Originally posted by skamalpreet Hello folks...I have been researching on which Macro lens to go in for....but I need to determine
1) what 'minimum focusing distance' constitutes a macro lens?
2) I have an MC Auto Focal (do not go by the name - it is a manual focus lens) 28 mm with minimum focusing distance of .20m. Does this qualify as a macro lens by present days standards
3) my pentax 18-250 has a minimum focusing distance of .45 m but at 250mm I seem to be able to focus as much lesser distance than .45m. How is this possible?
As Sean said, "Minimum focus distance" is measured from the image plane,not the lens. The distance from the lens is called "working distance".
Minimum_Focus_Distance=Focal_length*((1+m)^2)/m where m=magnification.
Working_Distance ~ Focal_length*(1+1/m)
The ~ sign means "approximately equal to"; it is approximate because it is true only for an ideal "thin lens"and does not account for any hoods, lens recesses, etc..
To answer your question about the 28mm lens, consider that to qualify as a Macro lens, the magnification should be 1/2 or more. Substitute 1/2 into the Focus distance equation,
28mm((1+.5)^2/(.5)) = 126mm, so it is not a macro lens.
Dave