The Canon EF 24-105mm f/4L IS USM ($999.99) is a pricey lens in the company's premium line of L-series glass. Its design is appealing on paper: a 4.4x zoom with a maximum f/4 aperture throughout its range, a tough, dust- and splash-resistant design, and a close focus range that projects images onto the sensor at about one quarter life-size. At wider angles it's quite sharp, albeit with dimmed corners, and if it maintained that level throughout its range we'd recommend it highly. But image quality falls off as you move beyond 50mm. If you're in the market for a 24-105mm, you're better off with our Editors' Choice, the Sigma 24-105mm F4 DG OS HSM.

Design
The 24-105mm measures 4.2 by 3.3 inches (HD), weighs about 1.5 pounds, and supports 77mm front filters. Like other L lenses it has a rubber gasket around the lens mount and, when coupled with a front filter, is protected from dust and splashes. A lens hood and carrying pouch is included, along with standard front and rear caps.

The zoom is at its shortest length at the 24mm position. The barrel extends in a telescoping fashion as you twist the rubberized zoom control ring, positioned at the base, toward 105mm. It's joined by a manual focus ring, which is also covered in rubber and located toward the front of the polycarbonate barrel.

A window shows the set focus distance in feet and meters. Two controls sit on the side—one switches between automatic and manual focus, and the other turns the image stabilization system on or off. Even when the lens is set to autofocus, you can turn the manual ring to fine-tune the focus to your liking after the AF system has locked onto its target.

The focus system is powered by a USM motor; it drives the lens across its full focus range and fires a shot in about 0.6-second. Canon offers another, less expensive zoom that covers this range, the EF 24-105mm f/3.5-5.6 IS STM. Its aperture narrows as you zoom, in contrast to the f/4 capture that the EF 24-105mm f/4L maintains throughout its range, and it uses an STM focus motor—a preferred choice for video, but generally not as powerful when shooting stills.

The 24-105mm can lock focus as close as 17.7 inches (45cm). That's good enough to capture photos at one quarter life-size at 105mm. The Sigma 24-105mm matches the Canon in close focus capability. If you want more magnification out of a zoom lens, consider Canon's unique EF 24-70mm f/4L IS USM, which can focus to 7.9 inches, netting 1:1.4 magnification—near life-size.

Image Quality
I used Imatest and the 50-megapixel EOS 5DS R to evaluate the 24-105mm's optical performance. At 24mm f/4 it delivers strong sharpness on the 50-megapixel body—3,535 lines per picture height averaged across the frame. That's better than the 2,200 lines we like to see at a bare minimum from a lens. As you'd expect, the center is the sharpest area (3,829 lines), with mid parts hitting just below the overall average (3,411 lines) and edges coming in at a strong 3,133 lines. Narrowing the aperture to f/5.6 bumps the overall score to 3,763 lines, and the lens peaks at f/8 (4,042 lines). Diffraction cuts into sharpness starting at f/11 (3,659 lines) and is more of an issue at f/16 (3,125 lines) and f/22 (2,347 lines).

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At 35mm the overall performance at f/4 drops slightly to 3,417 lines, but edges are still strong (2,913 lines). At f/5.6 the score jumps to 4,125 lines, and stays strong at f/8 (3,922 lines) and f/11 (3,608 lines). Quality drops at f/16 (3,087 lines) and f/22 (2,290 lines).

Results remain excellent at 50mm—3,156 lines at f/4, 3,493 lines at f/5.6, 3,619 lines at f/8, and 3,539 lines at f/11, before giving way to the expected drop in fidelity at f/16 (3,167 lines) and f/22 (2,433 lines). But it's at 50mm that we first see some weakness at the edges of the frame. At f/4 and f/5.6, the outer third of the image drops to around 2,200 lines, our threshold of acceptable clarity, before improving at f/8 (2,700 lines) and f/11 (3,000 lines).

At 70mm image quality takes a serious hit. The overall score at f/4 drops all the way to 2,528 lines, with edges that are noticeably blurry (1,412 lines). There's a bump at f/5.6 (2,819 lines), but the periphery is still weak, as it is at f/8, where the lens puts up strong enough numbers in the center to pull the average score all the way up to 3,093 lines. At f/11 the zoom shows 3,253 lines across the frame, and edges finally hit the minimal marks, notching about 2,276 lines. You don't lose much by shooting at f/16 here (3,069 lines), but there's a hit at f/22 (2,431 lines).

Clarity is weaker still at 105mm. At f/4 the lens hits 2,319 on average, again thanks to a sharp center, but the mid parts of the frame are too soft (2,020 lines) and the edges are a blur (996 lines). Performance hits the basement of acceptability at f/5.6—2,470 average, with the mid parts of the frame showing 2,218 lines, but with same blurry edges as you get wide open. At f/8 the average score improves (2,615 lines), and it's better still at f/11 (3,012 lines)—in both cases the center and mid parts of the frame are fine, but the periphery is still too soft, at 1,154 and 1,944 lines, respectively. It's not until f/16 that the entirety of the frame is crisp; the average score is about the same as at f/11, but edges jump to 2,400 lines there. Again, avoid f/22—there's a significant drop in image quality when the aperture is that narrow.

Sharpness is strongest at the wide end, but so is distortion. At 24mm the zoom shows a whopping 5.5 percent barrel distortion, drawing straight lines with the outward bow that resembles a mild fish-eye effect. Thankfully that drops to just 1.2 percent by 35mm, which is barely noticeable in the field, and the 1 percent pincushion distortion that sets in at 50mm is likewise negligible. You can notice the 1.3 percent inward pincushion effect at 70mm and 105mm, however. Thankfully lens distortion isn't a big deal for shutterbugs who post-process images—you can correct it with a single click in image editing software like Adobe Lightroom. But if you don't like to spend a lot of time editing photos, it's a concern.

The 24-105mm also has problems with even illumination at its widest angle. Corners and edges are noticeably dim at 24mm, showing about a 2.5-stop loss (-2.5EV) at f/4 and -2EV at f/5.6. It's better controlled at f/8 and narrower, within the 1-stop we want to see in a photo. When zoomed further in, the lens does a better job—corners lag behind the center by around 1EV at f/4 when tested at 35mm, 50mm, 70mm, and 105mm, and that delta is decreased when stopped down to f/5.6 and beyond.

Conclusions
There's no way around it, the Canon EF 24-105mm f/4L IS USM underperforms, especially when you consider its price tag and its place in Canon's illustrious L lineup. It promises a longer zoom range than the EF 24-70mm f/4L IS USM, but doesn't match it in optical quality. It does live up to L standards in terms of build quality. The less expensive Canon EF 24-105mm f/3.5-5.6 IS STM doesn't feel as sturdy, and it doesn't capture as much light when zoomed, but it delivers crisper results throughout its zoom range. For the same price, the Sigma 24-105mm F4 DG OS HSM delivers better image quality all around, while maintaining the same macro magnification, zoom range, and maximum aperture. It's our Editors' Choice for this type of lens.

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