Talk:Reciprocity (photography)

message: 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?
I think the issue is the same as in "normal" long exposition photography. But as I agree that your question will probabbly arrise again from readers, something short should really be added. --Bertoche (talk) 04:20, 13 October 2011 (UTC)
thread_title:  Fast Shutter Speeds 

message: 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
:which can be used as a rule of thumb for very low light exposures of standard color print film, metered for a second or more. A metered one second really required 2.5 to expose the film accurately, and a metered 60 seconds really required nearly 20 minutes.
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)
replies: text: If we can have a source, I don't mind citing it.  But it does seem a bit incredible to me that film is this bad at 1 second usually, and that such a power law ends up being adequate over a range of films and times.  Of course, it also needs to be cut off some place, such as at the point where it gives an answer less than the metered exposure. Dicklyon (talk) 01:42, 25 February 2009 (UTC)
replies: text: It's not incredible at all for a product to perform badly outside the use for which it's designed (~EV5 and up, ~1 second and faster). The utility of a single approximate model, no doubt, results from the great latitude color print film has in processing and printing. -65.246.126.130 (talk) 16:54, 27 February 2009 (UTC)
replies: text: Right, but most sources show that 1 stop of correction kicks in at around 1 to 10 seconds; your formula has a bit over 1 stop already at 1 second.  Anyway, I have a number of sources with tables for particular films; but no formulae.  Dicklyon (talk) 05:15, 28 February 2009 (UTC)

text: The model's utility no doubt derives from its simplicity and always being within a couple EVs of the exact exposure (OK for color print film) for all brands and ISO ratings of film. The old Shutterbug article should clinch it. -65.246.126.130 (talk) 19:33, 3 March 2009 (UTC)
replies: text: I see lots of variability in other data.  But if you can produce the article, we can cite it. Dicklyon (talk) 04:42, 4 March 2009 (UTC)
thread_title: Reciprocity model

message: 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)
replies: text: 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)
thread_title:  Digital 

message: *"The reciprocity law, enunciated by Bunsen and Roscoe in 1862, states that for any given photosensitive material, the photochemical effects are directly proportional to the incident light energy, i.e. the product of the illuminance and the duration of the exposure. For a photographic emulsion this means that the same density will be obtained if either illuminance or exposure duration is varied, provided that the other factor is varied in such a way as to keep the product H in the equation H = Et (encountered earlier) constant. 
: Abney first drew attention to the fact that the photographic effect depends on the actual values of H and t, and not solely on their product. This so-called reciprocity failure arises because the effect of exposure on a photographic material depends on the rate at which the energy is supplied. All emulsions exhibit reciprocity failure to some extent, but it is usually serious only at very high or low levels of illumination, and for much general photography the reciprocity law can be considered to hold. In the sensitometric laboratory, however, the effects of reciprocity failure cannot be ignored, nor in certain practical applications of photography." 
::
: Please use this as you see fit. Redbobblehat (talk) 01:59, 27 February 2009 (UTC)
replies: text: I started a history section, citing that and others.  It could be cited in more places in the article, too. Dicklyon (talk) 06:10, 27 February 2009 (UTC)
replies: text:  K. Schwarzschild "On The Deviations From The Law of Reciprocity For Bromide Of Silver Gelatine" The Astrophysical Journal vol.11 (1900) p.89  --Redbobblehat (talk) 17:17, 13 March 2009 (UTC)
replies: text: The latter is cited already; say more about it if you like.  The former is trivial; dictionaries are not generally useful sources when better ones are available. Dicklyon (talk) 21:13, 13 March 2009 (UTC)
thread_title:  Source references 

message: Red, if you're trying to learn about reciprocity from this article, I can see why you're confused.  I've tried to clarify.  The reciprocity law should not be confused with the definition of exposure, which it very much was.  Reciprocity says same exposure leads to same effect.  That's all. Dicklyon (talk) 20:29, 14 March 2009 (UTC)
replies: text: Dick, thank you for your sympathy, patience and continued efforts to help clear this up. :-) --Redbobblehat (talk) 21:22, 14 March 2009 (UTC)
thread_title:  Cleaning up confusion 

message: I've had the article semi-protected, meaning that anonymous IP editors can't edit it for a while; this should give time for the E-77 to arrive, and then I can verify what's in it and fix the section appropriately.  If you guys want to participate, I recommend making accounts.  Dicklyon (talk) 03:41, 17 April 2009 (UTC)
thread_title: Semi-protected

message: User:Mbhiii has kindly found and added a source for a power-law compensation model; so we can include it and no longer have to cite a forum for it.  Dicklyon (talk) 20:14, 17 April 2009 (UTC)
replies: text: "Just as those who practice the same profession recognize each other instinctively, so do those who practice the same vice." - Marcel Proust  —Preceding unsigned comment added by 65.246.126.130 (talk) 21:05, 17 April 2009 (UTC)
replies: text: Not sure what your point is, but nice quote.  Now Mbhiii is still adding some additional unsourced interpretation, trying to patch up an obvious deficiency in the model that is short at one parameter.  However, his fix pins the breakpoint at 1 second.  A reason alternative is to let the breakpoint float and measure time in units of it.  Then no explicit correction factor is needed.  If nobody has stated such a correction one way or the other, why should we make one up and put it into the article?  Dicklyon (talk) 23:53, 17 April 2009 (UTC)
replies: text: It's too easy to get confused about how to use an equation like this (just look at the last entry by Ryan Buckley in "Reciprocity tests of Kodak UC100 and Fuji Reala"). Better recheck the fix. There's no breakpoint. F^(p-1) is constant in time. -74.162.130.58 (talk) 04:23, 18 April 2009 (UTC)
replies: text: The breakpoint is what you need to add to make this power law model make sense and it's at t = 1, because for smaller t than 1 it predicts a correct time less than the metered time.  This can be 1 second, but could just as well be 1 minute or any other time unit, but all we know, since the model is not of a form to control this independent of the chosen time units. Dicklyon (talk) 06:32, 18 April 2009 (UTC)

text: OK, now I see what you mean by breakpoint; F^(p-1) just keeps it, in effect, in the right place. If p is derived for Tc and Tm in seconds, multiplying by 60^(p-1) guarantees Tc<Tm still occurs only for t<1 second, when using Tc and Tm in minutes. -74.162.151.39 (talk) 15:03, 18 April 2009 (UTC)

text: "just keeps it in the right place" would only be true if the right place was always 1 second; on the page you cite, the correct place is more like 50 seconds; you'd be better off changing the minutes.  Your correction just makes the model less accurate for the films described at that forum. Dicklyon (talk) 22:47, 24 April 2009 (UTC)
thread_title: Solved

message: It turns out that User:Mbhiii and User:65.246.126.130 are the same editor.  If this were to be admitted and dealt with, it would make discussions here easy.  If it's not, then we might suspect a violate of WP:SOCK.  Dicklyon (talk) 17:24, 18 April 2009 (UTC)
replies: text: Wrong, there are at least a few different editors from this IP, but not on this article, as far as I can tell. -65.246.126.130 (talk) 22:27, 21 April 2009 (UTC)
replies: text: Are you saying that you are not the same as Mbhiii?  Do we need to do a checkuser on you?  Dicklyon (talk) 00:47, 22 April 2009 (UTC)

text: I've started an investigation on the lot of you at Wikipedia:Sockpuppet_investigations/Mbhiii.  Dicklyon (talk) 01:22, 22 April 2009 (UTC)
thread_title: Sockpuppetry

message: One of Mbhiii's many socks asks that I "address he substantive issues".  OK, here it is: besides the fact that basing edits on random comments in forums is against policy, the particular point that Mbhiii is most recently trying to make is that the formula for corrected time as a power of metered time can be made to be more generally correct if an extra factor is included to compensate for using units different from seconds.  This is one possibility, but stating it simply gives more credence to the erroneous idea that the formula is somehow sensible in the first place.  In particular, it pins the "break point" between reciprocity region and reciprocity failure region at 1 second.  
A perfectly reasonable alternative  (also unsourced) would be allow different time units, without a factor to correct back to 1 second, such that the break point could be moved to 1 arbitrary time unit.  Or, as he tried to put into the article a few weeks ago, use a formula with an explicit second parameter in the form of a multiplier, so that the units and the breakpoint can be made independent.  
Even better, make the break point explicit and use the formula t_c = (t_m / t_{break})^p.  More sensible, but still unsourced, and still lame, so let's don't.  
An even more sensible formula would be this fix applied to the one in the forum that he links (here), which is not the same as the formula that's sourced; that is, the power law formula should be the extra time, not the corrected time; then no explicit breakpoint is needed and you get a continuous sensible model.  For the films studied, the "break point" times for both films are much longer than 1 second, and two different factors are needed.  This page does NOT provide an example of the sourced formula being wrongly used by using minutes instead of seconds – the sourced formula would be very wrong either way; actually closer in minutes, though.  
The formula t' = tm + 0.055*(tm^1.73) can be rewritten as t' = tm * (1 + (tm/tb)^0.73), for tb = 53.2 seconds; and if the equation is written that way, then the times can all be measured in whatever units you want (all the same, of course).  The breakpoint represents the point where the added time equals the metered time (1 stop of correction if looked at on the time axis); since it's nearly a minute, the one-parameter formula that's in the reliable source would actually work much better in minutes than in seconds; it would say no correction at 1 minute, the corner, which would be an error about about 1 stop, but then would be relatively accurate after that, instead of being way wrong for all times between 1 second and 1 minute.  So Mbhiii's attempt to "correct" the gross error only makes the formula work worse, by pretending it is sensible.  
Lacking any sources, adding material that shores up the worst of the models is just lame, and stupid, not just against policy.  Adding material to demonstrate any of these better models is not stupid, but still against policy, so let's just leave well enough alone unless we find more sources of stuff to report. Dicklyon (talk) 04:31, 24 April 2009 (UTC)
thread_title: "The substative issue"

message: Mbhiii asked for another third opinion, but didn't make a section for it.  Here's one.  The question is as discussed above: should we add an original "simple algebraic correction" to a model, based on a question about a different but related model, in a forum, even though the resulting corrected model is still not compatible with the data and model in that forum?  04:58, 24 April 2009 (UTC)
replies: text: What Dick thinks is sensible is some model other than the one sourced (as he details above). Any notes to make the current model more useful by avoiding a common error is something he opposes (ibid). Please review the following and render an opinion or find someone who will. Thanks. (Note, this is not affected by the 3O given earlier by User:AlekseyFy. That model has since been sourced to a book, ending that controversy, as discussed at "Solved". Rather, it has to do with a common misuse of that type of model, for which the only source, so far, is an error on a forum, and its simple algebraic correction. See also the history and recall citing a forum is not absolute; in this case, it's for someone's calculation error.) 21:04, 24 April 2009 (UTC)
replies: text: Mbhiii, what I oppose is not "Any notes to make the current model more useful by avoiding a common error", but rather any unsourced expansion on this very poor model, and particularly a change that as I argue above can be seen as making the model less useful by tieing it explicitly to the arbitrary and usually wrong break-point time of 1 second. Also, be aware that the "error" in the forum is about a completely different and more useful model that would not have this problem, where the correction would indeed make some sense; it is much less applicable to the sourced model that we have now. As I've said before, my own preferences here are not an issue if we stick to the policy of verifiability; what I really would prefer is to find more good sourced information that we can add, especially on better models; a lame comment in a forum, misapplied to a different model where it does more harm than good, doesn't qualify.  Dicklyon (talk) 22:32, 24 April 2009 (UTC)

text: Dithering about the break-point at 1 sec. is useless, because, as you know, already, reciprocity compensation is really only needed for times > 1 sec., with film latitude making up any differences. (...what microscope manufacturers seem to assume, too.) Also, as you know, as well, the exact same correction applies to the power law component in the forum model. -MBHiii (talk) 20:54, 25 April 2009 (UTC)
replies: text: I'm not dithering; I'm saying consistently that we must not add unsourced extensions, interpretations, and so-called "corrections" to the article. The rest was just to help you understand why if we did it would not actually be very helpful, and might even be very harmful in more firmly supporting the worst aspect of the sourced model, which is its peculiar tie to the 1-second time unit used.  Dicklyon (talk) 22:03, 25 April 2009 (UTC)

text: Repeating this, over and over again, shouldn't make it any more believable. -MBHiii (talk) 17:30, 27 April 2009 (UTC)
replies: text: What is it that you find hard to believe?  That we shouldn't add unsourced interpretive stuff to the article?  Or that the reasons for the details I presented here are to help you understand the issues?  Or that the model you are tying to shore up ties the break-point for the reciprocity failure region to 1 second, or 1 of whatever time unit is used?  Dicklyon (talk) 18:05, 27 April 2009 (UTC)

text: This is like dealing with "When did you stop beating your wife?" One has to waste too much time on your unacceptable assumptions. -MBHiii (talk) 18:44, 27 April 2009 (UTC)
thread_title:  Third opinion, second request 

message: I am concerned with the following paragraph
Since the Schwarzschild's law formula gives unreasonable values for times in the region where reciprocity holds, a modified formula has been found that fits better across a wider range of exposure times.  The modification is in terms of a factor the multiplies the ISO film speed:
:Relative film speed  = (t + 1)^{(p - 1)} \ 
where the t + 1 term implies a breakpoint near 1 second separating the region where reciprocity holds from the region where it fails.
The statement here is affected, in my opinion, by the following problems:
1 - The formula is not correctly described, since clearly the constant 1 in (t+1) is measured in seconds, but nowhere is stated that t has to be in seconds.
1.1 - Magic numbers are always suspicious: why one second and not a constant depending on the type of film? I guess that this formula won't work, for example, for printing paper.
2 - The value of p here is the reciprocal of the value of p in the next paragraph (well, this may actually be the next paragraph's fault, anyhow it is confusing)
3 - The source cited is an amateur book which, moreover, has its math wrong (in fact, the author states that two formulae are the same while they are just approximately the same).
3.1 - The author of that book is not an expert (at least in the academic sense), and, as far as I know, he may have invented these formulae himself.
4 - The modified model is just an obvious correction to the Schwarzschild power-law.  —Preceding unsigned comment added by 87.2.56.67 (talk) 14:05, 30 July 2009 (UTC)
thread_title:  Modified formula 

message: There seems to be little on the physics of sensitizing dyes in photography. Remember that silver bromide is naturally sensitive to blue and shorter wavelength. In semiconductor physics, that means photons with energy greater than the band gap. Sensitizing dyes allow for other wavelengths. Now, in addition to the question of how many photons it requires for the silver bromide, one must ask how many photons are needed for the dye. I suspect, but don't have a reference for, that much of reciprocity failure is related to sensitizing dyes. In the case of long exposures, the photon energy can be lost before the next one comes along. I also suspect, but again have no reference for, that the short exposure reciprocity failure is related to the dyes. The dye has to absorb one photon, adjust appropriately, then a second one. If the two come too close together the dye can't absorb them. That would explain the color shift in color film, too. It would be nice to have some references, though. Gah4 (talk) 08:20, 16 July 2013 (UTC)
thread_title:  Sensitizing dye