Sigma 50-500mm Aperture reading

I've just been playing with my new Sigma 50-500mm which so far I have found great!

I am puzzled by the maximum aperture setting displayed on my D60. The lens detail says f4-f6.3 and when I am at 50mm the f4 matches my D60. However, when zoomed out to 500mm, the display shows f5.6, not f6.3.

Can anybody tell me why please ?

The lens "lies" to the camera, so that the Autofocus will still work - if it reported f6.3, many cameras (including, I think, the D60) will refuse to even try to AF, since their spec requires a minimum open aperture of f5.6.

The correct aperture is used at the time of taking the shot, however.

KRs
Chris

I just bought the Sigma 50-500 2 days ago too. The image quality is good. The simple user manual comes with the lens says this regarding the exposure mode for Sigma AF and Canon AF:
"Although the camera will indicate f5.6 at tele side as the full aperture setting to allow autofocusing, the exposure will be correct. However, when you are in the manual exposure mode and are using an external exposure meter, increase the exposure by 1/3 step over the meter reading."

My understanding of the above statement: Sigma fools the camera to think the aperture is f5.6 to allow AF, but the camera meter still functions properly if you are in any auto mode. Compensation has to make if the camera is used in manual mode and set according to the reading of an external meter.

Thanks for the replies.

I must admit that I didn't read the manual .... oops! After 20 years of playing with PC's, you think I'd finally remember to RTFM once in a while with the camera stuff as well !