Learn more about how camera lenses work and how to fix common lens problems with this detailed primer.
My lens has an AF/MF switch on its barrel. My camera also has an AF/MF switch round the front. Do they both do the same thing?
Yes. Both let you select either autofocus (AF) or manual focus (MF). You may find it easier to keep the switch on the camera permanently set to AF, then use the equivalent switch on the lens to move between autofocus and manual focus.
Some lenses allow to you to use manual focus while the lens is still in AF mode, although most require that you set MF to disengage the autofocus motor first – otherwise you risk damaging the lens.
Why would I need to use manual focus?
The camera’s autofocus system can struggle to lock onto a subject in certain conditions, so you may have to take over and do the job yourself.
Manual focus can be more accurate for some types of photography, such as when you’re shooting extreme close-ups, or when you need to pre-focus at a precise distance – some lenses have a distance scale that makes this easier.
Rest assured that the camera won’t adjust the focus once you’ve set it, which is useful when you’re using a lens that lacks internal focussing.
What is internal focusing?
The front of an internal focusing lens doesn’t rotate or extend as the lens focuses.
For lenses without internal focussing, focus first and ensure the lens is set to MF before adjusting a filter.
Why does the image not look sharp when I zoom?
The majority of autofocus zoom lenses are what’s known as ‘varifocal’, which means that the focus shifts as you zoom.
In this instance, it’s better to use autofocus once you’ve finished zooming.
Alternatively, switch your camera to continuous autofocus and use this to keep an object sharp while you’re zooming.
How can I get sharper results?
Professional-grade lenses feature large apertures to let in lots of light, along with higher-quality glass and corrective lens elements to reduce optical flaws.
This generally makes them significantly sharper than kit lenses and cheap zooms.
That said, even an expensive, tack-sharp prime lens will produce a picture as soft as Brie cheese if it’s not used properly.
The key thing is to make sure you keep the lens stable when you take a picture, and to make sure the shutter speed is faster than the equivalent of the effective focal length of the lens when you’re shooting without a tripod.
For example, with a 50mm lens on a full-frame camera, aim for a shutter speed no slower than 1/50 sec – you may need to use a larger aperture or higher ISO sensitivity to achieve this shutter speed if light levels are low.
In fact, the improvements made by camera manufacturers to the high-ISO performance of digital cameras combined with image stabilisation technology have been a game-changer as far as handheld photography is concerned.
What is image stabilisation?
There are actually two types of stabilisation. Canon, Nikon, Sigma and Tamron favour lens-based stabilisation, while Panasonic, Sony, Olympus and Pentax use sensor-shift stabilisation instead.
They both do the same thing, though: detect camera shake and counteract its effects by moving either a floating optical element (in the lens) or the sensor (in the camera).
The key advantage of sensor-based stabilisation is that it works for any lens attached to the camera, even an old manual-focus one.
Lenses that have stabilisation built in, such as Canon’s IS (Image Stabilizer) lenses and Nikon’s VR (Vibration Reduction) ones, are more expensive, but the performance of each stabilisation unit can be optimised for each type of lens.
How will I know that image stabilisation is working?
Most stabilised lenses have an on/off switch on the lens barrel.
With IS on, the system whirrs into life once you touch the shutter release. And we do mean whirrs.
Optical image stabilisation can be noisy when it’s active, so much so that it can be picked up by the camera’s internal microphone when shooting video.
You’re also likely to see the image in the viewfinder or the Live View screen judder then become very still.
So why would I ever want to switch off a lens’s image stabilisation?
Image stabilisation draws its power from the camera battery, so you may be forced to switch IS off if you’re low on juice.
If you’re working from a tripod, it’s best to switch off IS altogether, so that it doesn’t attempt to detect vibrations that aren’t actually there, which can lead to rather soft pictures.
If you’re moving the camera to keep an active subject in the frame, you may need to switch off the IS system again.
If you don’t do this, you’ll probably find the system will work against you, attempting to correct for the camera movement that you’re intentionally making.
Some stabilised telephoto lenses have an IS mode switch that lets you toggle between the normal IS setting (Mode 1) and a special ‘panning’ setting (Mode 2) where the camera only corrects movement in one plane – horizontal or vertical.
Some stabilisation systems can even automatically detect when you’re intentionally panning or shooting from a tripod and will make any adjustments as necessary.
IS options differ between lenses, so there’s no getting away from it: you’ll have to refer to the individual lens’s manual to check how it will react in different situations.
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