What happens if a lens adapter is too short on a mirrorless camera?
Asked 12/12/2020
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When adapting an SLR lens to a mirrorless camera, the adapter needs to match the correct flange distance. If the adapter is too long, you lose infinity focus and effectively get closer focusing, similar to an extension tube.
What happens if the adapter is too short instead? Does it change the image size or reduce crop factor when mounting a full-frame lens on an APS-C camera? I’m not asking about a focal reducer/speed booster—just a plain mechanical tube or spacer that is shorter than it should be.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
5y ago
2 Answers
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A tube that is too long makes the camera shortsighted. A tube that is too short makes the camera farsighted, meaning that it wastes a good amount of its close focusing range, instead being able to focus behind infinity itself (which is rarely useful). If the tube is considerably too short, the camera will be unable to focus on any real distance including infinity itself. However, if you then add closeup lenses in front, you might bring its focusing ability back into the finite range.
Originally by user95069. Source · Licensed CC BY-SA 4.0
user95069
5y ago
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Generated from our catalog & community — verify before relying on it.
A too-short adapter does not reduce crop factor, enlarge the image circle, or change magnification in the way a focal reducer does. A plain tube has no optics, so it cannot shrink the image projected by the lens.
What it changes is focus range. If the adapter is too long, the setup becomes “shortsighted” and loses infinity focus while gaining closer focus. If the adapter is too short, it becomes “farsighted”: the lens focuses beyond infinity, which wastes part of the normal close-focus range. If it is short by a large enough amount, the lens may not be able to focus on any real subject distance at all, including infinity.
So a shorter mechanical adapter is not a way to make a full-frame lens behave wider on APS-C. To alter effective focal length/image scale, you need optics such as a focal reducer (speed booster), not just a shorter spacer.
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