No, sorry, that is not the case. Look at the equation again: there is a value (different for each film) called tc,1 which is the adjustment for a 1 second exposure (found by experiment) for that particular film. That value needs to be plugged in to the equation to get the final time.
Cheers, Bob.
Furthermore, the ^ symbol means "Raised to the power" of the number that follows it, not multiplied by it. There is no simple multiplicative factor to convert indicated time to corrected time. I am preparing to post a more complete exposition which may make it easier to use and understand.
It is not linear -For TRI-X, based on Kodak pub F4017, if the EV indicates a 10 second exposure, the correction is +2 stops - or 40 seconds - with a 20% reduction in development. At an EV indicating 100 seconds the corrections is 3 stops or 13.3 Minutes with a reduction in development of 30%. Even a one second exposure is supposed to be at +1 stop. (I generally don't start correcting till there is an indication for 2 sec or more.)
Kodak has been quoting those numbers for a long time. A problem is that view camera users do not usually consider aperture adjustment a viable option. The aperture is set for depth of field as a rule. Most of the majestic scenery has little need for f 64, but still there is the adage "f64 is where it's at."
Howard Bond's data show much less than 1 f-stop equivalent time extension for 400TX. Nevertheless, the ancient Kodak data I had from before 1950 does fit the equation that I presented quite well. My equation would calculate 41 seconds added exposure at 10 seconds indicated, or 51 seconds altogether.
[FONT=Courier New]There is an advantage to plotting the correction vs the indicated exposure. You need plot only one line, either on linear or log-log paper, which we will call "Basic correction". On log-log paper it is a straight line with a slope of 1.62 inches (or cm) rise per inch (or cm) run, as they say in surveying. It passes through 1 second correction at 1 second indicated. On linear graph paper, you calculate enough points to define the line for an imaginary film with a reciprocity correction of 1 second at 1 second indicated exposure. This calculation is easy on a TI-30 pocket calculator. Basic correction = indicated time ^ 1.618.
For each film there is a film factor that is multiplied by the basic correction to get the actual correction. This value is added to the indicated time to get the corrected exposure time. Film factors must be defined by experiment.
I have provided a table of values that you can plot on either form of graph paper. If you know any reciprocity correction for any indicated time for any film, you can calculate the film factor by simply dividing the actual correction by the basic correction that you read from the graph for that indicated time. For example, if I know that the exposure time for my film is 20 seconds at 16 seconds indicated, the actual correction is 4 seconds, the basic correction is 88.8 seconds, and the film factor is thus 4/88.8 or 0.045. This film factor then allows you to estimate the reciprocity correction for that film for any desired indicated time. In the example case, an indicated exposure of 32 seconds would have a basic correction of 273 seconds which multiplied by the film factor od 0.045 gives a correction of 12.3 seconds for a total exposure of 44.3 seconds.
Let me restate what we have so far. Given a plot of basic reciprocity corrections in the form of an increment to be added to the time indicated by your light meter, and a film factor for a particular film, you can read the basic correction for the indicated time, multiply it by the film factor and add the result to the indicated time to get the time you should use. For any film, if you know the reciprocity correction for a given indicated time, you can get its film factor by dividing the actual correction by the basic correction from the graph.
All of this is empirical of course. It fits the films for which I have data with practical accuracy. The fact that the same basic curve works with both traditional and tabular grain films of two different manufacturers lends some degree of confidence to the method.
For most users of view cameras, changing aperture to correct for reciprocity is not a desirable option. The exposure time adjustments determined by Howard Bond and represented by these charts and graphs will assure the retention of shadow detail.
CHART FOR ESTIMATING CORRECTIONS FOR RECIPROCITY FAILURE
Multiply the basic correction by the factor for your film and add it to the indicated time.For example, if your light meter says you need 16 seconds for 400TX, the corrected time would be 88.8 X .17 + 16 = 31 seconds.
That chart was in proper format when I submitted it. Now I can't even read it. I hope you can see what I was getting at from the words.
I'm trying again.
[FONT=Fixedsys]
CHART FOR ESTIMATING CORRECTIONS FOR RECIPROCITY FAILURE
Indicated.........Basic...........For this film:..................Multiply by
time, seconds...correction
1..................1................400TX......... ...................0.17
1.4................1.7.............400TMAX or 100TMAX.......0.07
2..................3.1..............HP5+.......... ...................0.11
2.8................5.4.............100Delta....... ..................0.05
4..................9.4
5.6...............16.5
8..................28.9
11................50.6
16................88.8
22................155
32................273
45................477
64................836
90................1465
128...............2567
180...............4497
256...............7880
Multiply the basic correction by the factor for your film and add it to the indicated time.For example, if your light meter says you need 16 seconds for 400TX, the corrected time would be 88.8 X .17 + 16 = 31 seconds
so according to my simple calculations
tri x indicated 90 second exposure would actually be a 265.05 second exposure?
I think my table is still not clear. The basic correction for 90 seconds is1452. The film factor is 0.17. The product of those is 247. Add 90 and you get 337.
Whoever said that FP4+ data would be enough, I found that Ilford thinks one size fits all. My equation fits their curve, but Howard's data do not. The difference is that I get a film factor of 0.51 when I use Ilford's curve. The same basic correction that I presented above fits both ends of Ilford's curve when I use that film factor. I would try the same correction for FP4+ as for HP5+, since Ilford seems to think they are all the same, including Delta400.
P.S.
I believe Howard's data. I also believe that HP5+ and FP4+ corrections are enough alike that it won't make a significant difference in practical work.
You can tell that Mr. Gainer used to design flexible, field-usable aids to navigation and other such things professionally. The attachment is a .pdf file that will make a little card to carry for reciprocity adjustments using Gainer's numbers. (I did a .pdf to make sure it holds formatting and can be printed and resized properly.) The first two columns are metered exposure times expressed as minutes in the first column or as seconds in the second column. These "just happen" to match the time markings on my light meter. The third column shows basic correction times in seconds, calculated by raising the metered time to the power of 1.62 per Gainer. The right hand box indicates Gainer's factor by which to multiply the "Basic Correction", dependent on which film you're using.
You get your final exposure time by metering, reading the corresponding basic correction, multiplying the basic correction by the film's Bond-Gainer reciprocity factor, then adding the result to the metered time. It's very simple if you have a calculator at hand and can do multiplication. An example is on the attached .pdf "card" to carry along as an aid in using the card. This is a preliminary stab at this, and I may work on making it more user friendly as time allows.
Someone with the time and inclination could make a slide rule version of this information for field use, with a chart of film factors or a place to list your own factors on the back. A circular version on card stock could be very packable and handy. Yes, I'm old enough to have learned to use a slide rule, and I still have a couple of them.
As for Ilford FP4+ and it's relation to FP5+, Reeves's data shows that FP4+ has marginally more reciprocity failure at 128 seconds of exposure, a loss of 1.2 stops as opposed to 1.18 for FP5+. This translates to a Schwarzschild exponent of .79 for FP4+ and .83 for FP5+. That points to Mr. Gainer's comments being right on target regarding exposing FP4+ at the adjustments for FP5+.
I also believe that Mr. Bond's data is accurate and Mr. Gainer's analysis is correct. I need to order the back issues so I can have a first-hand look and learn more. I have no way to prove this, but I think the manufacturers assume that anyone using their film for critical work will test under conditions of use, and that they don't think that testing under manufacturers' lab conditions will have a lot of bearing in the always unique circumstances in the field. Therefore, they only provide general guidelines that will get the users close enough on the first try to have something usable, and from which to make their own adjustments.
Originally Posted by Bob F.
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