[Footnote: We take from the Br. Jour. of Photo. the following interesting
paper read by W. Goodwin before the Glasgow and West of Scotland Amateur
Association.]
With good plates, and intelligent development, a practiced photographer
may within certain limits correct the effects of an over or under
exposure; but you have all, doubtless, found out that there is a correct
exposure, and that you cannot trespass very far on either side of it
without sacrificing something in the resulting negative.
MR. W.K. BURTON'S TABLE OF COMPARATIVE EXPOSURES
------------+--------------------+-------------------+-------------------------
| | Badly lighted| Portraits in bright
| | interiors,| diffused light
Aperture | +------------+ up | out of doors.
calculated | Landscape with | Fairly | to | /
on the | heavy foliage in | lighted | | / Portraits in
standard | foreground. | interiors | | / studio light
system +------+-------+ +------+ | | | /
of the | Sea |Open | | Under| | | | / Portraits
Photographic| and |land- | |trees,| | | | | in ordinary
Society. | sky. | scape.| |up to | | | | | room.
------------+------+-------+-----+------+-----+------+------+-----+------
| sec | sec | sec | m s | m s| h m | sec | m s| m s
No. 1, | 1/160| 1/50 | 1/8 | 0 10 | 0 10| 0 2 | 1/6 | 0 1| 0 4
or f/4 | | | | | | | | |
------------+------+-------+-----+------+-----+------+------+-----+------
No. 2, | 1/80 | 1/25 | 1/4 | 0 20 | 0 20| 0 4 | 1/3 | 0 2| 0 8
or f/5.657 | | | | | | | | |
------------+------+-------+-----+------+-----+------+------+-----+------
No. 4, | 1/40 | 1/12 | 1/2 | 0 40 | 0 40| 0 8 | 2/3 | 0 4| 0 16
or f/8 | | | | | | | | |
------------+------+-------+-----+------+-----+------+------+-----+------
No. 8, | 1/20 | 1/6 | 1 | 1 20 | 1 20| 0 16 | 1-1/3| 0 8| 0 32
or f/11.314 | | | | | | | | |
------------+------+-------+-----+------+-----+------+------+-----+------
No. 16, | 1/10 | 1/3 | 2 | 2 40 | 2 40| 0 32 | 2-2/3| 0 16| 1 4
or f/16 | | | | | | | | |
------------+------+-------+-----+------+-----+------+------+-----+------
No. 32, | 1/5 | 2/3 | 4 | 5 20 | 5 20| 1 4 | 5-1/3| 0 32| 2 8
or f/22.627 | | | | | | | | |
------------+------+-------+-----+------+-----+------+------+-----+------
No. 64, | 2/5 | 1-1/3 | 8 |10 40 |10 40| 2 8 |10-1/2| 1 4| 4 15
or f/32 | | | | | | | | |
------------+------+-------+-----+------+-----+------+------+-----+------
No. 128, | 4/5 | 2-2/3 | 16 |21 0 |21 0| 4 15 | 21 | 2 8| 8 30
or f/45.255 | | | | | | | | |
------------+------+-------+-----+------+-----+------+------+-----+------
No. 256, |1-1/2 | 5-1/2 | 32 |42 0 |42 0| 8 30 | 42 | 4 15|17 0
or f/64 | | | | | | | | |
------------+------+-------+-----+------+-----+------+------+-----+------
The estimation of this correct exposure is probably the greatest
difficulty in photography, and it is particularly discouraging to find
plate after plate useless because the guess has been wide of the mark.
There are some here to-night who have spoiled so many plates that at last
they are prepared by experience for almost any contingency, and to those
I nave very little to say; but there are also many who are still in their
troubles, and I propose to tell them how the amount of guesswork required
may be reduced to a minimum.
The factors which govern exposure are: the subject of the picture, the
lens and its aperture, the rapidity of the plate, and last, but not by
any means least, the quality of the light by which the work is to be
done.
Let us consider each of these separately, and see if we cannot reduce any
of them to rule. In this respect the subject will be found somewhat
intractable. Scarcely two subjects will be found to send exactly the same
amount of light through the lens. However, a broad classification may be
made, and this has been done by Mr. Burton in his Table of Comparative
Exposures. A glance at this table will show how greatly the character of
the view may influence the time of exposure. Thus, with full aperture of
a rapid symmetrical, the exposure for open landscape is given as
one-twelfth of a second; when heavy foliage appears in the foreground,
half a second will be required; while, under trees, as much as forty
seconds may be needed.
The first aid I have to suggest is the use of such a table as Mr.
Burton's. Before we do anything more in this direction, we must consider
the influence of the lens and its diaphragms. In theory the single
landscape lens is more rapid than the doublet of equal aperture, but the
difference is so little that it may be disregarded in practice, and my
remarks will apply to both.
The rapidity of a lens depends mainly on its aperture and its focal
length. Thus a lens of twelve inches focus will require four times the
exposure of a six inch, with an equal sized diaphragm, and a quarter inch
diaphragm will require four times the exposure of a half inch when used
in the same lens.
The Photographic Society of Great Britain have recommended that the
diaphragms of all lenses should bear such relation to the focal length
that each should require exactly double the exposure of the next smaller.
Now, if we turn again to Mr. Burton's table, we shall find that it is
constructed on this principle, and that each stop is numbered so as to
show its exposure. Obviously, the most sensible thing would be to get a
set of stops made to correspond with this arrangement, but we will see
how we can construct a table for stops of any size.
First, if possible, find the equivalent focus of your lens. If it is made
by a known maker, you will find it in his price list, and if not, you may
calculate it for yourself by the rules given in the various text books,
provided you have a camera of pretty long focus. However, it will be near
enough for our purpose if you get a sharp image of the sun on a piece of
paper, and while you hold lens and paper, get some one to measure the
distance from the paper to the diaphragm aperture, or, in the case of a
single lens, to the center of the lens. Note down this focal length, and
proceed to measure your diaphragms in sixteenths of an inch.
Then, with pen and paper, proceed to divide the diameter of each stop
into the focus, and state the result as a fraction of the focus, thus
f/8. For example, a Ross half plate rapid symmetrical has a focal length
of 7½ in.; for convenience reduce this to sixteenths=120. A diaphragm
measuring seven sixteenths will give the fraction f/17. Now let us see if
any of these stops correspond with Mr. Burton's. The first two in his
table will only be found in portrait lenses, but we shall probably find
one to correspond with the third, if we are using a doublet lens; with a
single lens we won't find any so large. Having picked out those that
correspond, and filled in the exposure for them, we have now to deal with
the odd sizes. Here is one, f/27, which is just half way between No. 16
and No. 32, but a moment's thought will show that as the exposure
increases as the square of the diameter, it won't do to take the exposure
half way between the two.
We have another factor to consider now: that is, the rapidity of the
plate. If you use plates by a maker who has a name to sustain, you may be
pretty confident that they are of fairly uniform rapidity, so after you
have got into the way of working any particular brand, the best thing you
can do is to stick to it. The exposures in our table are for plates of
medium rapidity in good spring light. In my own experience I find that
they just suit "thirty times" plates, or fifteen on the sensitometer; but
then I like a full exposure with slow development, and I know that others
find these exposures just right for "twenty times" plates developed in
the usual way. The most rapid plates in the market will not be overdone
with half the given exposures. It must always be borne in mind that an
error of a fraction of a second in either direction may be corrected in
development, and it is impossible to make a very serious error if you
refer to the table.
We come now to the light. If you depend on the eye entirely in judging
the quality of the light, it will sometimes play you tricks. The rays
which are most active on the plates are those which have the least effect
on the eye. We can, however, by chemical means arrive at an exact
estimate of the active power, and for this purpose an actinometer is
used. This is simply an arrangement whereby a piece of sensitized paper
is exposed and allowed to darken to a standard tint, and by the time it
takes to reach that tint the value of the light is judged. Capt. Abney
has, however, pointed out that ordinary sensitized paper is not suitable
for bromide plates, since there are conditions of light in which the
plates will be fairly rapid while the paper will be very slow. He gives a
formula for a bromide paper, which is treated with tannin in order to
absorb the bromine set free during exposure, otherwise the darkening
would be very slight. I used this paper for a while, but found it rather
slow. The tannin also turned brown on keeping for a week or so. I then
made some more, substituting for tannin potassium _nitrite_ (not
nitrate), which is colorless. This was an improvement, but still it was
just slow enough.
However, noticing in Capt. Abney's article the statement that the bromide
of silver should be as nearly as possible in the same state in the paper
as in the plate, I thought "Why not Morgan's paper?" This, of course, is
just bromide emulsion on paper, and if, as I suspect from its color, it
contains a trace of iodide, why, so do most commercial plates. A sheet of
this paper cut into strips, soaked for ten minutes in a fifteen-grain
solution of potassium nitrite, and dried, gives a sensitive paper which
darkens with great rapidity to a good deep tint, and keeps indefinitely.
Here is some prepared last summer, which is still quite good. To use this
paper make a little box so that a little roll of it can be stored in one
end, and drawn forward as required beneath a piece of glass.
Bearing in mind that your table of exposures is calculated for the best
spring light, go to the country some bright day next month with
note-book, actinometer, and the necessary appliances for exposing a few
plates. Select, say, an open landscape, and use your smallest stop. When
all ready to expose, get out your actinometer and expose it to the
reflected light of the sky for ten seconds (if the sun is shining, turn
your back to it, and keep the actinometer in your own shadow); then put
it in your pocket, expose a plate according to your table, and in case
the light or plate should not be just in accordance with the conditions
under which the table was prepared, expose other two plates, one a little
less and one a little more than that first exposed. Then note down
everything you have done--kind of view, stop, speed of plate, exposure of
each plate, and length of exposure of actinometer.
When you get home, the first thing to do is to get hold of a paint box
and paint the underside of the glass of your actinometer to match the
darkened paper. Do this by gas light. Then scrape away a little of the
paint, so as to let a strip of the paper be seen below it. After this
develop your three plates with a developer of normal strength, and see
which is best. If you have chosen a really bright spring day, and are
using plates of medium rapidity, you will most likely find that exposed
according to the table just about right.
Now let us see how we can use these aids in our field work. We have
ascertained the correct exposure with a given stop on one class of view,
with light of a given quality, but now suppose all these conditions
altered. Let the view have heavy foliage coming close up to the camera,
the stop be a size larger than that used in our first experiment, and the
day rather dull. The table tells us what the exposure would be with this
stop on this view, on a bright day; and if the actinometer take twenty
seconds to reach the painted tint, then we must double the exposure given
in the table.
You may sometimes find that the actinometer indicates a very different
exposure from what the eye would lead you to expect. For instance, one
day last September I went to Bothwell Castle, to get a picture I knew of
in the grounds. It was one of those strange yellow days we had then, and
the sun, though shining with all his might, was apparently shining
through orange glass. The actinometer indicated an exposure of thirty
seconds where in good light one would be right. I was rather incredulous.
Thirty seconds in broad sunshine! However, I gave this exposure, but for
my own satisfaction I gave another plate fifteen seconds only.
On developing, the latter was hopelessly underexposed while that having
thirty seconds gave a negative which furnished one of my exhibition
pictures.
I have shown you how to reduce the quality of the light to a certainty,
also how to reduce to rule the exposure with different lenses and stops
on certain classes of subjects, and it remains with you only to guess
correctly to what class the view you wish to take belongs; I can assure
you from my own experience that there is enough uncertainty about that
point to prevent good negatives ever becoming monotonous.
The only aid I can suggest in this case is the continual use of a
note-book. Note every plate you expose, and when you have a failure be
careful to record the fact, and you will gradually find these accumulated
notes becoming a great help in cases of doubt. One hint I can give to
beginners is that a great number of the pictures to be met with in this
part of the country are intermediate between "Open Landscape" and
"Landscape with heavy foliage in foreground;" and it is scarcely needful
to say that if you are in doubt, let the exposure be rather too much than
too little; you _may_ make a negative of an overexposed plate, but never
of an underexposed one.
* * * * *