Resolution comparison of three EF macro lenses 50mm 1:2.5, 100mm f2.8 L USM and 180mm f3.5 L USM at their maximum magnification. In addition, Canon Extender 2x II and (hollow) 65mm extention tubes were used. The total magnification effect is also listed in the table. Magnification indicates how many times the image of an object on a negative (or on the chip) is larger than the object itself. A resolution chart image recorded on a 35mm negative film was used for this test.  

EF Lens	Magnification	Res. at f:16 at center	Res. at f:16 at edge	Details (Magnification x Resolution)
50mm	0.5	26	-	13
50mm with ext. tubes	1.7	17	-	29
100mm	1.03	20	19	20
100mm with ext. tubes	2.02	13	-	26
100mm with ext. tubes and 2x extender	3.81	7	-	27
180mm	1.01	24	21	24
180mm with 2x extender	2.03	21	-	42
180mm with 2x extender and extension tubes	2.85	9	-	26
180mm with extension tubes	1.35	18	-	24

 Conclusions.

1. The macro 180mm f3.5 L USM lens seems to be designed specifically for macro photography. Its sharpness at full macro mode is better than sharpness of the macro 100mm lens.

2. Adding 2x Extender makes a still quite sharp 2:1 macro lens. One bad thing is that the auto-focus feature is disabled with it. It doesn't matter for extreme macro, but for shooting a butterfly from 3.5m a much better choice is EF 400mm DO IS USM lens. The 2x Canon extender is not compatible with the 100mm macro lens.

3. Use of extension tubes is not recommended for 100mm and 180mm lenses, however, dramatic loss of performance is observed only with the 180mm lens. The tubes are compatible with the 50mm lens.

Tamron macro AF 3.5 180mm SP DI lens
I've found on the photozone page that Tamron 180mm lens is rated the highest, and got a perfect score of 5.0 there (the Canon macro 180mm got only 4.2 points). I've tested the lens and the sharpness in the standard mode was indistinguishible from the sharpness of Canon 180mm macro lens, however, at their macro mode the Tamron lens performed slightly worse, at 22/19 (center/edge). The Canon 2x Extender is not compatible with it. Its plastic design and slower autofocus, compared to the Canon lens, is a fair trade off for the almost 2x lower market price.

 

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