If you are looking for a digital camera, do not miss Phil Askey's Digital Photography Review or you may regret it later. This site is a tremendous reference for all things related to digital photography and Phil's reviews -- objective, comprehensive and very well written -- are a tribute to an art of professional journalism that one has missed lately on big media houses.
I have been testing a few cameras lately, Canon's PowerShot G5 and the EOS 10D. DPR has a preview of the forthcoming Olympus E-1 SLR. Even if you are not interested in any of those, I recommend the article on the E-1 which explains very well the difference in today's cameras sensors and why size matters.
Image ratio and size of sensors
- 24 x 36 mm (2/3 ratio and size of the traditional 35 mm film). Sensors of this size are found on high-end digital SLR cameras aimed at professional photographers (at 10,000€ a pop without a lens!)
- 15.1 x 22.7 mm (2/3 ratio), sensor found on Canon's SLR (D30, D60, 10D)
- 13.5 x 18 mm, aka 4/3" Type or the "four thirds system" in Olympus' marketing jargon
- 6.60 x 8.80 mm, aka 2/3" Type, sensor found on top of the line compacts
- 5.32 x 7.18 mm, aka 1/1.8" Type, sensor found on low end compact cameras
The image ratio governs the shape of images. 2/3 is what you have been used to for a long time with 35 mm cameras. 4/3 is the image ratio of a standard TV or computer screen.
The sensor size, however has several impacts on the outcome.
Smaller sensors capture a smaller circle of light behind the lens. On SLR cameras this translates into a focal length multiplier factor that you need to take into account if you are used to traditional cameras. For example, a 24-70 mm lens mounted on an EOS 10D, which has a multiplier factor of 1.6, will turn into a 38 x 112 mm lens. The main drawback is that your expensive 16 mm wide angle lens is now a 26 mm for twice the price!
Smaller sensors however, because they need a smaller image circle, tend to cut out many of the faults you see in zoom lenses at wide angle (darker corners, distorted parallels). Manufacturers, notably Nikon and Olympus, are trying to build on that benefit by producing smaller and lighter lenses (and with wider angles) specifically designed for the sensors size.
Photodiodes count and size
You may have noticed that digital cameras marketing focuses a lot on how many millions of pixels there are on the sensor. The more is not necessarily the better, especially in the low end market of compact cameras. A very important factor is the size of each photodiode, which has a direct impact on the sensitivity and the noise. A small photodiode captures less light than a bigger one and therefore has a lower sensitivity. Noise is generated by many factor, notably the influence of photodiodes on their neighbors, and is also bigger with smaller photodiodes. This gives strange situations in the marketing war, like with the Olympus 4040 and 5050 models. The former has a 4M pixels sensor with 3.1µm photodiodes pitch, the latter 5M pixels at 2.8µm pitch. The trouble is that both sensors have the same size and therefore, capturing the same amount of light in the same conditions, the supposedly better (and more expensive) model at 5M pixels shows actually less sensibility and higher noise than its predecessor at 4M! Another issue with tiny photodiodes is that the lens must be of excellent quality to resolve details at such a small resolution. Digital SLRs have all both bigger sensors with bigger photodiodes pitch, hence improving sensibility and reducing noise.
You can probably sense by now which system I prefer. After much deliberation and despite the bigger size and weight of SLR cameras, I find them way better than the current compact ones. I was used to the SLR, and in addition to the indisputable difference in quality and versatility, I just can't get myself used to look at a ridiculously small LCD screen to compose a picture, shoot in strange positions, and pray that the camera has got proper focus (something you discover only once you've dumped the file on your computer, far away from your subject). If only they were not so outrageously expensive...