Re: Colorsync-users Digest, Vol 2, Issue 509 - filtering
Re: Colorsync-users Digest, Vol 2, Issue 509 - filtering
- Subject: Re: Colorsync-users Digest, Vol 2, Issue 509 - filtering
- From: Ray Maxwell <email@hidden>
- Date: Tue, 27 Dec 2005 08:36:59 -0800
I Wrote:
Can someone explain either the flaw in my logic or give other
evidence that would support how adding a blue filter will help the
signal to noise ratio.
Thank you,
Well, Gordon gave us evidence that something was wrong with my analysis.
I repeated Gordon's experiment with a few more controls. I did the
experiment with the following setup:
Canon 5D
ISO 1600
RAW file
Full manual control
Increased 2 stops to compensate for the blue filter
White Balance done in Adobe Camera RAW
The result was that the noise in the Blue channel was less with the
blue filter.
So, what was wrong with my Signal to Noise Analysis?
While signal to noise ratio is very important in many situations it is
not the critical element in this situation. Unfortunately the eye does
not look at both the signal and then compare it to the noise. It just
looks at the absolute value of the noise. The absolute value of the
noise does increase by the same amount as the signal.
I did a further experiment that showed that if you decrease the ISO
setting by two stops (1600 to 400) and do white balance in ACR, you
can achieve the same resulting decrease in noise. Of course you will
have to increase your exposure by two stops just like you did with the
filter.
I would also point out that the noise in the red channel does increase
slightly when the blue filter is used. However, this increase is not
as great as the blue channel without the filter. You can see this
effect in Gordon's examples.
The decrease in ISO setting does not cause the red channel noise
increase.
Hope this satisfies everyone.
Stefan and Gordon, I was wrong and will admit it.
I believe that I have explained the effect correctly this time.
BTW when I borrowed the last blue filter in stock from my local photo
dealer, he told me he no longer stocked these correction filters since
you can do the same thing in Photoshop...I am not sure if I should ask
him to read this thread : )
Ray
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