Which qualities matter most when you’re reading lens reviews?
If your camera comes from a leading manufacturer, there’s a good chance that almost any lens you want will be available, either from the camera maker or one of the third-party manufacturers. Compact systems camera owners may not be as well supported, although with more than 40 lenses to choose from, owners of Micro Four Thirds system cameras are better off than users of other compact systems cameras.
Focal length and price tend to dictate the final choice. Focal length is easy to deal with because most manufacturers split their lenses into specific categories: wide-angle for landscapes, short teles for portraits, standard zooms for everyday shooting and super-zooms for travel. There are special lenses for macro shooting and architectural photography and quite different ones for photographing wildlife.
Short telephotos are ideal for portrait photography.
Once you’ve decided upon the category you want to investigate, there are other factors to consider, relating to the ‘quality’ and performance of the lens itself. Different photographers will have different requirements for image quality, depending upon where their images will be displayed.
If 90% of your images are captured as JPEGs and end up on a Facebook page or in emails, the bar can probably be set quite low. In contrast, if you capture raw files and 10% or more of them are printed at A3+ size, the lenses you use need to be as sharp as possible and capable of acceptable edge-to-edge sharpness across the image frame.
We’ve identified seven key features that can be used as a basis for comparing lenses that are otherwise similar in focal length. In alphabetical order they are as follows: bokeh, chromatic aberration, distortion, flare, sharpness, speed and vignetting.
Uncorrected pincushion distortion caused by a 150mm telephoto lens shows up in the upturned ends of the horizon in this shot. This defect is easily corrected with editing software.
Vignetting (edge and corner darkening as shown simulated in this image) is often corrected with in-camera processing of JPEG files. It is easily corrected in raw files when they are converted into editable formats.
Some of these features, such as distortion, lateral chromatic aberration and vignetting, can be ignored because most cameras and raw converters can identify and correct them. That leaves us with bokeh, flare, sharpness and speed.
Let’s resolve the easiest one, speed, first. If you’re on a tight budget you can cross speed off the list; fast lenses are expensive. Larger lenses with larger elements are required to let more light into the camera. Because they use more glass and have complex coatings they cost more to make. They are also very heavy for the one- or two-stop advantage they provide.
Fast lenses provide a brighter image in optical viewfinders, which is advantageous for focusing and framing shots in poorly-lit situations. But if your camera has an electronic viewfinder, it’s easy to increase the gain and make the viewfinder image brighter without relying on the lens speed.
Flare can’t be corrected in-camera and it’s very difficult to achieve satisfactory corrections for most flare with image editors. Many factors come into play to produce flare: the lens design (how many elements, their position, coatings); the position of the aperture blades and the direction and intensity of the light, to name the most significant.
Wide-angle lenses are often flare-prone and some produce unusual flare patterns, including streaks, coloured circles and replicas of the shape of the aperture. All distract attention from the image and are, therefore, undesirable. Consequently, this feature is an important consideration when choosing a lens.
This example of forced flare was produced by pointing the camera towards the sun, although the sun wasn’t in the centre of the frame. The resulting streak and blotches are produced by internal scattering of light within the lens. Interestingly, the other characteristic of flare, veiling haze, is not present, indicating effective internal coatings. The shot was taken with the 18-55mm STM kit lens at 27mm on a Canon EOS 100D body. No lens hood was used.
More direct backlighting was used in this shot and the sun was close to the centre of the frame; but the intensity of the light was less as the shot was taken later in the day. Taken with a M.Zuiko Digital 12mm f/2.0 prime lens on an Olympus OM-D E-M5 body. A lens hood was fitted.
The effects of flare can be reduced for most lenses by fitting a lens hood. Unfortunately, some manufacturers don’t supply them at all, while others only provide them with their highest-priced lenses, so they are often overlooked. Lens hoods are especially valuable for preventing loss of contrast and colour saturation with strong side-lighting.
How much sharpness do you need? And how sharp should images be as a whole? You need to assess both carefully. Landscape and architectural photographers want their images to be sharp from edge-to-edge of the frame as well as in the centre. And they want lenses that remain sharp at apertures of f/8 or smaller so they can shoot with plenty of depth of field.
Landscape photographers need lenses that are sharp edge-to-edge.
Portrait photographers may not be as fussy. They need sharpness for the subject’s eyes and hair, but may not want it for the skin. And they usually want the ability to defocus backgrounds attractively at the same time.
There’s no lens with these characteristics so these photographers need lenses that resolve well in the centre of the frame to produce sharp eyes and hair. Smoothing skin tones is best done as part of post-capture editing.
As well as looking at resolution figures in lens tests, pay attention to how well high resolution is distributed across the focal length range of the lens. For most shooting you will require acceptably high resolution from the widest aperture to the point at which diffraction begins to reduce sharpness. Wide-aperture sharpness is more important for slower lenses (f/5.6 a stop or two down) as they provide a shorter range of usable aperture settings.
Few consumer lenses produce really nice bokeh because they don’t isolate subjects from backgrounds enough and seldom produce enough softening of background details. Bokeh is also assessed subjectively, which means what one person deems ‘pleasing’ may be considered ‘pretty average’ by another. We covered this topic in detail in the last issue of Photo Review.
When evaluating lenses, it’s important for the aperture to be symmetrical; circular apertures are preferable to polygonal and an odd number of blades (seven or nine) is better than even numbers. If all else is equal, choose the lens with the best bokeh characteristics.
Few consumer lenses can produce enough softening to isolate subjects from backgrounds. In this example, the maximum aperture of f/6.7 at the 300mm focal length with the camera’s M4/3 sensor wasn’t wide enough to produce smooth defocusing.
It’s worth putting effort into lens selection and investing as much as you can afford when you buy a new lens. Lenses outlast cameras. Ideally, the performance of the lens you buy today should meet the requirements of tomorrow’s cameras.
Decide whether each parameter we’ve listed here even matters to you. You may only need to consider seriously two or three of factors listed above, although they will probably have the same order of importance.
Avoid ‘protective’ filters. Even a high quality filter won’t perfectly match the optics of the lens. You shouldn’t use any filter unless its function is necessary for the end result you want to achieve.
Look for traces of false colouration that might be added by lenses. Different glasses used by the various manufacturers can produce very slight colour changes, which won’t be counteracted by the auto white balance. Once you’re accustomed to one manufacturer’s colouration, swapping to lenses from another manufacturer can be jarring if their colour bias is opposite to the one you’re accustomed to.
This is an excerpt from Photo Review Issue 57.