Reciprocity Factor

Hello. I shoot a lot of black and white film, and also a lot of infrared films. I keep hearing about the reciprocity factor, and I don't really

understand what it even is. I understand it can effect the image, and it appears with long exposure times, but I don't fully understand

what it entails. If anybody can explain this to me, that would be fantastic.

Thanks everyone!

JonPaul Hills

<p>The law of reciprocity says that you can use different combinations of shutter speed and aperture to achieve the same exposure.</p>

<p>For example, if you shoot 1/4 second @ f/8 or 1/2 second @ f/11 the film will react the same.</p>

<p>When you get to long exposure times (or very short ones), the law no longer holds, and you must increase the time beyond what you would normally expect.</p>

<p>This is generally only true for exposures over about 10 seconds, but it varies with different films. You'll find information about it on the manufacturer's data sheet for each film.</p>

<p>For example, with Fuji Acros you don't need to change the calculated exposure for times up to 120 seconds (2 minutes), and only increase by 1/2 stop (50%) for times from 120 to 1000 seconds.</p>

<p>- Leigh</p>

<p>On the short end of shutter speeds Kodachrome was the worst.:-)</p>

<p>The word reciprocity usually refers to social or economic situations. I give you this, you give me that; that's reciprocity.</p>

<p>But in photography, it is referring to the reciprocal function.</p>

<p>The amount of light can be expressed(generally) mathematically, though it is sometimes oblique. Example:...</p>

<blockquote>

<p>For example, if you shoot 1/4 second @ f/8 or 1/2 second @ f/11 the film will react the same.</p>

</blockquote>

<p>This is true. For an aperture at f/11 is one half the size(in area) of the f/8 aperture.</p>

<p>So, 0.25 second times f/8 is equal in the amount of light hitting the film or sensor as to 0.50 seconds at f/11.</p>

<p>'F' is the short hand for the focal length of the lens. So if you have a 50mm focal length lens, then the diameter of the aperture at f/8 is 50mm divided by 8, or 6.25mm. The diameter of the aperture on the same lens at f/11 is 50mm divided by 11, or 4.55mm. The smaller aperture(f/11) lets in almost half as much light as the larger(f/8) aperture.</p>

<p>If you did the math for the area of the circle, that is the diameter of the aperture, using the radius of the diameter, and Pi r^2, you would find that the area of the f/11 aperture is almost half the area of the f/8 aperture.</p>

<p>If you half the light, you have to double the time to get the same amount of light hitting the film or sensor. It is reciprocal.</p>

<blockquote>

<p>When you get to long exposure times (or very short ones), the law no longer holds, and you must increase the time beyond what you would normally expect.</p>

</blockquote>

<p>This is called "Failure of Reciprocity". The reciprocal nature of most film(especially) and some sensors drops out at the far extremes of shutter speeds.</p>

<p>Leigh's post is right on, of course. I just hoped to elaborate for clarification. And it's not exact, it's just good enough for government work. Or photographic work.</p>

<p>Example:<br /> F/8 of a 50mm lens the diameter of the aperture is 6.2500mm. The radius is 3.1250mm.<br /> F/11 of a 50mm lens the diameter of the aperture is 4.5454mm. The radius is 2.2727mm.</p>

<p>So the area of f/8 is 3.14 times the square of 3.1250mm or 30.66 square mm.<br /> And the area of f/11 is 3.14 times the square of 2.2727mm or 16.22 square mm.</p>

<p>On long exposures my understanding was that it was an educated guess. The same remains true with digital, but if anyone could narrow the guesswork it would be interesting. I certainly don't mind experimenting with various exposure times with digital, it is fun to see the results and get instant feedback.</p>

<p>The law of reciprocity tells us that we can manipulate aperture and shutter speed and so long as we maintain equal exposing energy, the results will be equal density and contrast. While true for moderate changes in exposure time, the law fails for unusually long or short exposure times and for corresponding unusually weak or brilliant lighting conditions.</p>

<p>During the exposure, photons are hitting the silver salts changing their chemical state. These crystals are under a kind of chemical inertia to remain stable. When exposure time is usually long, the interval between photon hits is long allowing some healing between hits. The result is more exposure is required than the planned amount. If the exposure time is usually short, the photon hits happen in rapid succession. Thus, the next photon hit occurs before the last has had time to register its full impact. The result is more exposure is required than the planned amount.</p>

<p>In other words: When the exposure is unusually long or unusually short, a loss of film or paper speed results. The countermeasure is to increase exposure to compensate. To compensate a reciprocity factor is applied. <br /><br /><br /> Note: Usually this phenomenon is called reciprocity failure.</p>

<p>I use Howard Bond's numbers for reciprocity failure. They are the times that work for me. The chart is at the end of the article.</p>

<p><a href="http://www.phototechmag.com/index.php/archive/reciprocity">http://www.phototechmag.com/index.php/archive/reciprocity</a></p>

<p>Before the current emulsions we have now, there were some pretty accurate reciprocity tables out there. I used them for all of the Kodak b/w films, daylight color films and even Polaroids(these same tables worked with Polaroid right up to its demise). The times were golden--except with color films you would also get color shift, which was actually handy when shooting daylight chrome film with tungsten light--it cooled off (color temperature rose) with longer exposures. (Tungsten balanced films were generally designed with a totally different response curve than daylight films, thus less adjustment for longer exposures).</p>

<p>Reciprocity failure has a couple of features to it that affect the outcome. First, any light sensitive material needs to get a certain minimum threshold of light before it starts recording. We saw this if we were testing for safelight fogging in the B/W darkroom. If we didn't pre-flash the paper to "energize" it, even a 20 second test with an opaque object on a portion of the paper could suggest no fogging. With a pre-flash of the paper, we might see fogging at only 10 seconds.</p>

<p>Basically, it is the same for film, thus with longer exposures, highlights start to record sooner than dark areas, thus there is generally an increase in contrast as part of "Reciprocity Failure". This is often a good thing, as many times these dark scenes, where longer exposures are required, have a shorter dynamic range.</p>

<p>Modern films seem to allow for about 10 seconds before Reciprocity Failure occurs whereas the older emulsions saw the effect at about 1 second. Also, the newer emulsions don't seem to have as steep a curve allowing less time to be added than the older emulsions as one experiences decreased sensitivity. Finding someone's table for this can save a lot of time testing to find the numbers that work but one should test the table's numbers to be sure they do give you satisfactory results.</p>

<p>(note: I haven't shot B/W film for some time, so the 10 seconds what I have found with current emulsions of color negative film--which is essentially what the manufacturer's claim in this regard)</p>

<p>It's called reciprocity failure and it is an effective loss of film speed at very low light levels. To compensate you need to increase exposure and adjust development. I have a table RF but I'm not at home at the moment. I'll email you a copy when I get back.</p>

<p>Alright thanks everybody! That all helps a lot!</p>

<p>Eventhough it is in Korean, You may be able to understand the graph which I studied.</p>

<p><a href="http://blog.daum.net/ohzart1/5">http://blog.daum.net/ohzart1/5</a></p>