close focusing with long takumars

Hi steve,

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Why is the minimum focusing distance for the 400mm only 8 meters?

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in the 35 mm world minimum distance of 10 times the focuspoint is common so a 50mm focusses down to 50cm.

for the same magnification on 6*7 we actually need ratio of 5!

so the 400 mm needs to focus down to 2 meters (6foot)

Is the reason mechanical or are there optical limitations.

Kind regards Gert Jan

This optical design is suited for infinity work. Spherical aberration

becomes a big problem in close up situations but stopping down will

solve that. So, there really is no optical reason why the 400 Takumar

has such a far close focus. I believe the reason is mechanical. This

lens is at about the limit of helical focusing, with my lens binding

slightly as I move the focus to closeup work. This binding is caused

by the overhang from the tripod. If the 400 were racked out further,

there would be even more binding. The 600mm uses a gear system to

move the rear assembly for focusing. The 400 could have been made to

close focus by either, changing to geared focusing or going to

internal focus.