T3D Re: Medium format macro needs 2 more stops?

[Tom Deering <tmd@xxxxxxxxxxx>]

On 10/6/98, Lme Kbee wrote:
>I dont' understand the thing John said about medium format needing 2
>more stops for macro.
>
>I understand that the film is larger, but isn't it still just as
>sensitive to light?

In a medium format camera, the film is farther from the lens than it would
be in a 35mm camera.  This increased distance is the key.

Imagine reading a magazine beneath a street lamp.  It may not be very
bright, because the light is so far away.  If you climbed up a ladder, the
light falling on the magazine would be brighter the higher you climbed.

The brightness of the light is related to the distance, squared.

Since the 80mm lens on a MF camera is about _2_ times as far from the film
as a 35mm lens, the light reaching the 35mm film is 4 times as bright (2
squared=4).

Or the other way around, the light reaching the MF film is 1/4 as bright
since it is twice as far away.  So the MF film would need 4 times as much
light, all else equal.

Does this make sense?  I pray I got this right.  :^)

Cheers,

Tom



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