Huck Finn said:
Hmmm . . . I thought that it was everything in the frame lines that are parallax corrected. At least that's what I see when I focus; the whole frame lines move. I assumed that everything outside the frame lines are not parallax corrected.
Oh my, I hope we're not increasing the confusion; this is a visual subject, yet we don't have visual aids that might show what's happening more clearly. In a quick Google search I don't find a nice graphic explanation for our situation.
Parallax refers to the difference between two views of a scene due to a difference in viewpoint. The camera rangefinder itself uses parallax to triangulate the distance to the desired focus point. That's not a problem.
🙂 What can be a problem is the couple of inches between the lens location and the VF location causing unexpected results on film.
Due to that difference in position, the lens and VF inevitably see the world slightly differently, and the difference is proportional to the distance to the subject. At long distances, that two inches or so is miniscule in comparison, so parallax error is so small it can be ignored. The closer you get to your focus point, the more significant it becomes. With an accessory viewfinder on top of the camera, the distance between it and the lens is greater than the camera's VF... more parallax error. The same problem exists for TLRs too, as the viewing lens is in a different location than the taking lens. View cameras and SLRs and digital LCDs are immune because the viewfinder "sees" through the taking lens, same as the film/sensor does.
Imagine you're lining up to shoot a pic of a flag hung on a wall 6m away, and at that distance your lens will just capture the whole flag but nothing else. Focused to infinity, the VF framelines are looking parallel to the lens but from a position 5cm up and left of the lens. If you frame the flag exactly in the VF, the film will record the flag missing its upper left 5cm, and show 5cm extra at the lower right. Parallax error!
As you focus the camera to the correct distance of 6m, the VF framelines shift down and to the right to show the area that the film sees. The view is still from a point 5cm away from the lens, but the framelines are just guiding your eye to look a little down and to the right so as to see the framing of the lens at that distance. Now both the VF and the film are looking at the whole flag... from a very slightly different angle. So there's still parallax, but one of the effects has been compensated for.
Huck Finn said:
Roman, I think I understand. I assume that if part of the mountains are still in the area of the frame, they will be corrected.
We've been talking about a single flat subject ... adding in other objects at varying distances throws a monkey wrench into the parallax compensation. Remember the correction is only correct for that one object at the focus distance. The lens and VF are still seeing the scene from very slightly different viewpoints.
You can get a feel for this by holding your index finger pointing upwards at arm's length in front of your nose. Close your left eye and line-up your fingertip with some object across the room. Now, keeping your finger in the same place, close your right eye and open the left. Your fingertip isn't lined up any more, and that's parallax. Things at different distances need different compensation for parallax, more for closer objects. If you have objects at several distances, you cannot compensate for all at one time.
If you have a person sitting 2m away, say, in front of the flag, it's not possible to compensate for parallax for both person and flag at the same time, and the the way the person lines up in front of the flag will always be a bit different in the view of the VF and the slightly differing view of the lens. If you move the camera a bit this way and that so the viewfinder shows one stripe of the flag exactly centered on the person's head, the fact the lens is 5cm lower and to the right means the picture will show the stripe slightly displaced from the alignment you saw in the VF. If you are focused on the flag, the framing will be correct for the flag; if focused on the person, then the framing will be correct for the person, and slightly off for the flag.
Huck Finn said:
More importantly, if I use a 28 mm lens with 50 mm frame lines, can I use the 50 mm frame lines to parallax correct the area in the middle of the picture & the rest of the viewfinder to get a general idea of the picture for composition? (I'm assuming that the full coverage of the viewfinder is about 28 mm as in the R2.)
Right, you can! If I understand you right... You can imagine some wider area centered around the 50mm frames, shifting in the VF as the framelines move for focusing, but I'd say this imagined larger area is going to be rather approximate, but it could help you avoid lopping off feet or heads... at the distance focused upon. (Not that for this purpose the focusing itself is important, but that the framelines move to compensate for parallax error at that particular distance.) This is what I end up doing with the 25mm Snapshot Skopar on the CLE, but I just try to imagine a modest additional area staying a set amount around, and following, the 28mm framelines.
