The Sigma 30mm f/1.4 is an interesting character. It is the only lens of its kind for any of the common mounts. For APS-C sensors it is very close to a standard lens with a focal length about the same as the sensor diagonal (28mm), and the f/1.4 max aperture is handy for those low light situations.
Construction quality is good. On the Pentax mount it uses the screw drive focus instead of HSM used for other mounts. It has a nice heft to it and everything works smoothly.
Size wise, it is a fairly large prime, but smaller than most zooms. It has about the same physical dimensions as the kit 18-55mm zoom lens, but weighs significantly more. However, given the f/1.4 aperture it is reasonably sized, and is noticeably smaller and lighter than most mid-level standard zooms like the Pentax DA 16-45m 17-70, or Sigma 17-70. It is more compact than many of Sigma's other primes (like the 28mm f/1.8 and 50mm f/1.4) or zooms, as it doesn't use their standard 77mm filter thread body, and instead uses a 62mm thread size. It doesn't have all the wasted space in the lens body like Sigma's other lenses.
Handling
The lens is a joy to use. Focus is quick and accurate with my K-7 with no focus adjustment. It locks quickly and accurately in all lighting situations. Occasionally it will focus confirm on an object completely out of focus, but this behavior can happen with any lens, and it is easily visible in the viewfinder. Usually this only happens with a strong back light or low contrast object. I usually just point the focus indicator to higher contrast edge or reduce the back light in the image and it focuses immediately.My only complaint with the handling is the focus ring turns during auto focus operation. The lens doesn't have manual override and it lacks a switch on the lens to disengage the focus ring. The user needs to be aware of this and never grip the focus ring tightly during operation. Manual focus is well damped, but a tad light. However, it is above average for an AF lens.
Performance
Optically this lens is bi-polar. It has a fair amount of barrel distortion (1.7% barrel) for a standard field of view lens, that can easily be viewed in field situations with straight lines. However, the lens has a strong emphasis on center sharpness that the design leaves the corners very weak at all apertures making the lens not so useful for architecture shots. Performance at f/1.4 near the center of the frame is also excellent, with good contrast even wide open.
There is a bit of focus shift at f/2 to f/4 or so and there is a bit of a trade off between corner sharpness and center sharpness depending on where you focus. The "Partway" graph shows best performance focus and was from a separate focus set as the center data. If you are using the 11 pt focus system, be sure to select the focus point closest to where you are focusing. Focus and recompose will give poor results near wide open with this lens due to the field curvature. The corners and extreme left and right borders are always soft and don't sharpen much beyond f/2.8.
Corner performance drops due to astigmatism and field curvature at f/1.4, but the lens is still usable at the corners for people and portraiture, and other work where high sharpness is not required, see the test picture of the cat with the eyes in the corner of the frame following the conclusion.
However, the images near the corners are not quite as pleasing, as the astigmatism leads to some less than pleasing blur and the chromatic aberration (CA) is higher near the corners, and the CA increases while stepped down.
Flare resistance is excellent. In contra light (a strong light that is shining at the lens) the lens rarely produces any flare spots from internal elements or the aperture, and if it does they are faint octagons. It also has little veiling glare, the reduction in contrast frequently seen in contra light.
Specular highlights are pleasant, but octagon when stepped down. They are fairly uniform in most situations and don't have strong rings around the highlights. See some samples in following the conclusion.
The lens has a strong hot spot if standing in bright light while taking infrared pictures using a Hoya R72 filter (hood was attached). See sample in images following. It helps standing in a shaded area to kill off any flare causing the hot spot.
Conclusion
The Sigma 30mm f/1.4 is a unique beast. It is the only lens of its aperture and focal length, and is relatively low price compared to similar offerings. It is a full f/stop larger aperture than the SMC Pentax FA 35mm f/2, and significantly wider field of view. It offers 1/2 an f/stop over the SMC Pentax FA 31 f/1.8 Limited, and Sigma 28mm f/1.8, and 2 f/stops over the SMC Pentax FA 28mm f/2.8, and none of these lenses surpasses the Sigma 30mm f/1.4 for center sharpness.However, corner performance on the test charts is dismal, and it is unfortunate they don't improve much stepping down the aperture. That being said, I find in real world applications I don't notice the corners being soft, as my subject matter is not usually architecture where the soft corners or barrel distortion become noticeable.
Strengths
- General Purpose (standard field of view)
- Available Light
- Portraiture
- Night (produces nice star bursts)
Weaknesses
- Architecture
- Cityscapes
- Panoramas
- Infrared
Let's face it: how many standard field of view lenses have an f/1.4 aperture for APS-C sensors? None. The SMC Pentax FA 31mm f/1.8 Limited is certainly an excellent lens, but costs nearly three times as much in the US and is 1/2 an f/stop smaller aperture. So even with the weaknesses of the Sigma 30mm f/1.4, I recommend it as a great available light portraiture and all purpose lens.
Test Images
(click on image for full size)Real world center performance at f/1.4
f/1.4, 1/125, ISO 400, K-7
Real world center performance at f/1.4 (image 2)
f/1.4, 1/80, ISO 400, K-7
Real world corner performance at f/1.4
f/1.4, 1/250, ISO 100, K-7
Scene showing soft left and right edges when stopped down
f/6.3, 10", ISO 100, K-7
Scene showing star bursts at f/5.6 (star bursts are natural, but image has other processing for curves, saturation, etc).
f/5.6, 0.8", ISO 100, K-7
Specular Highlights f/1.4
Specular Highlights f/2.8
Center focus shot with distracting background at f/1.4
f/1.4, 1/160, ISO 100, K-7
Center focus shot with distracting background at f/4
f/4, 1/80, ISO 400, K-7
Infrared image showing hot-spot, Hoya R72 filter on lens.
f/5.6, 5s, ISO 200, K-7
Infrared image without hot spot (was standing in the shade).
f/1.4, 1/30, ISO 3200, K-7 with Hoya R72, processed in camera
And a bunch more from my flickr photostream.
Sigma 30mm in the center with the Rokinon 85mm f/1.4 on the left and the SMCP A 50mm f/1.4 on the right. A great set of lenses for wedding photography.
might also be worth a look.