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Home » Reviews & Guides » Guides » Lens Sense
Lens Sense was written to give you, the user, a clearer idea about what different lenses do and how to decide which lens you should be using to best achieve the optimum photograph. Each of the eight sections are clearly explained and accompanied by illustrations to aid understanding. In Lens Sense, we cover everything from what different focal lengths mean to your photograph, to how to determine what depth of field to work with as well as looking at the compatibility of different cameras and lenses. All the terminology that you will be faced with is explained and practical examples are given. Read on to find everything you need to know to take your photography to a higher level!
The focal length of a lens represents the distance from its optical centre to the digital camera sensor when the subject of the photo is in focus. Focal length is measured in millimetres (mm). Put simply, a short focal length means you would need to be close to your subject to get a close up, while a long focal length means you can be far away and still get a close up.
This image shows the cropping factor of different focal lengths. The yellow 50mm crop roughly equates to human field of vision. Any focal length below 50mm is considered wide angle and above 50mm is telephoto.
The lower the focal length, the wider the field of view and therefore, the more of the scene can be fitted in to the frame. At the widest angles, some distortion may be seen towards the edges of the frame where vertical lines may appear curved.
Using a lens with a 50mm focal length, the field of view is approximately equivalent to what the naked eye sees. As focal lengths increase, the effect becomes more telephoto. The field of view is narrowed as part of the scene is magnified and has the appearance of being brought closer.
At the highest focal lengths, the narrow angle of view means that only a small portion of the scene is captured. The subject within the frame is greatly magnified and appears much closer. The magnification of a given focal length can be roughly estimated by dividing the size by 50mm (equivalent to naked eye). For example, a focal length of 300mm gives an approximate magnification of 6 x life size (300 / 50 = 6).
Lenses initially fall into one of 2 categories; Prime and Zoom. A prime lens has a fixed field of view and whilst it is possible to focus on both near and far subjects, it is not possible to alter the magnification of a prime lens as the focal length is fixed. Common examples of prime lens focal lengths are 28mm, 50mm and 85mm.
Zoom lenses have a variable focal length and therefore the angle of view and magnification of the image can be varied. Some zoom from extreme wide angle to slight wide angle (e.g. 14-24mm), some zoom from wide angle to telephoto (e.g. 24-70mm) and others vary from telephoto to extreme telephoto (e.g. 100-400mm).
These are all zoom lenses. Some people confuse the terms zoom and telephoto and assume they mean the same. This is not the case. As the examples above show, a zoom lens can be telephoto (covering focal lengths over 50mm), but it can also be wide angle (covering focal lengths under 50mm). In addition, a telephoto lens can be a prime lens.
The aperture controls the amount of light that passes through a lens. It is measured in f stops (e.g. f1.8, f5.6, f11). The diagram below shows how the aperture closes much like the pupil of an eye.
The higher the f stop, the more the aperture closes and the less light is permitted through the lens.
Traditionally the aperture was adjusted via a ring on the lens, but most moderncameras allow the f stops to be changed from controls on the camera itself.
When the specifications of a lens are quoted they usually state the focal length of the lens and the maximum (largest) aperture, e.g. Nikon 50mm f1.8. We can therefore see that this is a prime lens with a focal length of 50mm and a widest aperture of f1.8.
Zoom lenses often quote a variable aperture figure, e.g. Canon 100-400mm f4.5-f5.6. This is a zoom lens with a variable focal length, so the maximum aperture also varies. At 100mm the maximum aperture is f4.5 and at 400mm the maximum aperture is f5.6.
Some of the more expensive zoom lenses have what is called a constant aperture and only quote a single figure, e.g. Sigma 70-200mm f2.8. With these lenses the maximum aperture does not change as the focal length does. The maximumf stop is f2.8 at both 70mm and 200mm.
As well as controlling the amount of light let through a lens, the aperture also determines the amount of the picture that will appear in focus. This is known as the depth of field. The higher the f stop, the smaller the aperture. The smaller the aperture, the greater the depth of field.
A small depth of field means that only part of the image will be sharp. This may be the closest part of the scene, the middle part of the scene or the furthest point away, depending on where you have focused. A high depth of field means that more of the image, from front to back, will be sharp.
Taken with aperture of f3.5
Taken with aperture of f22
At f3.5 the depth of field is very narrow. The flowers are in focus but the background is not. At f22 the depth of field is much greater. The flowers are still sharp but the background is also much clearer.
In the image above the point of focus was the 27cm mark. The aperture used was f3.5 which has meant that only the focused central area is sharp. Both the foreground and the background are not in focus.
The same point of focus was used for the image below. However, because the aperture was changed to f13, the depth of field is greater and both the foreground and background appear more in focus.
These are lenses designed specifically for close up photography (e.g. flowers, insects, jewellery etc.). They are usually prime lenses but zoom versions do exist. A genuine macro lens such as the Nikon 60mm f2.8 or the Canon 100mm f2.8 will give at least a 1:1 reproduction. That is to say that image of the subject will be projected at life size or larger on to the camera’s sensor.
A true macro lens produces life size images on the cameras sensor
This reproduction ratio does not vary with focal length. A 50mm macro lens and a 180mm macro lens will both give a 1:1 reproduction, but the 180mm will do it from further away. Because of this, macro lenses with longer focal lengths are better for photographing insects which might fly away.
Some telephoto zoom lenses may claim to have a macro facility, but most only provide a 1:2 reproduction at best and as such are not true macro lenses (e.g. Sigma 70-300mm).
These are extreme wide angle lenses that give around a 180° angle of view. Standard fish-eye lenses give an almost 180° view both horizontally and vertically and as such produce a near circular image on the camera sensor.
Full-frame fish-eye lens
A full-frame fish-eye gives a 180° view diagonally and therefore a full-frame rectangular image is produced.
With both types of fish-eye lens the distortion towards the edge of the image is very clear with both straight horizontal and vertical lines being severely curved. It is this effect that gave this type of lens its name – fish-eye.
A very specific and technical type of lens, tilt and shift lenses allow the end of the lens to be moved up and down, side to side and tilted forwards and backwards independently of the lens mount and camera. They are primarily used for architectural photography as the movements of the lens allow photographs of buildings to be taken without the problem of “converging verticals”.
This image clearly shows the problem of converging verticals. The vertical lines of the building converge and appear to get closer higher up. This is clearly not accurate and happens when photographing tall buildings from the ground.
The second image shows the effect of a tilt and shift lens. The independent movements of the front element of the lens enable the plane of focus to be adjusted within the lens itself. This in turn allows the converging vertical lines to be correct, giving a far more natural and realistic image.
Another use of tilt and shift lenses is for the artistic focussing effects that can be achieved. Because the plane of focus can be adjusted it is possible to produce images where only a small part of the subject is sharp.
Canon’s TS-E range of lenses offer tilt and shift functions in focal lengths of 17mm, 24mm, 45mm and 90mm. Nikon only have one in the form of their 85mm PC (Perspective Control) Micro Shift lens. In addition to these, the Lensbaby range also offers similar functions, albeit in a far more simplified and manual way.
Canon TS-E 24mm f3.5
Nikon 85mm f2.8D PC Micro Shift
Because digital camera sensors vary in size, the visible effect of different focal lengths and their magnification also changes. Full-frame sensors are the largest and are the same size as a traditional 35mm film frame. However, at present most sensors found in digital SLR cameras are smaller than this, which has a direct effect on the requirements and performance of the lenses.
Full-frame sensor with a standard lens
Cropped sensor with a standard lens
The image projected from the rear of a lens is circular and the circle of light produced has to be big enough to cover the whole sensor.
As the second image above shows, with a cropped sensor only the central part of the projected image is used. Because the outer edges of the image are not recorded, the effective angle of view is reduced. In other words, the widest part of a scene is lost and because of this, a wide angle lens becomes more telephoto.
Image showing full-frame and cropped sensor comparison
The image above shows how the same scene will be recorded on both a full-frame and a cropped sensor. While the full-frame sensor records the widest parts of the scene, the same lens on a cropped sensor has the effect of a telephoto lens, seemingly magnifying the image and bringing it closer. The two images below show how identical sized prints from the two different sensors would appear. Compare these to the images in the Focal Lengths section and the telephoto effect of a cropped sensor is easily seen. Although the exact size of a cropped sensor varies from model to model, the effect is to increase the apparent focal length of the lens by 1.3x to 2x depending on the camera model.
To obtain a wide angle image on a cropped sensor the lens has to be extreme wide angle. Because these types of lenses are usually very expensive, manufacturers have designed smaller, cheaper lenses that only produce a big enough projected image to cover a cropped size sensor
Full-frame sensor with standard lens
Cropped sensor with digital lens
Full-frame sensor with digital lens
However, as the diagrams above demonstrate, these “digital” lenses are not suitable for digital cameras with full-frame sensors or film cameras as the projected image does not cover the sensor. These types of lenses are usually given a specific name or code to identify them from standard lenses, e.g. Sigma use DC, Canon EF-S, Nikon DX, Tamron Di-II.
Wildlife and sports: Long telephoto or zoom lenses with a wide maximum aperture, e.g. Sigma 70-300mm f4-5.6, Canon 100-400mm f4.5-5.6.
Portrait: Short-ish telephoto lens with wide maximum aperture, e.g. Nikon 85mm f1.8, Tamron 90mm f2.8.
Landscape: Wide angle prime or zoom lens, e.g. Sigma 10-20mm f4-5.6, Canon 17-40mm f4
Travel/Documentary: Slight wide angle or standard lens with reasonably wide maximum aperture, e.g. Canon 50mm f1.4, Nikon 35mm f2
General use: Mid-range zoom covering both wide and telephoto focal lengths, e.g. Nikon 18-70mm f3.5-4.5, Canon 24-105mm f4, Sigma 28-70mm f2.8
All in One Solution: Lenses covering very wide angle up to long telephoto, e.g. Nikon 18-200mm 3.5-5.6, Canon 28-300mm f3.5-5.6, Tamron 18-250mm f3.5-6.3.
In addition to these, independent lens manufacturers like Sigma, Tamron and Tokina manufacture lenses to fit a range of different cameras. Any given lens will usually be available in Canon, Nikon, Sony/Minolta, Pentax/Samsung and 4/3rds fit. Sigma also make lenses to fit their own range of cameras.
What's next: So now you know everything you need to know about lenses, and you can make an informed choice on the lens that will take your photography to the next level!
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