Why does my Nikon 55-200mm look more zoomed in than my 18-200mm at 200mm?

Asked 2/20/2021

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I tested several lenses on my Nikon D3300, including the AF-S DX 55-200mm VR and AF-S DX 18-200mm VR. When I compared shots at the overlapping focal length of 200mm, the 55-200mm gave a noticeably narrower field of view than the 18-200mm, as if it were more zoomed in.

I expected 200mm to match 200mm, so I’m wondering whether this is normal. Is the marked focal length only approximate, or can different lens designs change the apparent magnification at the same marked focal length?

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

5y ago

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Well, it's more of 200-ish mm being pretty much 200-ish mm at the best of times, unless you're testing two copies of the same lens design. The stated focal length is usually a nominal value, and only a few companies will tell you what the actual focal length of a lens is for general photography. (Hasselblad is notable in that regard; there's a "marketing" focal length on the lens and box that lets you make a rough, good-enough choice, and an actual measured focal length in the fine print you can use for critical applications.) But that's not what's going on here.

The stated focal length only tells you what's going on at infinity focus. How the lens achieves closer-than-infinity focus greatly affects the field of view. Old lens designs used the simplest method of focusing: all of the lens elements remained in fixed positions relative to one another, and the whole kit and kaboodle was moved farther away from the film/sensor plane. That's been true since focusing used either extendible bellows or two nested boxes back in the 19th Century, and was the way things were done until quite recently. When you focus close, the image projected onto the film/sensor gets larger.

There is more than one way to do it, though, and these days you're as likely to find that your lens adjusts the relative positions of various elements within the lens - internal focus. This allows the lens to stay the same physical length when focusing, but it achieves focus on the film/sensor plane by reducing the infinity focal length. As a side benefit, when this is done right, you can maintain exactly the same field of view throughout the lens's entire focal range, eliminating "focus breathing". It does mean, though, that when focusing closely, it will have a field of view similar to a much shorter lens that does things the old-fashioned way. There's no free lunch; if you want things to stay the same size when you focus, you have to give up closeup magnification.

With lower-end zoom lenses - kit lenses and the like - you're probably going to find a combination of the two methods in use at the same time. A strictly internal focus system is more expensive and complicated to build, especially when you add in zoom. You can't tell from the focal length printed on the lens/box or the stated close focus distance exactly what the behaviour is going to be like. One "200mm" lens may act very much like an old-school lens, making what you're focusing on considerably larger as you focus closer, while another may only give you about the same picture as you would have gotten with an old-school 135mm. Without testing (or reading/watching reviews), you have no way of knowing. No, not even if they give you a magnification number, since that depends on the subject distance as well as the focus method.

Originally by user97659. Source · Licensed CC BY-SA 4.0

user97659

5y ago

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Yes, this can be normal.

A lens’s marked focal length is nominal, and it’s most meaningful at infinity focus. At closer focus distances, different lens designs can change their effective focal length as they focus, which changes the field of view. Superzooms like an 18-200mm often use more complex internal focusing and can show more “focus breathing,” so at the same marked 200mm and the same close subject distance they may frame a little wider than a simpler telephoto zoom like the 55-200mm.

So “200mm” does not always mean identical framing in every situation, especially when focused closer than infinity. Small manufacturing tolerances can exist too, but the bigger reason is usually the focusing design and resulting effective focal length at near distances.

If you compare both lenses focused at a very distant subject, the framing should be much closer.

UniqueBot

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5y ago

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