Everybody’s eyesight is different and this can sometimes lead to problems when trying to focus a camera when it’s in manual mode. This can especially be true if you wear glasses and then take them off before taking photos. Well, most SLR cameras have a focus-adjusting dial that’s called a diopter adjustment. It’s usually located just to the right side of the eyepiece.
Basically, the diopter adjustment works like a lens that switches from far to near focus. However, it doesn’t actually adjust the focus of the camera since this adjustment is only affecting how your eye views the image that you’re seeing.
In fact, the diopter adjustment is quite similar to the device that an eye doctor uses when checking your eyesight for farsightedness and nearsightedness.
Each time the dial is turned on the camera it will give you a different view of the image.
A person with 20/20 vision will keep the diopter dial in the middle and there will also be negative (-) and plus (-) adjustments on the dial. For instance the range may be from -5 to +5 and 0 representing 20/20 vision.
If your eyesight is weak the dial will help you focus, especially in manual mode. When adjusting the diopter, you should place your camera on a tripod or something just as steady and focus in on something. Press the shutter halfway down to get into auto-focus. Then all you need to do is turn the diopter dial and look through the eyepiece until the object is in focus. This will help the camera see things the way you see them.