Equivalent focal length for MFT lenses

Started Apr 12, 2013 | Discussions thread
Detail Man
Detail Man Forum Pro • Posts: 16,789
Re: Equivalent focal length for MFT lenses

KenBalbari wrote:

Detail Man wrote:

KenBalbari wrote:

... 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.

What a piece of work that bloke was. The worst combination ever is dense as well as vitriolic.

Yes, he was trying to BS hs way through, with a "never admit you were wrong" mentality.  Basically the world's least successful con man.

When one refers to oneself as a "seasoned professional" after having been run out of town on a rail, it probably helps to have a portfolio of more than wretchedly exposed "bootay shots" to show.

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.

Puzzled a bit myself. Found this web-page (linked in the last post on the thread) to be helpful:


Regards, DM

I kept wanting to think there was a difference between reflected light and a source light, in that a source light is dispersing, whereas reflected light reaching the camera all tends to be coming off a surface in one direction (and even tends to be polarized for this reason, that only waves hitting the object in a certain way bounce in that direction), which I think is all basically true, but apparently wasn't really that critical here.

In my humble understanding (recently gained from researching that subject a bit), such (reflected) sources represent the integral (in space) of a large number of point-sources (the paths that happen to be reflected in a uniform direction) - similar to the case of a non-point-source radiator.

It's interesting that when the dimension of such a (non-point) source is around 1/5 of the distance from that source, the inverse-square law holds to with around 1% accuracy (or so some state). As ever, though, the area (from a single point-source) decreases with distance, so the intensity (from each individual point-source comprising the whole) remains constant.

It seems that the integral (in space) of a large number of point-sources (might) have different characteristics ? Seems that are still "superpositioned", though. Interesting stuff to try to envision.

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