This section is from the book "Modern Photography In Theory And Practice", by By Henry G. Abbott. Also available from Amazon: Modern photography in theory and practice: A hand book for the amateur.
The aim of all photographers, whether professional or amateur, is to make a negative which will produce a bright clear picture, (for with a poor negative this is impossible) and to attain this two things are necessary namely, correct exposure of the plate to the action of light through the lens, and correct development.
The first of these is the more important, though if the exposure is not correct it is sometimes possible to vary the development so as to make a fairly good negative from a very badly timed plate.
I find the main difficulty to all beginners is in not having a clear idea of the simple principles of what governs the timing of a plate correctly, and I shall therefore endeavor to give a few of the more important factors which come into consideration every time a picture is taken.
All plates of a certain make and sensitiveness require a certain amount of light admitted to them through the lens in order to effect the proper chemical change to make a good negative. Either more or less than this certain quantity will remove the quality of the resultant negative just so far from perfection. Our object then must be to learn just how much light to admit to the plate. No absolute rule can be laid down for this but each must learn by experience and careful observation after knowing the main principles.
♦ By R. D. Cleveland, Chicago.
Now suppose that instead of a camera and lens you have a tin box with a water tap running into it. You are told to let a pint of water into the box as quickly as you can. Of course you open the tap as wide as you can and let it in with a rush and then shut it off. If you are told however to let it in slowly you open the tap only a little way and leave it open much longer. And this is just what should be done with the light on a negative. The amount of light admitted is governed in two ways; by the length of time the lens is opened and by the size of the opening through which the light is admitted and the two must be considered together.
The stops or diaphragms to a lens are for the purpose of sharpening or equalizing the focus but they also limit the admission of light according to the areas of their openings.
The areas of circles are to each other as the squares of their diameters. The stops to the best lenses now are usually marked with the diameter of the circle proportionate to the focal length of the lens. That is, suppose your lens has an eight inch focus, then a stop of one inch in diameter would be marked f 8, and of half an inch in diameter f 16.
Now, no matter what the focal length of your lens, stops of similar marks bear the same relative size to their individual lenses and to each other. To illustrate, if you have one stop marked f 4, and another f 8, their areas would be as 16 to 64 or the larger one would be four times greater than the smaller, would admit four times as much light and therefore would require only one quarter as long exposure in the same light. Other size openings of course would be in proportion and the relative areas and times of exposure very easily calculated.
Suppose now for instance you know (or think you do,) that you could get a good negative in the 1-100th of a second with an f 4 stop; then with an f 32 stop you would get the same quality negative or admit the same amount of light to the plate in 64/100 of a second. This is one reason why time exposures with small stops are more likely to give good results than instantaneous with large stops. With the large stop just mentioned if you had given the plate 2/100 exposure, you would have doubled the time and might have spoiled your negative, while with the small stop to double the exposure you must give about 1 1/4 seconds. Thus with the small stop your latitude of exposure is vastly increased and your chances of spoiling a plate from either over or under exposure are correspondingly diminished.
Thus you see the time necessary for a correct exposure so far as the stops are concerned is always in proportion to their size and this you can calculate exactly by knowing their relative diameters. It will be found the least confusing in practice to confine yourself generally to the use of but two stops; the largest one that will cut the plate sharp all over for instantaneous, and one of the smaller for time. The relative areas of these two can be learned and applied quickly at any time, while more will simply be confusing. For those who do not fully understand the marks on their stops I will say the relative times of exposure are as follows: Assuming f 4 to be represented by 1.
That is f 64 would require 256 times the exposure that f 4 would in the same light.
The variations of intensity of the light at different times of day and different seasons of the year have also to be taken into consideration, but this too may be learned quite easily and carried in the head or the memorandum book.
You all know that the light of the sun is strongest in July and that it is much more intense at noon than at nine or four o'clock, and by learning just what this difference is it will help you very materially in correct timing. From the table in the American Annual for 1893 page 329 I quote a few figures, and any of you who care to can get more of them.
At noon in July with a given stop, if you require an exposure of 2/10 of a second, at 3 o'clock you require 1 2/10 seconds and at 6 o'clock 4 seconds.
In January under similar conditions it would take 1 7/10 seconds at noon, 4 5/10 at 3 o'clock and 9 at 4 o'clock.
Now we come to the point of how much we may vary over or under the correct time of exposure and still have a fairly good negative.
In order to test this roughly I exposed a number of plates on the same object, one after the other, with the came stop timing them from 1 to 40 as nearly as possible, and developed them all together.
They were made on Seed 26 plates on a cloudy day with a single combination Darlot lens with about an f 32 stop and developed with Hydrochinone developer. I numbered them in regular order, and assuming No. 3 as 10 they were timed as follows: No. 1,-2; No. 2,-5; No. 3, - 10; No. 4, - 20; No. 5, - 30; No. 6, - 40. No. 3 proved the best negative though No. 2 with 5/10 less exposure and No. 4 with double the exposure of No. 3 were also pretty good and with slight care in the development might easily have been made very good. From this I conclude roughly that we may have a latitude of exposure from one half below the best point to one half above and still be reasonably sure of a good negative.
If you take this latitude with a small stop and long time it is pretty great, but when you have got to measure it in the hundredths or fiftieths of a second as you have to with snap shots it is getting it down pretty fine for most amateurs and they need not wonder when they spoil plates with their kodaks.
In short it resolves itself into this. When you can get a good negative in the 1-100th of a second, if you give it 1/200 1/100 you may spoil your negative entirely. As a consequence of this knowledge no experienced photographer will take an instantaneous picture when he can possibly get anything else.
Now having exposed your plates you must develope them. If you have timed them right this will give you no trouble as they will almost develop themselves with any good developer.
Of developers there are so many good formulas any of which work well that I refrain from giving any.
I have gotten equally good negatives with oxalate, pyro, hydro., eiko., Rodinal and Amidol and all I can say is, get one that has been well tested by good judges, learn its peculiarites and stick to it.
On the continent the best photographers still stick to their first love-Oxalate of potassium and iron; while in England I believe the most popular is pyrogallic acid and ammonia. We Americans try every new patent medicine that the dealers advertise as "better than all others" and make ourselves sick if we were not before.
All plates have full directions for development contained in the box with them and they can generally be followed with good results. If you are uncertain about the correctness of your timing it is best to put the plate first into old or diluted developer and see how it acts. If it blackens all over quickly it is probably over exposed and should be taken out and washed and a different developer used. If it is a long time in showing the image it is under exposed and should be treated accordingly.
If very much either over or under exposed and it is something that you can take again, my advice would be, don't waste time on it but throw it away and take another. However, it may be your picture cannot be taken again in which case it is worth a little labor.
If it is under exposed weaken your developer and let there be an excess of alkali in it; cover your tray from all light and let it remain until all the detail is out that will come. Then to strengthen add the normal amount of pyro or hydro solution until the negative is quite black and then put in the hypo. If the plate is over exposed the simplest method is to add a few drops of a 10 per cent solution of bromide of potassium or ammonium and this will generally correct any but an excessively over timed exposure.
Practice of exposure with careful note of the time and the resultant negative will soon make you almost certain of your time in any light, and then the development will be very plain sailing.