Sigma 18-35mm F1.8 DC HSM "Art" Review - General Image Quality

Sigma 18-35mm F1.8 DC HSM "Art"

General Image Quality

The 18-35 F1.8 is peculiar because at a constant F1.8 aperture across the entire zoom range, it may certainly be described as "a bag of primes." Before we get to the sharpness and other image quality aspects to assess its "prime-ness," we wanted to see what this Sigma was all about in its most basic image quality facets.

18mm to 35mm Zoom Range

Should the Sigma 18-35 be your first zoom (as a prime aficionado) or "just another" lens you're feeding your Lens Buying Addiction with, we wanted to give a quick demonstration of what you can expect regarding the field of view differences throughout its zoom range from widest to narrowest (18mm to 35mm).

Additionally, this lens is going to be considered by many of you against other alternative lens options— namely the DA* 16-50mm F2.8, FA 31mm F1.8 Limited, and HD DA 20-40mm F2.8-4 Limited lenses from Pentax, all of which will be compared to in this review later on. As such, we will show the following zoom range not just using the min and max on the Sigma, but comparing also what the Sigma's zoom range looks like versus 16mm through 50mm per Pentax's premium standard zoom.

The below are from the same set of images (all shot on a tripod at F10, ISO 100, and 2s Timer using the K-3). Depending on what you prefer - the one photo or the animated .gif slideshow - we decided to include both. First, here's the consolidated photo, which you can view a much larger version simply by clicking on the image below:

*The lightpole really is crooked like that - not the lens's fault!

Next, here's the animated .gif:

While this may come across in bad taste to some, we couldn't help but be underwhelmed by the zoom range. Yes, it's a zoom lens with a constant aperture of F1.8. We aren't taking away from Sigma's incredible achievement there. It's just that, if we are going to use this lens as a standard zoom for what it was designed for— events, weddings, concerts, etc.— we can't help but feel 18mm is not wide enough. The two millimeters may seem trivial, but for those who have shot events and indoor settings (or even just landscapes), that extra wideness, however trivial it may seem, makes a tremendous difference.

Again, we are not trying to speak poorly of what Sigma accomplished - it's just that 16-30mm would have been far more versatile and preferred. 15-30mm would have been even more amazing, even if only 15-28mm. Also, because of how sharp this lens is (supposedly - we'll get to that!) and how high resolution digital sensors are these days (and will continue to become), cropping to a 35mm field of view would be of no significant consequence and detriment to image quality.

Color and Contrast

The next thing that we wanted to assess was how the lens produces contrast, tone, and color. We tested the lens this way in a variety of situations and found that no matter the focal length, the lens will produce contrasty and punchy colors. With one exception...

Contrary to what we've done in the past where we take a picture and compare "RAW output" vs "Auto Levels" for contrast and color, we noticed something peculiar between maximum and closed down apertures. In both extremes of the focal length range shown below, you'll notice that the output at wide open is significantly "colder" than when closed down. At first we thought we had made a mistake in the white balance settings, but even after double checking and even re-shooting the scene (including across multiple lens samples), the same results ensued. Also, both of the sets of images below (so all four images taken) were not only shot at the same white balance setting by the K-3, but they were also all set to "Daylight" (5500K, +10 hue) in Lightroom to check consistency. No matter how you adjust it, the difference is still there between wide open and closed down.

For the below images in this section, you'll notice a green icon in the middle of them, illuminating once you move your mouse over the image. Grabbing it and sliding the bar to the left or right will allow you to alternate between the two versions for a much easier and dynamic juxtaposition/comparison.

18mm at F1.8 and F10

As discussed above, 18mm at two different apertures. These were taken literally within seconds of each other, so any differences caused by daylight are irrelevant - this is certainly a property of the lens.

35mm F1.8 and F8

Same as above, only this time we used F8. Once again - these two shots were taken within seconds of each other (and about 15 seconds after the pair at 18mm):

Just to be sure, we made some tests with other lenses, and aside from the expected vignetting while open, the dramatic color cast shift was unique to this Sigma lens.

It almost looks like the 18-35 slaps on a polarizer lens with how much richer the skies and the foliage became. Not necessarily the end of the world, however something to be aware of, particularly for those whom intend to use this lens wide open most of the time (why else would you get it?).

Metering and Exposure

We found the Sigma 18-35 to meter very accurately no matter the camera it was mounted on. The accuracy and consistency of its metering and resulting exposures only increased when used with a K-3. We attribute this metering difference to simply the K-3's far superior metering system as compared to all its predecessors.

Lens Correction and EXIF Data

Because the lens is not a Pentax lens, it does not benefit from certain in-camera lens corrections, namely (1) distortion, (2) chromatic aberration corrections, and (3) vignetting (named "Peripheral Illumination Correction" in the K-3). This is because the software necessary for such functions are embedded in each individual lens, activating when the lens is mounted on a Pentax K-mount body. Because Sigma is a third party lens maker, such functions are not supported.

If you are confused by that and whether you'll accidentally trip one of the settings, then don't fret - Pentax made it fool-proof. You don't even have the ability to access the specific menu options (1, 2, and 3 from above, identified below). This is clearly seen in the side-by-side images of the K-3's quick menu screen. On the left the camera is fitted with a Pentax lens, and on the right the Sigma "Art" lens is mounted, with the latter clearly lacking the bright blue of the other options:

Regarding the exif, because it's not a Pentax lens, don't expect the lens ID to be transferred to your post processing software. It's not a deal breaker, especially since it still records the critical settings, however the lens does not transmit the lens ID:

For those who use Adobe Lightroom, the latest version now includes an embedded lens profile for the Sigma 18-35 to select from out of the hundreds of included profiles to apply Adobe's predetermined lens corrections. For those unfamiliar with Lightroom, it can be done in as little as four or five clicks, so it's not a huge deal breaker.

On-Board Flash Usage

Not exactly image quality per se, but one thing that we had forgotten to look at with our reviews until now was simply the ability of the lens to be used with the flash that's built on the top of your camera. We know, we know - this is an uber fast lens that defies the need for flash, especially the pop-up one. Nonetheless, we wanted to take a look in case this small caveat could make or break the usability of the lens for one of you. Because even pros sometimes use it for fill flash in a pinch.

The follow images were all shot from the same position (about 2m from the wall) using the 18-35 F1.8 on a K-3 and purposely underexposed to emphasize the effect.

Click on any image below to open an enlargement and cycle through them.