Low ISO Noise in Blue skies and water reflections?

[Nikon_Josh]

Quick question--- I am not a pixel peeper usually. I'm not usually interested in it as it is usually a hiding to nothing!

But I was stunned when I DID do some pixel peeping on images I took in Spain on holiday. I zoomed in on the the blue skies and alot of noise was present and this is at base ISO. I was quite stunned by this as it was not something I expected to see. Yes, I know you guys will say.. I have underexposed the images.. but it's not that.

My D90 is under warranty still, so I can still send it back. But I am thinking this may be more a fact of life in digital sensors. If you guys could explain I would appreciate it? I mean I always expect to see a tiny bit of grain, but red and blotchy noise taken in a well exposed part of the image?

 

[MissCream]

Almost impossible to tell if you don't post a sample

 

[jaomul]

Dirty sensor maybe?

 

[KmH]

It helps to have a good understanding of how a digital image works. IIRC, base ISO for a D90 is ISO 200.

For discussion sake - your image sensor has a 6 stop dynamic range. That 6 stops of dynamic range can render 4096 gradations of luminosity.

You are making a digital image (binary, base 2), so each darker stop of dynamic range has 1/2 of the luminance data the previous brighter stop of dynamic range.

Assuming a correctly exposed photo, the brightest stop of light in the photo has 2048 of the 4096 gradations of luminosity available. The next darker stop of luminosity has 1/2 of the remaining 2048 gradations of luminosity available, or 1024, and so on - 512, 256, 128, until the darkest of the 6 stops has only 64 gradations.

This is why for digital photography it's wise to do ETTR * Expose-To-The-Right.

If the sky or the water are in the middle stop and has only 512 gradations of luminosity available, noise is going to be more visible than in the parts of the scene that are in the brightest stop.

ALL digital photos have some amount of image noise. Plus, there are different types of image noise - shot noise, amplifier noise, thermal noise, etc.....

What was the air temperature? Had the camera been in the Sun long?

 

[Nikon_Josh]

It helps to have a good understanding of how a digital image works. IIRC, base ISO for a D90 is ISO 200.

For discussion sake - your image sensor has a 6 stop dynamic range. That 6 stops of dynamic range can render 4096 gradations of luminosity.

You are making a digital image (binary, base 2), so each darker stop of dynamic range has 1/2 of the luminance data the previous brighter stop of dynamic range.

Assuming a correctly exposed photo, the brightest stop of light in the photo has 2048 of the 4096 gradations of luminosity available. The next darker stop of luminosity has 1/2 of the remaining 2048 gradations of luminosity available, or 1024, and so on - 512, 256, 128, until the darkest of the 6 stops has only 64 gradations.

This is why for digital photography it's wise to do ETTR * Expose-To-The-Right.

If the sky or the water are in the middle stop and has only 512 gradations of luminosity available, noise is going to be more visible than in the parts of the scene that are in the brightest stop.

ALL digital photos have some amount of image noise. Plus, there are different types of image noise - shot noise, amplifier noise, thermal noise, etc.....

What was the air temperature? Had the camera been in the Sun long?

Thanks Keith, this is the sort of explanation I was hoping for! I have never even noticed this before because I rarely pixel peep. So you imagine my shock when I saw noise at ISO 200 in bright daylight conditions.. this really helps me to understand why I have noise and how I can work to avoid it in future.

Interestingly enough, the average temp was about 30 degrees and 25 degrees at the lowest. And I have heard before that hot weather can contribute to image noise.. So this would make sense! You say only 6 stops of dynamic range?? They say in the stats its something like 11... shows they overstate the stops of DR.