Diopters

Sorry I should have posted this here instead of general. If I have the inbuilt diopter set to -3 to be able to see clearly through my viewfinder with glasses on a 450D I am guessing I would need at least a -3 to view without. But since I want to make sure I have some leeway would I bet better off getting a -4 (Their strongest I think) and just dial back to +1 when using glasses?

Diopters all have the same range. The standard is +1 to -3, the next one is -4 to 0, etc. I think the standard one is -1, and the camera goes +/- 2. So a -4 would be -6 to -2.

The 450D goes to + or - 3 so if I add on a -4 surely it would make it a max of -7? But I can dial back the other way to bring it back to -3 (Which will now be +1 on the dial?)?

Slightly confused over it to be honest

I think you have it correct Dick, if you add a -4 you could dial in a total of -7.

It is a little confusing as the null point on Canon cameras (at least my models) is -1 and the diopter takes that into account. So when you hold the little bit Canon sells you and it says -2 it is really a -1 and you are adding it to the null point to make -2. So Canon sells a 0 diopter but not -1 as -1 would be a clear piece of glass but a -0 is really a +1 to neutralize the null point on the camera to 0.

As a slightly side point of interest you cannot just take a glasses prescription and go with that as you may be more comfportable with something else. For example I use a stronger diopter than my prescription would suggest. My opthamologist says it is because I am overcompensating. I get the general impression it is a bad thing but it is something I do. I suspect may mean some people may also undercompensate.