Wonky lens or wonky photographer?

[lowep]

I think I have a wonky lens but the problem may be me?

It is a 55mm Pentax 67 Takumar that I have mounted on my Contax 645 with a Zork panorama adapter that I am using with mixed success for MFDB panorama stitching. When it works the results are very satisfying but when it doesn´t that is often it drives me nuts.

First I manual focus and take an exposure with the lens centered more or less on the same axis as the camera body, then I slide the lens all the way to the left and take another exposure, slide the lens all the way to the right to take a third exposure, then adjust and stitch the three overlapping exposures together in Photoshop.

What I am noticing is the first two exposures (center and left) often turn out to be out of focus whereas the final of the three exposures with the lens slid all the way to the right is more often than not the only one of the three exposures that is in focus, even though I never readjust the focus after manual focussing the first (centered) exposure. So far it is never the other way around ie the exposure with the lens slid to the left in focus and slid to the right out of focus. Now why is that?

Do I have a wonky lens?

Maybe I should have dumped it in a dustbin long ago but heck sometimes (rarely) all three exposures turn out very fine.

[allegretto]

First, when you slide lens to L, you must slide camera to R. Then vice versa for the other way. In doing so you keep the same nodal point

Second, focus before each exposure. Not only can focus change, but it WILL change if you change the nodal point relationship. The changes you make in front of the lens are small compared to the changes that occur behind the lens

[Glenn NK]

First, when you slide lens to L, you must slide camera to R. Then vice versa for the other way. In doing so you keep the same nodal point

Second, focus before each exposure. Not only can focus change, but it WILL change if you change the nodal point relationship. The changes you make in front of the lens are small compared to the changes that occur behind the lens

Good points.

The first bit of advice prevents parallax shift (camera should be moved the same amount but in opposite direction that the lens is shifted).

Hopefully this will solve his problems.

Glenn

[lowep]

 

if i use the zork adapter to slide the lens to the left, then slide the camera to the right, am i not back where i started, in which case what is the point

[Glenn NK]

if i use the zork adapter to slide the lens to the left, then slide the camera to the right, am i not back where i started, in which case what is the point

No, the technique is absolutely correct and it works.

An exercise to try with the camera/lens on a tripod:

a)  set up camera and lens (in neutral shift) and place two objects that are in perfect alignment with the optic axis (line of sight).

b)  shift lens to the left (on the camera body).  Note that this is the same as moving your line of eyesight 12 mm.  The two objects that were in alignment are now of of alignment.

c)  shift the camera body to the right by the same amount - notice that the camera's lens is now back in the original position - the two objects will now be back in alignment in the viewfinder/LCD.

All you have lost in image coverage is 12 mm + 12 mm of the entire width of the scene (for full shift each way).  For a 24 mm lens, this loss on the edges is completely insignificant.

The beauty of this approach is that the problem of parallax shift error is completely eliminated.

Glenn