Amazing weird combination

th3_man89 · Dec 26, 2008

i was renting a 2x teleconverter and i tried to mount my kit lens 18-55mm. in the booklet, it says the closest range of focusing is 0.28m. and when I mounted it, I notice a few things:

1. my AF doesn't work on f4.5 onwards
2. my f-stops is not doubled, yet the shutter speed is greatly reduced, which are basically the same issue
3. the weird yet amazing thing is the focusing reduced to 0.2m (i think) and it becomes a 110mm macro. eventhough it was manual focus.

can anyone explain what happen here???

benhasajeep · Dec 26, 2008

A 2x teleconverter doubles your lens range. Max of 55mm normal to 110mm with 2x. But there is a penalty. Becuase of the additional glass and magnification you also loose 2 stops of light.

Cameras need enough light for AF. Some do better than others. So it may not be getting enough light for it to AF. Most say f/5.6 I believe. With the TC on, you are past what the camera requires, basically it was working a little better than they say it would.

Basically if your lens is 55mm f/4 zoomed out. With this TC it is now a 110mm f8. So you do loose 2 f-stops. Since you said it was showing f/4.5, it would actually be f/9.0

If you were to use a 1.4x converter. You lens would be same as a 77mm but act 1 stop slower. f/6.4 approx.

The focusing distance just depends on the build of the teleconverter and the lens used.

Garbz · Dec 26, 2008

Only onbrand teleconverters update the aperture displayed on the camera. No biggie though. Things work just as well with or without the correct display on the camera.

McQueen278 · Dec 26, 2008

teleconverters work with reduced image circle lenses? Could you mount a dx/ef-s lens to a TC and then to a D700/5D? I didn't think this was possible. It probably won't cover the whole sensor in any case, but it's interesting.

Big Mike · Dec 29, 2008

Could you mount a dx/ef-s lens to a TC and then to a D700/5D? I didn't think this was possible. It probably won't cover the whole sensor in any case, but it's interesting.

Yes, I think it would be possible to do it that way...but the reduced image circle would probably give you heavy vignetting.

There is also a way to 'hack' your EF-S lenses to mount them onto full frame cameras. I know someone who did that to a 10-22mm and uses it on his 1D mk II.

Garbz · Dec 29, 2008

Hmmm actually the way I see it if you have a TC value larger than the crop factor of the sensor then you will probably get no vignetting at all. Like a 1.7x TC on a 1.5x APS sensor may work.