Sigma AF 20mm f/1.8 EX DG RF (DX) - Review / Lab Test Report
Lens Reviews - Nikon / Nikkor (APS-C)
Article Index
Introduction
Analysis

Review by Markus Stamm, published March 2013

Introduction

Sigma offers a trio of fast wide angle lenses, featuring focal lengths of 20, 24 and 28 mm. All three lenses share a maximum aperture of f/1.8 and a very short minimum focus distance, which allows for interesting and often dramatic perspectives of close subjects.

The lenses are designed for full frame cameras, but can of course be used on DX DSLRs, too. In this review we'll have a look at how the 20mm lens performs on our current DX test camera, the Nikon D7000, where it is the equivalent of roughly a 30mm f/2.8 lens.

Due to the similarities of the lenses, the three reviews share large portions of text.

The lens belongs to the high end line of Sigma lenses (EX) and consequently the build quality is on a high level. Unlike its longer sister lenses, the lens features a rear focus group, which has two positive side effects: the physical length of the lens remains constant regardless of the focus setting and there is no wobbling inner lens tube. The lens therefore feels way more solid than the 24mm and 28mm lenses.

The broad focus ring works as a clutch to switch between AF and MF mode. When pulled towards the camera, the lens can be focused manually, while the AF still works as long as the camera remains in AF mode. This way the lens allows for FTM (full time manual focusing), a feature usually only available on Nikon AF-S or Sigma HSM lenses with ultrasonic wave drives.

Typical for many Sigma lenses the focus ring of the EX 20 rotates in "Canon" style, which is the exact opposite of how these rings work on Nikkor lenses. If you're used to the Nikon way of focusing, the Sigma lens may be a challenge for your muscle memory. In addition, you'll have to remember that the focus assist in the viewfinder assumes a Nikon lens and consequently suggests the wrong direction of focus ring movement.

The front element does not rotate during focusing, so using a polarizer is easily possible. However, because of a fairly large filter diameter (82 mm) a polarizer, especially a higher quality one, easily becomes a rather expensive accessory.

Our review unit still featured the old crinkle finish that was typical for Sigma EX lenses for many years. However, Sigma changed the outer finish to a smoother one for the whole portfolio. You can have a look at our review of the Sigma EX 28/1.8 to get an impression of what the current finish looks like. If you purchase the EX 20 new today, it will also feature this smoother finish, unless it has been sitting on the store's shelf for a few years already.

The lens has no built-in AF motor and relies on a drive screw operated by the camera (which makes it incompatible with entry-level DX cameras that no longer feature this screw drive). Thanks to a rather short focus path the AF works quite fast.

Specifications
Equiv. focal length30 mm (full format equivalent)
Equiv. aperturef/2.7 (full format equivalent, in terms of depth-of-field)
Optical construction13 elements in 11 groups including 2 aspherical elements
Number of aperture blades9
min. focus distance0.2 m (max. magnification ratio 1:4)
Dimensions88.5 x 89.5 mm
Weight520 g
Filter size82 mm (non-rotating)
Hoodpetal-shaped, bayonet mount (supplied)
Other featuresLens provides distance (D) information to the camera




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