Similarly priced to Nikon's own 70-300mm lens, is this third-party option any better?
Sigma 70-300mm f/4-5.6 DG OS Review
The Sigma employs a wide zoom-ring towards the middle of the lens and a forward-positioned manual focusing ring that rotates in AF mode, slightly restricting the grip that can be used when the lens is fully extended. There is a focused-distance scale and depth-of-field markings for the 70mm focal length. The AF/MF switch and the OS Off/On switch are at the rear of the lens.
Sigma’s Optical Stabilizer offers only a single mode but is effective and exceptionally quiet. The AF mechanism is a little noisy but at least it’s quick enough to capture most common subjects.
As is the case for other lenses with a forward-located focus ring, the reversible lens hood prevents manual focusing from being used below about 200mm. In addition, removing or fitting the lens hood makes the lens barrel rotate and this can be both inconvenient and disconcerting.
Sigma warns: ‘when you use the lens in rain or near water, keep it from getting wet,’ adding, ‘it is often impractical to repair the internal mechanism, lens elements and electric components damaged by water’. Use on the beach to photograph surfers during testing passed uneventfully so Sigma’s warning, albeit sensible, should not prevent careful use in a variety of environments.
MTF testing produced good results for the 70mm and 135mm focal-length settings but at 200mm there was an uncharacteristic dip in wide-aperture performance. Also, the lens narrowly failed to hit the critical 0.25 cycles-per-pixel level when set to its maximum focal-length, though this behaviour isn’t unusual for a lens of this type.