December, 1906.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



611 



Photography. 



Pure and Applied. 



By Chapman Jones, F.I.C, F.C.S., ice. 



Opacity Measuring Apparatus. — The British Journal 

 of Photography (page 873), quotes from tlie Revue dcs 

 Sciences thotographiqnes, the description of an " Opacity 

 Comparator," by M. Monpillard, which is illustrated by 

 M. Nachet. This instrument is so similar to my " Opacity 

 Balance," that I described in 1899 (Journal of the Royal 

 Photographic Society, xxiii., 99), that it may justly be 

 regarded as a modification of it. In both an incan- 

 descent gas mantle is the source of light ; two mirrors 

 give two beams of light, so that the plate to be meas- 

 ured may be interposed in the path of one ; these two 

 beams impinge on opposite sides of a prism and so are 

 noilected into the eye-piece for observation. So far as 

 the instruments differ, I believe mine tO' be the more 

 advantageous and less costly. But my main reason 

 for referring to this matter is that M. Monpillard claims 

 that his instrument will do what it cannot do. He 

 makes the same mistake that has misled many in- 

 \estigators both in England and on the Continent, in 

 neglecting the light that is scattered by the silver 

 deposit in the negative, or at least neglecting an un- 

 known part of it, while this scattered light forms a 

 considerable and variable proportion of the light trans- 

 mitted, variable even in the different densities of the 

 same plate. The claim, therefore, that such an appara- 

 tus will give " the absolute value of the density," is 

 quite in error. I have found that by neglecting this 

 scattered light the " density " readings may be in- 

 creased from twenty to forty per cent, or more, when 

 measuring ordinary plates, and up to as much as three 

 hundred per cent, if the deposit is bleached. The chief 

 claim that I made for my opacity balance was that it 

 serves to quickly and accurately compare the deposits 

 Oif the same density on any one given plate. 



M. Monpillard has adopted a pinhole aperture in his 

 eye-piece, to avoid parallax when viewing the final re- 

 flecting prism, to compare the beams of light reflected 

 intoi the eye-piece by its opposite sides. I tried modifi- 

 cations of this method, and although it is quite practic- 

 able, it is so ditlicult to keep the eye central while at- 

 tending tO' the matching of the two beams, that I sub- 

 stituted a small plaster plug on the surface of which 

 the twoi beams fall side by side and in actual contact. 

 This method entirely gets rid of the difficulty, and 

 serves for all the opacities generally met with, when 

 using an incandescent gas mantle. The direct ob.serva- 

 lion method is available, if required for very high densi- 

 ties, i)y merely withdrawing the [ilaster plug and [Hit- 

 ting in ;in eye-piece. M. JVIonpillard's apparatus will 

 give an aperture 3 m.m. wide for the measurement of 

 spectra, but I regularly use an aperture of one milli- 

 metre rmd can easily reduce this to one half the width 

 or less if desired. I camiot see, therefore, that M. Mon- 

 pillard has modified my mstrument in any way to 

 advantage. 



Depth of Field in Lenses. — In the photograph v of 

 solid objects it is obviously necessary to get sufficiently 

 good (k linition, at the same time and on a flat plate, of 

 objects ;it different distances from the lens. Theoreti- 

 cally, perfect definition in the same plane of objects at 

 different distances is impossible, but as no lens is 

 thcdrelically perfect, si. th:it a point in the object ahv:ivs 



gives an image of measurable size, and which is greater 

 when the object is not at the exact distance focussed 

 for, this becomes a purely practical matter. It is first 

 necessary to determine the extent of surface that the 

 image of an absolute point may be allowed to cover. 

 Of course, this must vary, and vary largely, according 

 to circumstances, but for purpo.ses of calculation, a 

 disc the one-hundredth of an inch in diameter is the 

 accepted standard in this country, and a disc one-tenth 

 of a millimetre (or the two-hundredth-and-fiftieth of an 

 inch) is the Continental standard. For small work 

 the English standard is too large, while the other is 

 often too difficult to realise, and unnecessarily small for 

 work even of moderate size. 



The hyperfocal distance is a constant that is of much 

 use in this connection, and may be described in various 

 terms. If a lens is focussed on an object that is so far 

 away that the image produced by the lens is not a 

 measurable distance from the principal focal plane of 

 the lens, the hyperfocal distance is the distance from 

 the lens of the nearest object that the lens will define 

 without exceeding the adopted standard. Then objects 

 between the hyperfocal distance and infinity are defined 

 without transgressing the standard of definition. If, 

 instead of focussing on a very distant object, the hyper- 

 focal distance itself is focussed on, then the distant 

 object will be just well enough defined, and there will 

 be also a considerable range of distance nearer the lens 

 than the hyperfocal distance which w-ill also be defined 

 within the accepted standard. For practical purposes 

 this nearer limit may be considered as half the hyper- 

 focal distance. If, for example, the hyperfocal distance 

 is thirty feet and the lens is focussed on an object thirty 

 feet away, all objects from about fifteen feet to the 

 horizon (or infinity) will be defined within the standard. 

 Methods of Calculating the Hyperfocal Distance. — The 

 hyperfocal distance is directly proportional to the focal 

 length of the lens and the diameter of its aperture, and 

 inversely proportional to the diameter of the permissible 

 disc of confusion. The usual formula for calculating 

 it is therefore //='—, where U is the hyperfocal dis- 

 tance, / the focal length of the lens, d the diameter of 

 its aperture, and e the diameter of the maximum per- 

 missible disc of confusion. If the aperture is expressed 

 in the usual way as a fraction of the fcn-al length (//8, 

 iS.c.), representing the denominator of the aperture frac- 

 tion as a, the formula becomes H — ^^- If the focal 

 length is measured in inches, and the disc of permissi- 

 ble confusion is taken at one-hundredth of an inch, then 

 for the hyperfocal distance in feet, H = ttt'- Dr. 

 Lindsay Johnson has recently pointed out, and it has 

 been pointed out before, that for an aperture of 

 //8, the denominator is so nearly 100, that, for this 

 aperture and disc of confusion, the error is negligibly 

 small if the formula is written H = f^. That is, the 

 square of the focal length in inches is the hyperfocal 

 distance in feet. The effect of changing the aperture 

 is easily calculated, the hyperfocal distance being 

 directly proportional to the aperture diameter. (At 

 //16 it will be the half of what it is at //8, at //32 a 

 quarter, and so on.) Dr. Lindsay Johnson also points 

 out that the hyperfocal distance may be calcul.'ited 

 exactly in Continental units, the diameter of the disc 

 of confusion being taken as one-tenth of a millimetre, 

 by multiplying the focal length in centimetres bv the 

 diameter of the aperture in centimetres, the resulting 

 figure being the hyperfocal distance in metres. 



Such figLM-es, like almost all other figures connected 

 with lenses, are not of the absolute value that they 



