Is 5D IQ that much better than 30D?

I understand that the 5D sensor is about 2.5 times the surface area of the 30D, however the 5D has many fewer pixels per square mm. I also realize that the potential for noise, in the 30D, is higher. At what size print can a difference be seen?

the fewer pixels per mm helps in the image quality

Well, 50% more pixels lets you go about 22-23% (linear) larger with the same pixel resolution. So figure how big you can make an acceptable 30D print and multiply the size by 1.22.

Under most conditions, you cannot tell the difference between an image from a 30D and an image from a 5D. Shooting at high ISO or printing to large sizes reveals the strength of the 5D

And more fine detail in WA images from 5D sensor

DOF is considerably less on the 5D also. The DOF on the 30D is tough sometimes. I can't imagine less.

I thought Depth of Field was a function of the lens, focal length, aperture/distance to subject. What did I miss here?

It changes the distance ratio between lens, subject and background.

Gerhard

Friend just bought the 5D after renting one for a weekend shoot. He owned an XT. Immediately bought the 5D because the image quality was so much improved - and he knew it wasn't his abilities.

cc10d wrote in post #2258433
I thought Depth of Field was a function of the lens, focal length, aperture/distance to subject. What did I miss here?

Well, that's technically correct. You've got to remember though that to shoot the same scene from the same position, the 30D user is going to choose a lens with a focal length that's 1.6 times less than that of the 5D user, to get the same field of view. Because the 5D user is using the longer lens at the same distance, the DOF for them is going to be smaller than for the 30D user (at the same aperture). Alternatively, if they are both using the same lens for a given scene, the 30D user will need to be farther away to achieve the same field of view... again this means that the 30D user has more DOF to work with at the same aperture than the 5D user.

I'd be surprised using the same ISO + same size image results one most likely couldn't tell the difference between the two cameras. The 5D holds the advantage from ISO 1600 up but not overwhelmingly so.

Image quality on the 5D is somewhat better than the 30D, but not so much that it makes much difference for most applications. The 5D makes use of newer technology while the 30D still uses the same sensor technology that was introduced with the 20D.

I read one article that claimed Canon was working on a 12 MP EF-S sensor that would outcompete the Nikon D2X, but decided not to bring it out yet because sales would hurt the 5D. The thought was that Canon will hold off releasing that sensor until after they bring out the next generation of the 5D which will have a 16 MP sensor.

Image quality of both sensors will continue to improve as new technology is released and full frame will likely always be ahead. The real question for the camera buyer will always be, "How much image quality do you need and how much are you willing to pay for it?"

ScottE wrote in post #2258661
I read one article that claimed Canon was working on a 12 MP EF-S sensor that would outcompete the Nikon D2X, but decided not to bring it out yet because sales would hurt the 5D.

Well that would indeed make me for go the the FF path even longer.

My 5D will attest to that - almost as good my old medium format .

dontblink wrote in post #2256742
...... Shooting at high ISO or printing to large sizes reveals the strength of the 5D

ScottE wrote in post #2258661
I read one article that claimed Canon was working on a 12 MP EF-S sensor that would outcompete the Nikon D2X, but decided not to bring it out yet because sales would hurt the 5D. The thought was that Canon will hold off releasing that sensor until after they bring out the next generation of the 5D which will have a 16 MP sensor.

Then you simply read someone' s guessing rumor. I am sure Canon has prototypes of several cameras they didn't release. The "they didn't release it or won't release it because it'll hurt 5D sales" thing is getting old for people talking about 10+MP dSLRs. People will buy the 5D because it is FF and the great things that come with that, plus the huge RAW buffer, bigger viewfinder. Users know the difference between cameras.

Again you're speculating the next 5D to have a 16MP sensor but you've stated it as "fact" in your post. No one knows what the 5D replacement will have!