Can exposure correction increase noise?

[sbloomer]

Hi all

Yesterday I took some nice pictures of our local Box Jellyfish (similar to Chironex but not as nasty). However, for some reason they were all about two stops underexposed (because I was klutz). I tried to adjust the exposure (as well as colour) in Photoshop, and the pictures came out unbelievably noisy. Using ISO100 I've never had noticably noisy pictures, but I was quite disappointed in this.

Is correction for gross underexposure likely to increase noise? I wouldn't initially have thought so. How does it happen? When I get round to "webbifying" the pics I'll post them.

 

[onbelaydave]

There is no substitute for proper exposure. The most I find I can "tweak" an underexposed shot during RAW conversion without adding any additional noise is 1/2 to 3/4 stop. But it is highly image dependent. Images with plain black backgrounds are the worst culprits for showing noise. A detailed background can sometimes be stretched 1- 1 1/2 stops and still look usable.

 

[Mike Veitch]

Definitely. What tools did you use in Photoshop to do your corrections?
Try not to use the brightness/contrast tool, it adds a lot of noise. Also any of the Auto features are not good.
Your best tool is the Curves tool. Inside this you will find 3 eyedroppers: white, black and grey. If you have anything that is white in your photo, use the white tool and click on that area. It will brighten up the whole image.
ALso you could try with the grey, then use the graph tool and bring it to the right until your exposure is decent but not too far.
Then go to the hue/saturation tool and bring the saturation up by about 20.
Make sure to follow all of this with an unsharp mask of 150% and radius of up to 3.0 but no more and with threshold at 0.
THis is the least noise making way to colour correct.

I think i will write a sticky for colour correction...

Good luck and let us know how it goes.
MIke

 

[sbloomer]

Definitely. What tools did you use in Photoshop to do your corrections?
Try not to use the brightness/contrast tool, it adds a lot of noise. Also any of the Auto features are not good.
Your best tool is the Curves tool. Inside this you will find 3 eyedroppers: white, black and grey. If you have anything that is white in your photo, use the white tool and click on that area. It will brighten up the whole image.
ALso you could try with the grey, then use the graph tool and bring it to the right until your exposure is decent but not too far.
Then go to the hue/saturation tool and bring the saturation up by about 20.
Make sure to follow all of this with an unsharp mask of 150% and radius of up to 3.0 but no more and with threshold at 0.
THis is the least noise making way to colour correct.

I think i will write a sticky for colour correction...

Good luck and let us know how it goes.
MIke

Thanks for the info - I originally started with the mandrake method but it was terrible, and started doing it all manually, particularly using levels and curves, but nothing worked particularly well. What made it more obvious that it was a uniform dark background.

I agree you can't compensate for crappy exposure, but I was just hoping... I'l just have to try again when I get the chance.

 

[segal3]

Thanks for the info - I originally started with the mandrake method but it was terrible, and started doing it all manually, particularly using levels and curves, but nothing worked particularly well.

That's not really *exposure* correction...

You can only achieve a semblance of exposure correction using (as onbelaydave mention) a RAW converter of a sort. What these converters do when you adjust exposure up or down is effectively raise or lower the "processing" ISO to gain a brighter or dimmer image. This is why it is always emphasized (with RAW) to shoot to the right of the histogram at the lowest ISO. You can lower the noise even MORE (effective ISOs of lower than what camera provides) by then decreasing the exposure to recover the highlights and balance the photo.

~Matt Segal