+ CANON 15-85mm f/3.5-5.6 USM IS + SHARPNESS TEST +

I recently had the following three lenses with similar focal lengths, so I ran a simple comparison test for the sharpness among them.
最近做了以下三支鏡頭的銳利度比較，提供大家參考。



From left to Right:
CANON 15-85mm f/3.5-5.6 USM IS
SIGMA 17-70mm f/2.8-4.5 MACRO
TAMRON 17-50mm f/2.8

In general, Canon 15-85mm has advantage of wider range of focal length and Image Stabilizer (IS). Sigma 17-70mm has advantage of macro capability. Tamron 17-50mm has advantage of large aperture of f/2.8.
All the photos were taken with the aperture of f/8 which is usually the sharpest aperture for a lens. Other aperture setting was not tested due to time restrain. The camera used for testing is CANON T1i (500D). Focus was set on the tree at the center of frame.

一般來說這三支鏡頭各有所長：
Canon 15-85mm 有最廣的焦距範圍及防手震IS。
Sigma 17-70mm 有最強的微距效果。
Tamron 17-50mm 有恆定大光圈 f/2.8。
以下測試光圈都設在f/8.0，相機是CANON 500D，對焦在中心點的樹。

FOCAL LENGTH = 15mm:


CENTER (100% CROP)

CORNER (100% CROP)


FOCAL LENGTH = 17mm:




CENTER (100% CROP)




CORNER (100% CROP)




FOCAL LENGTH = 50mm:




CENTER (100% CROP)




CORNER (100% CROP)




CONCLUSIONS:
1. Each lens produces good sharp image at the center.
2. Sigma 17-70mm seems to stand out for the sharpness at the corner.

測試的結果，三支鏡頭的銳利度已經在水準以上。
比較讓我驚訝的是Sigma 17-70mm這支鏡頭竟然比Tamron 17-50mm銳利，這跟各大網站出現的數據不符。
或許對焦無限遠時並不是Canon 與 Tamron 這兩支鏡頭的最佳光學狀態吧？


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