THE CAMERA 



101 



such as a bill hoard covered with print- 

 ed matter, making sure that the hill 

 board is also parallel with the plate 

 surface, you will note that no matter 

 where you look on the surface of the 

 ground glass that von will find the 

 same uniform definition, if, however, 

 you should repeat the same with a 

 rapid rectilinear lens at full opening 

 you will notice that the corners are not 

 sharp when the center is sharp. 



An anastigmat of the same focal 

 length will satisfy this test easily even 

 at apertures such as F 6.3, and F 4.5. 

 Here we have the first great difference 

 between the anastigmat and the rapid 

 rectilinear. Of course, if a test is made, 

 it is very essential that lenses of the 

 same focal length be used, although the 

 short focus anastigmat will often equal 

 the performance of a much longer 

 rapid rectilinear. Not only will the 

 anastigmat cover a larger field more 

 sharply than the rapid rectilinear, but 

 the aperture can be brought up so that 

 the opening may almost equal a quar- 

 ter of the focal length, producing lenses 

 which are capable of photographing 

 and arresting the most rapid motion. 



Now when a rapid rectilinear is 

 focused wide open, no position will be 

 found where the image is sharp all 

 over. This is because the field of the 

 lens is saucer shaped, consequently, 

 since the sensitive surface is of neces- 

 sity flat, they do not coincide, except- 

 ing in the very centre. Of course, the 

 image is improved by storming down, 

 but naturally w r e increase the time nec- 

 essary for exposure, which is the very 

 thing we do not want to do. It is, of 

 course, assumed that the image of the 

 bill board nearly fills the plate when 

 the tests are made. The anastigmat 

 under the same conditions makes sharp 

 images over the plate from corner to 

 corner although the stoo used may he 

 the very largest one the lens will allow. 



The speed of a lens is the F value 

 of the largest aperture at wdiich it will 

 make a sharp picture from corner to 

 corner of a flat test surface, such as a 

 test chart or a hill board as indicated 

 above. The tests are always made on 

 a flat surface and this, as indicated be- 

 fore, is always perfectly parallel to the 



plate and at right angles to the axis 

 of the lens. No "speed lens" will give 

 quick exposures if stopped down, there- 

 fore, the word speed should be used 

 with care in lens discussions. The mis- 

 use of the word "speed" is responsible 

 for more misunderstanding over the 

 value of lenses between lens-makers 

 and the public or between photograph- 

 ic editors and their amateur readers 

 than any other point. 



An anastigmat at F 6.3 or greater 

 apertures is known as a last lens. It 

 is six or seven times as speedy as the 

 ordinary rapid rectilinear, for the 

 reasons that the ordinary rapid rectilin- 

 ear, no matter at what speed it may be 

 listed must be stopped down in order 

 to obtain results which are satisfac- 

 tory. It should be noticed that stop- 

 ping down the lens is the negative way 

 of correcting sharpness. As you stop 

 down you approach the effect of a pin 

 hole. The pin hole is (using an ex- 

 pression like an "Irish Bull") a very 

 admirable lens, which has unlimited 

 depth, but no speed whatever. 



The optician modifies the correc- 

 tions of the surfaces of the glasses, and 

 the chemist changes the composition 

 and density of the glass until the image 

 which the lens produces is sharp even 

 at full opening. This is one of the 

 points we pay for in the manufacture 

 of the anastigmat. As stated before, 

 stopping down the rapid rectilinear 

 lens corrects defects, but only a nega- 

 tive manner. The badly corrected rays 

 we throw away, but in the anastigmat, 

 every ray of light gathered in by the 

 lens is forced to do some work in the 

 production of the image so that these 

 rays must be wasted at the diaphragm. 



It is, of course, understood that we 

 do not photograph hill boards con- 

 stantly, hut when we are called upon 

 to photograph a group we have a con- 

 dition very much like the bill board. 

 We find for instance in a group of one 

 row that the ends of the line are nut 

 of focus when the centre is sharply in 

 focus. Stopping down will bring a 

 sharp focus through the thickness of 

 the people in the group. If the group 

 is two or three rows or many rows, the 

 conditions are similar with an anastig- 



