A question on ISOs

[Star-Lord]

Hi everyone,

I'm getting started with astrophotography (kinda, I'm using my Huawei P9 smartphone).

I've a question about ISOs. I've heard that say it's good to take a shot at ISO 3200 30s shutter speed to capture a lot of light from the stars.

But I've also read that you can take shots of ISO 50 30s shutter speed too - why would you do this? What is the benefits of having ISO 50?

Broad question I know.

Thanks

SL

-EDIT-

also, I managed to take a few shots last night and saved them as DNGs. Is it possible to get a decent images cleaned up in Photoshop from a single DNG image, or do I need to stack some to get the best results? I've tried changing the levels and curves but the images goes to sh*t.

[D4N]

The gain setting you should use is very camera specific, if there is nothing specific for your camera available on the web then it will be down to trial and error.

 

The first thing to understand is that the ISO values are just representative of the type of result you would get using film, what is really happening is that you are applying gain to the data.  Now to be able to simulate low ISO films the software may need to apply negative gain to your data.  So for example if your camera has a sensitivity equivalent to ISO200 film with zero gain selecting ISO50 would apply negative gain to the data, this would not be ideal for astrophotography as it will destroy faint details.

Now when you are applying higher gains you are doing two things, you are increasing the signal and increasing the noise.  To get around the latter camera manufacturers often use software to reduce noise at higher levels of gain.  Unfortunately noise reduction software in cameras often mistakes points of light like stars for hot pixels and removes them from the image.

So what you want is an ISO that isn't below the parity gain level and isn't so high that you are inducing too much noise and damaging your data with built in noise reduction.

 

DNG is a raw format so you should be able to stretch the data without it turning into a mess but you will indeed get better results from stacking them, try stacking several sub frames in DSS then stretch in Photoshop.  Your images are probably too big to use dithering in DSS unless you tell it to crop to a region of interest.

 

[Star-Lord]

6 minutes ago, D4N said:

The gain setting you should use is very camera specific, if there is nothing specific for your camera available on the web then it will be down to trial and error.

 

The first thing to understand is that the ISO values are just representative of the type of result you would get using film, what is really happening is that you are applying gain to the data.  Now to be able to simulate low ISO films the software may need to apply negative gain to your data.  So for example if your camera has a sensitivity equivalent to ISO200 film with zero gain selecting ISO50 would apply negative gain to the data, this would not be ideal for astrophotography as it will destroy faint details.

Now when you are applying higher gains you are doing two things, you are increasing the signal and increasing the noise.  To get around the latter camera manufacturers often use software to reduce noise at higher levels of gain.  Unfortunately noise reduction software in cameras often mistakes points of light like stars for hot pixels and removes them from the image.

So what you want is an ISO that isn't below the parity gain level and isn't so high that you are inducing too much noise and damaging your data with built in noise reduction.

 

DNG is a raw format so you should be able to stretch the data without it turning into a mess but you will indeed get better results from stacking them, try stacking several sub frames in DSS then stretch in Photoshop.  Your images are probably too big to use dithering in DSS unless you tell it to crop to a region of interest.

 

Thank you for the reply!

Is it possible to stack the same image a few times and clean that up (or would that be defeating the object of taking multiple shots and then stacking them?)

[D4N]

You need to stack different images otherwise you are not adding anything to the image.

Think of the image as data, some of that data is signal that represents stars and nebulae, the rest of it is noise.  The idea is to get more data and remove noise.  Since the noise is mostly random and the data is not stacking increases data and reduces noise.

There is also image calibration for reducing the less random noise such as amp glow and hot pixels.

[Robd]

Stacking the same image will not improve anything.  In each image the target, stars etc., is constant. The sky, noise etc is random and will vary between images. In simple terms stacking and taking the average will retain the target values whilst reducing the values of the background, thus reducing the noise.

Stacking a single image X number of times just gives the original back again. 

One image ten times averaged - 1x10/10 =1

Pain for no gain.

Rob