THE MICROTELESCOPE 



" depth of focus." Let us explain exactly what the 

 phrase means. Once or twice in our pages, we have 

 recommended that the fine adjustment should be 

 rotated to and fro while certain objects are being 

 examined. When potato starch grains, for instance, 

 are magnified sufficiently highly to show their char- 

 acteristic markings, the whole of the grain cannot 

 be seen clearly at one time, because at that magnifi- 

 cation the " depth of focus " is slight. The higher 

 the magnification the less is the " depth of focus "; 

 when this quality is absent altogether only one 

 plane of an object can be viewed clearly without 

 re-adjusting the focus. With low magnifications, 

 we may, to a limited extent, have more than one 

 plane of an object in focus. 



The same question of " depth of focus " occurs 

 in photography and perhaps an example showing 

 how it affects the camera user may make the matter 

 clearer. Suppose we wish to photograph a land- 

 scape having, let us say, a tree in the foreground, 

 a cottage in the mid-distance and a hill in the dis- 

 tance. If our lens is one of large aperture, that is 

 to say admits a considerable amount of light and 

 is also what is known as a long focus lens we shall 

 find, when we view the scene on the ground glass, 

 that when the tree is sharply focussed, the cottage 

 and hill are not clear. When we rack in the camera 

 to get the cottage sharply focussed, the tree and 

 hill will be un-sharp. Similarly when we focus on 

 the hill the tree and cottage remain out of focus. 

 The reason is that the lens in our case possesses 



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