Tamron 18-270 AF at f/6.3 - Why is it Possible?

[Emergency Exit]

I've been reading a great deal about extenders. One of the side effects of using extenders is that it stops down your aperture by a stop and 2 stops, for the 1.4x and 2.0x respectively. So this means if I use my 70-200 f/4 with a 1.4 extender it'll essentially become a 98-280 f/5.6, and with a 2x it'll be 140-400 f/8.

Since I use a 60D, which is obviously a non-1D series body, AF will only work with the 1.4x extender when using the 70-200 f/4. Using a 2x will result in a max aperture of f/8, and AF does not work at this point.

This all made sense to me until it came to me that I have a Tamron 18-270 f/3.5-6.3. At the 270mm end, the max aperture becomes f/6.3, which is 1/3-stop below the minimum of f/5.6. Yet without being aware of this I've been using this lens with full AF functionality - even at 270mm, f/6.3.

So what's the science behind this? Or is it the lens that's reporting f/6.3 while actually being at 5.6?

[Lenty007]

It's a gimmik.
Like Sigma, Tamron uses the same (software)trick to let the body know the lens goes to F/5.6 instead of the actual F/6.3, hereby slightly underexposing the shot at the largest zoomlength.
I can live with that.
Greetings,
Alain,

[mykayel]

Originally Posted by Lenty007

It's a gimmik.
Like Sigma, Tamron uses the same (software)trick to let the body know the lens goes to F/5.6 instead of the actual F/6.3, hereby slightly underexposing the shot at the largest zoomlength.
I can live with that.
Greetings,
Alain,

I don't think it will actually underexpose the shot. It doesn't matter what aperture the lens is reporting to the camera as only a given amount of light is coming in through the lens, and this amount of light the camera uses to determine what shutter speed is needed to properly expose the shot.

[timberlandlh]

I don't know about the lens you speak of...however, my Canon 2X won't couple to my Canon 18-200. Only mounts to my white lenses.

[Emergency Exit]

So it is a software trick. But why does the camera still report (on the screen) f/6.3 at 270mm? Is the AF module independent from the LCD screen info system?

[Lenty007]

Your guess is as good as mine
A short google search showed me this result ... http://www.flickr.com/groups/sigma_l...7625815848449/
Under the picture trere's a phrase that says "(note some zooms that got to f6.3 trick the camera into not seeing that)."
I suppose the lens manufacturers are the clever kind and honestly I'm not worried how they put one and two together.
The lens itself I use on rare occasions and it keeps on surprising me each time.
I'm used to L-type lenses but this one provide good compromising qualities.
Good luck in the far east.
Greetings,
Alain,

[Emergency Exit]

Interesting read. Thanks for the link.

Sorry to add another question yet, but then why does the AF "not work" beyond f/5.6? Is this because the DoF is too wide so that the AF module gets confused as to focus on which object?

[sandpiper]

Originally Posted by Emergency Exit

Interesting read. Thanks for the link.

Sorry to add another question yet, but then why does the AF "not work" beyond f/5.6? Is this because the DoF is too wide so that the AF module gets confused as to focus on which object?

No, it's not due to the DoF, it's to do with the amount of light getting in. At smaller apertures than f/5.6 the image can be too dim for the AF to work properly. If you put a (non-reporting) TC on a slow lens, giving you f/8 say, the AF will still operate, as the camera still sees f/5.6. However, although it may well work reasonably effectively in bright sunshine, in duller conditions it is likely to hunt or fail to find focus altogether. It will certainly be a lot slower.

In order to prevent people moaning that the AF is faulty, Canon simply sets the camera to shut down the AF past f/5.6 (f/8 for 1 series).

From experience, manual focusing works better past f/5.6 than relying on a non-reporting combo and sluggish AF.