Copyright Michael Karbo, Denmark, Europe.


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    Chapter 34. Zoom lenses

    One of the really practical things with modern cameras is the zoom lens, which most of them have. A camera can then zoom in and zoom out in relation to the subject – some models more than others. If we are going to understand this technique, then we can’t avoid the technical details concerning focal length and viewing angle.

    The first digital cameras had either a fixed focus (1X zoom) or a limited 2X zoom. Since then 3X, 4X or even 5X zooms have become standard in the small pocket cameras, where a lens mustn’t take up much room. It has to be able to be drawn into the camera’s body, when you switch it off.

    You can always read the lens’ data, which usually is engraved on the front. Here we have a quite normal pocket camera with a 5X zoom lens. You can read that the lens’ length (focal length) goes from 7,8 mm to 39 mm . This determines how powerful the zoom effect is and by this the viewing angle of the image.

    If you could compare focal length (measured in mm) from one camera to another, then it will have to be converted to the standard measurements, which will be described later.

    This pocket camera has a 5X optic zoom. The lens can still be drawn into the camera body, so it stays completely flat and pocket-friendly. (Pentax 450)

    Figure 161. The effect of a 3X zoom lens.

    Zooming in and out

    When you zoom in, this corresponds to putting a telescope in front of the camera so that you come close to the subject (with the effect of a telelens). The opposite happens, when you zoom out. Then it is as if you are moving away from the subject (with a wide angle lens).

    Most cameras have a motor zoom, which is controlled with the help of two tilt buttons. In some of the bigger and more expensive zoom cameras, the zoom effect is controlled manually be twisting a ring on the lens. By doing this you are moving the objective’s lenses yourself and you can see the effect in the viewfinder. This is an extremely agreeable function, which is also found in Single Lens Reflexcameras.

    The length of the lens

    It is the length of the lens that controls the telescope effect. The longer the lens is (the bigger the focal length), the more powerful the telescope effect will be.

    You can’t always see for yourself that the focal length has become longer or shorter because the alteration can have taken place in the lens itself where the individual lenses can have moved by themselves in relation to each other.

    Figure 162. The tilt contacts to the zoom have to be easy to reach with your thumb.

    Zoom factor

    In practice, the focal length has an influence on which angle the camera photographs with and, therefore, on how much is included in an exposure.

    The lens here has to be twisted to get a zoom effect. The focal length is altered because the lenses are moved in relation to each other. Seen from outside, the lens becomes either longer or shorter. (Minolta A1)

    For practical reasons, the lens’ focal length is given in so-called standard measurements, which have been taken from analog 35 mm film cameras. In standard measurements, a normal lens has a focal length of 50 mm. When the focal length is shorter than a normal lens, then we are talking about wide-angle lenses.

    For everyday use it is easiest to keep to the zoom factor, which, for example, can be 4X or 5X. This tells us that the focal length will be four or five times longer, when the lens zooms from the one outside position (wide angle) to the other (telelens).

    Wide-angle and zoom

    With a wide-angle lens, we can get an enormous subject into a camera. This corresponds to the photographer moving away from the subject so that more is included in the image. You can photograph almost all types of subjects with wide-angle.

    It can be a big advantage indoors if you can get more of a sitting room into an image. If there is a large party with a lot of people, you don’t have to be 12 meters away to get all the party into the shot.

    Figure 163. The shorter the focal length is, the more you can get into a picture, because the viewing angle is bigger. Note the difference between the viewing angle with 28 mm and 35 mm wide-angle – it is, in fact, big.

    Figure 164. The two most ordinary sizes of wide-angle lenses have focal lengths of 35 and 28 mm. Their effect is obvious to see with indoor exposures.

    Attention on wide-angle

    An ordinary pocket camera with a 4X zoom lens will typically have a focal length from 35 to 140 mm , when we have converted to standard measurements. But a camera can also have a focal length of 28 to 112 mm, this gives the same zoom factor of 4X.

    Camera

    Zoom

    Focal length

    Nikon 5200

    3X

    38 - 114 mm

    Canon S60

    3,6X

    28 - 100 mm

    Olympus5060Z

    4X

    27 - 110 mm

    Pentax 555

    5X

    38 - 190 mm

    Figure 165. Examples of ordinary cameras in the 5MP class with rather different wide-angle lenses (converted here to standard measurements).

     

    There is rather a large difference in whether the ”shortest” focal length is 35 or 28 mm . This gives an angle viewing of 63 or 75 degrees respectively. A lot more of the subject is included when the focal length is down to 28 mm , so this lens is very useful and can be strongly recommended.

    Wide-angle distortion

    Ordinary cameras have zoom lenses with limited wide-angle effects. But if we look at a Single Lens Reflexcamera, it is possible to replace the lenses. And here you can find an even shorter focal length of, for example, 20 and 15 mm . With these sorts of lenses we get a fish-eye effect, because of a geometric distortion in the lens something like this:

    Figure 166. Fish-eye effect, which can be experienced with ultrashort wide-angle lenses.

    Check the lens

    There is also geometric distortion with a 28 mm lens – but only limited. It is easy to find out how powerful the distortion is.

    The classical test is photographing a brick wall. The joints between the bricks have to be parallel without distortion, but they are not completely if you photograph with a 28 mm wide-angle lens.

    Figure 167. Test result with 28 and 35 mm focal length. In the top image, there is an obvious distortion in the lowest and topmost rows. It is, however, not enough to mean lot in practice.

    A focal length of 28 mm is normally better than 35 mm because of the big angle view. A camera is more flexible when there is a good wide angle.

    Please note that you can produce panoramas by ”pasting” several photographs together. You can in this way create large wide-angle exposures of high quality. This requires serial exposures, which have been photographed, for example, from right to left and copied afterwards with special software.      

    Figure 168. Panorama exposure, where five single images have been copied together into one enormous wide-angle image.

    Large focal lengths

    When a lens has focal length of 50 mm , we call it a normal lens. When a focal length is longer, then we have telelenses. Their advantage is the telescope effect. If we, for example, have a lens with a focal length of 200 mm (ca. 6X zoom), then it will give a very fine enlargement revealing details, which couldn’t otherwise be seen with the naked eye.

    Camera

    Zoom

    Focal length

    OlympusC-8080

    5X

    28 - 140 mm

    Fujifilm S7000

    6X

    35 - 210 mm

    Canon Pro1

    7X

    28 - 200 mm

    Minolta A2

    7X

    28 - 200 mm

    Sony F828

    7X

    28 - 200 mm

    Nikon 8700

    8X

    35 - 280 mm

    Panasonic FZ-10

    12X

    35 - 420 mm

    Figure 169. Example of cameras with powerful zooms, which are all relatively big, heavy and of high quality.

    Figure 170. Zoom cameras are rather big and can seem a little clumsy. The lenses are big and usually of high quality.  Minolta’s models A1 and A2 are fine examples of these types of models. They distinguish themselves by having optic stabilisators, which effectively eliminate a great deal of the shaking, lenses experience.

    1X zoom

    2X zoom

    4X zoom

    7X zoom

    Be careful about shaking

    A zoom camera is a big camera but you have to be careful when taking photographs with such powerful tele-effects.

    You can, for example, take photographs of animals, which are otherwise difficult to get close to or take sports exposures from long distances. The problem is shaking. The unavoidable shaking of your hands, which hold the camera, is strongly magnified when you zoom powerfully into a subject. It is important here to be familiar with the camera’s means of action, which can be used to avoid shaken exposures:

  • Shortest possible shutter speed

  • Maximum luminosity (big aperture, high ISO value)

  • Optic stabilisator

  • Tripod

    Generally, the important thing is to use fast shutter speeds to avoid shaken exposures. If a camera zooms 10 times in on a subject, then you should expect at least 10 times as short a shutter speed, if shaking is to be avoided – unless you use a tripod.

    Lots of light and high light sensitivity is necessary for working with shutter speeds of, for example, 1/500 and 1/1000 second. This means that the lens has to be powerful so that it can, for example, photograph with diaphragm 2,8 with 10X zoom. Not all cameras can do this. It is in the same way an advantage if the exposure can be made with ISO 400 rather than with ISO 200.

    Optic stabilisation or a tripod

    If the light conditions are not right for a very short shutter speed, then the camera will have to be stabilised.

    This can be simply done by placing the camera on firm ground. Or you can use a tripod. Then you can easily take telephotos with longer shutter speeds, especially if you activate the camera’s self timer. In this way you can avoid shaking from the release button.

    If you haven’t got a tripod with you, the exposure can maybe be taken without shaking anyway – if you have invested in a camera with an optic stabilisator. This is a very effective technology, which is, unfortunately, only found at the more expensive end of the price scale. The idea of the system is that the lens and the image sensor in some way or another are installed in shock absorbing suspension.

    This stabilisator is also called anti-shake (or gyro), and it works very well. A great deal of the shaking is removed and the result is an advantage of 2-3 EV.

    This means, that if telephotos have required a shutter speed of 1/400 second before they were free for shaking, then they can now be taken with 1/100 or maybe 1/50 second without shaking, if the stabilisator is activated.

    You can also use the stabilisator to take exposures indoors without flash. If you, for example, hold the camera still with 1/8 seconds exposure, then you can often manage without a flash and get a better result with the natural lighting.

    Camera

    Zoom

    Luminosity

    Stabilisator

    OlympusC-8080

    5X

    F2,4 – F3,5

    No

    Fujifilm S7000

    6X

    F2,8 – F3,1

    No

    Canon Pro1

    7X

    F2,4 – F3,5

    No

    Minolta A2

    7X

    F2,8 – F3,5

    Yes

    Nikon 8700

    8x

    F2,8 – F3,5

    No

    Panasonic FZ-10

    12X

    F2,8

    Yes

    Sony F828

    7X

    F2,0-2,8

    No

    You have to be aware of a zoom camera’s light sensitivity and whether it has an optic stabilisator. This means a lot for the enjoyment you can get from having a powerful zoom lens.

    A tripod is a natural accessory for a zoom camera. (Sony F828)

    Digital zoom

    There is a variant of zoom functions, which is called digital zoom. Some cameras advertise, for example, with a text like ”22X zoom”. A zoom lens this powerful, however, does not exist.

    What they really are talking about is an optic zoom of 10X, but the camera’s software can artificially zoom further into the subject with the factor 2,2. That is how we get up to the 22X.

    So digital zoom is not a genuine zoom. The idea of the system is roughly to take out a middle section of the image and enlarge it until it fills as much as a whole image.

    In practice, digital zoom is not nearly as good as a genuine optic zoom. It creates more image details but this is done artificially (by interpolation). If you are in a special situation and need to zoom extra in, then you can try digital zoom. Remember to use a tripod.

    Figure 171. Digital zoom can be activated via the menu system. (Canon S50)


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