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Lenses (Part 1)
The sections on lenses is obviously going to be of more interest to users of DLSR cameras as most will at some point want to buy additional lenses and it is important to understand the different properties of these lenses and what they will offer in terms of creative possibilities.
The purpose of a lens is simple – it’s to focus the rays of light coming into the camera onto the image sensor. Different lenses, however, have many different properties - some of which are listed below:
- The focal length which, along with the physical size of the image sensor, governs the angle of view which is 'seen' by the camera.
- The range of aperture sizes available. The manufacturer's description of a lens will usually only list the maximum aperture.
- The minimum focusing distance.
- Whether it supports macro capability.
- The camera mount, usually specific to each make of camera. Some lenses are designed to work specifically with cameras with a smaller image sensor.
- Canon's EF-S lenses can't be used on Canon’s full frame sensor cameras.
- Nikon's DX lenses can be used on their full frame (FX) sensor cameras, although the camera automatically crops the image to a much lower pixel resolution.
- Auto focus / manual focus / internal focus / full time manual focus
You should also consider asking yourself:
- Does it incorporate image stabilisation?
- Does it incorporate a dedicated focusing motor, or rely on the one in the camera’s body?
There's a lot of information to cover in these sections, so let’s start with what it probably one of the most important.
Focal Length
There are two basic types of lenses:
Prime Lens - a lens that has a fixed focal length. These lenses are generally very high quality and have large apertures, however many people might find them a little restrictive for general purpose photography.
Zoom Lens - a lens in which the focal length can be changed over a range of values by either turning a ring on the lens (DSLR zoom lenses) or using a control on the camera to drive motors that move the lens (compact cameras).
The principle purpose for changing the focal length on a lens is to change the field of view. A short focal length will give a wide field of view and capture more of the scene whereas a long focal length will only capture a small part of the scene and therefore appear to magnify objects in the scene. The three pictures below, all taken from the same point, clearly illustrate this.
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wide field of view |
'normal' field of view |
narrow field of view |
As well as the focal length, the physical size of the image sensor is also a factor in defining the field of view. A 50mm lens will give a wider angle of view on a camera with a large sensor than it will on a small sensor.
Sensor Size
A 35mm film negative has a size of 36mm by 24mm. If the sensor in a digital camera has the same dimensions as this, it is called a full-frame sensor. A 50mm lens on a digital camera with a full frame sensor will have exactly the same field of view as it would on a 35mm film camera.
Due to the high manufacturing costs of full frame sensors, most digital cameras use smaller sensors - the table below shows some of the more common sizes. The 'crop factor' column will be explained in a moment.
| Sensor Size (mm) |
Name |
Crop Factor |
Some camera that use this... |
| 36.0 x 24.0 |
Full Frame |
1.0 |
Canon EOS 5D, 1Ds, Nikon D3 |
| 28.7 x 19.1 |
APS-H |
1.25 |
Canon EOS 1D |
| 23.7 x 15.7 |
APS-C (Nikon) |
1.5 |
Nearly all Nikon DSLRs, Sony and
Pentax are very similar in size |
| 22.2 x 14.8 |
APS-C (Canon) |
1.6 |
Most Canon DSLRs inc. 350D,
400D, 450D, 20D, 30D, 40D |
| 18.0 x 13.5 |
Four-Thirds |
2.0 |
All Olympus and Panasonic DSLRs |
| 7.2 x 5.3 |
1/1.8" |
5.0 |
Compact cameras |
| 5.8 x 4.3 |
1/2.5" |
6.2 |
Compact cameras (often models
quoting a high optical zoom) |
Crop Factor
For a moment let’s consider lenses designed to work with a 35mm film SLR. A lens with a given focal length will always cast an image circle of a certain size. This will not change whether the lens is on a 35mm film camera, a full-frame DSLR or a small-sensor DSLR. The smaller sensor, however, will only capture the middle portion of the image projected into the back of the camera by the lens which results in a cropped field-of-view. In the image below, the circle is the image projected by the lens: a 35mm piece of film or full-frame sensor would capture the image in the red rectangle. A crop sensor (as found on consumer level DSLRs) would capture the image in the blue rectangle.
Here it is illustrated another way. The image on the left is the full-frame sensor image – the one on right shows the picture as captured by a camera with a crop factor of 1.5 (the film border is there to give a sense of scale). You can see that it’s captured a smaller field of view but the image has not been magnified in any way as the focal length of the lens has not changed.
Left: a 50mm lens projecting an image which is captured by a 35mm film or full-frame sensor. Right: the same 50mm lens casts the same size image on the sensor, but the smaller image sensor is only capturing the middle part of the image.
To get the equivalent field of view on a 35mm/full-frame camera you would need to use a longer focal length - computed by multiplying by the crop factor.
Left: this is the image projected onto the 35mm film / full-frame sensor by a 75mm lens. The longer focal length gives greater magnification on the sensor but the field of view now matches the 50mm lens on the smaller sensor camera.
As 35mm film has been in existence for a long time, photographers are used to which angle of view corresponded to a particular focal length. Because of this familiarity with the film format, the crop factor is sometimes known as a focal length multiplier (FLM). We don’t like this term as it assumes that 35mm / full-frame is some divine format that everything should be referenced to. As you’ve seen in figure 4.2, a 50mm lens is always a 50mm lens whether it’s on a full-frame or crop sensor camera - it’s just the field of view that changes.
So a 28mm lens on a film camera is a standard wide-angle lens but becomes not such a wide angle lens on a crop sensor.
So if you are used to film SLRs, we would advise you to forget what you know about focal lengths and their corresponding field of view - and think in terms of actual focal lengths for your specific camera. So if you own a Canon, Nikon, Sony or Pentax, think of 18mm as wide angle, 35mm as your standard lens and 70mm as your portrait lens (and not 28mm, 50mm and 105mm as in the days of film).
With compact cameras however, it is common to talk about the 35mm equivalent zoom range as the physical sensor size can vary greatly from one camera to another. Doing this removes the sensor size out of the equation and makes it easier to compare one compact camera to another. For example, a Fuji S9600 has a focal range of 6.3mm to 67mm, whereas a Canon SX100 IS has a focal length range of 6.0mm to 60mm. So does the Canon give the wider field of view? Without knowing the size of the sensor, you just can’t tell. As it happens the Fuji’s lens is a 28-300mm equivalent (crop factor of 4.48) and the Canon’s is 36-360mm (crop factor of 6.0 due to a very small sensor).
If you must continue to use 35mm equivalents and want to buy a lens for your DSLR that gives you what you used to know as an x mm lens on your old film SLR, you should divide by your cameras crop factor. Using a Canon EOS 40D with a 1.6× crop factor as an example: if you want a lens the same as your old 24mm ultra-wide film lens, you will need a 24mm/1.6 = 15mm lens on a Canon EOS 40D.
Angle of View
The term 'angle of view' is often used to mean the same as 'field of view', although we tend to use it when specifically talking about quantifying the angle. The angle most commonly quoted is the diagonal angle of view - the angle of the scene captured from one corner of the picture to the opposite corner.
We won't bother going into the maths of how to compute the angle of view, however as a rough guide, doubling the focal length will halve the angle of view. The table below shows the angle of view for some common lens focal lengths and for the four most common DSLR sensor sizes.
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Full-frame
36.0 x 24.0 mm |
Nikon APS-C
23.7 x 15.5mm |
Canon APS-C
22.2 x 14.8mm |
Four-thirds
17.3 x 13.0mm |
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Crop factor: |
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Focal length |
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10mm |
130.4° |
109.5° |
106.3° |
94.5° |
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14mm |
114.2° |
90.6° |
87.2° |
75.4° |
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18mm |
100.5° |
76.4° |
73.1° |
62.0° |
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20mm |
94.5° |
70.6° |
67.4° |
56.8° |
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24mm |
84.1° |
61.1° |
58.1° |
48.5° |
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28mm |
75.4° |
53.7° |
51.0° |
42.3° |
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35mm |
63.4° |
44.1° |
41.7° |
34.4° |
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50mm |
46.8° |
31.6° |
29.9° |
24.4° |
|
55mm |
42.9° |
28.9° |
27.3° |
22.3° |
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70mm |
34.3° |
22.9° |
21.6° |
17.6° |
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105mm |
23.3° |
15.4° |
14.5° |
11.8° |
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200mm |
12.3° |
8.1° |
7.6° |
6.2° |
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300mm |
8.2° |
5.4° |
5.1° |
4.1° |
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500mm |
5.0° |
3.2° |
3.1° |
2.5° |
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