Using 2x converters
I've been trying to get my hands on a fast prime lens of between 85mm & 120mm for my OM system for portrait work with no luck. I do have a 50mm f1.8 and a 2x converter.
I know that using the converter will lose me 2 stops of light which isn't a big problem but will it also lose me 2 stops aperture?
I don't think it should. If anything I think I should actually get a shallower depth of field as I will be focusing at a greater distance and, if I remember correctly, DOF is affected by aperture, distance to subject & focal length of lens. Is this correct or am I doing dodgy science?
The f:stop is equal to the focal length of the lens divided by the physical size of the aperture. If you add a 2x converter, you are doubling the focal length of the lens but the physical size of the aperture stays the same, so your 50/1.8 lens becomes a 100/3.6 lens. Plug the numbers into a DOF table and see what you get.
Doublers also usually add some transmissive light loss--often around 1/2 stop. You don't notice it if you are metering through the lens, but if you use a handheld meter or non-TTL auto flash, you might want to test it, so that you can take it into account.
Davids answered this one better than I could. Remember to make sure the lens you get fits the converter or the glass in your lens might hit the glass in your converter when you focus OUCH!!
If you can find a 2x converter that goes in front of the lens you will maintain the f/1.8 as far as focussing goes - ie a 50 mm f/1.8 lens will turn into a 100 mm f/1.8 lens, albeit of less-than-perfect optical quality. There will be some extra transmission loss, as with a behind-the-lens converter, but that doesn't affect depth of field. The difficulty would probably be in finding such a converter that didn't vignette with a 50 mm lens. A lot of tele-converters were made for Super 8 mm movie cameras with non-interchangeable zooms, and they are available second-hand. New ones of reasonable quality are fairly expensive. The Century one, with a 58 mm mounting thread, is 500 bucks new.
and one of questionable quality 100-200 from your local digicam supplier. use probably less than $50
Olympus made a rather nice f2.8 100mm lens. It's slower than your 50mm, but faster than the 50 + teleconverter and the quality will be much better.
Good Evening, Sparx,
David's comment above reminds to mention that Olympus also made (makes?) an absolutely superb f2 100mm lens for the OM series cameras; it's probably the sharpest lens I have; some claim, though, that's it's too sharp for portraits.
one good aspect of teleconvertors (hope I've got this right as I own one but never use it as such, only as a extension tube cause it's guts unscrew and can be removed!) is, you maintain the minimum focusing distance of the prime lens, thus effectively increasing the magnification ratio obtainable. Something like that :)