AlanG Posted June 5, 2007 Share #41 Posted June 5, 2007 Advertisement (gone after registration) Here's an illustration. If you imagine parallel railroad tracks shot from above at an angle, maybe it will help. The perspective does not change by switching lenses because the camera position remains the same. But because the angle of view changes, the amount of convergence is of course greater at the edges on the wide shot. (Otherwise our lenses wouldn't work.) Welcome, dear visitor! As registered member you'd see an image here… Simply register for free here – We are always happy to welcome new members! Link to post Share on other sites Simply register for free here – We are always happy to welcome new members! ' data-webShareUrl='https://www.l-camera-forum.com/topic/25759-strange-distortion-using-the-cron-28/?do=findComment&comment=273485'>More sharing options...
Advertisement Posted June 5, 2007 Posted June 5, 2007 Hi AlanG, Take a look here Strange "distortion" using the Cron 28. I'm sure you'll find what you were looking for!
colorflow Posted June 5, 2007 Author Share #42 Posted June 5, 2007 Alan--wider lenses don't produce more keystoning than longer lenses. It is just that the keystoning is more noticeably with wides. Please try to understand what everyone has already said: This is specifically and only an issue of perspective and has nothing to do with distortion. (Yes, all lenses have some distortion but what this thread is about is that you rotated and tilted the camera and continue to say that perspective is related to distortion.) Please do that. And read up on optics or read what others have posted here. And get it thru your head that focal length doesn't cause variances in keystoning, even though keystoning is more evident in wide angle shots. --HC HC: Sorry you are wrong. Alan Link to post Share on other sites More sharing options...
Jack_Flesher Posted June 5, 2007 Share #43 Posted June 5, 2007 Thanks Rob. i posted this not for the subject or composition, just curious about the keystoning which we have beaten to death. As for color, I think you are right but I am a poor judge of that since I can't tell blue from green. Alan Alan/Rob: I'll certainly take your guys words for it! It just looked pretty darn warm compared to here on the West coast where we have lots of clear sky lighting our landscapes PS: Rob, I just finished the last of your Newcastle Brown from December. Really went down nice in the west coast heat (and light) Jack Link to post Share on other sites More sharing options...
ho_co Posted June 5, 2007 Share #44 Posted June 5, 2007 AlanG--Seems to me what you've diagrammed above in http://www.l-camera-forum.com/leica-forum/leica-m8-forum/26051-strange-distortion-using-cron-28-a-3.html#post273920 and what you said in http://www.l-camera-forum.com/leica-forum/leica-m8-forum/26051-strange-distortion-using-cron-28-a-2.html#post273894 indicate exactly what I've said several times: With a wide angle lens you can see more of the keystoning, but the amount of keystoning is determined strictly by camera location and angle. Seems to me you are simply repeating this fact, and if your doing so gets it thru to Alan, the better for it. Heaven knows, we have all said the same thing--Jack, Guy, you, I--but you've got Alan's trust, so go for it! Or if what I've said doesn't mesh with what you're saying, let me know; perhaps there's something here I'm simply not getting. I understand what you mean by "the amount of convergence" being limited by our field of view; but it's not really not the 'amount of convergence' as I see it, but the amount of visible convergence in the field. As Alan said, keystoning is less noticeable with longer lenses because of the reduction in field of view. Seems to me we're simply getting caught up over image size, saying that if you can see more keystoning then the tilt is producing more keystoning. My point is that perspective is perspective, whether you see outside the frame or not. Maybe that's too big a jump? Then I fall on my face. Aren't you repeating in the posts linked above what I said at http://www.l-camera-forum.com/leica-forum/leica-m8-forum/26051-strange-distortion-using-cron-28-a-2.html#post272714? And notice that you had to make only one diagram, not a different one for each focal length. That is because perspective is identical for all lenses at a given tilt from a given distance. And just as you say, by limiting the angle of view you're making the keystoning less noticeable, but it is still the same. Consider a thought-experiment of using a camera on which a 600mm lens is a wide angle. Then you'll see the same wide-angle keystoning as you do in your diagram above, even though the 600mm is no longer a long lens. Same with a 15mm. Perspective with a Minox is the same as with a 35mm film camera, but a 15mm lens is a 'normal' on the Minox but a wide angle with the 35mm camera. As we've all remarked, we're covering the same data again and again. You seem to have Alan on your side. Keep him there! And your explanations are all quite clear. Anyhoo, enjoy! --HC Link to post Share on other sites More sharing options...
AlanG Posted June 5, 2007 Share #45 Posted June 5, 2007 HoCo, Thre is some misunderstanding in what the other is saying by one of you. I'm not sure at this point if I can follow it. In my illustration, the keystoning on the wide shot will be about 34 degrees at the edges, and the keystoning in the tele shot will be about 15 degrees at the edges even though the perspective is the same. I think that is what the other Alan was getting at from the beginning. (I'm not even sure which Alan is being refered to in some of these posts) If that is what you've been saying then fine. But that didn't seem clear to me eiher. Perhaps these are complicated concepts to describe with words alone. Link to post Share on other sites More sharing options...
ho_co Posted June 5, 2007 Share #46 Posted June 5, 2007 Great, AlanG! We agree. That's what I meant by saying that keystoning would be less noticeable with a tele shot, though it was unchanged in reality. Your diagram illustrates that well, and I'm sorry if I confused or complicated the matter. My point was that the physics aren't changed by using a wider lens, though we can see the angles more acutely with a wider lens IF AND ONLY IF the image isn't blown up to fill the same field of view as we had when we took the pictures. In other words, if we make two 8x10's and hold the tele shot at arms' length, then we have to hold the wide angle shot only a few inches from our nose to reproduce the same perspective, just as your diagram shows. Again, sorry to have confused or annoyed. --HC Link to post Share on other sites More sharing options...
colorflow Posted June 5, 2007 Author Share #47 Posted June 5, 2007 Advertisement (gone after registration) Well I have finished some calculations based on elementary optics and geometry. It got pretty hairy in 3D so I can't gaurantee it. The amount of keystoning (how much a vertical line is bend horizontally) is proportional to the product of the tangent(horizontal angle of view) and the tangent(tilt angle) Alan G., please check the above, but I think it is pretty much correct though may not be precisely accurate. I have omitted several other terms involving the height of the objects etc. Obviously the taller the object the more keystoning. So if an object is near the edge of a photo the horizontal angle of view would be close to the angle of the lens, therefore the wider the lens the more keystoning. i.e. the same object placed at the edge of the frame would have more keystoning with a wide angle than a longer lens. HoCo, if you agree with that, then fine. Alan Link to post Share on other sites More sharing options...
colorflow Posted June 5, 2007 Author Share #48 Posted June 5, 2007 Alan G: Did not read your post witht he diagram before my last post. We agree. Alan Link to post Share on other sites More sharing options...
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