Camera Lenses

A Camera lens is a device that bends light, so that different light beams line up, preferably at the focal plane, which is where the film or sensor is located.  When this is true, it is said to be in focus, the earliest cameras had a single lens.  Since the film was so large in size, typically 4"x5", 5"x7", 8"x10", 11"x14" and 16"x20".  These numbers were not magical, they were simply standard window glass sizes, in the days of plate photography.  When sheet film came out, the same sizes were adopted.  However we are getting off track, if your negative is 8"x10" (20cm x 25cm) you can make things larger, by contact printing on 4x5" paper.

Eventually they developed  screw type lens mounts that allowed for changing lenses, and lenses of different sizes were developed.  It was quickly discovered that if you made the focal length shorter, the angle of view got wider, which meant more would fit in an image at a given distance from the camera, and depth of field increased.  News photographers would often use these benefits by fixing short lenses on their cameras.  Unfortunately there are also a disadvantage, straight lines would appear curved, and perspectives would change, an item that was close to the camera would appear much larger then normal, and an item further away would appear much smaller.  Photographers have used both of these traits to advantage. 

By the same token longer lenses would have a narrower angle of view, less would fit in the image at the same distance,  straight lines would not be affected, but the perspective changes would be reversed, flattening the image.  Portrait photographers learned quickly that using a slightly longer than normal lens, was more flattering, with a 35mm camera 85mm - 100mm is about right.   In the 1960's they developed the bayonet mount, which made lenses easier and faster to change.  Unfortunately where with screw mount lenses nearly all camera's used the same 49mm mount, each bayonet could be a little different.  You could no longer use a lens made by one manufacturer, on another makes camera.  Although third party lens makers would quite often make lenses with different mounts on them, and one company Tamron made adaptall lenses which could be used with different makes of cameras, using different adapter rings. 

A4601000

In the film days I used 4 lenses, a 28mm (wide, but not wide enough that the distortion wasn't livable), a 50mm, a 135mm and a 200mm, I have a 1.5x extender for that camera so it's really 28mm, 50mm, 100mm, 135mm, 200mm, 270mm and 400mm.  For digital, I have an 18-55 and a 70-300 which are a good combination, yes I realise that 56-69mm are not covered, but that isn't really an issue.   This camera uses a smaller sensor, giving a longer lens, for the Canon APS-C the 18mm is the same as a 29mm on the film camera, 55mm is the same as 88mm, the 70mm is the same as 112mm on a 35mm and 300 is more like 480mm on the 35mm. 

This weeks image of a white lion was taken at African Lion Safari, last year, from a car with the windows closed, with the 70-300mm lens at 85mm on the Canon 300D.  It's a nice image, of a cat on the prowl, it's funny to see that a house cat uses similar methods when they are hunting. 

I think that is all for now.... 

W.