Newbie question about 'on axis' issue

[john borrelli]

In the course of rereading Jack Dykinga's excellent book Large Format Nature Photography. I became confused by the term 'on axis' in his discussion of the use of the shift camera movement. For example,when discussing shift he states that "...it's best to leave the shift adjustments set neutral or zeroed. As long as the shift is zeroed, subsequent swing movements will be on axis." I am a landscape photographer who uses small amounts of shift quite often, frequently in combination with small amounts of tilts and on occasion with swings. Is this creating a problem I am not aware of? I then noticed that no less than Ansel Adams similarly seemed concerned about the 'on axis' issue when he wrote, in his book The Camera that "...I used the swing front to secure focus... In addition, I shifted the back to the left to keep the building on the lens axis, thereby avoiding the characteristic distortion of short focal length lenses..." Could someone explain the on axis issue and whether it is safe to use shift again? I Thank you all for this excellent forum! John Newbie

[Emmanuel BIGLER]

John.
I am not sure that I understand your point since there is a question a bout the lens optical axis and the rotation axis for tilts and swings. So this is what I am tempted to say, pardon me if what i suggest is not exactly your problem.
I see two points.
The first point is what happens when various tilt, swing and shift movements are combined.
If you use a lateral shift, and then apply a swing on the same standard, you move the lens or the film back in such a way that you'll loose focus quickly.
If your front standard is set to zero lateral shift, you can apply a certain amount of swing in front if the swing axis crosses near the centre of the lens, you'll not loose the focus too much. Ideally if you tilt or swing the lens arounf the rear nodal point when you take the image of a distant object, you won't move the image on film and the defocusing will me minimal. As soon as you rotate the lens far from the rear nodal point you'll move the image quickly and you'll lose the focus as well since the average distance between the lens and the film will change.

The second point is related to the perspective rendition of a building when the image is projected far from the optical axis. This is another problem and is not directly related to the question of focusing, it is related to how a 3-dimensional object is projected, slightly defocused, on a plane in the corners of the format for a wide-angle lens. With most view camera lenses which are quasi-symmetrical, this projection problem is similar to what happens in a pinhole image without lens and is not related to how movements can be combined to get a sharp image.

[Leonard Evens]

I agree with Emmanuel's interpretations. In particular, with a wide angle lens, if you use a significant rise/fall or shift, you are going to emphasize what are sometimes called wide angle "distortions". These are not actual distortions but result because the print is not viewed at the proper center of perspective. This will happen whether or not you tilt or swing, but it should be something you are doing on purpose with the possiblity of such distortion in mind. It it results from a shift, and you don't intend it, it would be better to recenter the frame.

[Jack Flesher]

I think what JD is trying to say is to zero your camera at the beginning of the process. This way if you are shooting a symmetrical camera (ie; NOT an Ebony SVU2 or a Sinar) then your swing (or center tilt) movement axes will indeed be in the center of your frame. IF you shift before your tilt or swing adjustments, you have in effect moved the the swing or tilt off axis with most cameras. This is not a significant problem in itself -- and may in fact be desired -- but only IF you know exactly where that new axis is located. The axis is the anchor point or line that stays in focus while you swing and then each side of that focuses closer or further away. Regardless, once everything is in proper focus, you can then make your shifts to fine tune composition.

[john borrelli]

Thank you all for your excellent and learned responses. I see these two points more clearly now. I am using a symmetrical camera Jack (an Arca Swiss Discovery and my only lens at this time is a 150 mm Apo Sironar N). I will definitely try to be more observent after using a shift. I think I have been assuming that small amounts of shift did not affect the area of focus. I will also try making shifts after swing and /or tilt, and again be more observent before taking the picture. Thanks again, John