What is the small +- wheel next to my view port for?

Question

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.

Asked by John Isaacks

Answer

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.

Answered by ahockley

Check more discussion of this question.

How can I accurately focus manually on an APS-C DSLR?

Question

My favorite portrait lens is the Nikon 50mm 1.4G. I love the light and what it is capable of. My problem with it is that on my D90 the AF is sometimes horribly slow, and in many cases (mainly low light) I can't auto focus at all. I use AF lock as much as possible, but for many shots I really wish I could just use manual focus. The problem is that I'm usually wrong about whether my subject is in focus or at least completely in focus. So my question is...

Are there any recommendations you can make for training my eye to be sure my manual focus is accurate? Any diopter adjustments or calibrating tools? Any eye exercises to train myself to recognize a true focus?

I know that I would probably have better AF performance on a more expensive body, but for now I think it's good practice to train myself in manual focus anyway. Thanks for any assistance.

Answer

One of your problems is that the matte screen in your camera (this is what you are actually looking at through the viewfinder, it's a semi-transparent plastic screen that sits at the top of the mirror-house, below the prism housing) is designed to give a nice bright useable image through a slow autofocus lens such as your average f/3.5-5.6 consumer zoom. This is good if you are using a consumer zoom, but very bad if you are trying to focus a fast prime lens. The reason is that the extra screen brightness is bought at the expense of focus accuracy - is unable to show you the small depth of field at larger apertures than about f/2.8-ish. In other words, if you look at the image from an f/1.4 lens through this matte screen, the depth of field you see will be that of f/2.8 and not f/1.4. Again, for a consumer zoom this is entirely irrelevant but for a fast prime it is highly misleading. You physically cannot focus a fast prime accurately through such a matte screen; you are just not getting the information needed.

A solution for manual focusing is to replace the default "fast" matte screen with an old-school, coarse-grained one which will show you accurate depth of field for a faster lens, and more "pop" when things come into focus. You may also get a screen with a split-prism center, or one with a prism-ring round the center.

I'm not sure if Nikon offers such alternative mattes for the D90 but third parties might although installation is likey to be a bit more fiddly in that case. The downsides to a coarse-grained matte screen is that it affects light-metering, and that the view from lenses slower than about f/2.8 will be darker (much darker, in the case of an f/5.6 lens) in the viewfinder. The matte does not affect autofocus performance in any way though.

Check more discussion of this question.

How do I calibrate the diopter correction on my DSLRs viewfinder?

Question

The viewfinder on my camera has diopter correction, but is there a "correct" way to set it up? In the past I've just allowed the camera to auto-focus on a distant point, and then make sure that is as sharp as possible, but this seems to be prone to error, and reliant on the auto focus of the lens. Is there a better way?

Answer

Remember that what you're looking at through the viewfinder is the image from the lens projected onto a ground-glass viewing screen. You want the diopter to be adjusted so you can see that most clearly.

Fortunately, this means that you can use the etched lines on the focusing screen, as well as the status information in the viewfinder, to adjust the diopter. Point the camera at a featureless bright-but-not-overwhelming subject (a white wall, say), and set the lens's focus such that the wall is just a blur. Now ignore that and set the diopter so the focus screen markings and indicator information are as sharp as you can get them.

Check more discussion of this question.