Talk:Reciprocity (photography)

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Contents 1 Fast Shutter Speeds 2 EV=5 Example 3 Reciprocity model 4 Digital

[edit] Fast Shutter Speeds

Reciprocity failure also exists at the other end of the spectrum -- when shutter speeds are faster than 1/1000 of a second. In that case the aperature has to be opened wider to compensate for the short exposure time.

Reciprocity failure is supposedly useful in astrophotography. Can someone elucidate?

[edit] EV=5 Example

Shouldn't the exposure time be 4.5 seconds in the example? I realize that it is nice to round off, but in this case, the round-off error is almost 10% of the exposure time. I'll change it in a few days, unless there are objections. (neffk <neffk ta ieee tod org> 9 April 2006)

[edit] Reciprocity model

The section says:

In the late 1980s, Shutterbug Magazine (see refs.) published tables of reciprocity factors for a wide variety of common commercial, color print films (Kodak, Fuji, ...) the type of film with the worst low light reciprocity failures. The films were the most common speeds, something like 50-1000 ASA/ISO, exposed for periods of seconds to hours in very low light. Generally the reciprocity factors, independent of brand and speed, fell along a rising curve that can be described as:

Best exposure time in seconds = 2.5 (Metered exposure time in seconds) ^1.5

Now, this makes no sense at all. It has a reciprocity factor of 2.5 already at 1 second, and getting worse. I've never seen film this bad. Can someone provide the exact reference, and maybe more details from it or a copy of it? Otherwise, we should take this out. Dicklyon 03:29, 9 April 2007 (UTC)

[edit] Digital

I think it should be specified whether Reciprocity is an issue with digital camera sensors. The section on astrophotography implies that it isn't, but doesn't say outright.--Tiberius47 06:34, 15 August 2007 (UTC)

Electronic sensors have a different kind of reciprocity failure. In silver grains, there's a "leakage" toward dark; that is, grains need several photons to turn, but after getting one photon and sitting a while they can forget, and leak back to an unexposed state. Photodiodes go the other way; they leak toward light. So with very long exposures, instead of less sensitivity you get more "fog" and "noise", which is sort of equivalently bad. But it has been possible to make some really excellent low-leakage CCDs, especially when cooled. That's the current best type of sensor for very long exposures. I wish had a good source on all this. Dicklyon 06:48, 15 August 2007 (UTC)