How do you calculate subject size at minimum focus distance for macro lenses?
Asked 6/18/2017
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I’m comparing Panasonic Micro Four Thirds lenses and want to estimate how large an object can be while still filling the frame at the lens’s minimum focus distance.
I tried using:
size = 2 × tan(angle of view / 2) × distance
But I’m not sure this works for macro distances. Panasonic lists minimum focus distance and angle of view, but I’ve read that close-focus and magnification can change the geometry.
For macro or near-macro shooting, is this formula valid? If not, what should I use to estimate the subject area that fills the frame at minimum focus distance?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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Your geometry is a reasonable approximation for distant scenes. At close scenes, the focal point to image plane distance is significant and must be taken into account.
At 1:1, the focal point 2x the focal length from the image plane and the scene. At higher magnifications, the focal point is closer to the scene than the image plane. You can't ignore this for calculating geometries of macro and near-macro shots.
Originally by user7603. Source · Licensed CC BY-SA 4.0
user7603
9y ago
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Generated from our catalog & community — verify before relying on it.
Your formula is only a rough approximation, and it breaks down for macro and near-macro distances.
Why: the published angle of view is usually based on the lens focused at normal distances, using the sensor size and focal length. At close focus, the effective focal length / lens-to-image geometry changes, so you can’t reliably use the standard angle-of-view figure with minimum focus distance to compute subject size.
Also, quoted angle of view is often diagonal, while “fill the frame” could mean horizontal, vertical, or diagonal coverage, which gives different results.
For macro work, magnification is the better spec. If a lens reaches 1:1 magnification, the subject area filling the frame is approximately the same size as the sensor. On Micro Four Thirds, that’s about 17.3 × 13 mm. At lower magnifications, the covered subject area is proportionally larger.
So: no, your calculation does not reliably mean a 0.076 m object will fill the image. To compare macro capability, use the lens’s maximum magnification (reproduction ratio), and then relate that to the sensor dimensions rather than minimum focus distance alone.
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AI9y ago
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