Canon T4i Optics

Zoom
The Canon EF-S 18-135mm IS STM lens offers more zoom than the typical kit lens, with good performance for its class.

This lens has a generous and versatile optical zoom ratio of 7.5x with a 35mm equivalent range of about 29-216mm, thanks to the T4i's ~1.6x crop factor. It's optimized for quieter, smoother focusing during live view and movies with a new stepper motor ("STM") design. Sharpness and contrast are pretty good across most of the frame at 18mm and f/8, though there's moderately high chromatic aberration visible in the corners and along the edges. (The Canon T4i can remove much of that, though. See below.) Corners are also just a little soft. At 35mm and f/8, sharpness across the frame is pretty good, but some chromatic aberration is still visible. Sharpness at 135mm and f/8 is also pretty good, though softness and chromatic aberration in the corners is a little higher than at 35mm. Overall, performance is quite good for a 7.5x consumer lens priced under $500.

Macro
A larger-than-average macro area with slightly soft detail. Flash throttled down fairly well.

The Canon EOS Rebel T4i's macro performance will of course depend entirely on the lens in use. With the 18-135mm STM lens set to 135mm, the Canon Rebel T4i captured a somewhat larger-than-average minimum area measuring 3.00 x 2.00 inches (76 x 51 millimeters). Details are a little soft in the center of the frame, and a little softer in the corners. (Most lenses have some softening in the corners at macro distances, however the 18-135mm STM performed better here than most.) Some minor vignetting can also be seen in the extreme corners. The Canon T4i's flash throttled down fairly well, just slightly overexposing the image (a matter of opinion, really), while coverage was even with no detectable shadow from the lens barrel.

Geometric Distortion
Higher than average barrel distortion at wide-angle and moderate pincushion distortion at telephoto with the 18-135mm STM kit lens.

The Canon EF-S 18-135mm STM lens produced images with about 1.0 percent barrel distortion at wide-angle, which is higher than average and noticeable in some shots. At the telephoto end, there was about 0.5% pincushion distortion, which is also higher than average though not quite as noticeable. The Canon T4i does not correct for geometric distortion in its JPEGs, as uncorrected raw files have identical amounts. Geometric Distortion is the tendency for the lens to bend straight lines outward (like a barrel -- usually at wide-angle) or inward (like a pincushion -- usually at telephoto).

Chromatic Aberration and Corner Sharpness
Moderately high to high chromatic aberration in the corners with the 18-135mm STM lens. Some minor softening in the corners.

Chromatic Aberration. Chromatic aberration in the corners with the Canon 18-135mm STM lens is high at wide-angle (18mm) and moderately high at full telephoto (135mm), with noticeable magenta and cyan fringes. The amount and intensity of the color fringing reduces as you move toward the center of the image, to the point where it is very low at the center. See below for how well the Canon T4i's optional chromatic aberration correction works.

Corner Softness. The 18-135mm STM lens produced some slightly soft corners when wide-open at maximum aperture. At full wide-angle, corners on the top of our test target are slightly softer than the bottom, and blurring extends pretty far into the frame. The center of the image has decent sharpness, just a touch soft. At full telephoto, the bottom corners are softest, but slightly sharper than wide-angle, and blurring doesn't extend as far into the frame. The lens is fairly sharp in the center, though not tack sharp.

Vignetting. Vignetting or corner shading is not an issue in the above crops, but that's because the Canon T4i's Peripheral Illumination Correction is enabled by default. See below for comparisons with it off.

Chromatic aberration in the corners is still moderately high to high, but sharpness in the corners and center improved when stopped down to f/8 at wide-angle. At full telephoto, sharpness only improved slightly. As you can see, the Canon T4i's Peripheral Illumination Correction over-corrected at wide-angle, producing a noticeably brighter corner crop than the center.

Chromatic Aberration Correction
The Rebel T4i offers optional Chromatic Aberration Correction to reduce lateral C.A. with Canon lenses.

As you can see, the Canon T4i's Chromatic Aberration Correction feature was very effective at removing almost all the lateral chromatic aberration in our 18-135mm STM lens test shots above. Bravo!

Peripheral Illumination Correction
Like other recent Canon DSLRs, the T4i features Peripheral Illumination Correction feature to reduce vignetting or lens shading with Canon lenses.

The Canon T4i provides what the company calls "Lens Peripheral Illumination Correction," which corrects for lens shading (commonly called "vignetting"), attempting to produce a more uniform exposure across the frame by compensating for the light fall-off seen with some lenses in the corners of the frame. Mouse over the Off and On links above to see the effect on the 18-135mm kit lens at wide-angle and telephoto at maximum aperture. PIC is enabled by default.

Peripheral Illumination Correction and Chromatic Aberration Correction are supported for over 85 different Canon lens models, in both raw and JPEG workflows. For JPEG shooting, the correction is made at capture time, while raw shooters can access the function in Canon's Digital Photo Professional software. From the factory, the T4i body ships with correction data for about 25 lens models. Canon's EOS Utility software allows correction data for lenses (including models as-yet unreleased) to be uploaded to the Canon T4i.

Accuracy
Average accuracy from the optical viewfinder, very good accuracy from the LCD in Live View mode.

Testing with a low-distortion mid-focal length macro lens to minimize the effects of lens distortion, we measured the Canon T4i's optical viewfinder's coverage at just over 95%. This matches Canon's specification, and is typical for consumer SLRs. The resulting image was slightly tilted and offset with respect to the sensor, which is unfortunately quite common with optical viewfinders in consumer oriented models (and many prosumer models). Live View mode using the LCD was more accurate, resulting in close to 101% coverage (a little loose), and of course the image isn't tilted or offset.