Sigma 28mm f/1.8 EX DG Aspherical Macro
Lab Test Results
May 19, 2011
by Andrew Alexander
Like its 24mm sibling, the styling of the Sigma 28mm ƒ/1.8 lens suggests that it isn't one of Sigma's recent designs. Also like the 24mm, it competes with lenses from Canon and Nikon, as well as being available in Sony and Pentax mounts.
The lens was designed to fit 35mm film or a full-frame sensor, and will provide an equivalent field of view of approximately 42mm when used on an APS-C-sized sensor. The Nikon version of the lens does not have a built-in motor, so it won't autofocus on consumer bodies such as the D3100 or D5100.
The Sigma 28mm ƒ/1.8 EX DG Aspherical Macro ships with a petal-shaped lens hood, accepts 77mm filters, and is available now for approximately $450.
Mounted on the sub-frame Canon 7D, when used wide open at ƒ/1.8, the lens provides fairly sharp results throughout the center of the frame, degrading to somewhat soft corners. Our sample of this lens appears to be slightly de-centered, favoring the lower part of the frame. Unsurprisingly, there is little difference in terms of sharpness between ƒ/1.8 and ƒ/2; however, stopping down to ƒ/2.8 provides an impressive increase in sharpness, with the same central area of the frame becoming sharp, and only slightly soft in the corners. Stopping down further to ƒ/8 provides further, but minor, gains in sharpness. At ƒ/11 there is some further reduction in corner softness, at the slight expense of central sharpness, but the numbers are fairly small at that point. Diffraction limiting sets in at ƒ/11, but there's very little to note; at ƒ/16 it's more noteworthy, with the image showing average sharpness across the frame. Fully stopped-down at ƒ/22, the lens is neither sharp nor soft.
Mounting the lens on the full-frame Canon 1Ds mark III tells a much different story. Shot wide open at ƒ/1.8, we note some impressive corner softness, though there is a small central ''sweet spot'' of sharpness. Again, stopping down to ƒ/2 provides very little improvement, but ƒ/2.8 transforms the lens into something much better: a large central sweet spot of moderate sharpness, edging to corner softness. Further sharpness gains are possible all the way to ƒ/11, but nothing that really gives you tack-sharp results from corner to corner - the center is good, but there's always some measure of softness in the corners. At ƒ/11 you get sharper corners at the cost of some of the central sharpness, and diffraction limiting has truly set in at ƒ/16. Fully stopped-down at ƒ/22, the lens is slightly soft across the frame.
As we've come to expect from very fast glass, in addition to noting with transverse chromatic aberration (color shifts usually found in the corners of an image, in areas of high contrast) the Sigma 28mm ƒ/1.8 shows signs of longitudinal chromatic aberration - magenta / green color shifts that appear on either side of the focus plane, when the lens is used at its widest settings. CA is fairly well-controlled for this lens, and the results are slightly more significant when stopped down - possibly because corner softness reduces the capacity of our testing softness to actually detect the CA in the first place. See our sample images for more detail.
Corner shading isn't an issue when the lens is used on the sub-frame Canon 7D, showing corners which are just over a third of a stop darker than the center, when used at ƒ/1.8. Otherwise, it's negligible. On the Canon 1Ds mark III however, using the lens wide open at ƒ/1.8 produces corners which are a full stop darker than the center, making corner shading quite noticeable indeed. Stopping down to even ƒ/2.8 drops this falloff to just a half-stop, and it isn't until ƒ/5.6 that corder shading drops to the quarter-stop level.
The 28mm ƒ/1.8 has about as much barrel distortion as you might expect from a wide-angle lens: around +0.4% in the corners, and easily correctable in image post-processing software.
The 28mm ƒ/1.8 uses an older-style electrical motor in the Canon-mount version we tested, producing a slight whine in focusing and average speed: about a second to go from close-focus to infinity. Sigma uses two switches to enable or disable autofocus, and they have to be set in the same assignment for either setting to work correctly. The Nikon-mount lens relies on a body-based autofocus screw, meaning you'll need a newer body such as the D90 (or pricier) to autofocus. In any case, the front element does not turn during autofocus.
The 28mm ƒ/1.8 is listed as a macro lens, producing an impressive 0.34x magnification at its minimum close-focusing distance of just under 8 inches (20 cm).
Build Quality and Handling
The lens is finished in Sigma’s matte black rubbery-feeling material, weighing in at a solid 500 grams (17.6 oz), with a metal lens mount and plastic 77mm filter rings. The lens features two controls, both of which are used to enable or disable autofocus. The first switch is what you might expect - a simple toggle switch which enables or disables autofocus - but the second and less intuitive switch is that to manually focus you must physically move the focusing ring rearward. You'll know you can then focus manually because you expose a macro magnification scale. However, there's no doubting that the arrangement is potentially confusing, and neither manual nor automatic focusing will work if the switches aren't aligned to the same setting. Otherwise, the lens is fairly bare, with only a distance scale under a window providing distance information in feet and meters. There is no depth-of-field scale, nor is there an infrared index (at least, on the Canon-mount version: Sigma's website seems to show a Nikon-mount version, with depth-of-field scale, and an honest-to-goodness aperture ring).
The focusing ring is fairly large at 1 1/4 inches wide, a rubber ring with ribs running lengthwise. The ring turns ninety degrees through its range, and ends at hard stops at the close and far points. The ring focuses slightly past infinity. There is some slightly lens extension, about a half-inch, as the lens focuses towards infinity.
The LH825-03 lens hood is a short petal-shaped hood, which can attach to the lens via a bayonet mount for storage.
Canon EF 28mm ƒ/1.8L USM ~$500
The Canon is sharper than the Sigma at all apertures, but exhibits slightly more chromatic aberration. Corner shading is slightly more substantial, especially on full frame. Distortion is about the same. Better build quality and more straightforward focus options; the Sigma costs slightly less.
Nikon 28mm ƒ/1.4G ED AF-S Nikkor ~$?
A legendary Nikon lens that we haven't had the opportunity to test, and only two-thirds of a stop faster than the Sigma, the lens used to be found selling for high prices in used markets until the introduction of Nikon's newer 24mm ƒ/1.4 AF-S, which offers significantly better results, but for a vastly larger price tag.
Pentax 31mm ƒ/1.8 AL Limited SMC P-FA ~$1,000
We haven't tested this lens, and you probably will have a fairly tough time finding a copy thanks to its Limited edition status. In Pentax mount, this is closest comparison. The user reviews of this lens speak very highly of it.
There's a lot to like about the Sigma 28mm ƒ/1.8 - while it doesn't offer its best results for sharpness or corner shading when used wide open at ƒ/1.8, especially on full-frame, it's actually fairly decent.
The VFA target should give you a good idea of sharpness in the center and corners, as well as some idea of the extent of barrel or pincushion distortion and chromatic aberration, while the Still Life subject may help in judging contrast and color. We shoot both images using the default JPEG settings and manual white balance of our test bodies, so the images should be quite consistent from lens to lens.
As appropriate, we shoot these with both full-frame and sub-frame bodies, at a range of focal lengths, and at both maximum aperture and ƒ/8. For the ''VFA'' target (the viewfinder accuracy target from Imaging Resource), we also provide sample crops from the center and upper-left corner of each shot, so you can quickly get a sense of relative sharpness, without having to download and inspect the full-res images. To avoid space limitations with the layout of our review pages, indexes to the test shots launch in separate windows.
Sigma 28mm f/1.8 EX DG Aspherical Macro
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