Is there any relation between the Focal length and the distance between camera and subject?

Hello,
I'm very new to photography. I'm confused in choosing fixed focal length lens.
I want to know if there is any relation between the Focal length and the distance between camera and subject?
Or I shall ask in the other way.
How would I know at what distant I can keep the subject(say a dog) when I use 30mm f/1.4 prime lens? What would be the maximum and minimum distant where I can keep my object for the above lens?
Some one please help.

Thanks
Raj

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Permalink Reply by Gary Wolff on September 19, 2008 at 7:54am

Hi Raj
I think you may be thinking about this in the wrong way. The focal length of a lens has no relevance to the distance you are away from your subject.

Your 30mm prime lens will always be a 30mm prime lens. Its focal length is fixed. The closer/further away you are from your subject will only decide how much fuller in the frame it is. The close you are the bigger the subject will appear, the further away the small the subject will appear.

If you have a zoom lens you can stand in the same place and make your subject appear closer or further away by zooming in or out. As you are new you may find a wide angle zoom of more use to you than a prime lens.

I hope this helps. I have tried to keep it as simple as I can.
Gary

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Permalink Reply by Mariraj on September 21, 2008 at 10:24pm

Hello Gary,
Might be. I was thinking about Depth of Field. I read that a shorter focal length lens has greater Depth of Field and vice versa.
This might be the one I asked in different way.

Thanks
Raj

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Permalink Reply by Mulgabill on September 25, 2008 at 5:14am

I believe you may have mixed your lens terms. There is no direct relation between focal length of a lens and distance from subject. With prime (non-zoom) lens you will need to move camera and/or yourself to and from the subject to fill the frame. A zoom lens enables you to fill the frame without moving (within limitations). There is a definite relation between the focal length of a lens and its aperture and this is (generally) related to the Law of Lux (light) Distribution.