(Note: I have updated this post several times, sometimes in the body of the original posting but in other cases by adding notes at the end. Also, it would be useful to update the illustration to include more current MP sensor dimensions, and I’ll do that at some point – but the point of the comparison is still useful even if you are looking for a camera with a 20+ MP sensor.)
Having a bit of free time this morning I decided to create a simple chart that compares digital camera sensors with different megapixel counts.
Keep in mind that this reflects the pixel dimensions but not the actual physical size of the sensor. For example a 12 megapixel APS sensor and a 12 megapixel full-frame sensor would look the same on this chart even though one is physically larger than the other. As such, a value of this way of looking at the differences is that it illustrates how much larger/small a print might be with the same resolution – to the extent that resolution is a result of pixel density.
All things being equal, it seems to me that moving from one size to the next larger (from 6 to 8 megapixels, from 8 to 10, from 10 to 12, etc.) is unlikely to make a significant difference for most people. I’m confident that a doubling of megapixel count is likely to be significant in many cases, though even here it will depend on a host of other factors. (Among them being is the size at which you print. If you print at, say, 12″ x 18″ you probably won’t see much difference past 8MP unless you look quite closely at your prints, and beyond 12MP there probably won’t be any significant in prints made from uncropped photos at this size.)
You might roughly extrapolate from the pixel count you are familiar with to figure out, for example, how large a print you might make using a higher pixel count sensor. For example, I feel that I can make good 12 x 18 prints from high quality 8 megapixel photographs. It seems that quality from a 12 megapixel sensor might be roughly the same at 14 x 21 or possibly 15 x 22.5 inches. A 16 megapixel sensor might provide similar resolution for a 16 x 24 or slightly larger print area.
As always, these assumptions are only general predictions. For example, if you assume that a larger print would be viewed from a greater distance, it would be reasonable to assume that you might get away with pushing the print size a bit further.
It is also important to keep in mind that pixel density is very definitely not the whole story when it comes to the questions of how large a print you can produce from a particular original photograph. Quite often the limitations in this regard are not based on the number of photosites on your sensor but are more likely based on other factors such as the stability of the camera when you made the exposure (e.g. – use a tripod), the accuracy of focus, the quality of your lens, your decisions about aperture, and even the subject of the photograph.
Update 5/21/08 – I still think this piece is generally on the right track, though after more experience using DSLR camera and making prints my views have changed a bit. Here are a few examples. I think I was a bit too conservative regarding print sizes that would be possible/acceptable from a given sensor. With good technique and care, I think it is possible – though not always quite automatic – to create a quite decent 16″ x 24″ print from a 8MP original. Although larger might be OK in some situations – e.g. an image in which fine detail is not the point – for me the details begin to get lost at about that point. I’m confident about going to 20″ x 30″ from a 12MP full frame original. These are not absolute boundaries, and there is a considerable amount of subjectivity at play.
I more or less stand by what I wrote about increases in MP count that might or might not be significant. I still think that the differences between, for example, 10MP and 12MP would rarely be visible, and I’m confident that the difference between 8MP and 16MP could be significant. But even here there are other factors to consider. To be more specific, if you make very careful and high quality exposures on the 16MP body and print quite large, the differences can be significant. I do not think that the difference would be at all significant in a letter size print, much less in a typical photo posted on the web
Finally, a smaller change in MP count accompanied by a switch from, say, a 10MP 1.6x cropped sensor body to a 12MP full-frame body could produce other image quality advantages in other ways that could make this a worthwhile upgrade for some photographers.