Aperture wont adjust on my canon rebel xt

The older Sigma lens will not work on the new EOS digital cameras...I've go two of them that will only work zoomed in at F4 and zoomed out at f5.6..... I shoot aperture priority with my Rebel XT and works out fine.... depth of field often shallow but, still nice shots.
There is a ton of info on the internet about "re-chipping" the lens....but, looks like too much to gain little.

The aperture can't be larger than the number printed on the lens. For example, if your lens says it's an 1:3.8 - 5.6, then the aperture can't be larger than f3.8. Since your lens is a zoom lens, it can only open that far at the 18mm setting. The farther you zoom out, the smaller the aperture will get.

Were these same lenses the ones you used on the old SLR? An aperture setting can't go lower than the lowest aperture of the lens itself.

Check to see that both cameras are set to the same settings. For example, turn both cameras to manual and set them both to the same aperture, ISO, and shutter speed. This could account for the difference in brightness if one is set to be more light sensitive than the other.

There is a minimal focusing distance for all lenses. If you move closer to the subject than that distance then the camera won't focus on it. Check you lens manual for minimal focusing distance. It is 11 inches for Canon EF-S 18-55mm f/3.5-5.6 USM, for example.

change your manual settings

Light up the scene! Even pro lens would have trouble taking pictures at night. Pro shoot in highly lighted stadium with 10 000$ lens with tripod or monopod. Tamron sells entry level lens, stop searching a trick, you wont find any other than lenlighting the scene!

This is something you normally only see when you are using a non standard lens that doesn't transmit the aperture to the camera.

If you are using the original 18-55mm lens it may either be faulty or the contacts need cleaning. A dry cotton bud should do this. Also, try another lens if you have access to one.

I agree with dovi. I actually emailed canon and here is what they said and how to do it; Thank you for your inquiry regarding your digital SLRs. We value you asa Canon customer and appreciate the opportunity to assist you, and I'm sorry to hear that you're running into the "error 1" message. From what you mentioned, it sounds like the lens might be having problems communicating with your cameras. One thing you could try is toclean the contact points on your lens and your camera. If you have not done this yet you can do so by turning the camera off, removing the lens, then brushing those copper points on both the lens and the camera body with something like a clean eraser. Should problems continue, I would suggest that you go to a local camera shop and try a different lens. That way you can determine whether the problem is due to the lens or to your cameras. I hope this is helpful Jason

whos make is the lens - some sigma/tamron etc lens will need to be rechipped and do not work on later EOS cameras also whats the aperture - there is a maximum that the AF will work with and again some 3rd party lenses have a problem