Wednesday, November 18, 2009

I often do a google search when I'm writing blog posts. Usually it's to check a fact, formula or site html. But sometimes I hit on something new that causes me revise what I plan to post.

Since decision time is galloping closer--the Young Shakespeare Player's dress rehearsals start on Friday--I must decide how best to photograph them. It's the last good time to show up with a camera. Then it's time off for the Thanksgiving weekend followed by two weekends of performances before the Julius Caesar cast disbands.

But instead of posting test shots of real people shot at a show opening as I promised I'll be hitting you with more posts on theory. This time I discovered a new site--

If the sight of a mathematical formula immediately sends you off to find a celebrity website you may want to skip this one. But if you are mildly mathematically inclined like me the site has the best explanation of the intricacies of camera noise I've found so far. It confirmed some of my suspicions, explained some of the mysteries I've worked on and set me straight on some matters I've gotten flat out wrong.

Like the number of photoelectrons a sensor can hold. My rule of thumb of 1200 photo electrons per square micron of pixel is too small. That number still fits the small sensors I've tested before. But with larger and better made sensors such as the one in my D60 there is room for far more photoelectrons and far more S/N.

Not that I won't be blogging about the show. It was put on by a group of collectors of found photographs-- antique or just plain old photos you find in flea markets or garage sales.

At the opening the speaker was a well known collector of folk art from St Louis. His talk was on the cream of his photo collection--the part that has been on display in a number of art museums. Afterwards he asked me to send him some of the photos I took during his talk. Another reason to work out how to best clean up low light images.

So far I've been concentration on how good a S/N I can obtain from the D60. I've been ignoring the other half of that question. How much S/N do I need?

The image of the Declaration of Independence provides some insight. (Click on it for a larger image.)

It was manufactured by taking a well exposed image and superimposing a gradient of Gaussian noise on top. The S/N varies from less than one on the left to eight on the right. From it you can see you don't need as much S/N to bring out the fine detail as you might have thought.

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