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:

As for the rest, I hate these equivalence debates and won't go further. You have been caught of guard stating that the F stands for focal length while it isn't and you are now trying to rationalize this, using the term aperture in a way most photographers won't use it, unless they are into optics and lens design. Just admit you have goofed with that minor thing,  like I admitted myself above.

But all I was saying is that the f in f/6.7 stands for focal length.  That is correct.

You are right though that I should take some of the responsibility if I am being misunderstood.  And especially that photographers generally think of the f-number as indicating the aperture, so I should have been clearer what I meant there.  For myself though, I think things become clearer when I stop thinking along those lines and remember that the aperture adjusts with the focal length in order to keep the f-stop the same.

For example, there was a recent thread where a less experienced user seemed to be confused about whether getting closer to a subject would change the exposure, with the idea that the inverse square law should apply to the reflected light the same way it does to the lighting source.  Now everyone could tell him that this was wrong, and that changing the distance of the camera doesn't change the exposure.  But some of the explanations of why this is so were quite confusing.  I read through a couple and was still baffled as to why.

But the simple explanation that clicked for me was that if you move the camera position and keep the focal length the same, the size of the area captured changes so that the total light captured is the same, and if you change the focal length to capture the same scene, the aperture diameter changes to keep the total light (and the exposure) the same.  But if you change the distance of your strobe light from the scene, nothing automatically adjusts in the camera, so you have to change your exposure to account for the difference in total light being captured.

So I think understanding what f/6.7 really means can be helpful in answering many questions photographers sometimes have, from why the F-number gets larger when the aperture gets smaller, to why moving closer to the subject and using a wider angle doesn't change the exposure, to why you need to adjust the F-number when you change focal lengths if you want to maintain the same depth of field.

But there's also a good reason we often take the shortcut of simply associating the F-number with the aperture.  It's because thinking in terms of relative aperture rather than absolute aperture makes the rules of exposure simpler, and makes picture taking easier.  This is why the F-number system emerged and became standard in the fist place.  In the past, other systems for designating aperture were used which led to even more confusion.

So in the future I'll try to use "aperture diameter" when that is what I mean, so as to avoid confusion.  I wasn't really wanting to get into one of those convuluted equivalence debates either, if anything I think I was maybe too brief in my initial explanation because I was trying to keep things simple.

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