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It is commonly known that macro lens are capable of focusing very close to the subject, but recently some other lens can also focus comparably close to some macro lenses, but with much lower magnification.

As a concrete example, the Tamron 28-200mm can achieve a min. focusing distance of 19cm at 28mm, but the maximum magnification is only 0.32x. Where the Laowa 25mm macro has a min. focusing distance of 17cm, but can achieve 5x magnification.

So what am I missing here? If the subject is at the same distance, and the focal length is the same, is it reasonable that one lens will produce a much larger image than the other? This seem contradictory to that focal length dictates the angle of view, so how can it be achieved? I know in my example the distance and focal length are not exactly the same, but they are pretty close, yet the difference in magnification is significant.

The question is not answered here because in my case the focal length of two lenses are much more close, and they are both full-frame lenses. It is not answered here either because the first line of the answer says:

I suppose it might be theoretically possible for a zoom lens design to have a shorter MFD at a shorter focal length, but I've never came across such a lens.

Yet here I am providing one such concrete example very close to matching his criterion.

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2 Answers 2

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The focal length in the specs it the focal length when focused on infinite. When you focus on close objects, you are increasing the distance between the lens and the sensor and in effect increasing your focal length (look at the pictures, a usual 25mm lens is much shorter than this).

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  • \$\begingroup\$ As you know, the focal length of a lens is a measurement taken when the lens is imaging an object at infinity (3000 times the focal length or more). At infinity, light from an object arrive as parallel rays. The lens alters the direction of travel causing these parallel rays to converge forming an image downstream from the lens. Because the lens has limited ability to refract (Latan bend back), objects closer than infinity converge further downstream. I believe referring to this elongated projection distance as back-focus distance is less confusing. \$\endgroup\$
    – Alan Marcus
    Aug 27, 2021 at 15:44
  • \$\begingroup\$ That's interesting. After seeing the length of the lens I guess you are right. I've heard of focus breathing but never realized that the focal length can change so significantly during focusing for macro lenses. \$\endgroup\$
    – Roy
    Aug 27, 2021 at 18:30
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    \$\begingroup\$ At magnification 1 (life-size called unity), the object to image place is focal length times 4. The object distance is two focal lengths forward, image plane is two focal lengths behind. Measuring points are front nodal to object (object distance). Back - focus is rear nodal to image chip or film, this measures two focal lengths. Position of the two nodal points are not easy to find, they can even be flip-floped. \$\endgroup\$
    – Alan Marcus
    Aug 27, 2021 at 21:31
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This lens has an ultra close-up capability. At 5X magnification the object to front element distance is 40mm not the 17cm (170mm) you quoted. Check out the specifications.

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  • \$\begingroup\$ Thanks. I find this confusing as well when I was reading the specifications. Why is there a discrepancy between the min. focusing distance and the min. working distance? \$\endgroup\$
    – Roy
    Aug 27, 2021 at 18:17
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    \$\begingroup\$ @Roy Focus distance is from sensor plane to subject. There's a mark on the top of many cameras that shows where the sensor plane is located (zero with a bar through it). Working distance is from the front lens element to subject which is the more interesting specification for me when doing macro work. \$\endgroup\$
    – qrk
    Aug 27, 2021 at 18:20

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