Re: [Leica] Focusing the M6

["Dr. Joseph Yao" <yaojkfdr@netvigator.com>]

Ken,

I call this 'cosine error' and I usually bracket focus if the condition
permits.  I do the same when shooting with the Noctilux at f/1.0.  Even when
I do not recompose, shooting at such close range with the lens wide open,
will still yield out of focus pictures at times: a slight movement on my
part (thus altering the subject to film plan distance) will render the image
out of focus.  I guess I still have a lot to learn....

Cheers,
Joseph 

on 25/10/00 11:00 am, Ken Lai at ken@compose.com.hk wrote:

> Assuming a 28 mm lens at an aperture of 2, the closest eye is at the edge of
> the frame (worse case) and is at a distance of 0.7 meter from film plane
> when you rotate the camera so the eye is in the center of the frame when you
> focus,
> 
> Now if you rotate the camera back to the original position so the subject's
> eye is now at the edge of the frame, the actual distance to the eye from the
> film plane is now 0.888 meter which means a 0.888-0.7=0.188 meter focus
> discrepancy.
> 
> However the 28mm lens at f2 and 0.7 meter has a DOF of around 0.026 meter in
> front and 0.028 meter behind the point of focus which means in this case the
> DOF does not cover the focus discrepancy and the focus is off by
> 0.888-0.7-0.026=0.162 meter or 16.2 cm.
> 
> So technically, the focus can be off if your subject is not in the center of
> the frame. But how to solve this, I have no idea.
> 
> Ken Lai