Why can a 90mm macro lens focus closer than 4× its focal length?
Asked 6/2/2020
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I’m looking at thin-lens magnification formulas that imply the focus distance d must be at least 4× the focal length f. But published specs seem to contradict that. For example, the Sony FE 90mm f/2.8 Macro has a minimum focus distance of 280mm, which is less than 4×90mm.
What explains this? Does it mean the lens is no longer effectively 90mm at close focus because of focus breathing or internal focusing?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
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The site you reference does not state that the effect of focus breathing is small. It states (emphasis mine),
The nominal focal length applies for the lens focused at infinity, but may be shorter for small distances.
For that Sony lens, indeed, it is experiencing focus breathing. Most internal focus lenses do — focus breathing is usually a side effect of that lens geometry.
Actually, 70 mm vs 90 mm is not all that much. For instance, the Nikon 70-200mm ƒ/2.8G VRII lens, when zoomed fully to 200 mm and focused down to its minimum focus distance, has a field of view of a 120 mm lens, a 40% difference.
Originally by user11924. Source · Licensed CC BY-SA 4.0
user11924
6y ago
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Generated from our catalog & community — verify before relying on it.
The 4f limit comes from a simplified thin-lens model. It works for an ideal single lens, or for lenses that behave roughly like one as they focus. Real photographic lenses are usually compound designs, and many modern lenses use internal focusing, where different element groups move by different amounts.
Because of that, the lens’s effective focal length at close focus can change from its nominal focal length. The stated 90mm is typically the focal length at infinity focus; at shorter distances it may become shorter. That is the main reason a lens can have a minimum focus distance less than 4× its marked focal length.
Yes, in practical terms, strong focus breathing means the lens may behave more like a shorter focal length at close focus. So a 90mm lens focusing very close can have a field of view closer to, say, a 70mm lens than a true 90mm lens at infinity. That is not a contradiction—it just means the thin-lens equations no longer apply directly to the full lens design.
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