Can the iris be manually adjusted at all? And if so, can you use this with an adapter on a camera that doesn't support auto iris?

The "iris" is adjusted by adjusting your aperture settings. You can do this either by setting your camera to aperture priority or setting it to manual and adjusting your ISO, shutter speed, and aperture settings. Where in manual or aperture priority, you choose settings according to a meter (either the camera meter or a desperately purchased and more accurate external meter). This is the case in any SLR camera/lens.

Can you shoot at f2.8 eventhough you zoom?

It does fine when you're closer to 70mm. But if you can't really use it when it's closer to 24 mm I would recommend saving up for the tamron 24-70mm f2.8

Real Angle Of View	84.1 Degrees
Maximum Aperture	2.8 f
Minimum Aperture	22
Zoom Ratio	2.92:1
Compatible Camera Models	Canon EOS-1Ds, Canon EOS-1Ds Mark II, Canon EOS 6D Mark II, Canon EOS-1D X Mark II, Canon EOS-1Ds Mark III, Canon EOS 5D Mark II, Canon EOS-1D, Canon EOS 5D, Canon EOS-1D Mark II N, Canon EOS 6D, Canon EOS 5D Mark IV, Canon EOS-1D X Mark III, Canon EOS-1D Mark III, Canon EOS 5D Mark III, Canon EOS D60, Canon EOS-1D Mark IV, Canon EOS 5DS R, Canon EOS D30, Canon EOS-1D X, Canon EOS 5DS, Canon EOS 10D
Photo Filter Size	77 Millimeters
Lens Mount	Canon EF
Number of Diaphragm Blades	9
Minimum Focal Length	24 Millimeters
Lens Design	Zoom
Lens Fixed Focal Length	70 Millimeters
Lens Coating Description	Super Coating
Lens	Standard
Compatible Camera Mount	Canon EF
Maximum Focal Length	70 Millimeters

Sigma 24-70mm f/2.8 IF EX DG HSM AF Standard Zoom Lens for Canon Digital SLR Cameras

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