Archived posting to the Leica Users Group, 2000/10/24
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]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