LED Display (aperture) with dedicated/third-party teleconverters

greenspun.com : LUSENET : Canon EOS FAQ forum : One Thread

I have both a Sigma 2x EX and a Canon 2 x MK I converter. Yesterday I was out with my 135 mm f2.8 lens and the Sigma TC and I noticed that the aperture in the LED display on my Canon 1n did not change to f 5.6 when the TC was attached. I could select any aperture down to f 2.8 and exposure readings would change accordingly ( I worked in Av-mode).

As I have not processed the slides yet I would assume that the slides where I (accidentally) used an aperture larger than f 5.6 are underexposed because the camera could not have set an aperture of - let's say - f 4 when taking the photo; it must have closed the aperture down to f 5.6 but with a shutter speed according to f 4. Luckily, I had taken just a few photos as I noticed this very early. I don't have my Canon TC at hand where I'm right now but I am pretty sure that the maximum aperture on the LED switches to f 5.6 - if used with a f 2.8 lens - immediately when the TC is attached.

I think, even if you are aware that the display does not change with Sigma's TC it is very easy to forget about it and set an aperture lower than f 5.6 and therefore get underexposed slides.

Do you have any experience with this problem?

Regards,

Sandra

-- Sandra Schänzer (SSchaenzer@agconsult.com), July 18, 2001

Answers

AFAIK 3rd party teleconverters simply connect the lens' electronic contacts to the camera's ones--Canon TCs read the lens information and modify them, so a 135mm/f:2.8 plus 2x TC would be a 270mm/f:5.6 for the camera. ("Would" because the only Canon 135mm/f:2.8 I know of isn't compatible with the OEM TCs.) With the Sigma TC and others, the lens data are transferred without modification, so the camera "thinks" it's dealing with a 135mm/f:2.8. Fortunately it isn't intelligent enough to wonder why suddenly focussing is so slow and f:2.8 is so dark.

Try the following: meter with the lens + TC combination at, say, f:2.8 and note the shutter speed the camera is setting; then remove the TC and meter through the lens only, using the same f-stop. If the camera now suggests a speed that is two stops faster, you needn't worry about underexposure.

-- Oliver Schrinner (piraya@hispavista.com), July 19, 2001.

Sandra,

Did you try Oliver's suggestion, and if so, what did you find out?

-- Hung James Wasson (HJWasson@aol.com), August 10, 2001.


All pictures were underexposed.

So with 3rd-party TC's you have to pay attention to the max. aperture you can set.

Sandra

-- Sandra Schänzer (SSchaenzer@agconsult.com), August 10, 2001.


Sandra,

Thanks for the feed-back! I'd been wondering whether or not going with one of the top end third party vendors would satisfy me -- I'd heard that the Canon TC's simply shut down AF when the converted f- stop rating is higher than Canon's recommendations, and that since other TC's lied the AF would try to continue funcioning. However, I'd rather have reliable exposures than a little more AF functionality! I'll save up for the Canon's. :)

-- Hung James Wasson (HJWasson@aol.com), August 11, 2001.


Sandra,

If you are using TTL metering, you should have obtained "proper exposure" when using the TC. In Av mode, the camera should compensate for the loss of light due to the presence of the TC with a slower shutter speed even though the correct f-stop is not being reported.

Oliver outlined an excellent experiment. He suggested setting the f- stop to 2.8. Then note the shutter speed when the TC is not attatched and compare it to the shutter speed when the TC is attatched. Since you have a 2x TC, the shutter speed should read two stops slower with the TC present in comparison to when it's not present. You could also confirm this by comparing shutter speeds between your Canon TC and Sigma TC when using the same f-stop and lens. You should find that under these condition each TC reports a different aperture, but the same shutter speed.

Therefore, when the Sigma TC is attatched and the reported f-stop reads 2.8, the actual f-stop is 5.6. When you adjust the f-stop so that it reads 5.6, you are actually shooting at f11.

Of course, you would run into problems when using a handheld meter or flash in non-TTL mode if you did not attend to the change in the f- stop when the TC is present.

-- Bill Boshoven (wcboshoven@netscape.net), August 12, 2001.



Moderation questions? read the FAQ