# Request from DPR - attention Andy Westlake

Started May 22, 2012 | Discussions thread
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 Exactly. f/2 = f/2 is nor more or less true than 50mm = 50mm. In reply to rovingtim, May 24, 2012

rovingtim wrote:

Great Bustard wrote:

I mean, if you can do 25mm x 2 = 50mm without shedding tears, then why is f/2 x 2 = f/4 such a painful complication?

f2 = f2 is an exposure related statement only and does not indicate what the final file will look like. Why not argue that 25mm = 25mm = 25mm ... because it does ... and it also does not indicate what the final file looks like.

Shall we compare all the different format lenses on the bases that 150mm = 150mm etc and see what we get?

It makes just as much sense.

While it doesn't matter whether or not you think in terms of exposure or total light when working within a format, the distinction is central when comparing between formats.

There are those that pooh-pooh the fact that the total amount of light that makes up a photo is central to the IQ of a photo, arguing instead that it's primarily the sensor. Furthermore, they make the false assumption that I am saying the sensor efficiency is irrelevant and/or that all sensors are equally efficient.

But if we compare the sensor efficiency of systems of the same generation, we will often find that they are not so different:

http://www.sensorgen.info/

In any case, if those that argue against f/2 on mFT is equivalent to f/4 on FF do so on the basis that the sensors are not exactly the same efficiency, how do they feel about f/2 on an EPL1 vs f/2 on an EM5, which also feature sensors with different efficencies.

Indeed, saying that f/2 on mFT is not equivalent to f/4 on FF because there is the variable of sensor efficiency, then we would also have to say that 25mm on mFT is not equivalent to 50mm on FF because there is the variable of pixel count.

So is f/2 on an EPL1 not equivalent to f/2 on an EM5 because the noise is not exactly the same? Is 50mm on an EPL1 not equivalent to 50mm on an EM5 because the pixel count is different?

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