Sigma makes good lenses for Canon and Nikon digital SLRs.   There are many terms used in Sigma lenses such as EX, OS, ASP etc.  These terms are  very confusing for beginners.  

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Best third party lenses for Canon DSLRs

Best third party lenses for Nikon DSLRs

   Aspherical lenses. 
This lens provides high optical performance while maintaining a compact size. 

   APO Lens

SIGMA’s APO zoom lenses minimize color aberration. 

   OS (Optical Stabilizer) Function
This function utilizes a built-in mechanism that compensates for camera shake. It dramatically expands photographic possibilities by alleviating camera movement when shooting by hand held camera.

   APO Teleconverter EX
This lens can be used with the APO Teleconverter EX. It can increase the focal length and will interface with the camera’s AE (automatic exposure) function. 

   EX Lens
The exterior of this lens is EX-finished to denote the superior build and optical quality, and to enhance its appearance.

   Hyper-Sonic Motor
This lens uses a motor driven by ultrasonic waves to provide a quiet, highspeed AF.

   Inner Focus
To ensure stability in focusing, this lens moves the inner lens group or groups without changing the lens’ physical length.

   Rear Focus
This lens is equipped with a system that moves the rear lens group for highspeed , silent focusing.

   DG Lens
The most suitable lenses for 35mm film single-lens reflex cameras, as well as for digital SLR cameras. Sigma’s development of the DG (Digital) range of lenses has concentrated on the correction of distortion and aberrations. 
   DC Lens
These are special lenses designed so that the image circle matches the smaller size of the image sensor of most digital SLR cameras. Their specialized design gives these lenses the ideal properties for digital cameras, the compact and lightweight construction is an added bonus!

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