Sigma's 24-105mm f/4 DG HSM OS Art is affordable, its build quality and excellent centre-of-frame resolution make it worthy of consideration, particularly for owners of 'full frame' DSLRs who want a capable general-purpose lens.
http://www.photoreview.com.au/reviews/lenses/full-frame/sigma-24-105mm-f-4-dg-os-hsm-art-lens
It delivers a pretty good overall performance too, with impressively high resolution in the APS-C image circle and very usable performance in the outer areas of a full-frame sensor. It certainly leaves one wanting in the corners of a 36 MP full-frame body like the D800 but with lower resolution bodies the deterioration in the corners is not as pronounced, and you can always stop down.
http://cameralabs.com/reviews/Sigma_24-105mm_f4_DG_OS_HSM_Art/
Sigma 24-105mm f/4 DG OS HSM Art Lens for Canon EF, $649
https://www.bhphotovideo.com/c/product/1009621-REG/sigma_635_101_24_105mm_f4_dg_os.html
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