DirtFarmer Loc: Escaped from the NYC area, back to MA

Many cameras have guide lines in the viewfinder that you can use to get the diopter adjustment set properly. If the lines are not in focus the camera needs to be adjusted.

billnikon Loc: Pennsylvania/Ohio/Florida/Maui/Oregon/Vermont

When you use manual focus on a camera, when the CAMERA is in focus, a green dot appears in the viewfinder of my Nikon camera.
It does not matter is Joe Six Pack uses my camera, when the CAMERA is in focus, the image is in focus.

Architect1776 Loc: In my mind

Architect1776 Loc: In my mind

Then why use manual focus?
The camera could have done it better and faster to get the green dot than anyone can manually.

DirtFarmer Loc: Escaped from the NYC area, back to MA

The green dot appears when the camera's autofocus indicates that the image is in focus. If you rely on that, you are using autofocus manually. You would get better results by having the camera's autofocus control the focus.

jlg1000 Loc: Uruguay / South America

Binoculars are an optical system that uses your eye lens and your retina as screen. So you use your eyes to focus.

In DSLRs there is a matte glass right under the pentaprism at the very same distance from the mirror as the sensor (or film), the lens focuses the image on that glass. Later the eyepiece works as a microscope to enlarge that image. So if the image us in focus on the matte glass it will be on the sensor regardless of who is watching.

The diopter adjustment is to help you focus your sight on the matte glass.

tenny52 Loc: San Francisco

It is simple. When you don't have an automatic lens.

StanMac Loc: Tennessee

Best explanation for the question yet, jlg. I should add that the optics in the viewfinder are designed such that the image you see on the ground glass, or the viewfinder’s LCD screen in the case of an EVF, is in focus at or near infinity. So if you need glasses for distance vision, you can adjust the viewfinder diopter adjustment such that you don’t need glasses to see the in focus image through the viewfinder.

Stan

Wanderer2 Loc: Colorado Rocky Mountains

My eyes are old and tired and I find that Back Button Focusing is at least as accurate for me as manual focus and I use that form= most of my shooting. That's for landscapes.

lamiaceae Loc: San Luis Obispo County, CA

Yes.

agillot

the manual say to adjust while looking at the display in the viewfinder, when the writing in there is sharp , you are there .

boberic Loc: Quiet Corner, Connecticut. Ex long Islander

Manual focus with auto lenses is made much more difficult by the very short "throw" of auto lenses. The tiniest difference will effect focus. Make sure that the slightest camera shake is not a problem. It is for me.

Jack 13088 Loc: Central NY

I’m not sure this is what the OP was asking but...

This is the first to hit upon the difference between a real image and a virtual image. In all cameras the optics forms (focuses or projects) a real image on an image plane which is either a sensor or film. A real image can be captured and used later. You must be able to see this real image to bring this real image into focus on the scene. In a reflex or view camera a “ground glass” is substituted for the film or sensor. Your eye looks at the real image projected on the ground glass to see the image. That is where the diopter adjustment comes into play. Even very young eyes cannot focus on an image less than a few inches away so the reflex camera folks put a magnifying glass in the view finder to “move” that real image out to a meter or so. Now folks over 40 or so cannot see that close. They have presbyopia ( literally old persons eyes) and need help to see that close. The diopter adjustment provides that help.

Binoculars form a virtual image that are seen by your eyes with the real image on your retina. You adjust that focus with the knob in the center of the two monoculars. Since it is unlikely that both of your eyes focus exactly at the same distance the diopter adjustment here fine tunes usually the right eye.

Before CH_CANON has a hissy fit I will mention that on a mirrorless camera the ground glass is replaced with a tiny TV screen in the viewfinder to show you a picture of what the sensor sees. “Obviously” superior.

In a camera you adjust the adjustment to focus on the lines on the viewfinder before looking at the image. (That sounds like double talk.)

In binocs you focus with the right eye covered then cover the left side and adjust the adjustment.

In the unlikely event you read this far I hope I have clarified things.

gvarner Loc: Central Oregon Coast

The diopter adjustment takes care of that. I set mine to be sharp with my glasses on.

Architect1776 Loc: In my mind

The problem is most UHHers Brag about having the latest whiz bang AF lens then brag they only do manual AF.
Incredibly silly.