Originally posted by Egg Salad
No the results are not the same. The way it is it is only useful with a 50mm lens.
Simply reversing this 50mm lens gives better sharpness than when on the TC.
But working with reversed lenses is quite uncomfortable as you can't focus and have to close the aperture yourself.
Without the glass - so as a simple extension tube I could also use the 135/2.5 as a macro lens....
A 2x teleconvertor will, effectively, give you some magnification (depending on the prime lens used) with ANY lens you can attach it to, without reducing the minimal focal distance, with the retention of infinity focus, and with the loss of two stops of light.
Were the same casing (the TC with glass removed) to be used as an extension tube, the magnification would depend on the length of the casing and the focal length of the prime lens used (the longer the focal length, the less magnification). There would still be loss of light (again dependent on the focal length of the prime lens) but there would also be a loss of minimal focal distance as well as the loss of infinity focusing.
Image quality would depend on the quality of the prime lens and the quality of the TC. Assuming that both are "decent", the impact on IQ for small enlargements, say up to 8"x10" would -to most observers- be insignificant
If you are a 35mm film user, interested mainly 4"x 6" prints, using a teleconverter for walkaround photography is a relatively cheap and very handy approach to close-focus photography.
This would be especially the case where the subject is difficult to approach or out of reach of combinations with a relatively short focusing distance: for example, many "Macro" or normal lens-extension tube combinations still require one to approach closer than 6" or 8", whereas, a 2x TC and a 100mm prime lens will give you a lens-subject distance of 39" for a 0.25 magnification and still more (subject distance), if a smaller effective magnification is acceptable.