Quote:
Originally Posted by LaPistola
MM stands for millimeters. I think what your asking requires a far from basic answer and unless you own a full frame camera it gets more complicated.
How it helps you is it will allow you to work out the min distance between lens and distance in front of the lens required before light starts to focus. Or think of it as how far away you need to be before the lens can focus on a subject.
With a cropped sensor camera (which ill assume you have?) you'll also need to factor in the crop factor. For this example ill assume you have a canon camera and either have a 7D or under which means you will have a crop factor of 1.6. This means your focal lengths will need to be times by 1.6 so with a lens with a focal length of 50mm will have a true focal length of 80mm meaning the lens (which is an internal part of the whole lens) will need to be 80mm away from the subject before it can focus.
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Please, LaPistola... the OP has no idea of focal lengths, so to confuse matters by bringing in crop factors and saying that a 50mm lens has a 'true' focal length of 80mm on a certain camera (which I hope you realise is complete rubbish) is not going to be helpful to him. He has no experience to base the 50mm or 80mm on. It only gets more complicated on a non-full-frame camera if someone decides you need to know something else that has no relevance to you.
I'm having trouble following your logic on the 'how far away before it starts to focus' thing, too. It is patently rubbish, anyway. That 50mm lens will 'start to focus' at the same distance regardless of what camera it's on. Different 50mm lenses will 'start to focus' at different distances.
theshortstack... The focal length of a lens, 15mm, 50mm or 500mm for example, show how far behind the lens rays of light from a distant point are brought to focus. This is a technical property of the lens, which determines how much of the scene can be recorded on the sensor. It's tricky to explain why a short focal length gives a wide-angle view, and the angle of view gets narrower as focal length increases. It's because, as focal length increases, the cone of light needed to cover a given sensor size becomes narrower, so it originates from a corresponingly narrower cone in the original scene.
Chris