This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.

**Determining Focal Length Of Lens Suitable For Operating Rooms Of Stated Length**. As it may be valuable to many photographers to know how they can determine the focal length allowable in operating rooms of limited length, we will give here the simple rules which govern the relations between focal length, size of image, and distance of the subject. When speaking of focal length we always refer to the principal or equivalent focus, which is the distance from the optical center of the lens to the surface of the plate when the object is at a very great distance.

784. Note: - For all Goerz lenses the optical center is situated in the plane of the diaphragm, and from that point the focus should be measured.

785. When making a picture n times smaller than the original, the object should be at a distance of (n+I)x focus from the lens. If this distance is limited to D feet=12D inches on account of the available space, it will be clear that

12D=(n+I) focus, or focus 12D/(n+1) inches. To illustrate: What should be the proper focal length for making a standing full-length cabinet portrait in a room 12 feet from wall to wall? This leaves, after allowing space for background and length of camera and moving space for operator, a clear distance of 8 feet between lens and sitter; thus, D=8xl2= 96 inches. A full-length cabinet picture measures 4 3/4 inches from head to feet. Now, assuming the person to be

5 feet 10 inches tall, n=70/4.75 say 14.5. We can now write f=12D/(n+1)=96/15.5=6.18, or say, 6 inches.

786. As there is still in the minds of some photographers a great deal of misapprehension regarding the presence of some occasional air bubbles in the glass of high-grade anastigmats, we think it advisable to publish the following communication from Messrs. Schott & Genossen, the manufacturers of the celebrated Lens Glass, the discovery of which has rendered possible the construction of the modern anastigmat.

787. Communication from the firm Schott & Genossen, Jena (Germany) Glass Manufactory for Optical and other Scientific Purposes:

The efforts of opticians, during the last few years, to improve lenses in their higher optical characteristics, have led to more extended use of glasses in the manufacture of photographic objectives, which differ widely in their optical properties and chemical composition from the crown and flint glass hitherto employed. Their manufacture is attended, to some extent, by far greater technical difficulties than are involved in the production of the optical glass formerly in current use. In the manufacture of the majority of the new kinds of glass which have taken front rank in the construction of improved photographic lenses, there are exceptional difficulties in securing perfect purity, i. e., freedom from small bubbles. The definite demands which have to be met to obtain relative dispersion and refraction differing from usual conditions, impose such stringent limitations upon the chemical composition of the glass fluxes, that no play is left to the manufacturer technically to provide suitable conditions for obtaining perfect purity. In consequence of this, it is practically impossible to supply these kinds of glass in uniform pieces free from a few small bubbles.

788. We would, however, point out that the presence of these small air bubbles, even under most unfavorable conditions, does not occasion a loss of light exceeding 1-50 per cent., and their influence upon the optical efficiency of a lens system is, therefore, of no moment whatsoever.

789. It is manifestly unfair to require that the manufacturer should reject nine-tenths of the glass which is made, simply because it shows a fault that is of absolutely no importance in practice, notwithstanding the fact that he is able to satisfy the higher demands of the optician with regard to all the really important properties the glass should possess for the functions of the lens.

790. If purchasers make the usual objection that a lens is "faulty" because of a few small bubbles, the optician must kindly explain that lenses of the highest class cannot be made of any sort of crown and flint, and that more important considerations have to be taken into account in selecting the glass than the absence of a few small bubbles.

GLASTECHNISCHEN LABORATORIUM,

Schott & Genossen.

791. Relative Exposures for Varying Proportions of Image to the Original, by W. E. Debenham. Reproduced from "The British Journal Photographic Almanac." - When an enlarged photograph has to be made, either from a negative or print, it is commonly understood that the greater the degree of enlargement the longer will be the exposure required; but I have generally found only the vaguest ideas to exist as to the amount by which such exposures have to be prolonged. Sometimes, indeed, it is assumed that the exposure will be in direct inverse proportion to the area covered, so that a copy of twice the linear dimensions of the original - covering, as it does, the area of four times the size - would require an exposure of four times that sufficing for a copy of the same size. This calculation, however, omits to recognize an important factor and leads to serious error, the actual exposure required in the case mentioned (assuming the same lens and stop to be used) being not four times, but two and a quarter times that of a copy of same size; whilst, when we come to high degrees of enlargement, the error would amount to an indication of nearly four times the exposure actually required.

792. To find the relative exposure add one to the number of times that the length of the original is contained in the length of the image, and square the sum. This will give the figure found in the third column of the annexed table. (See Paragraph No. 798.)

793. As examples: Suppose a copy is wanted having twice the linear dimensions of the original. Take the number 2, add 1 to it, and square the sum, 32=9. Again, if a copy is to be of eight times the linear dimensions of the original, take the number 8, add 1, and square the sum, 92= 81. Copies respectively twice and eight times the size (linear) of the original will thus require relative exposures of 9 and 81 - i. e., the latter will require nine times the exposure of the former.

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