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.