And those lenses have/had a split image focusing aid in them?

There are several ways that camera manufacturers do this depending what kind of camera you have.

In the case of mirrorless, there is a LCD screen that the image is presented to. There is only one place that the image will be in focus, no matter the condition of your eyesight. In the case of the EVF, its the same deal, except now you are looking through the viewfinder at a smaller LCD screen. There are optics in the viewfinder that allow you to focus your eyes on this small LCD screen, and a diopter adjust in case your eye can't reach focus on the LCD screen. Since you are focusing on the LCD screen, there is only a sharp image on that screen when the camera is in focus. You cannot shift focus on the camera and correct for it by adjusting the diopter adjustment. Remember, the diopter adjust is to obtain correct focus of the LCD screen. And if the image is in focus on the screen, it will look in focus to your eye.

Optical view finders are much the same principle. There is a surface that the image is projected that is sharp only when the camera is in focus. This surface used to be ground glass. Don't know if that is what it still is a I no longer have OVF cameras. And the diopter adjust is used to allow your eye to focus on this surface. No chance that there will some other point you can find focus.

Another type is range finder and that is the alignment used to indicate focus. I haven't had one of these cameras in a long, long time, and I am not up on what modern cameras of this type do now.

But be assured that it is not like your binoculars. It is designed to find the focus of the camera and not your focus.

Not quite.

Yes, the lens optimum focus is on one plane at a given distance. But that plane may cut many elements of the image. And the tolerance for “in focus” is set by the manufacturer.

The Nikon mirrorless cameras provide an awesome capability called “focus peaking”. When you twist the focus ring to engage manual focus it shows all the focus areas it detects in a color you have preselected.

Also note that the mirrorless cameras focus using the sensor directly so an alternate system (like your eye) isn’t involved. It obviates the need for lens fine tuning and engages the full sensor area.

The simple explanation that no one has layed bare (one reply came close, but was only close, and was overly verbose) is quite simply this:

With binoculars or such, you look THROUGH the device.

You do NOT look THROUGH the camera. You’ll often carelessly *say* that you do, but you really do not.

“Looking through the camera” is merely a figure of speech. There is really no similarity to using binoculars.

(End of simple version)

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Not so simple version:

With binoculars you actually DO look through the device. The nearest solid thing in your view is the object you’re looking at, up to miles distant.

With a camera, the thing that you are actually looking at is only 1 to 4 inches away. You ARE NOT looking at your object of interest. You are looking at a very small viewing screen right in front of your eye.

Manual focusing.

Of course, you need to set the eyepiece diopter adjustment so that you can simply correctly see the image on the focusing screen and the data in the viewfinder. You can make this adjustment with or without your eyeglasses as you prefer. Once you have it adjusted, unless there is another user of your camera or your eyesight changes, you can leave it in place.

If you have not used manual focus for a long time, or never used it before, your focusing technique is important. The method is to focus on the subject or yo selected point of focus- when you arrive at the sharp focus point, then go beyond that point and come back. It's called rocking in the focus and is especially important where there is shallow depth of field, with the use of long focal length or any situation where the critical focus is required. Once you practice and get used to it, you can do this quickly.

Most currently made camera bodies do not have many of the old-style focusing aids such as split-image or micro screen center spots.

Older lenses, that were intended for manually focusing film cameras. had more travel in their helical focusing mechanisms- this made of easier to focus control. Many of the lenses made of modern digital cameras have significantly less travel and may require more precise manipulation.

The absence of engraved depth of field and footage scales can make the scale hyperfocal distance techniques more difficult or impossible on modern lenses that are intended for autofocus usage.

I hope this helps.