Westminster fountain at sunset

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I had a scare last night when everything in my view port looked blurry on my brand new Canon 60D, even the digital display a red focus boxes.

The images photos came out clear, the live view was clear, the auto focus was working but everything in the view finder was fuzzy.

I realized spinning/adjusting the small wheel next to the view port fixed this issue.

So my question is, why is this needed at all, what is it for? Is there a way to turn it off? I don't want to accidentally spin it again and make everything fuzzy.

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1  
See also this question on what to do with the dial once you know what it is. :) photo.stackexchange.com/questions/6681/… –  mattdm Apr 6 '11 at 16:07

4 Answers 4

up vote 15 down vote accepted

That control adjusts the diopter, an optical adjustment to the viewfinder that allows folks with varying vision abilities to see things clearly. Think of it a bit like reading glasses - it allows for minor vision correction. The idea is that folks who have decent vision but might need minor correction can make an adjustment on the diopter so that as they look through the viewfinder, objects are clear.

As you note, for someone with perfect vision, throwing and adjustment in there can actually make things look worse and cause problems with manual focus.

As it is a manual/mechanical adjustment, I'm not aware of a way to disable the ability to adjust the diopter, although a bit of gaffer's tape over the dial might help avoid inadvertent adjustments.

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+1 Thanks, I do wear glasses, I have been shooting with them on though. If I am comfortable that way, is there any reason to shoot without them using the diopter? Thanks. –  JD Isaacks Apr 6 '11 at 17:11
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@John Isaacks: Yes, certainly. You can put your eye closer to the screen, making the corners (and status display) easier to see, and blocking more stray light. –  mattdm Apr 6 '11 at 18:19

The other posters explained what this dial is. To complete the answers, here's how and why to use it. Assuming you wear glasses with a small enough number, you can set the diopter dial to compensate for your vision imperfection. The way to do that is usually to set the dial such that the numbers and indicators in your viewfinder (like aperture and speed indicators) look sharp.

As to your question of why should you do that - many times viewing through the viewfinder with glasses actually reduces the apparent image of the scene so it degrades your ability to perfectly compose and frame your image. This is because the glasses further away your eye from the eyepiece (try looking at the viewfinder from afar and see what I mean). Removing the glasses lets you stick your eye to the viewfinder so you can see the whole frame. (Now, there is a technical term for that, but I just can't recall what it is.) If this is not a particular problem for you, then you can leave the diopter unchanged and stay with your glasses. I wish my number was low enough to allow me to remove my glasses.

Additionally, it eliminates yet another layer of (possibly dirty) glass between you and the scene.

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I'm not sure of the whole technical term, but there's something about "eyepoint". –  mattdm Apr 6 '11 at 18:20
    
+1 good info... –  JD Isaacks Apr 6 '11 at 20:20
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It's called "eye relief", I believe. Your typical tunnel-vision, crop-sensor DSLR isn't so bad; on a full-frame camera or a Leica-type rangefinder using glasses can be a real pain. –  Staale S Apr 6 '11 at 20:39

That is the dioptre adjustment wheel. A "dioptre" (or sometimes diopter) is the measure of optical power in a lens. Usually, cameras have a -3 to +1 dioptre control that adjusts where the viewfinder focuses. It is not a very powerful adjustment, and is generally intended to allow users with minor vision imperfection (very slight near or far sightedness) to use the viewfinder without needing corrective lenses of their own.

There should be a tick mark on both the camera body and the wheel, and when the two are matched, it indicates the "zero" setting, or default setting. A good way to set the dioptre correctly for your eyesight is to use Live View to get perfect focus on a subject, then look through the viewfinder and adjust the dioptre until it looks sharpest.

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Diopter adjustment.

If you need glasses, you can use this little wheel approximate the correction your glasses provide. Then, it's easier for you to shoot without your glasses on.

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