# The truth about fast glass on m4/3

Started May 21, 2012 | Discussions thread
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 Re: The truth about fast glass on m4/3 In reply to topstuff, May 21, 2012

The main problem in this "f-number" discussion is that people use the f-number for a lot of things because they are basically ignorant about anything else. I've see it put to roughly three uses, and will now explain what my knowledge says about these:

1) Some use f-numbers to express absolute light gathering of the entire camera + lens combination. This is downright absurd. What these people are looking for is probably exposure value, or EV:
http://en.wikipedia.org/wiki/Exposure_value
http://en.wikipedia.org/wiki/Exposure_value#Tabulated_exposure_values

It's the kind of thing you need to actually put in some effort with to understand, so it's not very popular in these forums. Fundamentally people are trying to express the full performance of a camera system with one number, which no one is about to succeed with.

2) Others use the f-numbers to measure depth of field. They say "this f-number is equivalent to that and that on another system". This is less wrong, but also wrong. All you need is to know that the f-number expresses the relative aperture, whereas depth of field comes from the absolute aperture.

So let's say we have two F/1.0 lenses, meaning the aperture diameter and focal length are the same. Both give 35mm equivalent field of view, one on 35mm and one on Micro Four Thirds. This means the first one has a 40 mm focal length/aperture, the second 20 mm. A 40 mm circle has four times the area of a 20 mm one, so the light gathering is doubled twice - meaning two stops more. You only use the f-numbers to calculate the aperture in millimetres, not for anything else.

But even better: we now take two 40mm f/1.0 lenses. We know that one is for Micro Four Thirds and one is for 35mm, but we don't know which is which. Both have 40mm focal lengths and 40mm diameter apertures. How will we know what the depths of field are? Answer: from the diameter of the aperture. They will have the same depth of field, doesn't matter what the sensor is. It's all right there in the lens specifications. You can use a rutabaga as your camera and the depth of field will still be the same.

3) Finally, people use the f-number to gauge correct exposure: basically to calculate the shutter speed and ISO for correct exposure, and to see how good light gathering a lens offers. This is the only correct use. Stick to it.
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