Problem No. 12: I tried using a lens with a built-in optical stabiliser for the first time the other day, but it didn’t seem to have much effect. Is it really worth having?
Image stabilisation is a complex, computer-controlled process, in which a group of elements in the lens is moved by servo motors to ofset the effect of any vibration. The latest generations of optical stabilisers in Canon, Nikon, Sigma and Tamron lenses work well, generally living up to their claims of a three- or four-stop advantage.
You can expect consistently sharp results, but bear in mind that stabilisation can’t counteract any movement on the part of the subject. For maximum benefit, you usually need to allow about a second after half pressing the shutter button (or AF button) before stabilisation reaches optimum performance.
Some lenses feature multiple stabilisation modes. a panning mode applies correction in only the vertical or horizontal plane, depending on whether you’re panning with the camera held in landscape or portrait orientation.
Many lens designs now feature automatic detection for panning or tripod use, and Nikon VR lenses often include an ‘active’ mode for shooting from a vibrating or unstable platform.
Problem No. 13: I’ve used the smallest aperture on my lens to get everything sharp, but the picture looks soft – have I done something wrong?
Most lenses have a minimum aperture of f/22, although some (such as macro lenses) offer an even smaller setting of, say, f/32. But why is the minimum aperture of a lens rarely listed alongside the focal length of a lens, as its maximum aperture is?
This is because the smallest aperture is rarely recommended to be used, as this setting leads to softer, lower contrast images caused by an optical phenomenon known as diffraction. Diffraction occurs when light waves entering the lens are ‘bent’ by the hard edges of the aperture. Every aperture setting causes this, but the bending is generally minimal. However, as the apertures gets smaller, the effect becomes more significant.
At the smallest aperture, the light waves are bent and spread out so much by the aperture blades that the image appears fuzzy, even though it’s been focussed correctly.
So, while a lens’s smallest aperture might enable you to maximise the depth of field when you’re photographing landscapes, the resolution will deteriorate. Try opening up a half or one full stop instead to improve picture quality.
Problem No. 14: What’s the ‘sweet spot’ of a lens, and how do I use it?
Lenses generally struggle to produce pin-sharp images when used at their maximum aperture. At very narrow apertures of around f/16 to f/32, diffraction also becomes more of a problem, again impairing sharpness.
The sweet spot, or point of optimum sharpness varies from one lens to another, but it’s usually around f/8. To make the most efficient use of this, switch to aperture-priority or Av shooting mode and set the aperture accordingly (for more on this, see our tutorial on How to find your lens’ sweet spot).
Problem No. 15: What is the significance of the second f-number listed on a zoom lens (such as f/5.6 on a 28-135mm f/3.5-5.6 zoom) – and do I need to worry about it?
Most zoom lenses have two f-numbers quoted, because as you zoom in from a wide-angle setting towards the telephoto end, progressively less light is able to pass through the lens. In this case, its maximum effective aperture at a 28mm setting is f/3.5, but it’s only f/5.6 at 135mm.
This has implications if you’re handholding the lens – as the aperture gets smaller as you zoom, the exposure time gets longer, increasing the chances of blurred shots.
Problem No. 16: I’ve read that petal-shaped lens hoods are more effective than round ones for preventing ghosting and flare. So why can’t I find a petal-shaped hood for the 18-55mm lens that came with my camera?
Most ‘kit’ 18-55mm lenses don’t have internal focusing, so the front element of the lens rotates as you move from one end of the focus range to the other. This means that the hood also revolves, so a petal-shaped lens hood can’t be used. Instead, you’ll need a regular, round-shaped hood.
Problem No. 17: I’m a bit confused about which lenses are designed for APS-C cameras and which are full-frame compatible. Is there any sure-fire way of telling, both for own-brand lenses and for third-party manufacturers?
You can use full-frame lenses on APS-C cameras, but you usually can’t use APS-C lenses on full-frame bodies. The most common APS-C lens designations are Canon EF-S, Nikon DX, Sigma DC and Tokina DX. The respective full-frame lenses are Canon EF, Nikon FX, Sigma DG and Tokina FX (to learn more about lenses and their different abbreviations, see DO or Di? Your lens markings explained).
Problem No. 18: What are the differences between standard autofocus motors, ultrasonic motors and ring-type ultrasonic systems – and which is the best?
Most modern lenses have built-in AF motors, rather than being driven from one in the camera body. The simplest use gears driven by an electric motor for actuating autofocus. Ring-type ultrasonic AF uses two large rings to create a rotational force for autofocusing the lens.
The main advantages are very fast AF performance with near-silent operation. The manual focus ring doesn’t rotate during autofocusing either, providing full-time manual override in single or one-shot autofocus mode. There’s a less advanced type of ultrasonic autofocus, using a small ultrasonic motor in place of the basic electric one.
This uses gear wheels to drive AF, which make a little noise. These systems are often as slow as basic motors, and don’t enable full-time manual focus override. Canon USM (UltraSonic Motor), Nikon AF-S (AF-Silent Wave) and Sigma HSM (HyperSonic Motor) apply the same designations to lenses that feature either basic or ring-type ultrasonic autofocus, so it can be hard to tell which type is used.
Tamron is more up front, differentiating between PZD (Piezo Drive) for its basic system and USD (Ultrasonic Silent Drive) for ring-type AF.
Problem No. 19: I’ve often read about chromatic aberration, but I don’t know exactly what it is. Can you explain?
Chromatic aberrations are caused when a lens focuses different wavelengths of light at different points. There are actually two types of these aberrations. With longitudinal chromatic aberrations, different wavelengths are focused at different places along the path in which the light is travelling. With lateral chromatic aberrations, different wavelengths are shifted in a transverse or sideways direction.
The effect is often called ‘colour fringing’, because it produces either red/cyan fringing or blue/yellow fringing around high-contrast edges within images. You can also get green/ magenta fringing, which is a mix of the two primary types (see our illustration of What is chromatic aberration?).
To reduce chromatic aberrations, lens 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 specialised hybrid glass to minimise the dispersion of light, such as Nikon ED (Extra-low Dispersion) and Canon UD (Ultra-low Dispersion) glass. Current DSLRs from Nikon and Pentax have built-in features for automatically correcting chromatic aberrations, therefore placing less demand on lens performance in this respect.
However, these corrections won’t work if you shoot in RAW and convert files to JPEGs using Adobe Camera Raw. Canon DSLRs have no built-in corrections for chromatic aberrations, although you can shoot in RAW and tune them out using the Digital Photo Professional software supplied with the camera.
Problem No. 20: I’ve often heard about barrel and pincushion distortion on zoom lenses – what is it exactly, and can it be corrected during editing?
Zoom lenses often suffer from barrel distortion at the wide-angle end of the zoom range, and pincushion distortion at the telephoto end. if you shoot a rectangular box that fills most of the frame, its edges will appear to bow outwards with barrel distortion and inwards with pincushion distortion.
Most image-editing programs include tools to correct distortion (see our tutorial on How to correct distortion in Photoshop Elements).
Problem No. 21: I’ve bought a new superzoom lens, but when i shoot into the light I’m getting a lot more ghosting and flare than usual. is there any way of reducing this effect?
Superzoom lenses tend to have rather more groups and elements contained within them, so light bouncing around inside the lens is more of a problem and ghosting and flare becomes an issue. The easiest way to minimise this is to fit a lens hood, because this will instantly cut down the amount of light that enters the lens from oblique angles.
Next, ensure that the front element of your lens, and any filters that you have fitted to it, are as clean as possible, without any specs of dust, smears or droplets of water on them. Anything that can catch the light and add spurious reflections adds to the risk of unwanted artefacts.
Unless you’re shooting directly into the light, you can often position the front of the lens so that it’s in shadow. Try using a baseball cap or a sheet of black card to shield it from the sun. Just be careful that it doesn’t creep into the resulting image.
Problem No. 22: Will changing lenses mean I end up with dust on the camera sensor?
So much has been said about ‘dust bunnies’ (small particles of dust that can land on your camera’s sensor and cause dots in images) that many DSLR photographers seem paranoid about changing lenses. Yet being able to use the best lens for any particular shot is one of the best attractions to DSLR photography.
Don’t be afraid to change lenses when you need to, but take a few precautions. Always switch the camera off, as this removes any static charge from the sensor that can attract dust particles. Where possible, choose a dust-free, non-windy location when changing lenses.
Next, get the replacement lens ready to fit so that you don’t have to waste time with the camera internals exposed. Finally, keep the camera’s lens opening pointing downwards when changing lenses, to minimise the risk of anything falling into it.
Problem No. 23: My zoom lens says it’s ‘macro’ but I can’t get anywhere near as close to subjects as I can with my compact camera. What’s going on?
True macro lenses have a reproduction ratio of 1:1, which means they can image a subject the same size as the camera’s sensor; on most D-SLRs, this is about 24x16mm. Many ‘macro’ zooms boast ratios nearer 1:4, so the smallest possible subject is about credit-card size.
It’s true that many compact cameras do allow close focusing, but they only do so with the lens at its wide-angle setting. So the smallest subject they can capture isn’t as small as you might think. It can also be hard to get enough light on the subject, which ends up in the camera’s shadow. A slightly longer working distance is much more practical (find out How to set your autofocus for macro photography)
PAGE 1: General Photography Problems
PAGE 2: Using lenses
PAGE 3: Digital camera accessories
PAGE 4: Digital camera settings and controls
PAGE 5: Camera exposure
PAGE 6: Using flash
PAGE 7: Photography technique
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