Why doesn’t a Canon 5D Mark II look much better than a 600D in my comparison?

Question

I upgraded from a Canon EOS 600D to a used Canon EOS 5D Mark II and expected a noticeable jump in image quality, but my test shots don’t look much better. I mainly shoot HDR and wanted fewer limitations.

For my comparison, both cameras used a Canon 50mm f/1.8. The shots were around ISO 125–400 and taken at f/1.8. Because the 5D Mark II is full frame, the field of view is wider, so I had to scale/crop the image to compare framing.

Why might the difference be smaller than expected, and how should I test these two cameras more fairly?

user75526 · Accepted Answer

You expected it to be better because people tend to think, bigger = better. People talk about how great full frame is without understanding what is really different, and your expectations are too great.

Sensitivity and Noise

Full-frame sensors have larger sensels than crop sensors that produce the same number of megapixels. For sensors of the same generation, full-frame sensors are expected to have an advantage in ISO sensitivity and noise. However:

The advantage is reduced on full-frame sensors that use smaller sensels to produce more megapixels.
The advantage applies primarily to high-ISO. Noise is minimized at low-ISO, such as those you used to take your test shots.
Sensible lens choices, with faster apertures, reduce the need to push ISO.
Newer technologies increase light sensitivity and reduce noise. However, Canon tends to develop technologies slower than molasses, and the EOS 5D mk2 was current throughout the 600D life-cycle. (This is why Sony has a 5-year head start on full-frame mirrorless.)

To see the noise difference between crop and full-frame, you need to take multiple ISO-matched images throughout the ISO range. View them at 100% without resizing. You should see noise become unacceptable at lower ISO settings on the crop sensor vs the full frame.

The "Full-Frame" Look

What your test images demonstrate is, when taking photographs from the same position, with the same lens, with the same settings, everything is the same between crop sensor and full frame, except field of view. Changing sensor size just changes the portion of the (exact same) imaging circle that is recorded.

What creates the "full frame look" is what people do to compensate for the different field of view. To fill the frame with the subject on a full-frame sensor, the camera has to be closer or the lens has to have a longer focal length. The difference can be reduced on crop sensors by using faster apertures, when available.