# F number and light reaching the sensor

Started May 31, 2012 | Discussions thread
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 Re: F number and light reaching the sensor In reply to dodgyexposure, May 31, 2012

dodgyexposure wrote:

Graystar wrote:

dodgyexposure wrote:

For a practical example, for a given sensor, is f2.8 on a 200mm lens the same amount of light reaching the sensor as f2.8 on a 50mm lens?

Not "amount", but luminance...which is luminous intensity per area. One analogy is water in a hose. The water has a pressure in pounds per square inch (force over an area.) You don't have an "amount" of water until you actually open the hose and collect water in a bucket. In the same way, you don't have an amount of light until the shutter is open and the light is collected on a sensor.

As for the process of collecting light...a good analogy is collecting rain. Think about rain...it's falling at the same rate in all places. If you try to collect the rain with a bucket, you're limited by the size of the mouth of the bucket. Get a bucket with a much larger mouth, and you can collect more rain in the same amount of time. This is exactly the difference between large sensors and small sensors. For any given area, such a 1 millimeter square, the same luminance (f-stop) will produce the same amount of light regardless of lens or sensor. But large sensors have more area, and so collect more light in the same time, just like the bucket with the wider mouth.

So is this the reason why a cropped sensor (APS-C) is said to have an 'effective' f-number higher than a FF sensor with a given lens?

No, that's something entirely different.

I think I get what you were thinking...because the larger sensor collects more light then maybe the "effective F-number" is what equates them...something like that...but no, it's not like that.

The "effective" F-number relates purely to the optical effect of depth of field. It doesn't relate to exposure at all. When the same scene is framed equally between two different size sensors at the same distance and same F-number, the DOF will not be the same. The DOF of the smaller sensor will be deeper. This is simply because the smaller sensor will use a shorter focal length to get the same framing. A little time spent playing with a DOF calculator will show you that for the same distance, same F-number, but different focal lengths required for the same field of view, the small-sensored camera, with its shorter focal length, will have a deeper DOF.

That's why compacts have such deep DOF...they start at focal lengths of around 5mm.

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