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Part 3 : Introducing the lens

*Disclaimer: Notes written by Alistair Keddie.


Introducing the DSLR Camera lens, photography notes



Part 3 : The lens

One of the big advantages of the SLR system is the interchangeable lens. No one lens will be adequate for every situation or subject we encounter and the ability to swap around makes the DSLR hugely flexible. It allows us to equip ourselves for particular types of photography such as landscape, wildlife, sports, portraiture or travel and will have the single biggest impact on the ‘look’ of our photographs.

Its a good idea initially to collect a selection of lenses which cover a range of overlapping focal lengths. I say overlapping because not all lenses are at their best at all focal lengths. For this reason, having one lens that covers, say 18 - 200mm is not the ideal as it may not perform as well as, say having three lenses that cover 17 - 40mm, 24 - 70mm and 70 - 200mm. However, its horses for courses and the 18 - 200mm lens solution may be the one for your particular brand of photography.

Lenses, particularly zoom lenses, can be quite complicated instruments and are made up of many parts, or individual elements. These elements are arranged inside the lens and attempt to compensate and correct for any distortions introduced as the light passes through. The lens also house some sophisticated electronics and motors which work with the camera’s autofocus system, aperture and, possibly image stabilisation. Letters such as USM on Canon lenses for instance refer to Ultra Sonic Motor.

To understand our lens better, we need to know about two main factors. These are the focal length and the maximum aperture which, as we will see, describes the ‘field of view’ and the ‘speed’ of the lens. These are usually expressed in two sets of numbers written somewhere on the lens. For instance, our standard kit lens is normally around 18 - 55mm with a ratio of 1:3.5 - 5.6.


Focal Length

The focal length of a lens is measured in millimetres and is used to describe the field of view visible through the lens between wide angle and telephoto. The actual focal length is a measure from the optical centre of the lens, which, in a zoom lens changes as we zoom in and out depending on how the individual elements inside move around. The focal length is a measure of the distance from this ‘centre’ to the sensor.

You’ll see the focal length written somewhere on the barrel of the lens, or perhaps on the front. A typical ‘kit’ lens would read something like 18mm - 55mm giving, at first glance a measure from extreme wide angle to ‘normal’ zoom (see the note later regards sensor size). Field of View literally describes how much of a scene we can see through the lens. The ‘lower’ the number, the ‘wider’ the field of view. The ‘higher’ the number, the ‘narrower’ the field.

As you zoom in, that is, narrow the field of view you also magnify and flatten the scene, bringing distant objects closer and also shortening or flattening the perspective. This has the effect of pulling the parts of our scene together, compressing them if you will. Zooming out has the opposite effect, making distant objects appear smaller and lengthening the perspective, effectively stretching the distance between parts of our scene. Wide angle lenses can create quite dramatic distortions especially when in close to your subject.

A list of focal lengths would include these descriptions and typical use.

  • 21mm and under : extreme wide angle : landscape, architecture
  • 21mm - 35mm : wide angle : landscape
  • 35mm - 70mm : normal : street, documentary, travel
  • 70mm - 135mm : medium telephoto : portraiture
  • 135mm - 300mm + : telephoto : sports, wildlife, birds

‘Normal’ is described as such because the amount of distortion introduced by the lens is similar to what we see with the human eye.

When using longer lenses there’s a useful rule of thumb to bear in mind regarding camera shake. When we zoom in, we magnify our subject but also magnify any hand held shake. Its therefore generally a good idea to consider using more support if the shutter speed begins to fall below the relative focal length of lens you are using. That means, for instance, when using a 300mm zoom we would start worrying about shake if shutter speeds begin to fall below 1/300th of a second. Likewise at 200mm, our rule would suggest a minimum speed of 1/200th of a second shutter before becoming concerned.


Maximum Aperture

The other number or set of numbers found on our lens describes its ‘speed’ where speed refers to the amount of light collected at a given focal length. This is known as the maximum aperture value of the lens and essentially tells us how wide the aperture can open in that particular lens.

The maximum aperture value is usually written as a ratio. Typically on our kit lens we’ll see the values 1 : 3.5 - 5.6 or something similar. This tells us that the maximum width the aperture can open to is f3.5 at 18mm and, f5.6 at 55mm. It also tells us that the aperture value shifts as we vary our zoom. That is, as we zoom in and change our focal length, our lens gets slower. The aperture gets smaller and lets in less light than when zoomed out. This has a direct effect on our shutter speed, which will fall as we zoom in. Bad news if we need to maintain fast shutter speeds in low light on a long zoom. For instance, on the Sigma 70 - 300mm lens the ratio is 1 : 4 - 5.6 making it quite a slow lens across all focal lengths but particularly when zoomed to 300mm.

Some lenses however, have a fixed aperture value across all focal lengths. In this case our lens would have a single value such as 1 : 2.8 indicating that the maximum aperture value will stay the same regardless of the focal length. This is good news as it means there is no fall off in light being collected and hence no fall off in shutter speeds to compensate as we zoom in. These lenses tend to be more expensive to reflect their higher optical qualities.

The maximum aperture of our lens also has an impact on controlling our depth of field. Remember that the wider you can open your aperture, the shallower the depth of field you can create (see notes on aperture in the exposure triangle).


Prime Lenses

So far, we’ve only looked at telephoto zoom lenses but there is another type called the prime lens. Prime lenses work at a fixed focal length and don’t have the ability to zoom. Their advantage is often in cost and weight savings whilst offering the same or better optical quality. You might find that you eventually favour particular focal lengths in your photography in which case prime lenses would be well worth considering.


Sensor size and apparent focal length

The size of the digital sensor in our camera has an effect on the ‘apparent’ focal length or field of view. Unless the lens has been designed for the particular sensor size inside our camera, which increasingly, they quite often are, the focal length of a lens is traditionally measured against the size of 35mm film. You’ll often hear a lens being referred to as 35mm and is easy to confuse with its focal length, but in fact refers to the 35mm camera system. It is important to be aware of this as our actual focal length, though not changed because of the sensor, will ‘behave’ differently according to this size. For instance, the sensor inside the Canon 5d MkII is the same size as 35mm film, making it a ‘full frame’ camera. Using a 35mm lens (that is, designed for a 35mm camera) with an 18mm focal length will give a true 18mm field of view. However, if we switch this same lens to the Canon 400d this is no longer true. The focal length is still 18mm but because of the smaller APS size sensor I need to multiply by 1.6. I do this because the smaller sensor ‘crops’ the field of view by this factor. Effectively it means that our 18mm lens gives an actual field of view closer to 29mm which is nowhere near as wide. 55mm will ‘behave’ more like 88mm and so on.


Megapixel count

Talking about sensor size, also requires a note about megapixel count. Megapixels are a measure of how many pixels the manufacturer has managed to squeeze onto the sensor. For instance, a 10 megapixel image is made up from approximately 10 million pixels and in general, the more pixels your image has, the bigger the print you can make. However, it doesn’t necessarily hold true that more megapixels will give better image quality, particularly when working with small sensors. As we squeeze more pixels onto smaller sensors, these pixels become smaller which often leads to more noise being generated. The technology is of course developing all the time but don’t be fooled by ‘wow’ megapixel counts without considering potential issues around noise.

DSLR Camera lens example of noise


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