Understanding Focal Length

It might sound a bit complicated to start with, but there really isn’t anything to be scared of when it comes to defining the focal length of lenses and their relationship with your photographs. Read on to grasp a full understanding of focal length.

Understanding Focal Length

Have you ever wondered how a camera can take a picture of a tree in the distance and have the subject fill the whole of the frame despite how far away it is?

Or how a picture can be taken close to a subject yet include a whole host of things either side of it? Stay with us and we’ll tell you all you need to know.

In the world of photography the focal length describes the distance in millimetres between the lens and the image it forms on the sensor (or film) when it is sharply focused at infinity – the farthest possible distance. This distance, simplistically, determines the angle of view – how much the lens sees – which controls what portion of the scene will be captured. And this portion in turn is dependent on the size of the sensor (or film). The lens’s focal length determines the magnification, that is, the size of the image that the lens forms. And to achieve the pictures you want you will have to employ lenses with different focal lengths – but more on that later.

 

Understanding Focal Length – A note on Sensor Size

When digital photography hit the scene, focal length on lenses played a major part in the final outcome of the images. The reason? Sensor size. Rather than having the historical 24x36mm recording area of 35mm film, the image sensors on most cameras were, and still are, smaller.

The outcome of this is that the equivalent focal length of digital camera lenses is numerically longer (more telephoto) than their film equivalents. For example, a 50mm lens, having an angle of view of about 47°, is considered to be a normal lens for a 35mm film camera, ie. it produces an image that through the human eye would be recognised as normal, not distorted in any way.

So for a digital camera with a sensor that is smaller than the customary 24x36mm recording area, the lens will in effect become more telephoto, creating a smaller angle of view. As digital photography has progressed, so more companies have made DSLRs with full frames whose image sensors are equivalent to the 35mm film size, making their angle of view the same as  the 35mm film cameras.

 

Understanding Focal Length – How Does it Work?

Understanding Focal Length - How does it work

When a ray of light passes from a less dense to a more dense medium (such as from air to glass) it slows down. If it strikes the glass surface at an angle, it is also bent a little, and this is called refraction. When it passes back into air, it speeds up again, and is again refracted if the surface is at an angle.

Thus a curved piece of glass will focus a parallel beam of light (arriving perpendicular to the lens, that is along its axis) to a point. The interesting thing happens when the light rays are parallel to each other but not parallel to the axis through the centre of the lens. This same lens will also focus these, but to a point above, below, or beside the focal point for rays along the axis, and all these points of focus of parallel rays will form a plane, called the focal plane of the lens. So, you put the sensor at this focal plane, and you’ve now used the lens to concentrate the light on the sensor.

Focal length is fairly easy to understand with a lens that has a single element, but most camera lenses are made up of lots of separate individual elements. These compound lenses have an effective distance from the image plane, somewhere among all the elements and groups, and the further away from the image plane that is, the longer the focal length. And so when you focus on something closer than infinity, and the lens is moved further away from the sensor (film), the lens will get longer.

This is not the case for all lenses; indeed many have fixed focal lengths that cannot be adjusted. This means that technically a 400mm fixed-focus lens should be 400mm long. But, if you were to get a ruler out, you would see that this is not the case. This is because of all the individual glass inside that makes it behave as if it is longer than it is.

Different compound lenses house varying characteristics – such as the angles at which they refract and disperse light, which ultimately affects image quality.

 

Understanding Focal Length – Focal Length Equivalents

Darren CrushDarren Crush explains

In terms of today’s DSLRs, most sensors are the APS-C-sized format, smaller than that of a 35mm (ie full) frame. What this results in is a magnification of a lens’s focal length, whose value is determined by the sensor’s size in comparison to a standard full frame. Manufacturers refer to this magnification as a ‘crop factor’ – or, how much of the image is cropped due to the smaller sensor.

The APS-C format, measuring around 24x16mm, is smaller than a full-frame sensor by a factor of 1.5x. So, the focal length of a lens must also be multiplied by this amount to arrive at its effective (35mm equivalent)focal length – so a 28mm lens becomes a 42mm lens, a 50mm lens become a 75mm lens and so on. The APS-C format does vary slightly according to different manufacturers, with Canon’s APS-C sensor at a factor of 1.6x and Sigma’s at 1.7x, but in any case it’s simply this figure that needs to be multiplied by the focal length to arrive at its effective length.

The most obvious way manufacturers have accounted for this is by adapting their lenses. As the traditional wideangle lens used to have a focal length of around 28mm, most kit lenses start from 18mm to meet this length (ie, 18 x 1.5 = 27mm). This is equally the case for the Four Thirds format, whose 2x multiplication factor means that Olympus’s kit lenses start at 14mm. As only the central part of the image is used by the sensor, it allows digital lenses to be both lighter and smaller, as less glass is needed in their construction.

Full-frame 35mm sensor

Understanding Focal Length - Sensor

A comparison of the different-sized sensors. While the full-frame format is fixed at 36x24mm, the APS-C format comes in slightly different sizes.

 

Understanding Focal Length – The Focal Length Rule

You can tell how focal length dictates the pictures you can take by memorising the following rule. As the focal length of a lens increases, its angle of view decreases because the magnification increases, which results in the object becoming larger in size.

Focal length affects perceptual perspective too. As the focal length and magnification of a lens increases, the image appears more compressed, resulting in less visual distinction and separation between the foreground, middle ground, and background.

Wideangle – Standard – Telephoto

Understanding Focal Length - Example

 

Understanding Focal Length – What Lens Should I Use?

There is a plethora of lenses out there for the budding photographer. You’ll need to choose one that’s suitable for the type of photography you practice, but when it comes to specifics, here are a few pointers on what lens is best for what scenario:

Understanding Focal Length - Standard LensStandard Lens

On full-frame DSLRs, standard lenses fall between 40mm and 55mm, though 50mm is the accepted norm. Closest to the field of view of the human eye, standard lenses offer an undistorted perspective and are often used for flattering portraits. The closest equivalent for APS-C sized sensors is a 35mm lens. Most DSLRs come with a standard zoom, which spans from moderate wideangle to short telephoto. Consumer lenses tend to have a lower maximum aperture.

Understanding Focal Length - Standard Test

Understanding Focal Length - Wideangle LensWideangle Lens

With shorter focal lengths and wider angles of view than standard lenses, wideangle lenses are employed by landscape and reportage specialists. Remember you’ll need a shorter focal length on many DSLRs to get the equivalent field of view if you don’t have a full-frame sensor. There’s a plethora of wideangle lenses available, from 8mm ‘fisheye’ lenses to 28mm lenses. Wide zooms are increasingly popular and effective.

Understanding Focal Length - Wideangle Test

Understanding Focal Length - Macro LensMacro Lens

If you’ve ever wondered how photographers fill the frame with small subjects such as petals and insects, the answer is the humble macro lens. Allowing for 1:1 (life-size) reproduction and focusing from as close as two inches, true macro lenses are specifically constructed for close-up photography. They are commonly available in focal lengths between 50mm and 180mm. The macro lens is not restricted to purely close-up photography – many photographers employ their services for portraiture as well.

Understanding Focal Length - Macro Test

Understanding Focal Length - Telephoto LensTelephoto Lens

Any lens that weighs in with a focal length in excess of 50mm is said to be a telephoto lens. Traditionally, short telephotos (between 70mm and 120mm) are ideal for portrait photography, but that 50mm standard lens you used on your old film camera offers the equivalent angle of view to a 75mm lens (in 35mm terms), so is now an ideal lens for portraiture. Longer focal lengths (between 135mm and 300mm and above) are perfect for sports and wildlife photography.

Understanding Focal Length - Telephoto Test

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