50D vs. XTi = IQ tie

Started Oct 8, 2008 | Discussions thread
JerryG1 Veteran Member • Posts: 3,232
Re: Thanks John (and a work intensive suggestion)

Johns a geek wrote:

I've seen people suggest these black exposures, but I guess I don't
quite get it. If you're trying to measure signal / noise, don't you
need some signal? With the lens cap on, your signal is zero, and
zero / anything = infinity... I agree that bright exposures can
swamp the noise, but it shows up in the shadow areas.

I would think the camera-generated noise is independent of the signal. Think of teh lens cap exposure as a photo of the night sky between the stars That's a worst case "real" situation that has ultra low signal and lets you measure the noise.

After I got off my post, I tried it myself with my XTi. Had to modify the procedure a bit because DPP only goes to magnification 400% which is not enough (for me) to see individual pixels. I made tiffs, set the magnification in PS to 3200% and filled the screen with image, which gave me about 1000 pixels (36 x 28). I did a few counts at various locations in the frame and averaged them.

My results for my XTi
ISO 100 no visible noise--less than 1 pixel per 100
ISO 400 minimal noise -- about 1 pixel per 1000
ISO 800 noticable noise-- about 8 pixels per 1000
ISO 1600 obvious noise-- about 30 pixels per 1000

It was less work than I thought. I didn't clock it, but it seemed like the whole job took maybe half an hour at most for the one camera.

I didn't enhance the noise and I didn't filter it. I didn't differentiate between bright and dull pixels, If I saw it, I counted it. I think if the same person did this for two cameras, it would give a pretty good feel for the relative noise at a given ISO for the two cameras. If you wanted to carry it further and rescale the 50D image down to XTi size, that would probably better represent the appearance of same size prints from the two cameras.

John, if you get to do any of this, be sure to let us know what you find. I plan to get the 50D in a couple of months, (if there is any money left in the world).

Thanks for all you've done so far.

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