Why does zooming from 170mm to 200mm on an 18-200mm lens change framing so little?
Asked 1/20/2012
2 views
2 answers
0
On a Canon 18-200mm lens, the framing difference between about 170mm and 200mm seems very small compared with changes at shorter focal lengths. Why is that? If the lens is rated to 200mm, should there be a clearly visible change at the long end, or can close-focus use make it appear shorter than 200mm?
Originally by user7264. Source · Licensed CC BY-SA 4.0
user7264
14y ago
2 Answers
4
Are you really seeing no difference, or is it just a slight difference? Take a look at this chart from this tangentally-related answer (thanks to Nick Bedford):
It's dramatic at the beginning, but flattens out at higher focal lengths.
The basic relationship between dimensional field of view and focal length is linear. That means 30mm is a huge difference between, say, 15mm and 45mm, because the linear view is cut to a third. But when you go from 170 to 200, that's just a change of ¹⁷⁰⁄₂₀₀, or 85%. That means 7.5% off each edge, and that's not very much at all.
Originally by user1943. Source · Licensed CC BY-SA 4.0
user1943
14y ago
0
Generated from our catalog & community — verify before relying on it.
Yes, this is normal in most cases. Changes in framing become much less dramatic at longer focal lengths. Going from 170mm to 200mm is only a modest proportional change: 170/200 = 85%, so the field of view only tightens a little compared with large-looking changes at the wide end.
That’s why a 30mm difference is very obvious from 15mm to 45mm, but much less obvious from 170mm to 200mm.
Also, lens focal length is specified at infinity focus. If you tested the lens at a closer subject distance, the effective focal length at the long end may be different, making the 200mm end appear even less distinct. Indoors or at short focus distances, this effect can be more noticeable.
So: at infinity, there should still be a small but visible difference between 170mm and 200mm. If there is truly no change at all, the lens may be faulty, but a subtle change is expected.
Recommended products
UniqueBot
AI14y ago
Your Answer
Related Questions
Why isn’t the Canon MP-E 65mm macro considered a zoom lens, and what does the 65mm mean?
Nikon 18-200mm won't focus at the telephoto end — is there anything I can try?
Why do some zoom lenses have a constant maximum aperture while others are variable-aperture?
How do 200mm, 300mm, and 500mm focal lengths compare?
Does a mount adapter change a lens’s focal length or field of view?