ISO Over Exposure

[Langy]

Last night I had a quick go at some DSO stacked photography with my DSLR and 300mm lens on a tripod as in the post here http://stargazerslounge.com/topic/206274-first-attempt-dso-images/

All the images were taken at 1.6 seconds so increasing time in that case is not too much of an issue.

However this has brought me to asking the question.

On a DSLR at what point do you start getting the noise where the DSS software will not deal with it properly or it generally appears in the image too obviosuly?

I know with the old film days that even on the B&W film I used that grain would become very obvious on ISO 400 but if you shot with ISO 800 or faster it was great for certain images. The rule of thumb on film, was to use the slowest sensible speed that was possible for the clearest image. However as these are dark scenes with taking images of the sky at night the noise shouldn't be so obvious as you slightly increase the speed. My preferred choice was the 125 Ilford film.

So realisticly how fast should you be setting the ISO as a minimum and only go beyond when the exposure times are too slow?

Thankfully all items are too far to have to start worrying about DOF and setting various apertures.

[IanL]

This explains about ISO and long exposure imaging:

http://www.blackwaterskies.co.uk/2014/01/do-high-isos-make-dslrs-more-sensitive.html

TL;DR: You won't get any more light in your image using a high ISO, digital is not film.  You will increase the appearance of noise however, and potentially lose detail in bright parts of the image. So use a low ISO (depends on the camera model, but ISO 400 is usually a safe bet).  Stack your image, then if it is still too dark use a histogram stretch to bring out the details.  The only good reason to use high ISOs is if you need to see the image on the back of the camera immediately (i.e. not using a laptop for camera control).

[Langy]

Thanks IanL

That's a really interesting article to read. I've only ever used the ISO setting the same way you would use a film camera, i.e. in poorer light conditions when you needed the faster shutter speed then you used the faster film.

I always knew that digital could never replicate the grain of a nice fast film like ISO 1600 or 3200 as they just became messy and noisy. I just didn't realise that the sensor was set to a level around 200 or 400. That changes the way I will be taking normal images in the future by not setting the camera at the lowest of 50 or 100.

For those of you who don't understand using the grain on a film look for a nice rugged image, usually an old person with far from perfect skin that is shot with B&W ISO 1600 or above, digital can not reproduce that.

[Ryan_B]

This explains about ISO and long exposure imaging:

http://www.blackwaterskies.co.uk/2014/01/do-high-isos-make-dslrs-more-sensitive.html

TL;DR: You won't get any more light in your image using a high ISO, digital is not film.  You will increase the appearance of noise however, and potentially lose detail in bright parts of the image. So use a low ISO (depends on the camera model, but ISO 400 is usually a safe bet).  Stack your image, then if it is still too dark use a histogram stretch to bring out the details.  The only good reason to use high ISOs is if you need to see the image on the back of the camera immediately (i.e. not using a laptop for camera control).

That was all new to me, and an interesting article too, thanks for sharing!

[ChrisLX200]

You have not said what camera you're using? A good quality camera can produce noise-free (almost) at high ISOs due to superior amplifiers and internal processing, my Canon 70D is certainly good for 1600ISO although I tend to stay with ISO800 (having generated a bank of darks, flats and bias files at various temperatures with that ISO). 800ISO is a happy medium for a new-ish quality camera but you may have to drop that after some test exposures if it appears to suffer from excessive noise. The remaining noise - and there will always be some - is dealt with by stacking subs and during post-processing. My elderely Canon 350D exhibits a lot more noise than the 70D at similar ISO so I either have to process it out or shoot lower ISO.

ChrisH

[RikM]

It also depends on the actual camera and optical system. I have used both Canon 1000D and Canon 1100D with an f/5 Newtonian. I get much better results with the 1000D. More Ha sensitivity, lower noise at the same ISO, it seems easier to process the images. I use ISO1600 unless there are very bright stars in the field and as long an exposure as necessary to lift the peak on the histogram to between 30 and 50%. All the technical specs suggest this is the wrong way round re. camera choice and that ISO800 should be better than ISO1600 but I find that with my camera at 1600 it has better colour depth and lower noise than an image at ISO800 stretched to the same screen brightness.

[Langy]

Chris, it's a Canon EOS 300D. I originally had the EOS 300 so when I went to Digital opted for the 300D as they were exactly the same with the exception of the digital element and magnified focal length.