Increase maximum focus distance?

Started Sep 21, 2012 | Discussions thread
Dave Martin Senior Member • Posts: 2,160
Re: Increase maximum focus distance? -negative lens

Dave Martin wrote:

PS I just noticed I misread op's problem specification so will recalculate & repost in a bit.

OP specified a 50mm lens fixed so it had a focus distance of about a foot.

A 50mm lens, 250mm from the sensor has a magnification of 4, and a 62.5mm focus distance (image plane to object plane). This is a total of 312.5mm, a little more than a foot.

Since the lens can't be moved and we want to focus further than 62.5mm from the lens a negative lens must be added to increase the focal length. To focus at infinity, the effective focal length must be increased to 250mm. The formula to stack lenses separated by a distance d is:

f=f'f"/(f'+f"-d) or 250 = 50*f"/(50+f"-d); solving for f" yields; f"~ -(62.5-1.25d)

The net effective focal length becomes f ~ 250-5d

This is a 250mm IF lens; when d=0, it is focused at infinity and its magnification is zero - when d is extended to 10mm, the focal length is reduced to 200mm, the magnification is increased to 0.25, and the total focus distance is 1.25meters.
(in theory that is - in real life, not so easy).


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