Sigma 17-70mm f/2.8-4 DC Macro OS HSM Contemporary

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Macro lens

A macro lens by Sigma for APS-C cameras, 17-70mm f/2.8-4. The Pentax version has no optical stabilisation and relies on the camera's in-body stabilisation.

Maker	Sigma
Focal length	17-70mm (macro)
Maximum aperture	f/2.8-4 (it gets darker as you zoom in)
Made for	APS-C sensors
Mounts	Canon EF, Nikon F, Pentax K
Stabilised	yes
Autofocus	yes

What it frames like

Its full-frame-equivalent focal length on the sensor it was made for.

How wide 17mm sees at its wide end on each sensor it covers.

f/2.8 wide open, to scale against f/1.4.

On APS-C (Nikon F, Pentax K): like 26-105mm on full frame; f/2.8-4 gives about the depth of field of f/4.2-6 there.
On APS-C (Canon) (Canon EF): like 27-112mm on full frame; f/2.8-4 gives about the depth of field of f/4.5-6.4 there.
Not usable on the larger-sensor bodies of Canon EF: they have no crop mode (and Canon's EF-S lenses don't even mount on them), so it is for APS-C bodies only.
On the larger-sensor bodies of Nikon F, Pentax K: it works with the camera in a APS-C crop mode, at fewer megapixels.