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THE GUIDE TO NATURE 



mat. It will be noticed, however, that 

 some stopping" down will be necessary 

 when there are several rows in the 

 group, but if a rapid rectilinear lens 

 were used on the same group, (a rapid 

 rectilinear lens of the same focal 

 length) it would be found that al- 

 though you may have stopped down 

 and attained a sufficient equality of 

 focus from the front row to the back, 

 that you have not yet attained a suf- 

 ficient sharpness on the ends and cor- 

 ners. So we stop down further, losing 

 more speed. This is a very practical 

 illustration of what anastigmat lenses 

 will do. 



We often hear that lenses of the 

 same relative aperture require the same 

 exposure. This is perfectly true, but 

 if the definition of one is not good 

 and we have to stop down further to 

 make it as sharp, we can then say that 

 one lens is faster than the other. 



Another great point of superiority is 

 the critical sharpness which the anas- 

 tigmat produces. The opticians stan- 

 dard of sharpness is his "circle of con- 

 fusion," that is, the disc of light in the 

 image corresponding to a point in the 

 object. In theory, the points of the 

 image should be mathematical points 

 but as a fact they are of considerable 

 size. When they are but i/iooth inch 

 in diameter, the eye perceives them as 

 points and we say the picture is sharp. 

 But if enlargements are to be made or 

 lantern slides which will be projected 

 enlarged on the screen, the standard 

 of definition must be raised to I /200th 

 inch, etc. As the anastigmat gives 

 better definition than the rapid rectilin- 

 ear, its negatives are capable of greater 

 enlargement. By stopping down the 

 "circles of confusion" become smaller 

 and the enlarging possibilities of the 

 Anastigmat are correspondingly great- 

 er. 



For standing figure work, the ad- 

 vantages of the anastigmat are self- 

 evident. We have often endeavored 

 to make a standing figure with the or- 

 dinary portrait lens or rapid rectilinear. 

 Unless we make a figure which is very 

 small, we find that when we focus on 

 the head or the feet, the body of the 

 subject will not be in focus ; and if we 



try to focus on the body, we will lose 

 the sharpness on the head and feet. We 

 are, therefore, forced to stop down on 

 account of the curved field of the lens 

 and the result is that when we have a 

 satisfactory focus on the standing 

 figure, that we have reduced the light 

 to such an extent that a much longer 

 exposure is required. 



As the depth of a moderate focus 

 lens at F 6.3 is often ample, the anas- 

 tigmat can often be worked at this 

 aperture. Herein lies the speed of an anas- 

 tigmat from a practical standpoint. It 

 is a cloudy day, perhaps a rainy day, 

 or perhaps by force of circumstances 

 the photograph can only be made at 

 late hours of the day. The anastigmat 

 with its large opening gives a full timed 

 negative, whereas the rapid rectilinear 

 must be stopped down beyond possi- 

 bility of hand exposure, except with 

 aid of the tripod. 



When light permits, stop down and 

 gain the depth you want. When you 

 can't have all the depth you want, you 

 will be forced to put up with what you 

 can get. Many times with an F 4.5 

 lens, you can get enough depth for the 

 exposure of the important subject you 

 want. If light permits, stop down. 

 This brings up a peculiarity of the B. 

 & L. Zeiss Ic Tessar. When stopped 

 down, the definition is not impaired. 

 This falling off of quality in high speed 

 lenses stopped down was very marked 

 with the early fast lenses, but is entire- 

 ly overcome with the Ic type. 



We are often asked, how can you get 

 speed with depth? This is a photo- 

 graphic impossibility. If your fast 

 lens has not enough depth for your 

 individual taste, at its full opening, sell 

 it and get a shorter focus one. A 6 

 inch F 4.5 lens has much more depth 

 than an 8 inch F 4.5 lens, in fact a 6 

 inch F 4.5 lens has more depth at F 4.5 

 than an 8 inch at F 6.3. 



We will give a practical problem. A 

 man wishes to make a photograph of a 

 house, and the time of day is late. He 

 has no tripod. By using the 6 inch 

 lens he can obtain a sufficient depth of 

 field to render the different objects he 

 wishes to be sharp. The man with the 

 8 inch lens is stuck. If he stops down, 



