ULTRA WIDE-ANGLES

THE VFRSATILITY OF AN ULTRA WIDE-ANGLE ZOOM MAKES IT A MUST-HAVE OPTIC FOR EVERY DSLR USER. WE TEST FOUR POPULAR ZOOMS THAT CAN BE USED WITH FULL-FRAME AND APPS-C SENSORS

WITH A WIDE-ANGLE LENS, your first thought is probably landscapes — and they’re certainly great for capturing broad, sweeping views. Another good time to grab a wide-angle is in tight spaces, particularly interiors when there just isn’t room to move back.

Wide-angles can also play to your creative side, making everyday subjects look excitingly dynamic with exaggerated perspective effects. This is not a characteristic of the lens itself, as perspective is determined by distance, but wide-angles allow you to get very close while still taking everything in, so foreground objects appear much larger and jump out of the image.

Wide-angles go particularly well with filters, such as grads (graduated filters) for darkening skies in landscapes, though their field-of-view is so wide that sometimes the edges of the filter can protrude into the corners of the image. A wide-angle holder will prevent this, and for the same reason it’s best to use slim-profile screw-in filters. Also think about an extreme ten-stop ND filter, for those milky water and streaking clouds effects with long exposures.

Some say you shouldn’t use polarisers with super-wides as they create a darker patch in blue skies. But that doesn’t always happen, usually only when the sun is low, and the effect of a polariser is just too good to miss. Any darker zone can be moderated with rotation, or simply remove the filter.

Depth-of-field is usually generous with wide-angles, and hyperfocal-distance focusing is the technique to maximise it, delivering sharpness right from your feet to the horizon. Check it out with an online depth-of-field calculator such as www.dofmaster.com or download the DOFMaster app for your smartphone.

In this month’s group test, all four lenses are designed for use on full-frame cameras. They can be used on APS-C models, too, of course, though that doesn’t make the most of their potential and you’ll be paying for larger format coverage that you can’t use.

Not only is their field-of-view substantially reduced on crop-format DSLRs, you’ll also miss out on focal length range compared to, say, 17-55mm zooms designed specifically for APS-C (zoom range is optically easier to achieve on smaller formats). Not to worry, though, as we have an even bigger selection of super-wides for DSLRs with APS-C sensors coming up next month.

How we test the lenses

OUR TESTS LOOK at the key factors influencing image quality — sharpness, distortion, chromatic aberration and vignetting. We also test performance of the AF and image stabiliser (IS) systems. For IS, we use an oscillating platform custom-made to mimic handholding the lens. The platform’s great virtue is that it’s an absolutely standard test, so all lenses are tested i n exactly the same way for accurate comparisons.

• Sharpness: What we call sharpness is a combination of resolution (the fineness of details) and image contrast (how clearly those details are shown). Resolution and contrast are directly linked and when one goes up, the other goes down. We use Modulation Transfer Function (MTF) analysis to measure sharpness, as do lens manufacturers. Results are shown as % MTF at 24 lines-per-mm for full-frame lenses and at 36 lines-per-mm for APS-C to adjust for the crop factor, so sharpness is generally slightly lower than full-frame, in line with actual use. Multiple readings are taken and averaged, and edge readings are taken from points l-2mm from the sensor edge. Peak resolution shows maximum cpmm at 20% MTF, which is the lowest level where light and dark tones can be reliably measured, though the difference is faint.

Unlike most lens test procedures that use a relatively small test target that often has to be shot at very close range, eg wide-angles, we use multiple individual targets of different sizes to keep distances realistic. A focusing wedge ensures absolute accuracy and automatically adjusts for field curvature and focus shifts that can skew other methods. A Nikon VI is used when possible (all Canon and Nikon lenses) with a precision custom adaptor to position any area of the test image over the centre of the sensor. This provides both a level playing field and also an extremely high resolution (equivalent to 74 megapixels on full-frame) to ensure the camera is never the limiting factor.

Distortion: Distortion makes straight lines towards the frame edges appear curved. It changes with focal length and is less noticeable on APS-C. Barrel distortion lines curve outwards (indicated as a positive percentage value, eg +1.5%); pincushion distortion curves inwards (indicated as a negative value, eg -0.5%). The lower the stated figure, the better: 1% or less is good, 2% or more being increasingly noticeable.

Vignetting: Vignetting is darkening towards the corners. It changes with focal length and is also less prominent on APS-C. Vignetting reduces quickly as the aperture is closed down. It’s easily removed in post-processing, but when it’sstrong this results in greater noise. Vignetting is measured in Exposure Values (1EV equals one stop).-1EV is usually not a problem.

Chromatic aberration: Also known as CA or colour fringing, is usually only slightly reduced at higher f/numbers, and is more noticeable on APS-C format due to the crop factor. Distortion, vignetting and CA can all be substantially reduced or eliminated with post-processing software.

Autofocus: Autofocus speed and accuracy is a game of two halves — half camera.half lens. On the lens side of things, most of the differences relate to the mechanics and build quality, and this is commented on in the reviews.

Image stabilisation: We use an oscillating platform, custom-made to hold a DSLR body in place and mimic handholding characteristics, and in our testing it has proved very realistic. The platform’s great virtue is that it’s an absolutely standard test.

AF motors

Micro-motors are the older and more conventional type of system used to focus lenses and remain a fast and reliable option. Better still are motors using ‘sonic waves’ to rotate the lens, which are faster and quieter. Most brands offer lenses that boast sonic motors, including: Canon Ultrasonic (USM), Nikon Silent Wave Motor (SWM), Sigma Hypersonic (HSM), Sony Super Sonic Motor (SSM) and Tamron Ultrasonic Silent Drive (USD).

Aspherical lens element

High-quality elements designed to improve performance, particularly towards frame edges. Aspherical lens elements are usually made from glass, but there are also many moulded glass/plastic hybrid elements, too.

Image stabilisation

Many lenses have a ‘floating’ element linked to sensors that move it to counteract movements when handholding. Most brands offer lenses with lens-based stabilisers, although with some it’s camera-based. They include: Canon Image Stabiliser (IS), Nikon Vibration Reduction (VR), Sigma Optical Stabiliser (OS) and Tamron Vibration Compensation (VC).

Internal Focusing (IF)

Also called inner focusing, this system rotates elements within the lens during AF so that the front of the lens doesn’t rotate. This is useful when using filters as you don’t have to adjust the filters following focus.

Premium glass elements

High-quality elements ensure the best possible image quality, delivering maximum sharpness, superior colour reproduction and contrast, and minimal chromatic aberration. Common types include: Canon Fluorite &Ultra Low Dispersion (UD), Nikon ED (Extra-low Dispersion), Sigma Extraordinary Low Dispersion (ELD) & Special Low Dispersion (SLD), Tamron Extra Refractive (XR).

SQL - 16 | 0,625 сек. | 6.94 МБ