# Sensor sizes and f stops

Started 11 months ago | Discussions thread
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 Re: No correlation In reply to darklamp, 11 months ago

darklamp wrote:

There is a correlation between sensor size and f-number.

No there isn't.

f-numbers are defined precisely to remove any correlation between aperture size ( i.e. the absolute measurement ) and the sensor.

You can calculate an equivalent f number by multiplying by the crop factor.

That's not a correlation.

http://en.wikipedia.org/wiki/Correlation_and_dependence

That's you defining another value using two others. Your new value has a dependence on sensor size, but the f-number you start from does not.

As I've already explained, that kind of equivalence is limited to depth of field.

I answer to the op who wonders what really means a f number when comparing with different sensor size.

Different sensor size should be taken into account, of course ! And you can calculate an "equivalent" aperture taking into account the crop factor.

There is a relation which "correlates" these equivalent apertures, I really think this is what the op wants to know.

It is very important to be aware of this. The op could buy a f1.8 with a1/2.3" thinking that it is better that a f2.8 with a 1" sensor. The 1" sensor is better for dof AND low light conditions.

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