Physics of Camera Sensors

The last few days I've been writing about camera sensors (EF-S Mount, 1.6x Zoom). There is a pretty important point that I haven't touched on yet. It is:

 

You always want more light to hit your sensor.

 

Fundamentally the quality of the image is limited by the number of photons that hit the sensor. Now photons are really small so they have no problem fitting through the lens, but there are a few things that get in their way. The glass in the lens and the diaphragm block some of the photons. This is measured by the f-stop value. A good heavy lens will have a lower minimum f-stop value which means more light gets through. These lenses have large glass elements and also higher quality glass. You can also pick the amount of light that gets through the diaphragm. Closing the diaphragm increases the depth of field, but at the cost of image quality. To compensate you need to change something else to get more photons to hit the sensor.

 

The distance and brightness of the subject also effect the amount of light. Brighter subjects have more photons coming off them which is good for image quality. Subjects that are farther away need to be brighter (or larger) because the number of photon that happen to enter your camera decreases rapidly as you get farther away. You can imagine a single photon coming off the subject in a random direction. It is much more likely to hit the sensor when the camera is really close than really far away.

 

The other really important factor is the time of the exposure. More time means more photons. This number is linear so if your exposure is twice as long you get twice as many photons. However long exposure can cause things to blur. Moving subjects and camera shake will blur the image. There is a really delicate balance between time and aperture to get the best depth of field, best blur, and best image quality. Fortunately for me my digital camera is smart and figures out the balance for me. I could put it in full manual mode, but I'd rather think about the framing and other creative aspects than setting the aperture/time ratio correctly.

 

There is another value in digital cameras that effects image quality. It's called the ISO setting. The name is left over from film photography where it mean something completely different, but has a similar effect. The ISO setting in digital cameras changes the amplification boost when reading the image sensor. This is just some stupid thing you have to do because sensors suck. You see, you need to guess the number of photons that hit your sensor in order to read it. In low light conditions you can increase the ISO setting which makes the camera read more details from the sensor. This is just a detail with the current sensor technology more than a fundamental light issue. However, back in reality this is a really important number when taking pictures and I wish the stupid engineers added a new switch and display for it. On my camera you can't see or change the value unless you take your eyes out of the view finder and press an button to display it on the exterior LCD.

 

As I've written this I've realized that the original point I was going to make isn't really true. I was going to say that you always want a bigger sensor so that more light hits it. After thinking about it more thought, more photons aren't going to hit a bigger sensor. If everything else stays the same but you magnify the image on the sensor, you get the same number of photons that are just spread out a little farther. You know what... I think full frame sensors are doomed. There isn't a strong physical reason why they need to be so big.