Teleconverter question

[jeffde]

I have Canon 7D camera
I have a 1.4 and 2 x Kenko Teleconverter and when i put them on the 70-200 f2.8 i loose 1 stop and 2 stops as i knew i would. (i've had them several years)

I put the 24-105 F4 with the converter on the other day (first time) (i'm going on a cruise next month and want to keep the weight down and am thinking of leaving the 70-200 behind)

but i didn't loose anything in f stops - i still get F4 with both converters - the focussing slows but i still get maximum aperture.

But i don't know why ?? any suggestions ?

[Xenedis]

I suspect that there isn't communication between the Kenko TC and the camera to indicate the actual f/stop.

The light reduction is a property of increasing the focal length, so even though your camera still reports f/4, you'll find that in reality you're at f/5.6.

As a simple experiment, take a photo with the lens natively at f/5.6, and then take an identical picture with the TC attached and the lens opened to f/4.

You should find that the exposures are more or less identical.

[ameerat42]

Even easier: meter a (uniformly illuminated) subject with and without the teleconverter both times while in Aperture priority mode (and obviously without changing ISO). Does the shutter speed increase/decrease 1 stop, ie, go from, say, 1/250s (without tc) to 1/125s(with tc)?

If it does, then the camera is not reporting the right f-stop.

[etherial]

I didn't think the 24-105 was compatible with teleconverters? Be careful you don't damage the lens!

[Xenedis]

I didn't think the 24-105 was compatible with teleconverters? Be careful you don't damage the lens!

It's certainly not compatible with Canon tele-converters.

I'm not familiar with the third-party offerings.

[Bennymiata]

I have the Kenko Pro 300 2X converter and it fits on my 24-105, but it won't autofocus, but it still works well with my 60D.

I mainly use the converter on my 100-400 to take shots of the moon and other far-away subjects, but it will even fit on my Tokina 11-16.

The Kenko doesn't have any glass sticking out the front of, in fact, the glass is recessed by around 5mm from the front face, so i think almost any lens will fit except for EF-S lenses which have elements sticking out behind the main body of the lens.

I haven't tried any other converters, but I'm very happy with the Kenko, and the price is pretty good too in comparison to others I've seen.

[jeffde]

Thanks guys for your replies - did the test as suggested and you are right shutter speed decreased - therefore aperture is NOT reporting correctly - so i think its best not to use the teleconverter...

Cheers and thanks
Jeff

(PS : such a simple solution - slapping myself for not trying that first ...) LOL

[Xenedis]

Thanks guys for your replies - did the test as suggested and you are right shutter speed decreased - therefore aperture is NOT reporting correctly - so i think its best not to use the teleconverter...

You can still use it if you need the reach; you just need to be mindful to add +1EV to your exposure to compensate for the light reduction.