Equivalent focal length for MFT lenses

Started Apr 12, 2013 | Discussions thread
KenBalbari Regular Member • Posts: 276
Re: Equivalent focal length for MFT lenses

rrr_hhh wrote:

This is where you are in error : you can change the focal length of the lens without changing the F number which is a standardized ratio. Things are simple : set the ISO, set the shutterspeed and the F number you need to expose your scene correctly : whether you are using a 28mm wide angle, or a 90mm tele you will have to use the same aperture on both lenses, although you have changed the focal length.

No, this is where you are in error.  Yes, it's a ratio, but the focal length is the numerator of that ratio.  So if you change the focal length, you also change the ratio, unless you also change somthing else.

And if as you say, you use the same aperture on both lenses, that leaves nothing else in that ratio that you can change.  The ratio is focal length to aperture diameter.

Now take two cameras with a different sensor : use the same ISO and same shutterspeed, you will need a 50mm on FF or a 25mm on MFT in order to keep the same angle of view, but in order to get the same brightness, you will have to use the same aperture, aka the same F number on both (Well there will be some difference in reality, because each camera manufacturer design their jpegs engine differently, but that won't be linked fdirectly to the size of the sensor).

The confusion seems to come from you believing that F-number is the same thing as aperture.  Clearly it isn't.  Did you read the wikipedia article you linked?  The F-number is the ratio of the focal length to aperture diameter.  When I say aperture here, I am always refering to the actual aperture diameter.

In fact, you don't even need a lens to measure the light and the aperture you need : you can find it using a separated lightmeter; it will be valid for all lenses on all formats.

You have three parameters to consider in the equivalence :

Here's the thing.  If all you care about is equivalent angle of view, then you only have to consider one parameter, focal length.  Go with that if you want to keep it simple and avoid confusing anyone.

  •  the exposure producing a certain level of brightness : F5.6 will remain F5.6 whatever the format is;
  • the DOF : F5.6 on FF correspond to F4 on APSC and to F2.8 on MFT
  • the different kinds of noise produced during the process of capturing the light; the MFT sensor being four times smaller will produce more noise than a similar sensor on full frame; in theory this could correspond to two stops difference in matters of ISO, but in practice it is much more complicated (the smaller sensors are usually packing more pixels than the larger sensor and their efficiency are usually different too). This is better determined by experience and by reading results of tests like those of DXO. 
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Once you bring these other parameters into it though, then what on mFT is most equivalent to 100mm f5.6 at ISO 800 on a 135 film sized sensor?  I think there is no denying that 50mm f2.8 at ISO 200 will be more equivalent than 50mm f5.6 at ISO 800.  In the later case you only have equivalent field of view and image brightness.  In the former, you have equivalent field of view, equivalent image brightness, equivalent depth of field, and roughly equivalent noise and dynamic range characteristics.

I'm not saying there is only one way to do equivalence.  By all means if you like keep it simple and limit it to angle of view, as DP Review generally does when discussing it.

But, much as I hate to argue semantics, when I think you do understand the priciple, in this case I think it's important.  Because once people bring f-stop into it insisting that f5.6 is always f5.6 and doesn't change, and then on top of that use the word aperture to refer to the f-stop, that's bound to cause in some readers the kind of misunderstanding that leads some to make posts like this:

"If I have an MFT 300 mm f/6.7 lens, and a full format 600mm f/6.7 lens that is of a worse manufacturing quality and just happens to be twice as soft as the MFT lens. Then, on equal ISO values, those two lenses will be an exact match. Right?"

Now maybe I would also have been clearer if I said "aperture diameter" rather than "aperture", but I thought that much would be understood.  In any case, to be clear, all I have been saying all along is that f/6.7 means that the aperture diameter is equal to the focal length divided by 6.7 and as a consequence, the f-stop (6.7) is equal to the focal length divided by the diameter.   So you can't change the focal length without also changing either the aperture diameter or the f-stop, it has to be one or the other.

So if you are going to fix the f-stop, I think you should be clear that this means you are changing the aperture (and thus depth of field).

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