Will two similar-quality zoom lenses give the same image at the same marked focal length?
Asked 12/7/2012
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If I shoot at the same marked focal length and with the same camera settings on two similar-quality lenses, should the image be identical? For example, how would a 24-70mm f/2.8 at 70mm compare with a 70-200mm f/2.8 set to 70mm? I'm assuming similar optical quality and exposure settings, and I'm wondering about both theoretical and practical differences.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
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A perfect lens would give the same exact results but there is no such thing as a perfect lens. Even so, this would only be true when focused at infinity. At closer distances, there is shift in focal-length that depends on the lens design. So with two different lenses, one may shift differently than the other.
Practically, you will certainly see a difference, not just between a 24-70 and 70-200 but two 24-70 or two 70-200. There is sample variation in lenses far more than in sensors. I wish they could build lenses with the same precision as sensors but they cannot yet or not at prices anybody but NASA can afford.
The labelled focal-length is also often rounded. Some manufacturers go with even numbers, multiples of five while some have used half-millimeters. So a 24-70mm, could easily be a 23-69mm or 25-72mm when measured, again with the focus at infinity.
Originally by user1620. Source · Licensed CC BY-SA 4.0
user1620
13y ago
0
Generated from our catalog & community — verify before relying on it.
In theory, a perfect lens set to the same true focal length, aperture, focus distance, and camera position would produce the same image. In practice, lenses are not identical, so differences are common.
Key reasons:
- Marked focal lengths are approximate. A lens labeled 70mm may be slightly above or below that.
- Focal length can change with focus distance (“focus breathing”), so two lenses set to 70mm may frame differently, especially at closer distances.
- This is most noticeable away from infinity focus; at infinity, similar lenses should be closer in behavior.
- Sample variation exists even between two copies of the same lens.
So yes, you can see practical differences in framing and rendering between a 24-70 at 70mm and a 70-200 at 70mm, even if both are high quality and use the same settings. Usually the difference is small, but some lenses show stronger focus breathing than others, which can make the effective focal length noticeably different at close range.
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