Reciprocity and metering?

[Xia_Ke]

Okay, stupid question time... I'm starting to play around with longer film exposures now and have discovered the oh so nice Reciprocity Law. I understand the basic principal of the law but, if all films suffer from reciprocity, and need exposure compensation, then why don't meters just show the adjusted exposure? Does the rate vary from film to film or something?

[Greg McCary]

It looks as though it varies form film to film. I am not sure I understand it completly anyway. If I am reading it right, longer exposures have to be adjusted to compensate for film breakdown? This topic has came up before. Check this link Aaron.

http://en.wikipedia.org/wiki/Recipro...photography%29

[Xia_Ke]

Thanks Greg

Each different film "emulsion" has a different response to long exposure. Some films are very susceptible to reciprocity failure, and others much less so. Some films that are very light sensitive at normal illumination levels and normal exposure times lose much of their sensitivity at long exposure times, becoming effectively "slow" films for long exposures. Conversely some films that are "slow" under normal exposure duration retain their light sensitivity better at long exposures. Compared at very long exposure times, Kodak's T-Max 100 speed film is faster than nominally 4 times faster Tri-X 400.

[Xia_Ke]

..I am not sure I understand it completly anyway. If I am reading it right, longer exposures have to be adjusted to compensate for film breakdown?...

Good question. I'm not it's that or if it has to do with each film having it's own optimize range of sensitivity and then once out of that range, it needs to be compensated for or...?

[photophorous]

Good question. I'm not it's that or if it has to do with each film having it's own optimize range of sensitivity and then once out of that range, it needs to be compensated for or...?

I don't really understand why it happens either, but for any specific film type, you can go to the manufacturer's website and the data sheet will have recommendations for how much to compensate and when it's needed. Those are still probably just starting points from which you'll have to experiment to find the best times.

Just as an example, here's the data sheet for TriX. Scroll down a little and there's a table.

http://www.kodak.com/global/en/profe...17/f4017.jhtml

[Xia_Ke]

Thanks Paul Data sheets are actually how I found out about reciprocity. I was looking at the data sheets for Pan F Plus and saw a graph talking about compensating for it. I've tried looking for more info on it but, can't find anything telling the why, only the how

[another view]

I can't explain the "why" either, but film is optimized to work within certain shutter speeds and when you use a shutter speed outside that range (faster or slower), you'll need to compensate. Usually the "fast" end of reciprocity failure happens at speeds faster than most cameras can shoot - 1/10,000 or faster so you only have to worry about the slow ones. Besides, how many people have actually used 1/4000 or faster?

Colors can shift, and with slide film you might need CC filters. The original Velvia was already a slow film at ISO50, so I'd run into that once in awhile, but the color compensation was very minor and probably not cruical to most situations. With color neg film, I wouldn't worry about it as any color cast could be taken care of with printing or scanning.

Reciprocity means that you can shoot 1/500 at f5.6 or 1/125 at f11 and get the same exposure. If the camera and lens are working properly, the negative will look the same (except for DOF and movement because of those settings, obviously). Since the film isn't as light sensitive at slow speeds, you might not have the same exposure with 2 seconds at f5.6 as 8 seconds at f11 - hence reciprocity "failure" because it's not exactly a reciprocal relationship between shutter speed and aperture anymore.

[reverberation]

I think it has to do with the amplification of the exposed silver halides during development. Essentially, it takes time and light to affect the silver halides (compound salts of silver) present in the film emulsion. The chemical process of developing film amplifies the exposed areas of silver halides which would take much longer for light to decompose by itself. By using different compounds in the emulsion and different salts of silver, film companies are creating a "window" of time and light that delivers an optimal image under a pre-determined chemical development. The problem starts when not enough silver halides are affected to be multiplied by development (the fast end) or when so many are affected that when they multiply, they dramatically alter the image (giving the impression of a change in film iso).

[jeffp]

http://www.camerabooks.com/custom.aspx?id=27

Ideally, a given amount of light should produce the same photographic effect regardless of whether it is bright and arrives over a short period of time, or is faint and arrives over a long period of time. This yields the "law" of reciprocity, which states that the photographic effect of an exposure should be proportional to the product of the light intensity (image irradiance in photons/mm²/second) multiplied times the exposure duration (in seconds). In particular, the photographic effect should be independent of the individual values of intensity and exposure duration.

In normal photography, this law holds reasonably well. Photographers know that 1/250 second at f/4 yields the same exposure as 1/60 second at f/8. But very high and very low light intensities requiring very short or very long exposure times are a different matter. At these extreme intensities, there are significant departures from this law. These departures are referred to as failure of the law of reciprocity, or reciprocity failure.

It is interesting that the most widely accepted explanation of photographic reciprocity failure is completely wrong. This misconception asserts that during a long exposure (such as in astronomy), the sensitivity of an emulsion decreases, that is, the emulsion speed slows down over time. Somehow the ISO/ASA rating supposedly goes from, say, 1000 at the beginning of an exposure to something like, say, 200 or 100 or 50 after an hour or so.

This description of what happens is so widespread and repeated so often that few question it. However, the theory of latent image formation refutes it. Something else is happening. The correct explanation is actually quite different.
Low-Intensity Reciprocity Failure

The formation of a developable latent image, as described above, actually occurs in two distinct stages. In the early part of the first stage, the clump of neutral silver atoms contains only one or two atoms. At this time, the process forming the latent image is reversible. The silver aggregate is unstable because thermal agitation (Brownian motion) can reionize one of these atoms. If this happens, then the reionized atom is lost from the latent image.

Not until the clump contains three or four atoms is it finally stable against thermal disintegration. When this number is reached, this stable, but not yet developable, latent image is called a latent sub-image.

During the second stage, the stable latent sub-image is converted into developable latent image. To do this, more exposure is necessary. The additional photoelectrons add one or two more silver atoms to the growing silver clump to finally give it the required threshold number of four to six atoms.

Thermal instability during the first stage of latent image formation has consequences for photography of faint objects. Because the photons arrive slowly during the long exposure, there is plenty of time for the reionization process to work. The potential latent sub-image can therefore fall apart almost as fast as it is being formed.

Not only are there fewer photons/mm²/second at low light levels, but the emulsion's net response to these photons is less. In other words, the efficiency of the process is reduced. This loss of efficiency (and thus sensitivity) at low intensities is the cause of low-intensity reciprocity failure.

Note carefully that this process is unchanged throughout the length of an exposure. In fact, the properties of an emulsion remain (nearly) the same from the time of its manufacture until it is placed in the developer. The emulsion has no built-in clock that decreases its responsiveness with time.

The controlling variable is the intensity (brightness, irradiance) of the incident light. Reciprocity failure is a light intensity effect, not an exposure duration effect.

It is of further interest that reciprocity failure can be occurring differently at different locations across the emulsion at the same time. Under the image of a bright star, emulsion efficiency is relatively high. But simultaneously in a nearby region under the image of a fainter star, emulsion efficiency is lower.

These differences in efficiency yield a rule of thumb: if it is desired to photograph stars one magnitude (2.512 times) fainter than those that can be barely recorded with a given exposure, then the exposure length must be increased about three times (neglecting the sky fog limit).

In addition to reciprocity failure at low intensities, photographic emulsions also exhibit reciprocity failure at high intensities. The mechanisms producing the two effects are entirely different.

If the light intensity is very high during an exposure, then the finite mobility of the silver ions becomes a source of inefficiency. When photoelectrons are liberated so rapidly that the sluggish silver ions cannot neutralize them fast enough, then more than one photoelectron can be active in a given crystal at the same time. Because of their mutual repulsion, no more than one photoelectron can reside at a given crystal lattice dislocation at once. Thus with high intensities, the forming latent image is dispersed to several dislocations at different places throughout the crystal.

But only one sufficiently large clump of silver atoms is needed to make the crystal developable. Any others that may be formed are redundant and useless. This lack of concentration of the latent image requires extra photon events to create at least one clump having the minimum-required four to six silver atoms. The resulting reduction of efficiency is the cause of high-intensity reciprocity failure.

This effect is well known to photographers who use very-high-intensity electronic (strobe) flash units having flash durations of about 1/10000 second. When using these super-bright flash units, extra exposure is required to compensate for the loss of emulsion efficiency.

[reverberation]

Thanks for looking that up. That is an excellent explanation.

[Xia_Ke]

Bravo jeffp, good find! :thumbsup: