I have an exam coming up and need some help please?with a single lens reflex camera, when is it recommended to use an assessory dioptric corrector lens?

  • When the viewfinder doesn't have built-in diopter correction?
    – xiota
    Jun 6, 2019 at 16:10
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    In general, you'll get more help and far better responses when you attempt to answer the question yourself and point out where you still have some confusion. Asking the straight up exam question with no effort is ... lazy at best.
    – OnBreak.
    Jun 6, 2019 at 17:00
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    An "exam" as in academic test? Or as in a visit to your optometrist or opthamologist?
    – Michael C
    Jun 7, 2019 at 2:53

2 Answers 2


I'm inclined not to answer homework questions, but I can help you on your way..

A camera's viewfinder displays a sharp picture for those with good eyesight. If you're a near- or far-sighted photographer however, the glasses may get in the way or the VF may scratch your glasses.

With that in mind, what do you think a dioptre does?

Additionally, why would a viewfinder with a focussing aid such as a split image somewhat render the use of a dioptre as a means to help focussing unnecessary?

Next to that, what could be a problem when a near-sighted photographer using a dioptre is not looking through his lens momentarily?


Far-slightness plagues most of us as we age. Reading glasses to the rescue. These corrective eyeglasses are commonly available in drugstores worldwide. Reading glasses are labeled as to their power. The unit of measure used is called “diopter” (Greek to see through). The diopter unit is just another way to express the focal length of a lens. 1 diopter = 1000mm --- 2d = 500mm – 3d = 333mm – 4d = 250mm etc.

SLR (single lens reflex) cameras feature an eye-level viewfinder. This is an optical presentation of the scene to be photographed. Such a scheme allows the photographer to pre-view an accurate projection of what the film or digital sensor will see during the actual exposure. Many digital cameras sport an electronic version of the eye-level viewfinder.

If your vison is less than optimal, some adjustment is possible and likely provided via a wheel adjacent to finder’s exit lens. This adjustment might be labeled in diopter units to correspond to the power of corrective reading eyeglasses. In other words, if you’re reading glasses are labeled +2d, then likely a +2 setting of the camera’s eyepiece corresponds.

If your camera’s eyepiece adjustment does not do the trick, a visit to an optometrist might result in a custom corrective lens to supplement the camera’s eyepiece.

Most of us “gray-hairs” remove our spectacles, adjust the eye-piece diopter corrector so we can clearly see the screen and the letters and numbers presented by the reticle (Latin for scale seen in optical instruments).

  • "visit to an optometrist might result in a custom corrective lens to supplement the camera’s eyepiece" – Had no idea that was possible. Good to keep in mind.
    – xiota
    Jun 6, 2019 at 16:13
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    I purposely didn't give a thorough answer so OP had to do some thinking himself too, and added some further questions to add some food for though. This is a homework question. Now your answer completely defeats that...
    – timvrhn
    Jun 6, 2019 at 18:05
  • He is studying for an exam, not asking for a yes or no homework answer. I am helping him understand the concept. In my opinion, those who ask for explanations are making a huge effort when they post questions here. I admire that effort, and I disagree with those who would discourage learning opportunities. Jun 6, 2019 at 19:27
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    Is it possible the "exam" is the kind that will be done by an optometrist/opthamologist?
    – Michael C
    Jun 7, 2019 at 2:56
  • Most SLRs and DSLRs are designed with a "built-in" -1 diopter correction to make the viewfinder image appear to be about 1-1.5 meters away (instead of a few mm). Most manufacturers mark this -1 diopter setting as the "neutral" or "zero" point on the diopter dial. Canon, however, has for decades designated the "center" index mark as -1. Thus, if one needs a +1 diopter pair of reading glasses when viewing things at a distance of 1-1.5 meters, one should dial the camera's diopter correction from the centered -1 setting to the '0' setting. That is also why Canon sells -2, 0, +1, and +2 diopter...
    – Michael C
    Jun 7, 2019 at 3:01

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