When the object is 50 meters away, how do you know that the camera is focused at a distance of 50 meters? Is it that we can only focus at about 50 meters?

  • @MikeSowsun I modified it. Isn't fixed focal length the same as depth of field? – enbin Apr 16 '20 at 5:40
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    Fixed fixed focal length is not the same thing as depth of field, in the same way that an eggplant is not the same thing as an elephant. – mattdm Apr 16 '20 at 6:31
  • If you take a picture, you can find the focus distance information in the EXIF data (if the camera/lens combination allows it). – xenoid Apr 16 '20 at 11:58
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    Because there are camera aids that tell you when things are in focus without ever telling you the distance: rangefinder, stigmometer, microprisms, and just a ground glass (not even mentioning the camera AF). In general photo usage, you focus first and this determines the distance, in movies, they usually measure the distances before shooting the scene to determines focus. – xenoid Apr 16 '20 at 13:38
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    @enbinzheng With an AF yes,the camera will focus automatically, with a good camera the focus will be "sufficient" (which doesn't mean "pinpoint" but close enough). The distance reported in the EXIF is evaluated from the focus position of the lens and other lens characteristics but is not meant to be an exact measure, it is an indication to the photographer. – xenoid Apr 17 '20 at 6:28

When the object is 50 meters away, how do you know that the camera is focused at a distance of 50 meters?

That object will be the sharpest. Focus aids like autofocus systems can help, and there are various ways to ensure accurate manual focus.

Is it possible to be laser-measurement precise with a camera designed for photography? No. That's not what they are for.

Is it that we can only focus at about 50 meters?

If you mean "is it only possible to be approximate", yes. If you mean "is 50 meters the only possible focus distance", obviously not.

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    That is, roughly, the definition of focus, yes. – mattdm Apr 16 '20 at 14:52
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    @enbinzheng Usually, you don't know and don't care. You just care if the object you want to be sharp is sharp. You can use autofocus confirmation, or camera features which highlight high-contrast edges (this is called "focus peaking"), or just use your judgement. After all, the photographic goal is for your subject to be sharp, not to measure distance. If you are trying to measure distance, you're using the wrong tool. If it's not sharp where you want it to be, reactivate the autofocus, or use manual focus and turn one way or another until the subject becomes acceptably sharp. – mattdm Apr 16 '20 at 22:56
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    There are a number of ways to get accurate focus manually. Usually, cameras provide tools to do this. For example, many mirrorless cameras include a feature called "focus peaking". You can read about how to use that in this question. – mattdm Apr 17 '20 at 13:59
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    @enbinzheng Your eyes are plenty accurate enough to produce a photograph. What's the point of your question? What are you actually trying to do? – OnBreak. Apr 19 '20 at 0:51
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    It is impossible to achieve perfect precision. However, it is well within human capability to achieve focus that is for all practical purposes absolutely accurate. We know this is true because there are millions of photographs in the world with good focus. – mattdm Apr 19 '20 at 1:23

Perhaps a restatement of the question is: How accurate are focus markings on lenses?

For all but very long lenses, focusing at 50 meters is very close to focusing at infinity. The actual difference from infinity, on a 50mm lens, will be just a degree or two different from the infinity stop on the lens.

So practically, a lens setting of 50 meters would have a large error, to begin with.

The second issue is how accurate are the focus settings on a lens? For most consumer lenses, the focus settings are not spot on. Close, but again, there is a resolution issue on the scale. If the difference between being in focus and out of focus is less than a degree of rotation of the focus ring, how fine can one print the scale? And is the focus ring position established by normal manufacturing tolerances, or is it individually calibrated? Most likely by manufacturing tolerances.

Having said all of this, it is possible to measure distance optically, and a common method is using two perspectives, and using the angle differential of a point on the object, as seen through the two optical points (lens placement). However, the more accurate and less costly method today is to use the time of flight of an emitted pulse of light and it's return (eg LIDAR).

  • For example, I do n’t know the actual distance. Can I focus on the actual distance manually? – enbin Apr 16 '20 at 23:11
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    You can always dial in the distance on the lens, but the accuracy of doing so is not perfect enough for discerning photographers. Consider the lens markings approximations, which are fine tuned by the human eye, or if the camera has one, the auto-focusing system. – mongo Apr 17 '20 at 12:45
  • Without these tools, you can only focus inaccurately, right? – enbin Apr 18 '20 at 22:00
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    You can focus accurately enough for the purpose. – mattdm Apr 19 '20 at 1:22

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