Does anyone know how adjustable diopter view finders work?

Just like the headline. I tried to google this, but if it is out there it is buried deep.

The viewfinder lens is much like the camera lens except usually it can be made smaller.  Adjusting the diopter is exactly the same as adjusting the focus on the camera lens and I would guess that it is done in the same way: by moving some internal elements of the lens.

Why do you want to know?

Tom Axford wrote:

The viewfinder lens is much like the camera lens except usually it can be made smaller. Adjusting the diopter is exactly the same as adjusting the focus on the camera lens and I would guess that it is done in the same way: by moving some internal elements of the lens.

Why do you want to know?

Partly curiosity. Although I don't see how changing focus would allow for negative diopter.

I am short sighted. My eye doctor gave me two pairs of glasses. One for driving, and one for computer work. The driving pair don't allow me to focus clearly on things in the car, like a GPS unit.

I was thinking a concave lens over the GPS might allow me to see it clearly.

On a tangent to this thought, I wondered how EVFs achieved a range from positive to negative diopter.

Jeffry7 wrote:

Tom Axford wrote:

The viewfinder lens is much like the camera lens except usually it can be made smaller. Adjusting the diopter is exactly the same as adjusting the focus on the camera lens and I would guess that it is done in the same way: by moving some internal elements of the lens.

Why do you want to know?

Partly curiosity. Although I don't see how changing focus would allow for negative diopter.

The same way a camera lens can focus beyond infinity.

I am short sighted. My eye doctor gave me two pairs of glasses. One for driving, and one for computer work. The driving pair don't allow me to focus clearly on things in the car, like a GPS unit.

I was thinking a concave lens over the GPS might allow me to see it clearly.

You mean a convex lens.  A concave lens will make it appear closer and even harder for you to focus on.  Any large magnifying glass will work if placed the correct distance from the GPS.  A fresnel screen magnifier may be your best bet.

Otherwise get bifocal glasses, which is what I use (you can also read all the other dashboard information).

On a tangent to this thought, I wondered how EVFs achieved a range from positive to negative diopter.

The zero position is for people with normal eyesight.  The viewfinder screen then appears to be at infinity.   Positive diopter makes it appear closer for people who are shortsighted. Negative diopter makes it appear beyond infinity for people who are longsighted.

It is just like using a magnifying glass.  You have to hold it at the right distance from the subject.  Too close, and the subject appears too close for your eye to focus on it (assuming your eye is up against the magnifying glass).  Too far, and the subject appears beyond infinity and your eye again cannot focus on it).