sensor size

[Many Hats]

OK, I don't get it. Can someone please explain sensors to me? The new Canon Rebel is 12mp...the Canon 5D is 12mp and 4 times as expensive. I'm guessing the 5D is superior cause it has a "CMOS" sensor, but I'll admit I really don't get what that means. Can someone please explain?
Thanks, Laura

 

[JustAnEngineer]

http://www.the-digital-picture.com/Canon-Lenses/Field-of-View-Crop-Factor.aspx

The EOS 5D has a 36x24mm full-frame sensor (same size as 35mm film). The EOS Rebel XSi has a 22½x15mm APS-C size sensor (causes a field of view crop factor of 1.6 compared to full-frame). The individual pixels on the 5D's sensor are larger than the pixels on the Rebel XSi's sensor.

P.S.: Take a look at this other thread:
http://www.thephotoforum.com/forum/showpost.php?p=1336635&postcount=19

 

[Garbz]

And because less sensors the size of a 5D fit on a silicon wafer, they cost 4 times as much to make.

 

[Big Mike]

Basically, the larger the sensor, the better. The 5D has what we call a 'full frame' sensor which is the same size as 35mm film. Most other DSLR cameras have a smaller sensor (APS-C size, as mentioned above). Most non-SLR digital cameras have very small sensors. As mentioned, it's a lot more expensive to make larger sensors, so there is the price difference.

All Canon DSLR sensors are CMOS sensors, by the way.

For the most part, a larger sensor will give you better image quality with less digital noise. The photosites are more spread out, which is desirable. There are other factors which influence image quality...so image quality is constantly improving, even as they cram more photosites onto sensors (making them less spread out).

 

[Many Hats]

So basically, megapixels is a relative term?

 

[Big Mike]

Not really...megepixels are pretty finite. A sensor will have an exact number of 'photosites'...which relates directly to the megapixel number.

That being said, the number of megapixels isn't a direct reflection of the image quality.

 

[Many Hats]

OK, so "photosites" spread out is better? I'm completely confused, but I guess the bottom line is that a 5D is going to beat out the new rebel on quality when they're both using the same lens. Same with 30D or 40D? And L series lenses will be better than other models? Sorry if I'm driving you all crazy. Just trying to learn.

 

[Bifurcator]

Yes! The less dense the cells the better resolving power and color fidelity. Definitely true.

If you'll notice in spreads like this: http://www.dpreview.com/reviews/compare_post.asp?page=1&remove=5

Probably the most useful specification in determining image quality is the "Sensor Density" row. In that one it reads:

1.4 MP/cm²
4.0 MP/cm²
4.0 MP/cm²
4.0 MP/cm²
3.1 MP/cm²
2.4 MP/cm²
1.9 MP/cm²

But across not down. The ones that are 2.x mp and under are the 1st ones to look at IMHO if you're looking for the best image quality.

 

[Bifurcator]

I'm finding that's a bit of hog-wash though. Sure it looks clean but it just ends up being mud. It depends what you're doing but if all you're doing is selling wedding prints, or shooting for a magazine (which are all terribly low resolution!) then what's the point? Besides ISO speed of course I mean!! That one counts allot. But the images from my little 2/3" A2 which has a pixel density of OVER 20 MP/cm² can be displayed side by side after post processing here at ThePhotoForum with the most expensive kit not excluding MF digital backs and no one can tell the difference. So considering post processing and considering the target output such specs as we all love to discuss are many times under many circumstances, only academic.