The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Lens
columns in the focussing screen will be just over five-eighths of an inch. With a good portrait lens having an aperture of one-fourth of the focus, the standard adopted as unity by the Photo-graphic Society ot Great Britain up to, say, a 12-in. lens, at all events, the definition in the centre of the field should be such that the small type will, on examination with an eye-piece, be quite legible. The same test may be employed for all lenses, and those professing to be aplanatic will for the most part be found to pass it, though with different degrees of perfection. Of course, by stopping down, almost any lens may be made to define well, at all events, in the centre of the field ; but we are speaking of lenses worked with the largest apertures with which they are supplied, and which, in most lenses of the Steinheil aplanatic type - rapid rectilinear, rapid symmetrical, Euryscope, etc. - will be found from In the newer forms of cemented
aplanatic lenses, in which the glass from the Jena factory has been utilised, somewhat larger apertures have been practicable, but whatever the aperture is, it is to be supposed that good definition is obtainable with it, and therefore the test mentioned may be fairly employed. When we come to examining the defining power of the lens at some distance from the centre of the field, we shall find with rapid lenses that there is a very great falling off. Taking again as an example a portrait lens of 12-in. focus and of apertureand slewing round the camera
so that the image is rendered at a distance of 2 in. from the centre of the focussing screen, we shall find that the small type of our newspaper column is no longer readable. The larger type used for the heading "Marriages," etc., should still be legible, but nothing more. Now insert diaphragmNo. 4 on the universal system, and the small type again becomes distinguishable. With a Steinheil type lens ofaperture the definition at this angle may be expected to be still satisfactory without further stopping down. This then is Mr. Debenham's method. It is convenient to cut out several of such columns as he mentions, and paste them on a piece of stout millboard, which is framed to keep it flat. Diagonally across the centre is pasted an ordinary inch tape. The whole arrangement is then hung up flat against the wallfand the camera set up and the image focussed till reduced to one-eighth in size, as suggested by Mr. Debenham. We may remark here that focussing is not effected in the ordinary way
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