_Test for a good Lens._--The generality of purchasers of photographic
lenses can content themselves with merely the following rules when they
buy. It ought to be achromatic, _i. e._ consisting of the usual two pieces
of crown and flint glass, that its curves are the most recommended, and
that it is free from bubbles: to ascertain the latter, hold the lens
between the finger and thumb of the right hand, much as an egg-merchant
examines an egg before a strong gas flame, and a little to the right of it;
this reveals every bubble, however small, and another kind of texture like
minute gossamer threads. If these are too abundant, it should not be
chosen; although the best lenses are never altogether free from these
defects, it is on the whole better to have one or two good-sized bubbles
than any density of texture; because it follows, that every inequality will
refract pencils of light out of the direction they ought to go; and as
bubbles do the same thing, but as they do not refract away so much light,
they are not of much consequence.
I believe if a lens is made as thin as it safely can be, it will be quicker
than a thicker one. I have two precisely the same focus, and one thinner
than the other; the thinner is much the quicker of the two. An apparently
indifferent lens should be tried with several kinds of apertures, till it
will take sharp pictures; but if no size of aperture can make it, or a
small aperture takes a very long time, it is a bad lens. M. Claudet, whose
long experience in the art has given him the requisite judgment, changes
the diameter of his lenses often during the day; and tries occasionally, in
his excellent plan, the places of the chemical focus: by this his time is
always nearly the same, and the results steady. As he is always free in
communicating his knowledge, he will, I think, always explain his method
when he is applied to. The inexperienced photographer is often too prone to
blame his lens when the failure proceeds more from the above causes. The
variation of the chemical focus during a day's work is often the cause of
disappointment: though it does not affect the landscape so much as the
portrait operator. {556}
If any one has a lens, the chemical and visual focus being different, his
only remedy is M. Claudet's method. And this method will also prove better
than any other way at present known of ascertaining whether a lens will
take a sharp picture or not. If, however, any plan could be devised for
making the solar spectrum visible upon a sheet of paper inside the camera,
it would reduce the question of taking sharp pictures at once into a matter
of certainty.
All lenses, however, should be tried by the opticians who sell them; and if
they presented a specimen of their powers to a buyer, he could see in a
moment what their capabilities were.