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Lenses (part 3)
Fast Lenses
This is a term you will come across quite often when talking about lenses. It is used to describe a lens that has a larger aperture than normal and therefore has the capability of gathering more lights thus allowing faster shutter speeds.
Bear in mind, these lenses can be very expensive for what to many may seem like a very small increase in aperture (sometimes less than one stop).
Canon makes three 50mm prime lenses:
- Canon EF 50mm f/1.8 II £90
- Canon EF 50mm f/1.4 USM £310
- Canon EF 50mm f/1.2L USM £1,150
Here are three similar Canon zoom lenses:
- Canon EF 55-200mm f/4.5-5.6 II USM £230
- Canon EF 70-200mm f/4 L USM £610
- Canon EF 70-200mm f/2.8 L USM £1160
The more expensive lenses offer more than just the larger aperture (low dispersion glass, more circular apertures, etc). The large apertures, however, are one of the main reasons photographers buy them as it not only allows faster shutter speeds, but can create a very shallow depth-of-field.
The Canon 50mm f/1.2L is a classic group portrait lens which, due to the very large aperture, can throw the background completely out of focus.
Image Sharpness, Lenses and Apertures
In general, a prime lens will produce sharper images than a zoom lens as it will most likely have less glass elements and less moving elements, thereby allowing the lens to be manufactured to higher tolerances.
Image sharpness will also vary depending on the aperture selected. If you’ve read the section on depth-of-field, you may expect a lens to produce the sharpest images at the smallest aperture as this will have the greatest depth-of-field. This is not actually the case. A lens's optimum aperture for sharpness is usually around two to four stops down from the maximum aperture depending on the lens. So an 105mm f/2.8 lens will most likely produce its sharpest images at around f/5.6 to f/11. This is one of the reasons professionals opt for fast lenses – Canon’s 50mm f/1.2 lens will be sharper at f/2.8 than the 50mm f/1.8 lens as it is stopped down more from its maximum aperture.
Macro Lenses and Extension Tubes
In photography, macro generally refers to the ability to focus on an object very close to the lens. See the section later in this chapter for more information on this. It is important to point out that to take macro photographs requires a special type of lens that is capable of projecting a much larger image onto the sensor (often life-size) and focusing closer than a normal lens. Many zoom lenses available have a macro feature although these will not be as good as a dedicated macro prime lens.
Any lens can be converted into a macro lens through the use of extension tubes, the purpose of which is to move the lens further from the image sensor and as a consequence of this, allow you to focus much closer.
Tilt and Shift Lenses
Sometimes known as perspective correction lenses, these are specialist bits of kit designed primarily for architectural photography. When taking a picture of a tall building you often need to tilt the camera upwards to get the whole building in. This leads to the top of the building being smaller than the bottom due to perspective.
The only way to avoid this is to keep the camera pointing straight at the building (the image sensor plane parallel to the building). The problem here is that you will often not be able to get in the top of the building.
A tilt and shift lens allows you to move the lens upwards relative to the image sensor which places more of the building within the sensor frame.
Although you can achieve perspective correction in software after the picture is taken, it is at the expense of optical resolution.
You can clearly see on this picture where the lens has been shifted out of its normal position.
A side effect of a tilt and shift lens is that it has a very strange depth-of-field where the plane of focus is also no longer parallel to the plane of the image sensor. If used with a large aperture, it can create photographs that make people think they are looking at a toy model.
Image Stabilisation
One of the problems all photographers suffer from is holding the camera perfectly steady while trying to take a photo with a slow shutter speed. The longer the focal length of the lens the faster the shutter speed needs to be to create a steady image.
Many of the more high-end lenses produced now incorporate some form of image stabilisation (IS). This will be indicated by some letters in the lens designation (Canon use IS, Nikon use VR, Sigma use OS).
These systems enable you to use shutter speeds two or more stops slower than you would using a lens without any image stabilisation (eg. with a 200mm lens you would not normally shoot with any shutter speed slower than about 1/250th second). If the lens is fitted with image stabilisation, however, you should be able to get shake-free images at 1/60th or even 1/30th second.
Note that if hand-holding the camera is your only option, IS is invaluable on long lenses, however there's no beating a sturdy tripod.
Some manufacturers (including Sony and Pentax) include image stabilisation inside the camera body by shifting the image sensor rather then elements in the lens. This gives you the benefits of image stabilisation no matter which lens you have connected to the camera. The downside of this method is that the stabilisation is not usually as effective as lens-based systems which can be optimised for each individual lens, and can provide a greater number of stops in stabilisation (up to five stops on the latest lenses).
Secondary Lenses (e.g. teleconvertors)
A secondary lens is a lens designed to be used in conjunction with another lens, either by attaching to the front of the main lens, or between the main lens and the camera body. They do not usually incorporate any focusing or aperture control as this is still carried out by the primary lens.
The most common types of secondary lenses are:
The ones in blue typically screw onto the filter ring in front of the lens - the ones in green are positioned behind the primary lens.
Wide angle and telephoto adaptors allow you to extend the apparent focal length range of your lens by acting a little like a magnifying glass in front of the main lens. For example, a telephoto adaptor might be called a 1.5× adaptor. If put on a 28-200mm lens, it would give the same magnification as a 42-300mm lens.
A wide-angle adaptor works in the same way but will have a scaling factor of less than one (for example a 0.8× adaptor) thereby making the apparent focal length of the lens smaller.
Any adaptor that goes on the lens will usually decrease the amount of light getting to the sensor.
Teleconvertors (Canon calls these extenders) do the same job as an adaptor however you attach this to the back of your primary lens before attaching to your camera. These are generally of a higher quality than the adaptors that screw onto the front of the lens.
The effective maximum aperture of the primary lens is also reduced (usually by one stop for a 1.4× convertor and two stops for a 2.0× convertor. For example a 200mm f/2.8 lens fitted with a 2× teleconvertors will become a 400mm f/5.6 lens.
Cross-system adaptors allow you to attach a lens designed for one camera system to the body format of another that uses a different lens mount. Not all lenses can be adapted to every camera body, and often the lens can only be used with a limited functionally (eg. manual focus and aperture control).
A close-up lens (sometimes called a close-up filter as cheaper ones are usually just a single piece of glass) allows you to reduce the minimum focusing distance of the primary lens and are therefore ideal for macro photography. They are sometimes called diopters and are often sold in kits of three (+1, +2 and +4) that can be used individually or in combination to produce a range +1 to +7. Manual focusing is recommended. Canon make some close-up lenses with two optical elements which are of a much higher quality than the single element ones.
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Extension tubes (shown right) are distinct from other secondary lenses in that they have no glass in them - their purpose is to move the lens further from the focal plane (ie. the image sensor). The effect of this is to reduce the focusing distance and increase the magnification. Therefore they are ideal for macro photography and considerably cheaper than buying a dedicated macro lens. They can be bought individually or as a set which can be used in combination if required. |
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If you are purchasing extension tubes, make sure you get ones that have the electronic contacts. Some cheaper extension tubes do not have these contacts so you will not be able to use the autofocus, and may only be able to shoot with the aperture wide open (unless the lens has a manual aperture ring). Two other downsides are that you lose the ability to focus to infinity, and as with most lens accessories, there is a loss of light when attached.
Other stuff about lenses
Full time manual focus means you can manually fine-tune the focus of the lens while it is still in autofocus mode (you don’t need to switch to manual focus mode).
On budget lenses (usually kit lenses) the front element of the lens (ie. the piece of glass at the front) is responsible for the focussing. This means that the front element will turn when the focus changes, not something that is ideal when you have certain types of filter on the lens. Higher quality lenses often use an internal focusing system whereby the lens elements responsible for focusing are inside the lens. As the internal glass elements are generally smaller and lighter than the front elements, the auto focus system requires less power and can focus much more rapidly. It also means the lens does not change length during focusing.
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Lens Hoods are the plastic tubes that optionally screw onto the front of the lens. Apart from making your lens look even bigger and therefore, if you are male, making you even more of a man, they actually reduce lens flare by blocking light entering the lens from an angle way outside the field of view. They also offer some level of protection for the lens if it is dropped or bumped against a hard object. The picture on the right shows a lens with and a lens without the lens hood attached. |
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