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Author Topic: Angle of View at less than infinity  (Read 2324 times)

jimhuber

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Angle of View at less than infinity
« on: May 01, 2006, 07:35:07 pm »

It was pointed out to me recently that the angle of view of two different lenses of the same focal length can be different at less than infinity focus. Specifically, my 135 f/2 L prime is much "closer" or "tighter" at about 2.5 m than my 28-135 at full telephoto zoom, also 135mm, and on the same camera, a 5D. Yet at infinity focus they're almost exactly the same angle of view. Why? And is there a way to predict this phenomenon from lens specifications?

Obviously I either missed something along the way or have forgotten it since my last photography class those many years ago (let's just say more than twenty). If it's a long answer, perhaps an addition to the excellent "Understanding Series"?
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Peter Jon White

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Angle of View at less than infinity
« Reply #1 on: May 01, 2006, 08:32:48 pm »

Quote
It was pointed out to me recently that the angle of view of two different lenses of the same focal length can be different at less than infinity focus.

Well, the actual focal length of a "normal" lens always changes with focus distance. We rate a lens' focal length at infinity. So, a "50mm" lens is only 50mm at infinity focus. At closer distances, the focal length is longer. As the lens is moved away from the film to focus closer, the image magnification increases as a result. It's not just the focus that is changing.

Here's another way of looking at it. If you have a 50mm lens, and it's focused at infinity, the size of an image on the film will be the same as the size of an image made with a pinhole camera with a 50mm focal length, in other words, the actual pinhole is 50mm from the film. As you take your 50mm lens and move it forwards to focus closer than infinity, the image size increases, and it increases at the same rate as it would if you had a pinhole camera with an adjustable pinhole and you moved the pinhole forward by the same amount. With the pinhole camera (camera obscura, remember those?) the image size is determined by the distance between the pinhole and the plane upon which the image is projected. Increase the distance (focal length) and you increase the image size.

Now, as to why two lenses would have the same magnification at infinity focus but different magnification at a closer focus, it can only be because one lens (or both) is playing tricks with focal length. I've heard that lenses with internal "floating" elements for focusing don't change focal length as they focus. I don't know much about modern lens design so perhaps someone else could clarify this. But if it's true that these "floating element" lenses don't change focal length (or perhaps change at a different rate), that could explain why your two lenses behave differently at different distances.

I tested two of my lenses. I don't have the 28-135mm zoom. But I do have a 70-200mm f/2.8L IS zoom and the 200mm f/2.8L lens. At infinity they are identical and at 2 meters they are also identical, all with the zoom at 200mm of course. But I don't doubt what you say about your two lenses as it would seem that at least one of them has this "floating element" system for focusing.
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jimhuber

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Angle of View at less than infinity
« Reply #2 on: May 01, 2006, 09:25:10 pm »

Yes, as I focus the 28-135 I can see a significant shift in magnification, so that's apparently related. I'm really curious to know if this effect and it's magnitude can be predicted from lens specifications.

In other posts I was more specific: at about 2.5 meters my 28-135 has just barely tighter angle of view than my 100mm f/2.8 macro and nowhere near as tight as my 135 f/2 L. It's that dramatic a shift. My guess would be the zoom is 105mm or less equivalent AoV. I would say it's obvious, but then I didn't notice it until earlier today when I was repeating some testing.
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