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Old Jun 20, 2007, 11:11 PM   #1
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I am soon going to purchase my first dSLR and have been reading on the subject. I am left with just a few questions that I am not sure how to answer.

The quality and type of the sensor seems to be one of the biggest factors affecting image quality, and from what I have read, it seems the bigger the individual photosites on the sensor, the better. How can one determine this value as it is not in the product literature. I get nervous reading about a 10 megapixel camera when I think maybe the same camera with 6 megapixels could take a better picture, if smaller in resolution.

Also, when the camera converts the analogue information from the sensor into digital image files it must use an A/D converter, which can be 8-bit, 10-bit, 12-bit or even 16-bit. The higher the bit count of the converter, the higher theoretical dynamic range the camera can achieve, but again, this converter information isn't included anywhere that I have looked.

I am looking at the Rebel XTi, so information on this particular camera would be helpful, but any information would be welcome.



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Old Jun 21, 2007, 2:41 AM   #2
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This is all a bit complex, so here's my understanding of the issues (and I may well have some of it wrong.)

All things being equal you're right, larger photosites are better. Of course with succeeding generations of sensor all things are not equal and often they can reduce noise even with smaller sites. Canon is quite adept at doing this in successive generations of their DSLRs.

Dynamic range is most closely associated with the size of the sensor photosites. The larger the site, the more photons it can capture before the well becomes full.

A "better" picture is hard to tie down exactly. Sharpness is very much an interaction between the size of the photosites (and associated diffraction limits), the efficiency of the microlenses, the noise off the sensor, the strength of the anti-aliasing filter and so on.

Bit depth and dynamic range are not the same thing; higher bit depth can increase dynamic range, but only if the photosites are large enough, so it's not always the case that more bits will give more DR. Most DSLRs including all the Canon's prior to the 1Ds MkIII use a 12-bit colour space. The 1Ds MkIII uses a 14-bit colour space. Both use the larger 16-bit address space for encoding, but the extra bits are "wasted" containing no useful information.

Some of the medium format digital backs capture in true 16-bit colour.

This is the page you need:

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