How do close-up filters work, and what do +1, +2, +3 diopters mean?
Asked 1/13/2017
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2 answers
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I’m trying to understand close-up filters for macro-style shooting. I’ve seen explanations like “+1 = 1000 mm, +2 = 500 mm, +3 = 333 mm” and I’m confused about what those distances actually mean.
If I put a close-up filter on the front of a lens, does that mean the lens can now focus at about that distance when the camera is set to infinity? For example:
- +1 ≈ 1000 mm
- +2 ≈ 500 mm
- +3 ≈ 333 mm
- +4 ≈ 250 mm
- +5 ≈ 200 mm
How does this relate to the lens’s normal minimum focus distance? Does a stronger close-up filter simply shift the focusing range closer, so the lens can focus on nearby subjects but no longer at infinity?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
2
The numbers, +1, +2, +3, etc., are diopters—units of refractive power equal to the reciprocal of the focal length in metres for any given lens.
In simple terms:
When placed in front of a lens, the effective focal length is divided by the amount of diopter power.
Practically speaking…
Parallel rays of light (from an object at infinity, say) focus through a
+1 lens at one metre (1000 mm) from the lens
+2 lens at one-half a metre (500 mm) from the lens
+3 lens at one-third a metre (333 mm) from the lens
… etc.
The take-away here is that your results will be similar to this by these ratios. You probably won't be exactly a metre away from the object; but, if you're a couple of metres away you can expect proportionally similar image magnification.
Important NOTE: The use of a meniscus lens/supplementary lens/close-up filter is not an optimal optical solution and there will be chromatic aberration and linear distortion in your images; but, they will appear predictably bigger. Use of extension tubes will give an improved image over that of supplementary lens although less compact and requiring increased exposure times.
Originally by user21789. Source · Licensed CC BY-SA 4.0
user21789
9y ago
0
Generated from our catalog & community — verify before relying on it.
Yes—those +1, +2, +3 numbers are diopters, a measure of optical power. For a close-up filter, the rough distance in focus when your lens is set to infinity is the reciprocal of the diopter value in metres:
+1 → 1 m (1000 mm) +2 → 0.5 m (500 mm) +3 → 0.33 m (333 mm) +4 → 0.25 m (250 mm) +5 → 0.2 m (200 mm)
So your understanding is basically right: adding a stronger close-up filter shifts the usable focus range closer to the camera. With the lens set to infinity, focus will be near that distance instead of at true infinity.
Then, by turning the lens’s own focus ring away from infinity, you can usually focus a bit closer still. The exact near/far limits depend on the lens’s normal focusing range, so the quoted distance is an approximation, not a guaranteed exact minimum focus distance.
In short: close-up filters let a lens focus closer than it normally can, which increases magnification, but they also prevent normal infinity focus while attached.
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