Before I start the discussion, I would like to refer you to my previous posts because I have already covered this concept extensively:
If you have read and understood those posts above then you can save yourself time by not reading this one…although I don’t mind if you do read this because this looks at sensors in a different perspective. I will try to cover some technical aspects in the form of analogies.
I noticed that not everyone who carries a camera actually understands photography so I’m hoping that my explanations here would help them. I will use rain as an analogy. I hope everyone has experienced rain and understands rain.
Do you know how they measure the amount of rainfall? They use a device called a rain gauge. It’s a very simple device and anyone can make their own. The simplest rain gauge is that of a basic straight container that looks something like this:
All you have to do now is wait for rain over a period of time and then measure the height of the collected rainwater. Really simple. Now you might wonder why I did not specify any measurements. How big should the container be? Surely, a larger container will gather more rain! That is correct. A larger container will collect more rain but the rainwater level will remain the same. Why is that? Because a larger opening also needs to fill a larger volume. That’s why rain is measured in mm (height) and not ml (volume). Here’s a very simple math that explains this:
Volume = area of opening x height of rainwater
height of rainwater = Volume / area of opening
Notice that if you increase the opening you also increase the volume and the height remains the same. They are proportional.
What does this teach us about photography? The concept of a rain gauge is analogous to that of photographic exposure. First you have your container opening which is your aperture. Then you wait for rain to fall over a period of time which is your shutter speed. The proportion of the container opening to its volume is your f-stop. A rain gauge is like photography where everybody has agreed to shoot at the same f-stop. The measured rain level is your exposure and determines how bright the image will turn out. As we have mentioned before, the size of the opening is proportional to the volume and therefore the measured rain levels are the same irrespective of container size. Recall that a f-stop is the ratio of the lens focal length (container height) and aperture diameter (container opening). That’s why a f-stop is a f-stop. A f-stop is the same no matter how big your lens is. A 35mm at f5.6 will have the same exposure as 100mm at f5.6 although the latter has a much much larger opening.
It follows that if we use the same lens on different sensor sizes we get something that looks like this:
The red circle is your lens’es image circle (area of rain). The rectangles in the middle are your different sensor sizes (rain gauges). From our rain gauge analogy above, you will realize that both sensors will have the same exposure (measured rain levels). A f-stop is a f-stop irrespective of sensor size.
Before you continue with the rest of the discussion, make sure that you understood the very basic concepts covered above because I will start explaining something that is often hotly debated in forums: sensor noise!
The full frame proponents will tell you that because of sensor size advantage over APS-C and 4/3rds and other “crop” sensors that it will have lesser noise and therefore cleaner output. The basis for this conclusion is the fact that larger sensors gather more light. Let’s discuss this in detail…
If you go back to our rain gauge analogy, it’s quite obvious that a larger container will gather more rain although the measured rain levels remain the same. Therefore a larger sensor will gather more light although the exposure remains the same. Now since noise is affected by the amount of light, therefore a larger sensor has lesser noise. Therefore, full frame is superior.
Well not so fast Pedro. A camera’s sensor isn’t really like a rain gauge. A sensor is actually composed of smaller components called sensels which are like smaller containers within a bigger container. It looks like this:
So now we will have to narrow down our analogy to those smaller containers (sensels) instead of the whole sensor. You could probably see where this is going. You will notice light is gathered by the sensels and therefore noise is NOT affected by sensor size but by SENSEL size.
Again, sensor size does not affect exposure (rain level analogy). A f-stop is a f-stop (rain gauge analogy) and is not affected by lens focal length or sensor size. Therefore a smaller sensel will have the same exposure as a larger sensel. However, a larger sensel gathers more light therefore it will have lesser noise. This is why a 12Mp full frame has better noise performance vs a 12Mp APS-C sensor. This is also why a 12Mp full frame Nikon D700 has way better noise performance vs a 36Mp full frame D800 by virtue of the larger sensels. This is also why a 16Mp APS-C D7000 has the SAME noise profile as a full frame 36Mp D800.
And thus, we arrive at the following conclusions:
1. SENSOR size has no effect on exposure.
2. SENSOR size has no effect on noise.
3. SENSEL size ultimately affects noise.
Again, it follows from above that the same sensors will have the same noise profiles (e.g. Nikon D7000, Pentax K5/K5II and Nikon D800) even if the sensor sizes are different as long as they are exposed in the same way; same f-stop, same shutter speed. You will find shot comparisons between those sensors in http://dpreview.com and they are in agreement with the conclusions above.
Now you might have read from others about something called equivalency. They say that unless different sensor sizes are exposed equivalently then they will have different noise profiles. For example, a APS-C sensor with a lens set to 35mm/f5.6 is equivalent to a full frame sensor with a lens set to 50mm/f8. Although they have different focal lengths and f-stops, they are equivalent in terms of angle of view, aperture and depth of field, all because of the crop factor of approximately 1.5. While it’s true that AoV and DoF are equivalent, they will certainly have different noise profiles. Firstly, it’s quite obvious that if you use the same shutter speed, the 35mm/f5.6 will be overexposed by a stop. So if you use the same shutter speed, you will have to stop down the 35mm to f8 which will decrease the aperture size and somehow this will affect noise?! We know from the rain analogy and sensor design discussion above that this is simply UNTRUE! I don’t know why the equivalency proponents keep pushing this concept when photographically it does not make sense. This equivalency-fu is like using rain gauges that do not adhere to the same standards and they will end up looking like this:
Notice that they have the same opening area of 6.25mm:
35mm / 5.6 = 6.25mm
50mm / 8 = 6.25mm
But because of this equivalency brouhaha we now have skewed rain gauges. Notice that you will have to gather rain over a longer period of time for the 50mm/f8 container to arrive at the same rain level as the 35mm/f5.6 container. Full frame proponents think that they are the standard so the illustration above would probably look like this from their perspective:
So now the crop sensors will have to expose at a shorter period of time just so they could abide by the standards set up by the elite full frame shooters.
For me, this is just silly. I feel that all these comparisons between full frame and smaller sensors are nothing but silly justifications for the perceived superiority of a particular sensor. Discussing equivalency is fine as long as it’s still about photography. It’s ok if you explain crop factor in terms of AoV or DoF but when you start using this as a tool to push the perceived superiority of your more expensive equipment then it’s really just bullshit. Bullshit and downright very wrong and misleading. Stop it.