that my camera is fairly universally recommended to be used at ISO 800 but I found 1600 was better in a dark sky. I found that out by experimenting for a couple of hours

Dave.