underexposed with teleconvertor

[Happy chewer]

Hi, I have a pentax k-x and a Tokina 260mm manual lens (no A), I have just purchased a vivitar MC 2x TC and having a little trouble with light levels, cam set to aperture exposure automatically and using the mode dial,I set iso to auto 100-6400,green button set to (Tv in M mode). I would have thought that as the camera is metering automatically it would be able to set the shutter and or iso and expose correctly, no different to the sun going behind a cloud and reducing the light,it's as if it's metering at wide aperture but capturing at reduced aperture, but the aperture is manual, I thought I was getting to grips with the exposure triangle but I'm baffled, I am having to manually change the exposure to +2.5, please explain what I'm missing, thanks in advance.

 

[Derrel]

The TC does cut down the light level by one full f/stop, and possibly a bit more. it is NORMAL to lose light, but why the meter is not compensating is a bit perplexing.

 

[Happy chewer]

Thanks Derryl ,that's what I thought, I've downloaded a 400 page pdf manual, I must have missed a setting somewhere , oh when I attach lens normally it asks for focal length and suggests 75 for my 75-260 lens, when I use TC2 should I put 150?

 

[Overread]

Focal length shouldn't affect it and whilst the teleconverter does take out about 2 stops worth of light (give or take a bit) its light taken out before it hits the sensor so the camera should still be able to compensate for it.

It could have been a fluke that you were metering off a bright subject and thus the camera underexposed (same as it will when confronted with subjects such as extensive snow cover)

 

[Happy chewer]

I've tested again and took two identically framed pics at iso 200, first one with +2 EC it shot 1/120 and it fitted histogram ok in medium sunshine, I set Ec to 0 and took next pic, it took next pic at 1/640 and it came out dark (50%), so I do have to compensate even though the metering is TTL, baffled.