I bought my first digital SLR in 2006, an 8 MP Canon EOS 350D (Digital Rebel XT). Since then I have been watching the megapixels increase in each successive model, up to 15.1 MP in the 500D (EOS Rebel T1i). Is more better and what is the practical maximum?
In this review I compare pixel densities for Canon digital SLRs although the analysis is applicable to other manufacturers and cameras. Even if noise can be managed, I find that pixel densities on APS-C sensors are reaching the optical limit of the lens.
Pixel density is what counts
A good sensor has both good per pixel sharpness (detail when viewed at 100%) and low noise (usually only a problem on DSLRs at high ISO sensitivity). Like film, image sensors come in different shapes and sizes and the general rule is that bigger is better.
Big sensors mean low pixel densities. We can quantify density as megapixels per square centimetre or, resolution-like, pixels per mm. Popular Canon DSLR sensor formats are APS-C (dimensions 22.3 mm x 14.9 mm) and “Full-Frame” (dimensions 36 mm x 24 mm, same as 35 mm film). The area of a Full-Frame sensor is 8.6 cm2, more than double the 3.3 cm2 of APS-C.
For the same number of megapixels and equivalent sensor technology, larger sensors have greater distances between pixels and less noise. Take note, I’m talking about true sensor noise, which can be seen in RAW files processed with noise removal off. Although modern noise reduction algorithms and Canon’s DIGIC in-camera processing is good, it’s always better to capture a clean image than to try and correct a noisy one.
Resolution is different, we need to consider the lens as well. Full-Frame sensors present a bigger target and are less demanding of lens sharpness than APS-C sensors with the same number of megapixels. I would like to see some tests to demonstrate this.
Figure 1 compares pixel densities for a selection of Canon Digital SLRs. The highly respected 5D at 12.8 MP, Full-Frame has lower density than the 300D (Digital Rebel) at 6.3 MP APS-C. The 5D Mk II has an astonishing 22.1 MP Full-Frame and still should be silky smooth, like my 350D at 8 MP, APS-C.
The only way is up
Figure 2 plots the growing number of megapixels in Canon’s consumer line of DSLRs. Observe that the increment for the 500D was 3 MP rather than the usual 2 MP and that it was released 1 year after the previous model and not at the usual interval of 1.5 years. This is Canon competing in a Megapixel race. Consumers don’t care much for pixel density, they don’t view images at 100% and more pixels mean more sales.
I don’t consider the 500D to be serious about image quality. DPreview agrees and noted that “the 500D’s RAW images are also slightly lagging behind some of the competition and surprisingly even the 450D in terms of high ISO noise and to a smaller degree in terms of pixel level detail” (DPreview, June 2009). They liked the 12.2 MP 450D (Digital Rebel Xsi) for image quality. It’s a good choice for smart shoppers. I would check out 450D body-only prices and spend the savings on a high quality lens.
What about the Canon 7D?
I have recently read DPreview’s in-depth review of the new Canon EOS 7D, posted in November 2009. The 7D has an 18 MP APS-C sensor and pixel density is very high at 5.4 MP/cm2. By some miracle of sensor technology, the 7D is said to deliver “excellent per-pixel sharpness” and “performs very well in low light situations” (DPreview, November 2009). DPreview were a little sneaky however and didn’t run any comparisons versus the 21.1 MP Full-Frame 5D Mark II. I would like to see that!
Accepting that the 7D has a near-perfect sensor, the reviewers acknowledge that “in most situations the lens, rather than the camera, is likely to be the limiting factor” (DPreview). Laboratory tests are not real world photography. They use a stopped-down Canon EF 50 mm f1.4 prime lens, ISO100, tripod and mirror lock-up. If you are shooting candids at f/2.8, ISO800 and hand-held then don’t expect to get your full 18 MP worth of image quality from your 7D. You might do better with a 5D Mark II.
The 7D apparently exceeds the practical limit of lens resolution. What’s more, there are no professional L series Canon lenses in the EF-S mount. Now the 7D is surely a good camera but not because it sports 18 MP. The 450D already has enough resolution for the best lenses (DPreview, May 2008) and 12.2 MP may well be the practical megapixel limit for APS-C DSLRs, in terms of resolution. It follows that 3.7 MP/cm2 is a practical pixel density limit in general. Higher pixel densities do not achieve more usable resolution.