Lately I've been getting interested in the principles behind photography (think physics). One key part of this is focal length and the idea that focal length is the distance from the front of the lens to the image sensor (for digital cameras) or film (for film cameras). The focal length is also the point at which light passing through the lens becomes focussed. This should mean that lenses of a given focal length should only be able to focus on objects at a particular distance. To put it another way lenses of a given focal length should not be able to have variable focus.
Yet we know this isn't true. All of our lenses have a focus ring and they all allow us to control where focus is without changing the focal length...
Or do they?
I decided to test this out by slapping my 50mm prime lens onto my camera then setting my camera up on a tripod in my lounge. I then took two pictures, 1 with the focus ring set to the furthest distance possible (infinity), the second with it set to the closest distance possible (in this case 45cm). The results were surprsing and gratifying at the same time.
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| Photo taken with focus set to infinity |
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| Photo taken with focus set to 45cm |
This is surprising because:
We expect fixed focal length lenses to maintain a constant focal length. Clearly this is not the case
This is not surprising/is gratifying because:
It fits with the basic physical principles of how light can be focussed on a surface
It fits with what you SEE happening when you adjust focus on a lens (the lens moves away from the sensor when focussing on close objects (increased focal length and a narrower angle of view) and towards the sensor when focussing on distant objects (decreased focal length and a wider angle of view)).
Well explained. Newbies will find it very useful
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