Digital Photography 101 - Part 3

Photography 101 – Part III

The Lens System

The Lens System – as we learned in the previous two episodes, the camera is a very complicated device, but it is nothing without a good lens. The lens enters the focused light into the camera's body and onto the sensor. In this part we'll walk through the lens system and its parts.

a. Lens system – a lens is only one optical element and a combination of more than one creates a system. The purpose of the lens system is to determine the focal length, amount of light and the focus of the light on the sensor's plane.

b. Aperture - the aperture is a diaphragm; its main purpose is to determine the amount of light that enters the camera. The aperture consists of a number of overlapping blades, which determine the opening of the aperture, much like the iris in the human eye. The aperture values are a result of a simple equation that means the "amount of time the aperture diameter enters in the focal length" or F/x=focal length/aperture diameter. And so a 200mm focal length lens and an aperture diameter of 50mm will result in an F/4 aperture opening. So the bigger the aperture's diameter – the smaller F number (reverse ratio). The aperture's values increase and decrease by fixed stops, in which the amount of light is multiplied by 2 or by 1/2 accordingly, and each stop increase or decreases the F number is by a multiple of 1.4 F (1.4 being the square root of 2.) The common stops are: F/1 F/1.4 F/2 F/2.8 F/4 F/5.6 F/8 F/11 F/16 F/22 F/32 F/45 F/64. Of course not all lenses can reach any of these aperture openings and most cameras also have intermediate F numbers between each stop. An F number will pass exactly the same amount of light on all lenses – 200mm lens F/4 will have the same exposure parameters as 50mm lens and F/4. A lens with a wide open aperture is also called a fast lens, as it allows more light in. Most lenses lack some sharpness in their most open aperture and the "Optimal aperture", which results in sharpest image, is usually X2 the widest aperture and so in an F/2.8 lens the optimal aperture will be F/5.6; this is just a guideline and each lens has its own "sweet spot".

c. Aperture and DOF – Depth of Field is the depth of sharpness in the image in front and behind the focused subject; The aperture affects the Depth Of Field (DOF) : the wider the aperture the shallower the DOF, and vice versa. The shape of the aperture affects the "Bokeh" (Japanese for blur) of the lens; the rounder the diaphragm - the nicer and smoother the bokeh will be. A round diaphragm can be reached by using rounded blades. It's recommended to read the article I wrote about DOF.

d. Focal length – the lens' focal length represents the distance between the front element of lens and the film or sensor. It doesn’t indicate anything about the physical length of the lens. Wide-Angle lenses utilize a small focal length, while Telescopic lenses utilize a large focal length, in order to take shots of distant objects. We divide Focal Lengths into several groups: less than 20mm - ultra-wide lens; 24-35mm – wide angle lens; 50mm normal lens; 75-300mm – telescopic lens; larger than 300mm – super telescopic lens. Fish-eye lenses use very small focal length, typically 10mm. The larger the focal length, the harder it is to manufacture, the lenses become heavier and more expensive and it's more difficult to achieve wide open aperture. For example, a lens of 500mm (focal length)F/4 would require front elements of at least 125mm in diameter. . There are lenses of fixed focal length and changing focal length (zoom). Usually the fixed focal length will produce higher optical quality since they hold less optical elements and are more precise. In DSLR lenses we find a crop factor multiplier, mentioned earlier in the Sensor size section.

e. Focus – in order to get a focused image, the light must precisely reach the sensor. To accomplish this, several elements must be moved inside the lens mechanism. Automatic focus works with a linear sensor that has several "pixels" that searches for contrast in the selected focus area. Why search for contrast? Because when an image is out of focus you can't find edges and contrast. The electronic engine in the camera or inside the lens mechanism signals the focus elements in the lens to move back and forth until such contrast is found and then we can take the picture. So, in order to easily focus the camera, we should point the selected focus area at a contrasting object. If the sensors are aligned horizontally, I will point them at something with vertical contrast such as a frame on the wall, and not at horizontal shades. In DSLR cameras, the central focus area is crossed by horizontal and vertical sensors, while the side areas are linear vertical or horizontal. That's why the central area is usually much more sensitive and faster. There are three ways to focus: manual – in which we move the focus ring on the lens; AF single – the camera locks focus while we keep the shutter half pressed for us to recompose; AF continuous (or AI servo) – the camera never locks focus, this setup is good for objects in motion. Lenses with fast focus motor (USM, SWM, HSM etc.) will focus faster. Wide aperture lenses will focus faster under dim light conditions.

f. Filters – almost every lens has an option for filter threading. The filters allow us more creative results and protection of the lens from scratches and dust. It's recommended to read the "filters" guide for more information. Read Filters Article for more information

g. Stabilization – especially effective for shaky hands, dim light and large focal length. The stabilizer mechanism usually allows up to 3 stops slower shutter speeds. Nikon calls it VR (Vibration reduction), Canon calls it IS (Image Stabilizer), Sigma went for OS (Optical Stabilizer) and Konika-Minolta's AS (Anti-Shake). The first three use the same principal of a floating element with gyroscopes and a fast motor to reduce to movement. Konika-Minolta's mechanism is based on a sensor on a moving platform instead of a moving lens element.

h. Perspective – lenses of different focal lengths have different perspectives. A wide angle lens will have a deeper perspective and objects will appear further away from each other; lenses with larger focal length will result in shallower perspective and objects will appear closer to each other. So if we want our swimming pool to look larger, we will use a small focal length.

i. DOF – Depth Of Field – I explained shortly before, but read the article I wrote about it where everything is explained. DOF Preview, a feature of advanced DSLRs, is a small button near the lens which, when pushed, shows how the DOF will appear in a smaller aperture. In today’s digital cameras, we don't really need it as we can take a picture and see the result directly on the screen.

j. Distortions – when it comes to lenses and optics, there are many kinds of distortion. There is Barrel Distortion which is created in wide angle lenses and 'rounds' the frame like a barrel, and Pincusion which is the opposite to the Barrel and 'draws' the corners of the frame inwards. Chromatic Aberration is caused when different light wave-lengths are focused on different planes than the camera’s sensor, resulting in red-ish or blue-ish glow around contrasting edges. Vignetting is darkening of the corners, usually occurring in cheap lenses and wide open apertures. Flare is caused usually by direct sunlight and due to reflecting of light between lens elements, due to poor coating.

I Hope you enjoyed part III of Photography 101.

Join me in the next and final episode – The Digital Image

Previous Parts – The Camera, The Digital System

Yours truly,

Roie Galitz

For more photography articles – composition, Depth of field, Filters, Flash Photography, infrared photography, Sunset photography, Blue Channel, Panning