Photography - The Lens

 


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Photography is all about capturing light on a photographic emulsion or electronic sensor. And as such the lens, film or sensor are the most important components affecting image quality. Essentially, the camera itself is just a light-tight box with a shutter!

You've seen the wonderful pictures from the rovers sent by NASA to Mars. All detailed and colourful. You might think that they're from some huge megapixel space-age techno-beast and you'd be surprised to hear that the sensor is a paltry 1 megapixel. Bigger pixels mean less noise which is always a good thing, but where NASA put all its money was into a very high quality lens. The results show it was worth it!

A camera that allows you to change the lens will obviously give you the greatest flexibility to pick the appropriate lens for the situation. An ideal camera like this is the SLR. You can choose from super wideangle to super telephoto. Macro for close ups. Bellows for even closer close up. Attach it to a microscope. Attach it to a telescope.

PRIMES VS ZOOMS

Prime (fixed) focals have the advantage of being fast (bright) and very high quality by virtue of the fact that they can be designed just for that focal length. A zoom lens allows a choice from a continuous range of focal lengths. They are useful where you require a range of focal lengths but want the convenience of a single lens, whether for weight (only one lens), always being ready to take the picture or shooting in a dusty environment and you want to keep debris entering the camera to a minimum (you also need to think about how to change films).

This all sounds great but there are drawbacks. Zooms are slower than primes (smaller minimum aperture) and can thus make hand holding and focusing (whether manual or auto) problematic. Also due to their complexity zoom lenses suffer from more abberations than primes. Lenses from the major camera makers tend to be very good. Third party zooms vary considerably. Like everything else, you tend to get what you pay for.

Zooming is more than just getting closer. It alters the focal length and affects the perspective and depth of field of the picture. Consider whether you should zoom in and use a longer focal length, or get closer and use a shorter focal length?

FOCAL LENGTHS

Standard Lenses (~50mm)
A standard lens is the usual lens supplied with an SLR. They are good general purpose lenses having an angle of view close to the human eye. They are sharp, compact and lightweight.

Small “standard" zooms have a range of typically 35-70mm (2x), 28-85mm (3x) or 24-105mm (4x). These zooms often replace the 50mm lens.

A typical compact has a zoom lens with a focal range of 35-100mm.

Wideangle Lenses (<50mm)
The natural choice for landscapes, sweeping panoramas and other outdoor scenes, group shots and generally anything requiring strong perspective. In some situations a wideangle might be the only way to capture the entire scene without excluding an important element in the frame. A characteristic of wideangle lenses is a deep depth of field making constant refocusing less critical. Good when you're in a hurry or the subject is moving fast such as photojournalism.

Medium Telephoto Lenses (85-135mm)
These lenses are perfect for portraits. Compared to a 50mm lens they isolate the subject from the background more and the increased focal length slightly flattens the image and gives more a natural and flattering perspective. Popular for candid photography.

Long Telephoto Lenses (>135mm)
Used for sports, nature or other types of documentary style photography that requires you to be close to the action but cannot be close physically be it dangerous or timid. Like portrait lenses they are great for picking out the subject from the background.

OTHER SPECIAL LENSES

Macro
Macro lenses can focus very close allowing real size, 1:1 image ratios, ie an object 10mm in size will appear 10mm on the 35mm frame. Excellent for nice close ups of insects or flowers.

Fisheye Lenses
Distort the perspective to create a circular “fisheye" 180° image. A very specialised lens. Picking the correct subject is necessary but when you do can produce some memorable images. Focal lengths vary, 7~16mm.

Super Wideangle Lenses (<24mm)
Like wideangle but more so, but not as much as the fisheye. Great for exaggerated perspective or scenes from restricted vantage point. Favoured lens of the estate agent!

Super Telephoto Lenses (>300mm)
Longer telephotos and an eye-watering price tag to match. Can be heavy due to the amount of glass they contain. Often they have a tripod mount on the lens. You will need to tripod mount to reduce camera shake and weight of lens (unless you're after a work out!) Favoured by tabloid journalist when spying on celebrities!

OTHER TERMS

Fast Lenses
A fast lens is one that has a large minimum aperture and is often a good thing. The minimum aperture might be f/1.4 or f/2.8 or whatever is appropriate for the lens compared to other lenses of the same focal length. Obviously the larger minimum aperture requires larger glass elements and is consequently heavier and maybe bulkier than a lens one or two slops slower. They are often higher quality as a side-effect of the lens maker justifing the extra expense.

Mirror or Reflex Lenses
It is possible to make lenses using mirrors to fold and focus the light rather than glass and are also known as catadioptric lenses. Many telescopes are like this. The advantages of this type of lens are compactness and reduced weight. Long glass telephotos are big and heavy beasts. The reflex equivalent is compact and lighter making hand holding possible. Like big telephotos, they usually have built-in rear-mounted filters. Catadioptrics also produce characteristic doughnut shaped out-of-focus highlights, or bokeh, which can be quite pleasing.

Apochromatic Lenses
An apochromatic lens is designed to focus three wavelengths of light, corresponding to the colours red, green and blue, onto the film plane. This reduces chromatic abberations, or the phenomenon of different wavelengths being focused at different distances or different point of the film plane. Chromatic abberation appears as coloured fringing around high contrasts objects typically a red fringe on one side and a purple fringe on the other. Normal lenses are called achromatic and they are designed to focus two wavelengths (red and blue) onto the film place and the designer assumes that everything between will be similarly focused. Apocromatic lenses are also designed to focus two wavelengths at the edges to reduce spherical abberations. Spherical aberrations show up as unfocused portions of the frame usually at the edges and at larger apertures. To achieve these feats some or all of the optics in an apochromatic lens are made from special (expensive) glass. Apochromitic lenses can be expensive!

SPECIALISED LENSES

Varisoft Lenses
Allows the photographer to adjust the amount of spherical aberration to create a distinctive soft focus effect. The lens has a control ring to set the amount of softness. Perfect for portraits. Creates more reproducible results than the alternative, but much cheaper, version of smearing vaseline on a skylight filter.

Shift Lenses
With a wideangle lens the exaggerated perspective can make tall buildings look like they are curving inward (or outward) if the camera is tilting slightly upward (or downward). Having the camera perfectly vertical (specifically parallel to the buildings) fixes the distortion but might not be the picture you are after. The shift lens allow the photographer to correct the distortion so that the buildings are straight again. Great for architectural photography and for panoramic shots intended to be stitched together.

LENS CARE

Don't use tissue to clean your favourite camera lens as it only redistributes the oily dirt and leaves tiny scratches. Use a blower brush, cleaning fluid and a lint free cloth.

HAND HELD PHOTOS

You might ask is: what is the slowest shutter speed I can use and still hand hold and get acceptable results? If you've ever used a telephoto before, you'll know that the longer the focal length the more difficult it is to hold the camera steady. That is why binoculars with ridiculous magnifications are impossible to use hand held.

A reasonable rule-of-thumb seems to be you can allow the shutter speed to drop to the inverse of the focal length. So a 200mm lens would be 1/200" and a 28mm lens would be 1/30". Naturally, all this depends on your own steadiness.

Of course, nowadays, electronics takes all the fun out of trying to hold the camera steady after a night on the pop. Anti-shake sensors and CCD scanning tricks can easily cope with moderate shaking and they seem to work well.

NOTES FOR DIGITAL CAMERAS

Comparison with 35mm
The sensor in a digital camera (CCD, CMOS etc) can vary in size. As new technology arrives they can get smaller or bigger and so the focal lengths of the lens can be difficult to relate to. To solve this the focal length is often specified as a 35mm equivalent, This is as if the sensor was scaled up to 35mm frame size (36x24mm) and focal length accordingly.

Digital Zoom
The most useless and over-marketed feature of a digital camera. I mean, what were they thinking? Most places quite wisely tell you to ignore the digital zoom. It is nothing more than cropping and enlarging a portion of the image with a resultant loss of resolution. It does not (and cannot) alter the focal length. Switch it off and use imaging software on your desktop PC to achieve better results if you need to crop.

Danny Hartley is a photographer and moderator at: http://www.ImagesClub.com

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