No - be careful. I struggled with this for some time till I got the correct answer.
Apart from the inbuilt diopter adjustment, the M8 (in fact all Leica M cameras) are set up so that when you look through the finder the apparent (or I suppose you could say optical) distance to the subject is (I think) 2 meters. If as I am you are long sighted, this means you will need less adjustment than that specified by the optometrist before even allowing for any inbuilt adjustment in the Leica finder.
If the eye examination revealed that a -1.0 diopter correction is required, the chances are that the optometrist is referring to the correction required at reading distance - which is certainly not 2 meters, more like 450mm.
Ring your optometrist and ask this question... What correction would I require for correct adjustment at a distance of 2 meters? They can calculate this - or should be able to from your eye test and should not need you to re-attend. This is what I did basically. And the optometrist scribbled a few notes made a few calculations and then told me the figure that I had already arrived at by trial and error ....see below. But of course I still had to make the further adjustment for the fact that M eyepieces have the -0.5 inbuilt adjustment.
This is what I did to double check this.
Go to your local pharmacist and look for the stand where they sell cheap non-prescription reading glasses. These usually cost between $5 and $20 depening on quality of the frames. Try them on, one after another by looking through the viewfinder of your camera with different glasses on till you find the one that gives clearest vision. Of course you will need to focus on some suitable object each time. This will give you pretty close idea of what diopter is required in practice. (In a pinch, until your diopter lens arrives you can even just by a pair and use them as your shooting glasses, but its not ideal as it puts your eye further away from where it should be for best view through the finder.) This is the strength diopter you buy-the same as the glasses that give the best result. And if you check it against what the optometrist tells you, you should find its -0.5 from the strength he / she specified.
A final option. Go to this e store and buy their viewfinder magnifier for the Leica M. It comes in two magnification strengths ........1.35 and 1.15. The link below says one is out of stock but when you click on it it says that have now restocked.
http://www.japanexposures.com/shop/product_info.php?products_id=109
http://www.japanexposures.com/2009/02/02/ms-mag-x115-magnifier-for-leica-m-sold-out/
The beauty of this option is that the magnifier also has infinite diopter adjustment between -3.0 and +1.5 diopters (assuming your requirement is in this range.) It works well, although it is said to give a slightly less bright image than the Leica single diopter lens - which is to be expected as there are more elements. The upside is that the increase in magnification also helps with focusing as well by increasing the apparent rangefinder length and making the rnagefinder more accurate. It costs about double what you would pay for a Leica single diopter adjustment. But I think its worth it as if your eyes change over time - you just manually change the setting on the magnifier to compensate - no need to buy new dioper lenses.
cheers and good luck.
