Picture angle versus focal length versus distance to subject

[ATJ]

I'm hoping someone can explain an anomaly to me with regards to picture angle and focal length. I would expect that a using a 70mm lens would cover a smaller area of the subject than a 60mm lens would when taken from the same distance from the subject.

While doing some experiments with 3 of my lenses, I find while this holds at a fair distance from the subject, the closer I get, the more difference I see. I'm guessing it has something to do with the optics and the distance from the front of the lens rather than the "film" plane but I don't really understand it.

The three lenses:

Nikkor AF Micro 60mm f/2.8D
Nikkor AF 35-70mm f/3.3-4.5
Nikkor AF-S DX 18-70mm f/3.5-4.5G

Both zoom lenses were set at 70mm for the test.

At ~4m from the subject, all three lenses gave me images I would expect.

60mm


35-70mm @ 70mm


18-70mm @ 70mm



At ~2m from the subject, the picture angle of the 18-70mm at 70mm is wider than the 60mm. The 35-70mm at 70mm is still the most narrow.

60mm


35-70mm @ 70mm


18-70mm @ 70mm



At ~1m from the subject, the picture angle of the 60mm is about the same as the 35-70mm at 70mm. The 18-70mm at 70mm is still the widest.

60mm


35-70mm @ 70mm


18-70mm @ 70mm


If I go closer again, the 60mm has the most narrow picture angle.

[pippin88]

Focal lengths are often not perfectly accurate, especially in zooms.

[shakey]

For lens specifications focal length relates to focus at infinity. Shorter focus points may have different focal lengths. Here's the long explanation

http://www.bobatkins.com/photography/te ... ength.html

HTH

[ATJ]

I am sorry I wasn't clearer with my question. I'm less concerned about the actual focal length and even what happens at infinity (while I realise that by definition focal length is determined at infinity). What I am more concerned about is the effect of focusing for close subjects on the picture angle. I was surprised to see such a difference between my three lenses when focusing on closer and closer subjects, especially that the 60mm lens had a much narrower picture angle than both zooms at 70mm.

I do understand that to focus on a close subject the "lens" effectively moves further from the focal place which I would have thought would have increased the focal length and decreased the picture angle. I just don't understand why this is not consistent between lenses - although now that I type this maybe it does make sense that different lenses would behave differently.

This is all part of my quest to find a more versatile lens for underwater use:
http://www.dslrusers.com/viewtopic.php?t=24679
http://www.dslrusers.com/viewtopic.php?t=24388
http://www.dslrusers.com/viewtopic.php?t=20331

The picture angle becomes very important underwater as it determines what subjects I can get good images of and which I can't. When I have taken a zoom lens, I end up seeing something small and can't get good images. e.g.
This photo was taken with the 18-70mm but could have been much better if I'd had the 60mm lens.



And when I take the 60mm lens, I end up seeing subjects which are too big for the picture angle. e.g.

[DaveB]

I am sorry I wasn't clearer with my question. I'm less concerned about the actual focal length and even what happens at infinity (while I realise that by definition focal length is determined at infinity). What I am more concerned about is the effect of focusing for close subjects on the picture angle. I was surprised to see such a difference between my three lenses when focusing on closer and closer subjects, especially that the 60mm lens had a much narrower picture angle than both zooms at 70mm.

What people have been saying is that as the focus distance decreases, the actual focal lengths of your lenses change and thus the "picture angle" (more-commonly referred to as the "field of view") changes. So you are concerned about the the actual focal length, at least indirectly.

While your 60mm prime is probably close to an actual 60mm throughout, it seems that your "18-70" is nowhere near 70 at close distances. The 35-70 has a similar effect going on, but not as strong.
This is dependent on the optical design of the lens, and the only way you'll know what the behaviour of a given lens design is will be to measure it (if you measure the distance and the field of view you can calculate the effective focal length). By the way, this does affect things like DOF calculations.

[ATJ]

OK, so bottom line I'm not going to be able to find the lens I want just by looking at the specifications for the lenses and even most lens reviews aren't going to tell me what I want to know. Reproduction ratio (if published) is probably going to give me the best idea of suitability of the lens for the macro work and I guess I can hope that field of view is wider than suggested by the listed focal length.

[DaveB]

 BTW, I just remembered that this is a "feature" of rear-focus lens designs, and that the Nikon 105mm macro isn't anywhere near 105mm at macro distances. It's not just zooms...