Bronica SQ-A: Confused about diopter in waist level finder.

I recently started shooting with a Bronica SQ-A, fitted with a waist-level finder.
Can anyone explain why the standard flip-up magnifier has a diopter of -1.5? (This value is clearly stated in the manual.)
I would have thought that a magnifier would have a positive diopter not a negative one.
I need to understand this because I am trying to work out which correction lens, if any, I should buy. I have a small distance prescription and also need a +1 diopter for reading).
I'm not having trouble focusing but i assume I should take my prescription into account in order to ensure sharp images... right?
I assume that it is my reading prescription of +1 that I need to take into account?
Should i simply add my reading prescription of +1 to the exisitng -1.5 and therefore buy a -0.5 correction lens - or is that not right?
Can anyone give any advice?

Thanks in advance.

Hmmm -- have to say, as an SQ-A owner and wearer of progressive bifocals, I've never given this any thought! I would expect if you use the viewfinder while wearing your glasses, the view finder lens should stay unchanged. You would only change it to avoid wearing your glasses. (As one who has noticeable astigmatism, I want to keep my glasses on.)

In addition, I always thought negative diopters compensated for near-sightedness [NOTE: I am not an optician, optometrist or any related profession ....!]. As such, I would expect a -xx lens to not show a focused image. Think about closeup lenses. The lens in my WLF actually focuses at about two or three inches -- that says to me it is a + diopter -- now maybe that number in the manual is some final number including the overall system, not the actual lens. Maybe a Bronica owning optometrist will show up here and fill us in!

Meanwhile I am inclined to wonder if the manual has the signs reversed! After all, we are trying to focus on a ground glass only a few inches from our eye. To do that with any object, we need very youthful eyes or a +diopter -- I have a couple of pairs of cheap reading glasses, a +1.5 and a +3 that I use over my regular glasses when I need to get down into near eye loupe distances when working on tiny mechanical objects -- or removing splinters! Since I already have about +3 built in the reading portion of the glasses, that's like +6 total.

Ver-r-r-ry i-i-interesting!

The chart on page 22 lets you compare your eyeglass prescription with an optical corrective eye-piece. http://www.bhphotovideo.com/FrameWork/Product_Resources/SourceBookProPhoto/Section01MediumFormat.pdf

Brilliant! Thanks macfred - that's perfect.
The chart also confirms that the standard magnifier for those with perfect vision is -1.5 which is odd... but at least I now know which correction lens I need.

macfred said:

The chart on page 22 lets you compare your eyeglass prescription with an optical corrective eye-piece. http://www.bhphotovideo.com/FrameWork/Product_Resources/SourceBookProPhoto/Section01MediumFormat.pdf

Click to expand...

Thanks -- that's a rather interesting document to have handy for many references.

I'm still not convinced those diopter numbers make sense, but that's likely because I know just enough to be dangerous!