Sigma 30mm f/2.8 on the NEX-7.
The Sony NEX platform is still relatively new on the market and the lens range is still relatively modest, so I was quite excited to hear that Sigma announced two new primes for the NEX mount, a 19mm and 30mm f/2.8. This is a review of the 30mm, which gives the same angle of view as a 45mm on a full frame 35mm camera. Sigma is at present the only third party lens manufacturer producing primes for the NEX mount.
The lens comes in a dark gray finish (a tap lighter than the NEX-7 black and less shiny). It is delivered with lens caps and a lens case but no shade. Sigma argues that their new coatings prevents flare without the need for a shade. The shade from the Sony kit zoom does not fit. Filter mount diameter is 46mm. Diphragm is 7 blades, but I have not tested the bokeh. Minimum focus distance is 0.3m (0.98ft) and, unfortunately for the video inclined, focus is quite loud with a distinct click when focus is attained.
One may wonder what is the use for a 30mm prime lens when NEX cameras come with a 18-55mm standard zoom. The advantages are as follows:
- some photographer prefer prime lenses (I do)
- prime lenses are usually faster, this lens is a stop faster than the standard zoom at 30mm (which, frankly is not that much)
- prime lenses can be smaller, so called "pancake lenses" are of great interest for a system selling itself on size
- prime lenses are easier to correct than zooms and can therefore be sharper.
So is the Sigma 30mm f/2.8 that small? Here are some comparison views between the Sony 16mm f/2.8 pancake, the Standard NEX 18-55 zoom and the Sigma 30mm f/2.8. As you can see, it is more compact than the zoom but not as small as the pancake. The gain over the zoom is real, though.
|The standard zoom, pancake and Sigma 30mm on the NEX-7||The standard zoom, pancake and Sigma 30mm size comparison|
What about optical quality, then? After all, the NEX platform already has the Sony / Carl Zeiss 24mm f/1.8, which is as big as the standard zoom and principally sells itself on aperture and quality. The Sigma is a fifth of the price, it does not have the f/1.8 aperture, but does it have the quality? Considering that the NEX-7 puts quite a strain on lenses with its 24 mpix resolution, what can we expect from a 200€ lens?
Designing compact lenses for the NEX platform is not easy, because of the small registration distance. The Sony Pancake, in particular, is known to have its share of problems when combined with the NEX-7. People using lenses designed for rangefinder have also noted that some of them give poor results, exhibiting extreme corner darkening or color shift across the frame. My understanding is that the sensor response is poor when light rays hit it at a steep angle, as is the case for rangefinder lenses. Sigma said that they have addressed this issue by designing the lens as telecentric and indeed the rear lens is surprisingly large compared to the front one:
|Sigma 30mm rear lens||Pancake, Sigma 30mm and 18-55 zoom front views.|
But is the lens sharp?
Testing sharpness for an amateur is not very easy, because the common methods like photographing a newspaper hung on a wall are fraught with potential errors. The method is used is different: is take a picture of a subject far away. Everything is at the same distance, since everything is far away. There are no consideration of the target needing to be perpendicular to the lens axis or flat. There is little risk or errors. As a bonus, lenses are tested at distance more akin to the distance they are used in practice: some optics give poor results at the typical distance used for a test target, but are good when the subject is further away or vice-versa. To insure that there is a target from the center of the lens till the corners of the frame, I took a picture of the city skyline and oriented it corner to corner, like this:
|Horizon skyline as diagonal, corner to corner target (taken with the Sigma 30mm at f/2.8)|
By examining the difference between the corners and the center, or the difference between the results at different apertures, or the difference between different lenses, we can see which lens is sharp and which lens is… less sharp. It is a very simple procedure, but quite informative for the photographer. For example, here are the results for the 16mm pancake at f/2.8:
|Pancake center crop||Pancake lower corner crop|
While the pancake has good center sharpness it exhibits very poor corners wide open. The difference is obvious.
And the Sigma?
This is the good surprise: at f/2.8 already, it is sharp corner to corner. It vignettes a little bit wide open (disappears when stopping down) and has a little bit of lateral chromatic aberration (which does not disappear when stopped down but is easy to correct), but apart from that it is close to be a perfect lens. Recommended.
|Sigma 30mm f/2.8 center crop||Sigma 30mm f/2.8 lower corner crop|
You may download the full resolution pictures at f/2.8, f/4, f/5.6, f/8 and f/11 from my flickr account: http://www.flickr.com/photos/jerome_munich/sets/72157629651034759/detail/
The views and opinions expressed in this article are those of the author and do not necessarily reflect the views and opinions held by dpreview.com or any affiliated companies.