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Lenses (part 2)
Digital Only Lenses
Most manufacturers have a range of lenses that are designed specifically to work with image sensors smaller than full-frame (e.g. Nikon’s DX lenses and Canon’s EF-S lenses). The only difference between these and a lens suitable for a full-frame sensor is the size of the projected image circle. This image is only big enough to cover the smaller sized sensor (the blue rectangle in the image at the start of the previous section). Nothing else is different – the magnification is the same. So a 50mm EF lens and a 50mm EF-S lens (or in Nikon terminology a 50mm FX and a 50mm DX lens) will give exactly the same image on the crop sensor camera.
A mistake we have seen people make in online forums is to assume a digital lens is already adjusted for the crop factor and therefore a 50mm digital lens on a crop factor sensor would be the same as a normal 50mm lens on a film camera. It is not. A 50mm digital-only lens will give the same image as a 50mm non-digital lens. 50mm is 50mm whatever the lens type.
 
The picture on the left shows the image circle projected by a full-frame 50mm lens. This lens is suitable for use on film, full-frame and crop sensor cameras. The picture on the right is a digital only lens designed for crop sensor cameras only. The image circle is much smaller, only just covering the smaller sensor (blue rectangle), however, notice that the image taken with a small sensor camera with either lens will be exactly the same.
The reason digital lenses are quite popular is:
- they can be made much lighter as the glass elements are smaller
- they are cheaper to manufacture
- some incorporate a telecentric or near telecentric design, meaning the light rays leaving the back of the camera lens are parallel and therefore perpendicular to the image sensor. This enables a greater proportion of light entering the camera to be detected by the image sensor and reduces the vignette effect (where images can be slightly darker in the corners). All lenses for cameras supporting the four-thirds standard are telecentric designs.
Digital-only lenses usually have a slightly different mount than the full-frame lenses so you can’t attach a digital-only lens to a camera that is not designed to support it. For example, an EF-S lens will not fit on a Canon EOS 5D. If you are thinking of upgrading to a professional full-frame camera, you are probably best advised to stick with lenses that work on these cameras.
Types of Lens
The table below shows the focal length of some common types of lens on different crop sensor cameras. Terms like 'ultra-telephoto' are vague so the figures are just to give you an idea if you come across the terms.
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Lens type |
Full-frame
no crop factor |
APS-C
crop factor 1.5/1.6 |
Four-thirds
crop factor 2.0 |
|
Fisheye (circular) |
8-10 |
5-7 |
4-5 |
|
Fisheye (full frame) |
16 |
10-11 |
8 |
|
Ultra wide |
16-24 |
10-16 |
8-12 |
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Wide |
24-35 |
16-24 |
12-18 |
|
Standard |
50 |
33 |
25 |
|
Telephoto |
50-300 |
33-200 |
25-150 |
|
Portrait |
80-120 |
53-80 |
40-60 |
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Ultra-telephoto |
300+ |
200+ |
150+ |
Lens Descriptions
Once you decide the basic type of lens you want you need to understand what all the numbers and acronyms in the lens description mean. For example if you browse a camera brochure you might see the following lenses:
- Canon EF 28-300mm f/3.5-5.6 L IS USM
- Nikon 300mm f/2.8 ED-IF AF-S VR
Here we'll look at some of the acronyms used by Canon and Nikon in their lenses.
Canon
| EF |
Electronic Focus – used to denote Canon’s range of autofocus lenses and the associated mount. An EF-S lens (‘S’ stands for short back focus) is one designed specifically for Canon's range of DSLRs with an APS-C sized sensor. EF-S lenses won’t work on full-frame camera like the EOS-5D or 1D range.
|
| Focal length |
This is either a single value (eg. 50mm) for a prime lens or a range of values (eg. 28-300mm) for a zoom lens.
|
| f/n.n |
The maximum aperture of the lens is given as an f-number. For a prime lens there will only be one value. For a zoom lens, there may be a single value or a range of values (eg. f/3.5 - 5.6). If there are two values it specifies the maximum aperture at the two extremes of the zoom range (eg. f/3.5 at 28mm, f/5.6 at 300mm). It is not the maximum and minimum apertures as people sometimes think. If a zoom lens only has a single aperture number (eg. 17-50mm f/2.8) it means it is able to maintain the f/2.8 aperture throughout the whole zoom range. Fixed aperture lenses are usually more expensive.
|
| L |
Luxury (or maybe it's 'Ludicrously Expensive'). These are Canon's professional and usually rather expensive range of lenses. They also include special optical materials such as low dispersion glass, and include better protection against weather and aging.
|
| IS |
Image Stabilisation technology is incorporated into the lens - it detects and compensates for unwanted movement allowing for the use of slower shutter speeds.
|
| USM |
Ultra Sonic Motors - makes auto focusing precise, very fast and near silent. Electronic vibrations power the mechanical movement of the lens. USM focusing is able to stop as soon as focus is achieved without overshooting.
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| II or III |
Indicates a revised version of the lens - a Mark 2 or Mark 3 version.
|
| DO |
Diffractive Optical element – a combination of lens elements used to combat colour fringing (chromatic aberration).
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| TS-E |
Tilt and Shift Electronic: non-autofocus lenses with tilt and shift capability (see section later).
|
| MP-E |
Specialist macro-only lenses. It only focuses between magnifications of 1-5×.
|
| FTM |
Full Time Manual focus – you can override the autofocus at any time by just turning the focus ring without needing to switch the lens into manual focus mode.
|
Nikon
| DX |
Lenses designed for Nikon’s digital SLR cameras – works with any of Nikon’s cropped sensor cameras (which at the time of writing is all of them except the Nikon D3, D3X, D3s and D700)
|
| FX |
Nikon’s name for a full-frame sensor – these lenses work on all Nikon DSLR cameras including the D3.
|
| AF-D |
The lens contains a microprocessor that provides distance information to the camera to assist in exposure metering.
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| AF-I |
Internal autofocus motor – ie. does not rely on a motor in the camera body. These lenses can be used on a Nikon D40, D40x and D60.
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| AF-G |
Same a AF-I but features a newer internal drive motor
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| AF-S |
Lens features a silent wave motor (SWM) – similar to Canon’s USM.
|
| ED |
Extra low dispersion glass to minimise chromatic aberration (colour distortions).
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| IF |
Internal focusing - designed for smoother focusing and better lens balance. It also means the lens does not change length while focusing.
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| G |
A type of AF-D lens without an aperture ring, and therefore must be controlled by the dial on the camera body.
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| VR |
Vibration Reduction – sensors detect camera movement and use giro-motors to rapidly move a special piece of glass in the lens to reduce the effect of camera shake. Similar to Canon’s IS system.
|
Sigma (make lenses to fit most DSLR cameras)
| ASP |
Aspherical Lens
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| APO |
Apochromatic - special low dispersion glass used to reduce colour aberration.
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| OS |
Optical stabiliser - Sigma equivalent to Canon's IS and Nikon's VR
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| HSM |
Hyper Sonic Motor - equivalent to Canon's USM and Nikon's AF-S
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| IF |
Inner focus - an internal lens group is moved to focus the lens, therefore the lens's length does not change
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| EX |
Excellent - equivalent to Canon's 'L' series lenses
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| DG |
A lens optimised for digital, however, it retains compatibility with 35mm film cameras and therefore also full-frame sensors.
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| DC |
Optimised for cameras with the smaller sensors (APS-C sized). The smaller image circle enables Sigma to make these lenses more compact and lighter. |
We would advise you to familiarise yourself with all the acronyms for your camera lenses. Most manufacturers give full details on their websites.
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Apertures throughout the zoom range
A little nerd point here that you may have already realised while reading the section above. The actual diameter of the aperture in a zoom lens will change as you move through the zoom range. This is true even on a fixed aperture lens as it is the f-number that is fixed, not the aperture diameter.
On a 28-300mm f/3.5-5.6 – the diameter of the aperture is:
- wide angle: 28/3.5 = 8mm
- telephoto: 300/5.6 = 53.6mm
The aperture's actual diameter is actually much bigger at the telephoto end than at the wide-angle end despite what the f-number may have you believe. If it was to maintain an 8mm diameter throughout the range the lens would have a maximum f/37.5 aperture at the 300mm end (which is pretty poor).
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Photographs
This is a site about photography so I'm sure you are expecting to see plenty of pictures.
For now, why not take a peek at the flickr galleries belonging to the two authors of this site.
Colin's Flickr Page
Phil's Flickr Page
"Sometimes I look with telephoto eyes, sometimes I look with wide-angle eyes."
- Alfred Eisenstaedt
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