Am I supposed to keep my glasses on?

This morning I went for a dawdling walk to play with my new toy: an adapter that lets me put my old, manual Minolta lenses on my shiny, automatic Sony camera. (There's a set on Flickr, completely unprocessed, if you want to see more). It was fun, for all sorts of reasons. Like, having to fiddle with aperture, shutter speed and focus. The camera does a brilliant job automatically, but it is also gratifying to make decisions.

Of the three, focus is far and away the hardest. The viewfinder provides a perfect preview of exposure, and you can even bring up a histogram if you're keen to shoot to the right. But focus, especially when the lens is from a different camera, is a different matter entirely. No little split image to align. No help but your own eyes.

And that, frankly, is where I admit I am totally ignorant. And confused.

I wear glasses. Out and about, they are varifocals. And I Understand that looking through the viewfinder using different parts of the spectacle lens is going to affect the focus of what I see. What I don't get is why I need to wear spectacles at all. So I don't, and some people say I'll never get a proper focus.

I'm looking through the viewfinder at a screen, and the screen is some distance from my eye. But as long as my eye can focus on the screen, and it can, why would the focus on the sensor be any different from the focus on the viewfinder screen? The camera has a dioptre adjustment, and as I move that, I see the grid lines go in and out of focus. So too does the image. And when the grid lines are out of focus, I can't bring the image into focus. But I really do not understand why.

With its own lens, the camera has a neat little trick when you set it to manual focus. Touch the focus ring, and it zooms the image in the viewfinder, making it that much easier to see whether the bit you are interested in is sharp. Touch the shutter button and it zooms out, allowing you to compose the picture. So helpful. If it can do the same when there's a manual focus lens on, I have not yet discovered how. You can see how it would help on a picture like this one, where I thought I had focussed on her eyes, but it is her nose that is sharp. Was that me, or did she move?