Focusing a canonet

[Michael I.]

recently recieved a canonet 17 gIII QL and as a SLR(and TLR) user have problems to compose photoes where the focused point is not in the center.
When the lens is stopped down there is no problem(focus on the object with the rf patch and then compose)but when the lens is wide open this metod won't work.

Any advice?

 

[peter_n]

Michael surely if a lens can keep a fairly flat plane of focus in its viewing area, you should be able to focus in one area on a subject, then pan the camera, and whatever was in that plane of focus should remain in focus unless it moves, right?

 

[dmr]

I'll often do this in 3 steps. First focus on the subject, or what I want to be in focus. Second, meter on what I want to expose for, holding the shutter half down to lock it. Third, compose and shoot. 🙂

 

[Pherdinand]

Michael surely if a lens can keep a fairly flat plane of focus in its viewing area, you should be able to focus in one area on a subject, then pan the camera, and whatever was in that plane of focus should remain in focus unless it moves, right?

It's not really like that, if i'm not mistaken... You can see it in the following way: Plane of object focus and image focus (aka film) are two parallel planes. If you focus on an object and pan the camera, just as you rotate the plane of film, you also rotate the plane of object focus. That means, the object on which you've focused will be in front of the object focal plane. Easiest to "model" it is with a box - pin down the centre of the face close to you and rotate it. The centre of the opposite face, the one that represents the plane of object in focus, rotates keeping the plane-to-plane distance constant (that is, the distance on which the focus is set) but the plane will move BEHIND (further) the original object, therefore the focus will be behind the object.

Of course, for very small rotations this is not too important - but with a repositioning of the objec from the centre to the 1/3 of the image, it CAN become blurry. Remember, an 50mm lens at f/2 has a DOF of less than an inch. EDIT: at close focus, say, 80-90 cm, of course🙂

Sliding the camera parallel with the film plane, to recompose, will however keep the object in focus, it's just much harder to do it on a large enough distance than panning the camera.

 

[Pherdinand]

I made a small drawing to make my point clear. All the distances are kept constant - only the "camera" (two parallel lines and the double arrow "lens"
between them) is panned w 25 degrees.

 

[peter_n]

I sort of followed what you were saying Pherdi but the drawing makes it clearer. 🙂 I was actually trying to model it drawing a radius with my finger on the edge of the desk last night 🙄 and figured out that you might have to step back...

I think moving sideways may be more reliable.