-0.5 and -1 diopters

[cynan]

Hello all, I hope I've posted correctly. I want to increase the effective focal length of an old brass lens from 210 to >300 with diopters. Where can I get hold of -0.5 or -1 diopters over 65mm in diameter

 

[cynan]

Sorry should have posted this in misc

 

[Maris]

I get my 75mm diameter minus dioptre lenses from my local optometrist. These lenses, like any simple accessory lenses, introduce small amounts of all the classic abberations. At long focal lengths and small stops on a large format camera its not a problem.