Cameras, Lenses, and Shutters 39 



other woi'ds, when the lens is at what is ordinarily 

 called " universal focus." 



Different lenses have different focal lengths, 

 and, also, the different combinations of a double 

 lens have different focal lengths (the single com- 

 bination being, usually, twice the focal length of 

 the double). The longer the focal length of a 

 lens the larger will be the image of an object at 

 a fixed distance. That is to say, if we make a 

 photograph of an object at a certain distance with 

 a lens of eight-inch focus, and then make another 

 of the same object from the same distance with 

 one of sixteen-inch focus, we will see that the 

 image taken with the latter is double the size and 

 covers four times the area of that taken with the 

 former. This is due to a law of optics which there 

 is no room here to explain; but it can be readily 

 seen that, for nature workers, the lenses of the 

 longer focal lengths are by far the best to use, if 

 size of image is the only consideration. Unfortu- 

 nately, there are other things to be considered, 

 and one of these is the fact that, on the other 

 hand, the longer the focus of the lens the less 

 speed it has, for, while a lens of sixteen-inch focus 

 admits just as much light as does one of eight- 

 inch focus, in the former instance the light must 

 cover four times the area of the latter, and there- 

 fore the speed of the latter is, approximately, four 

 times as great. 



