2X TC creates larger max aperture...?

[A.M.D.A.]

When I use my 300mm F4 MF-only lens/TAMRON 2X TC/PENTAX *ist D, I am somehow able to take pictures at a maximum aperture of F1.7! Also F2.8, F4, F5.6 etc.. This lens supposedly has a maximum aperture of F4, and when used on its own (no 2X TC), F4 is as far as I am able to "open up".

Full name of 300mm lens: PENTAX SMC PENTAX-A* F4 300mm.
Full name of 2X TC: 2X Pz-AF BBAR MC7 TAMRON-F AF Tele-Converter.

I have not experienced this issue when using the TC with my other two lenses (28-80mm F3.5-4.5 and 100mm F2.8), only with the 300mm combined with the TC. Attached is a sample photo with EXIF data showing an aperture value of F1.7.

Attachment 63285
Excuse the quality, it is just a "grab shot" for an EXIF data sample. For those of you without an EXIF reader handy, or somehow find the data unreadable, click here, then scroll down a bit.

Camera Brand: PENTAX Corporation 
Camera Model: PENTAX *ist D      
Exposure Time: 1/400 sec.
F-number: F1.7
ISO Speed Rating: 1600
Flash Fired: Flash did not fire, compulsory flash mode.
Metering Mode: Center-Weighted Average
Exposure Program: Aperture priority
Focal Length: 0.0 mm
Contrast: Normal
Saturation: Normal
Sharpness: Normal

Any insight as to why this f-number-issue would be great...

[freygr]

Your lens must be an older lens and TC the combination the aperture link/connections most likely are not compatible, giving errors. The aperture of a lens is fixed in stone at the time of manufacture.

[A.M.D.A.]

Thanks for the reply, freygr.

So the lens is so old that it and the camera can't communicate properly. That sure would logically explain this f-stop-issue !

[brmill26]

As freygr said, maximum aperture is physically fixed, and therefore cannot ever be increased in any way. The off brand extensions do some odd things in communicating with the camera, that some times are beneficial (though increasing the aperture isn't one of them). I've read that some off brand TC's used with Canon cameras can sometimes actually allow for faster AF because the camera doesn't know about the extra light loss. Your mileage may vary...

[OldClicker]

It may also lie to the camera to keep the f-stop below what the camera requires to autofocus. - TF

[Ron Kruger]

Looks like your equipment has a "failure to communicate," because for focal length, it says: 0.0. I know that if I put an older or different brand of lens on my K20D, it asks me to dial in the focal length. Did you get that message?

[A.M.D.A.]

Looks like your equipment has a "failure to communicate," because for focal length, it says: 0.0. I know that if I put an older or different brand of lens on my K20D, it asks me to dial in the focal length. Did you get that message?

No, I don't get any message of any kind.

And of course I don't get AF, because neither the 300mm or the 28-80mm have gold contacts on the mount. But the 100mm MACRO does and I do get the appropiate focal length in the EXIF. With the 28-80mm and 300mm the focal length always reads 0.0mm.