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Do you know your IS from your VR (Vibration Reduction) and understand the main types of vignetting? Can you spot a converging vertical? No? Then read our ultimate lens Frequent Asked Questions.
From the outside they look simple enough, but what’s really going on inside your camera lens?
Well, a lens can house up t0 20 or so individual lenses in a dozen or more groups, Other moving parts include the aperture blades, which are often curved to give more pleasing depth of field effects, and an autofocus motor. Then there are all the electronics needed for communicating with your camera. Clearly, DSLR lenses are anything but simple devices, but you don’t need a PhD in optical engineering to understand how to get the most from them.
1. Why is there such a huge price different in lenses?
Expensive lenses generally deliver much better optical quality than cheap models, because they usually include advanced low-dispersion glass, highly customized aspherical elements, exotic coatings to reduce ghosting and flare, and are manufactured with greater precision to deliver the very best standards of image quality.
In addition, professional zoom lenses tend to offer faster maximum apertures, which remain fixed throughout the zoom range and are referred to as ‘constant aperture’ lenses.
These will also have a much more rugged and robust build quality, capable of standing up to demanding use on a daily basis. They often feature splash-proof and dust-resistant seals, too, so they can be used in any weather conditions and survive the consequences.
2. what is the ‘sweet spot’ of a lens, and how do I find it?
Practically all lenses fail to give their sharpest detail and greatest contrast when shooting ‘wide open’ at their maximum apertures. To achieve the best image quality possible, it’s usually best practice to ‘stop down’ by two or three stops so, for example, with a lens that has a maximum aperture of f/2.8, you would shoot at f/5.6 or f/8. To find the sweet spot of your lens, take a series of test shots throughout the whole aperture range and bear in mind that, for zoom lenses, the sweet spot aperture may be different at varying zoom settings.
When reviewing the results, you will probably notice that while the quality initially increases as you start stopping down, it will decrease again at very small apertures, such as f/22 or f/32, due to the effects of diffraction.
3. What is ‘fall off’, and how do I avoid it?
There are three main types of vignetting associated with camera lenses, each of them producing images that are darker towards the corners than in the center of the frame. With optical vignetting, light entering the lens from directly in front of the camera illuminates all elements equally, whereas elements towards the rear of the lens are more shaded from light entering at an angle than the front elements. Optical vignetting can be overcome by reducing the aperture from its largest setting by two or three stops.
Mechanical vignetting is common when using very wide-angle lenses, where the corners of the field of view are physically obscured by objects such as thick screw-in filters or lens hoods that extend too far in front of the lens. You can normally cure the problem by using screw-in filters that have an ultra-thin construction! and by using petal-shaped lens hoods that are specifically designed for the lens.
Natural vignetting, also known as ‘fall off’, is a function of the light from the lens striking the sensor at more extreme angles in the corners, compared to being more direct in the center. This is more of a problem in compacts than DSLRs, and can be corrected in image-editing programs.
4. What is the slowest shutter speed I can use when hand-holding my camera?
The rule of thumb is that the slowest shutter speed you can use without suffering from camera shake is the reciprocal of the focal length. However, this is for full-frame cameras, so with most DSLRs, you also need to take the crop factor into account. For example, most Nikon cameras have a 1.5x crop factor, so a 60mm lens will give an effective focal length of 90mm. This means the minimum shutter speed you would use is 1/90 sec, rather than 1/60 sec.
Shutter speeds rarely tally exactly with focal lengths so, where a 70mm lens would be equivalent to 105mm, it’s best to err on the side of caution arid use a minimum shutter speed of l/125 sec.
5. How do I stop my lenses steaming up and clear the moisture when this happens?
Steaming up is caused by a sudden change from a cold environment to a warm one. For example, if you leave your camera in the boot of your car on a cold winter’s night, then bring it into a warm building in the morning, the outer surfaces of the camera arid lens can be covered in moisture almost instantly.
The best plan is to apply temperature changes gradually, but if this isn’t possible, simply leave the camera to come up to the ambient temperature naturally. If you really can’t wait, use a clean cloth to gently wipe the front lens element, using a circular action.
The danger here is that if there’s any grit on the lens you could easily scratch the element, so for safety’s sake, have a screw-in UV filter fitted permanently.
6. What is chromatic aberration?
This is also called ‘color fringing’, because it produces either red/cyan. blue/yellow or even green/magenta fringing, which is a mix of the two colors around an image’s high-contrast edges. It is caused by the lens focusing different wavelengths of light at different points, either at varying distances along the path of light (longitudinal) or shifted in a transverse direction (lateral).
To reduce chromatic aberration, manufacturers typically combine pairs of elements that have different refractive indexes. called ‘achromatic doublets’, which work together to cancel out refraction.
High-quality lenses also often include elements made from specialized hybrid glass to minimize the dispersion of light, such as Nikon ED (Extra-low Dispersion) and Canon UD (Ultra-low Dispersion) glass.
7. What are ghosting and flare? And how can I avoid them?
These are effects caused by stray light bouncing around inside the lens, reflected between the various elements and the inside of the lens barrel. A loss of detail and color as well as the creation of ghost images are most commonly caused by intense light sources within the field of view, or close to its periphery.
The most effective solution is to avoid shooting into the light, but this isn’t always possible. For example, when shooting portraits on a sunny day, it’s common practice to position subjects with their backs to the sun and to use fill-in flash to illuminate their faces. Ghosting and flare can then be a huge problem so, in scenarios such as this, high quality lenses come into their own, featuring highly specialized coatings not only on the front element but also on internal elements to minimize the effects. The best way to reduce ghosting and flare is to fit a lens hood, which cuts out extraneous light falling onto the lens from an oblique angle.
8. Why would I need to use a prime lens?
Zoom lenses are wonderfully convenient and versatile, but unfortunately there’s almost always a compromise to be made when it comes to image quality.
Common degradations include barrel and pincushion distortions, increased chromatic aberrations and vignetting at the wide-angle end. Prime (fixed focal length) lenses typically suffer much less from these unwanted attributes, as well as delivering sharper images into the bargain.
Another bonus is that prime lenses usually offer larger maximum apertures, being referred to as ‘fast’ lenses, because the bigger apertures enable faster shutter speeds. This is extremely useful for avoiding camera shake under dull lighting conditions, so there is less reliance on flash, as well as for freezing the action and avoiding motion blur.
Another bonus in portraiture is that large apertures tend to give a smaller depth of field, which makes it easier to blur any distracting backgrounds and focus the attention on the subject instead.