f/1.8 on m4/3, ASP-C, FF; are they the same?

Started Nov 1, 2010 | Discussions thread
mjohnston Regular Member • Posts: 195
Re: f/1.8 on m4/3, ASP-C, FF; are they the same?

Maybe someone who knows for sure can verify my understanding, but I think it works like this:

Ultimately, the ISO measures should account for any differences in fundamental sensitivity between sensors. ISO's purpose in the digital world is to act as a constant for use in the algebra that determines exposure. ISO 100 on one camera should be the same as ISO on another, even if one has to amplify the signal from the sensor more/less than the other to get the same result. That's why a tiny sensor P+S will have the same exposure times as a FF on a given ISO.

Now of course that says nothing about how good the picture will look. That depends on a number of factors that are taken into account when designing a sensor.

Consider a full frame sensor with 12mp vs a micro 4/3 sensor with 12mp. Each pixel of the micro 4/3 sensor is much smaller. While the intensity of light through the lens doesn't change, the amount of light per-pixel is less, since each m43 pixel is smaller. Does this mean the sensor's signal has to be amplified more? That's a good question that I don't know the answer to. It sounds like less total light per pixel might mean that more amplification is necessary. On the other hand, perhaps it's more like paper blowing in the wind - a big piece of paper will be blown just as far as a small one because the pressure per area is the same regardless of paper size. Sensor physics experts, please comment!

Other effects come into play as well. Except for backlit sensors, pixels in a sensor are not at the surface - they're in little pits. As a sensor gets smaller, it doesn't necessarily get thinner. This means that each pixel is deeper and light has to get deeper into the pit to hit the sensor. If the light isn't coming at the sensor at a perpendicular angle, the light might hit the side of the pit and not get picked up. That's why sensors have microlenses and why telecentric lenses are desirable on small sensor cameras, such as micro 4/3.

Depth of field is a different issue.

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