Photons Missing In Action

Started Nov 24, 2009 | Discussions thread
OP Marianne Oelund Veteran Member • Posts: 7,746
They're Relative to Wide Open

noirdesir wrote:

I presume all your measurements are relative to wide-open. If you see a consistent under- or overexposure, could it be that the f-stop wide open isn't exactly the nominal one and that thus the stopped down values are the correct ones and the aperture wide open is the "inaccurate" one?

In other words, can the aperture error compensate for the efficiency loss when the lens is stopped down? In some cases yes, but unfortunately that is purely academic, and of little help to the user, as the image exposure will still be thrown off. If a manufacturer were to intentionally do this to "correct" for light loss, they would be doing a disservice to their customers!

Another thought, could it be possible that the ratio between the t-stop and the f-stop varies with aperture, eg, less reflection losses for the light rays passing through the center than for those passing through the outer regions?

Possible, but I have no means of separating that effect from the diaphragm stop-down. In any case, the data do correctly infer the exposure errors which will result for images taken.

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