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Comparing lens sharpness at 100mm:

Camera: Canon EOS 5D
Lenses: Canon EF 100mm f/2 USM, Canon EF 24-105mm f/4L IS USM, Canon EF 70-200mm f/2.8L IS USM
Conditions: Indirect mid-morning sun, tripod-mounted, ISO 100, Zoomed to 100mm
Processing notes: ACR 3.3, Sharpness 25, Identical settings for entire batch

Original, cropped square (70-200, f/8)

f/2.8 detail, 100%:

	N/A
100mm f/2	24-105mm f/4L	70-200 f/2.8L

f/4 detail, 100%:

100mm f/2	24-105mm f/4L	70-200 f/2.8L

f/8 detail, 100%:

100mm f/2	24-105mm f/4L	70-200 f/2.8L

f/11 detail, 100%:

100mm f/2	24-105mm f/4L	70-200 f/2.8L

f/8 crop, ~30%:

100mm f/2

24-105mm f/4L

70-200 f/2.8L

The Lineup

Conclusions:

Canon EF 100mm f/2 USM

• Noticably sharper than the 70-200 at f/2.8. Nearly indistinguishable at f/8 and slower
• Great all-around performer
• Good value

Canon EF 24-105mm f/4L IS USM

• Not quite as sharp as the 70-200
• Although zoomed to 105mm for all shots, FOV is still wider than the 100mm prime and 70-200mm at 100mm. Appears closer to 95 mm by my estimate.

Canon EF 70-200mm f/2.8L IS USM

• Strongest performer at f/4 and slower
• Almost as good as the prime wide-open
• Slightly cooler color rendition than its rivals