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Get information and user reviews for this lens from Amazon: Sigma 135-400mm f/4.5-5.6 APO DG Aspherical AF Zoom Telephoto Lens for Nikon AF
The Sigma AF 135-400mm f/4.5-5.6 APO Aspherical RF is not a perfect lens nor can you expect that a budget lens reaches high end quality levels. Nonetheless it is capable to provide very decent results especially in the 135-300mm range. At 400mm @ f/5.6 the quality is quite soft but from f/8 and up the quality is very respectable. The distortion characteristic is almost perfect and vignetting isn’t really field relevant on an APS-C DSLR. CAs are a quite pronounced at 135mm but the overall level is still acceptable. Mechanically the lens leaves more to be desired. The quality of the used materials is fine but the friction of the zoom ring is way too high and the worst implementation that I’ve seen so far. There’s no such thing as a free lunch but admittedly the Sigma offers quite a bit of possibilities for your bucks. READ FULL REVIEW