LOW-POWER WOEK 23 



1. Suppose first we want to magnify an object, say a house-fly, four diameters on 

 our ground glass so as to make a lantern slide from our negative, and we do not want 

 to use a camera length of more than, say, to inches ; what lens is required ? — 



Let F=focal length of lens required; let M = magnification ; let L=camera 

 length. 



L 



The formula here is, F = nr]r~. — ^ 

 ' (M+i) 



. , lO lO . 



This becomes F = , — ; — > = — = 2 inches 

 (4+1) 5 



Therefore we require a 2 -inch lens. 



If we desire to work in millimetres, then : — 



-r, ^ 250 



^ = (MT7) = T =^ 50 mm. 



This means that we must have a lens of 50 mm. focus to give us a magnification 

 of 4 diameters, with a camera extension of 10 inches (250 mm.). 



2. Suppose now we want to know whether our camera will require the additional 

 front to obtain a picture of 10 diameters on the screen, using the 50 mm. lens; as 

 we know the limit of our lantern slide is three inches, so the object it is evident must 

 be small or it will not all go on the plate. Presuming we are satisfied with a portion 

 only showing, then the formula becomes — 



L = (M + i)F = (10+1)50 = 550 mm. or 22 in.; 



our camera will need the front it is evident. 



3. Lastly, we want to know what magnitude in diameters would our object be 

 reproduced on the plate with a given lens and a given camera length. Say, for 

 example, what magnification can we obtain with our 13-inch (325 mm.) camera, and 

 no additional front, using still a 2 -inch (50 mm.) lens. The formula here is — 



(M + 0 = ^ = = 6-5 



as M + I = 6-5 .-. M = 6-5 - I = 5-5 diameters. 



Or in inches — 



(M + i) = ^ and 6-5 - I = 5-5 diameters. 



If one wanted to use any other lens it is obvious that the same formula would apply. 



Hitherto we have made no especial mention of the support for the camera and 

 the slide holder ; we merely mentioned that the camera and object-holder should be 

 firmly supported, and that to obtain the right amount of magnification the camera 



