If you are confused about the effect the sensor size makes to your image quality you are not alone. In this post, I try to explain a good part of these problems. I try to do that without any math. But I assume some basic background in photography.
We wish to learn about the effect of a smaller sensor size to the depth of field, the background blurriness and the noise. Those are important ingredients for the image quality. For portraits, we wish to have a noise free and blurry background. For night shots, we wish to have as little noise as possible.
Let us do a thought experiment to understand the topic. Imagine, we could make the world smaller by a factor of 2 in all dimensions. We would shrink a full frame camera to a 4/3 camera. The shrunken camera is then said to have a crop factor of 2. Note that we shrink everything to half its size: the sensor width and height (e.g. from 36 x 24 to 18 x 12), the focal length (e.g. from 50mm to 25mm) and the open width of the aperture at any given F stop (e.g. from 5mm to 2.5mm).
First of all, the angle of view remains exactly the same for our 25mm lens compared to the 50mm in full frame. So the image shows the same things as before, even if we would not shrink the world.
Moreover, the light that comes through the lens is only 1/4 of the light, since our aperture has only 1/4 of the area than before. But the distance to the sensor plane is only 1/2, and so we get the same amount of light per square mm. If we used film we would get the same exposure. That explains why we compute the F stop by dividing the aperture by the focal length. This stays the same in our shrunken camera. Thus we use F8 no matter what size of camera we have.
But some important things change.
(1) Our light is on the same level for the smaller camera. So each pixel gets only 1/4 of the light. Remember that the pixels are smaller too. But the pixels produce the same amount of noise. The noise does not depend much on the size of the pixel. Thus the noise level in relation to the signal strength for each pixel increases by quite an amount. We can say by a factor 4. The image gets noisier.
(2) The objects do not shrink or get closer by a factor of 2. If they would we’d get the same depth of field and blurriness in our image. But a man in 10 meters distance is still the same in the same distance. So we have to change our focus by moving the lens further to the sensor plane. Of course, the camera will do that for us automatically. The effect of this is known to any photographer and is quite obvious: If we focus on something further away the background gets sharper. For practical purposes we can say it gets twice as sharp when the distance is doubled.
What that means is that you get a less blurry background and more noise at the same settings.
That’s bad. The good point is that you also get a larger depth of field around the object if you need that.
To compensate for this you can double the aperture size, i.e., increase by two stops (e.g. from F4 to F2), and decrease your ISO by two stops to compensate (e.g. from ISO 400 to ISO 100). Your sensor will then get the same amount of light and your noise will be the same. Moreover, your background blurriness is the same too. It is a bit complicated to compute, but your depth of field around your object will also be approximately the same. You get the same image quality.
In terms of image quality for images with an object and a background, you can replace a full frame 80mm lens at ISO400 and F4 with a 40mm lens for your 4/3 system at ISO100 and F2.
But, of course, your full frame lens might allow you to shoot at ISO100 and F2. You have no chance to cope with this on a 4/3 system. For you would need ISO25 and F1. The full frame camera offers more opportunities. It is also more bulky and expensive.
If you do not care about the background or you want more sharpness all across the frame the small camera seems to favor you. But you still have more noise at the same settings.
In terms of noise for night photography or landscapes, you get the same noise with ISO400 on the full frame and with ISO100 on a 4/3 system. To compensate, you need more light by increasing the aperture or the exposure time.
By the way, measurements at various camera sites in the net confirm this finding. With the same F stop on the same image, the full frame camera can be shot at ISO 800, the APS-C camera at ISO 400 and the 4/3 camera at ISO 200, and all will produce the same noise. Pictures will still look different due to the different background blurriness.