If you read just one article on my blog, let it be this one. In Size Matters, Robert Atkins explains what happens when you shrink the imaging surface to a fingernail-sized sensor, as most digicams do. If you read between the lines this speaks volumes about the design, purpose, and limitations of these cameras. There is a physical limit to how much resolution you can achieve on a small scale, no matter how many megapixels you pack into it and how good a lens you have. The limit is caused by diffraction, the tendency of light to spread out when forced through an aperture. It turns out that beyond a certain limit it makes no sense to increase the resolution of tiny sensors, because no lens can ever deliver this resolution. And in fact, more sensors per millimeter can actually degrade performance:
"So what" you might think, "if you have enough pixels, what does the sensor size matter?". Cameras with smaller sensors use shorter focal length lenses to get the same angular coverage as cameras with larger sensors do with longer focal length lenses. So if you have a 28-105mm zoom on a Canon D30, a 10-37mm zoom on a Nikon 995 or a 7.4-28mm zoom on a Minolta Xi, you get approximately the same shot. What's the big deal about the physical size of the sensor? Why does it matter?
[L]ets take an example of an 8x10 print and let's say we want a fairly sharp print, so we are going to need a resolution of at least 3 lp/mm in the print. First, by looking at line 7 we can see that that's possible with all three cameras. Now for 3 lp/mm in the print, what resolution do we need from the sensor? Well, that's given in row #8 of the table. Since the smaller the sensor the more the image needs to be enlarged, to get the same resolution in the same sized print we need more resolution from smaller sensors. The table shows that for the D30 we need to record the image at up to 40.4 lp/mm on the sensor. For the Coolpix 995 we need to record 115 lp/mm on the sensor and for the Xi we need to record a whopping 152.4 lp/mm on the sensor.
In fact we could not obtain the desired 3 lp/mm in an 8x10 print if we were able to stop the lens down to f16 on those camera. That's why the smallest aperture available on these and most other small sensor cameras is f8.
So now you know why "bigger is better" when it comes to image quality and digital sensors. Of course bigger is also more expensive, and bigger means bigger (hence heavier and more expensive) lenses, so you can see why many digital cameras stick with small sensors. It's cost, not quality that keeps sensors small.
An f2.0-2.8/7-50mm lens on a small sensor is NOT equivalent to a f2.0-2.8/28-200mm lens on 35mm format! We are all used to apply a "cropping factor" to calculate the equivalent focal length, but we should do this also to the aperture value, in order to get an "equivalent f-stop". Then, the lens would be an equivalent f8.0-11.2/28-200mm lens. It's just that no manufacturer would like to name it this way. - Achim Schumacher, in a comment to Bob Atkins' article.