User:Esquilo/Photo guide

The word photography comes from the Greek words foto (φωτό) which means “light” and graphic (γραφικ) which means “to draw” or “to paint”. This is exactly what photography is about; To paint with light. Your canvas is either photographic film or a digital image sensor, the camera is your brush and the light is your paint. Sometimes, the light is just right and even an unexperienced photographer can take a fantastic picture with a simple camera set on full automatic mode. But most of the time, conditions are more demanding. In such cases, the photographer must be able to control the light that hits his canvas.

Nikon D700 is a typical modern DSLR (Digital single-lens reflex camera).

Canon IXUS 850 is a typical modern compact camera.

The camera

The main parts of a camera is always the same, regardless of the age and price; The lens assembly, the aperture, the shutter and the light-sensitive area. The aperture regulates how much light that is let in, the shutter regulates for how long time the light is let in and the film/sensor records the image.

The lens

A zoom-lens from Nikon with 18 to 105 mm focal lenght.

The lens is a piece of shaped glass or (more rarely) a curved mirror, whose purpose is to focus the incoming light. The distance from the lens to the point of focus is called focal length and is the most important property of the lens. A lens with a long focal length is called a telescopic lens and works somewhat like a spyglass; It magnifies distant objects. A lens with a short focal length, on the other hand, is called a wide angle lens or fisheye, and has a wide angle of view. A lens assembly with a variable focal length is called a zoom lens.

By varying the distance from the lens to the film or sensor, you also vary the focal distance. That is the distance from the lens where objects are depicted as sharpest. Objects closer or further away from the lens appear blurred and are said to be out of focus. A lens made to allow a very short focal distance is called a macro lens.

Summary

long focal length = magnification, small angle of view

short focal length = diminution, large angle of view

Mirror lenses

Lenses with very long focal lengths are usually both long and heavy. One way to deal with this is to use curved mirrors instead of lens elements made from glass. In a mirror-lens the light is first bounced from the primary mirror in the back and then again from the secondary mirror in the front. This way to "fold" the light-path provides for a light and compact lens while still having long focal length. Another advantage with mirror lenses is that they do not suffer from chromatic aberration. Disadvantages are no autofocus and fixed (and often quite small) aperture (see The aperture below).

The aperture

A big (f/2.8) and a small (f/16) aperture in a camera lens.

The aperture is a barrier that can block the incoming light. The aperture has a hole through which a portion of the light can pass. In most cameras the diameter of the hole is variable to adjust the amount of light let through. The diameter of the aperture is not measured in inch or milimetre, but in fractions of the focal length of the lens. So if the focal length is 35 mm and the diameter of the hole is 12.5 mm, the aperture is said to be f/2.8.

A small aperture does not only reduce the amount of light, it also reduces the angle of light for objects out of focus and thus creating a longer depth of focus. This means that objects out of focus appear sharper than they would have been with a larger aperture.

Effect of aperture on blur and depth of focus.

Some lenses, notably mirror lenses but also low-end conventional lenses do not have an adjustable aperture. This often means that the rim of the front lens also acts as aperture. It can sometimes be confusing that the aperture value varies with the zoom on a lens with fixed aperture, but remember that aperture value is the focal length divided by the aperture diameter. So, when the focal length changes the aperture value also changes, even though the aperture diameter is constant.

Summary

large aperture = more light, shorter depth of focus

small aperture = less light, longer depth of focus

The shutter

The shutter is a curtain that protects the light-sensitive film or sensor from light. When the trigger is pressed the shutter is raised and lowered again when enough light has been let in. Depending on the light conditions the time the shutter is open can be anything from less than one millisecond to several minutes. The amount of time the shutter is open is called exposure time because it is the time the film or sensor is exposed to light.

The appearance of the shutter at various speeds.

If the motif and the camera are static, a long exposure time does not cause any problems. But if either the camera or the subject moves, the pattern of light projected on the film/sensor changes causing blur in the picture. This can be used creatively to express the motion, but often the result is unwanted and disappointing.

Summary

long exposure time = more light, higher risk for motion blur

short exposure time = less light, lower risk for motion blur

The film or sensor

A cassette with photographic film.

When light strikes a sheet of photographic film, salt of silver is reduced to metallic silver. The stronger the light and the longer the exposure, more salt is reduced leaving an invisible pattern of silver on the film. This pattern is made visible in the development of the film. When light strikes an electronic sensor element, electrons are knocked free and accelerated by the applied voltage, causing a current that is measurable. Thousands such elements are arranged in an array and the different currents through each of them form the pattern which is processed by the camera's image processor to create an image file.

Film is manufactured in different grades of sensitivity measured by the ISO-scale where a higher number indicates a more sensitive film. A sensitive film has larger silver salt grains causing the resolution of the picture to be lower. The sensitivity of an image sensor is tuned by adjusting the voltage. The resolution of an image sensor is fixed, but high sensitivity also makes the sensor more sensitive to noise caused by other factors than incoming light.

Summary

low sensitivity = requires more light, higher picture quality

high sensitivity = requires less light, lower picture quality

Extra features

Autofocus

Autofocus means that the camera, and not the photographer, adjusts the focal distance. Modern digital cameras almost exclusively rely on passive phase detection to determine the focus and use a small electrical motor to move the lens-groups. Traditionally the motor has been located in the camera body with some sort of mechanical coupling to the lens, but with the advent of ultrasonic motors it has been more common to have the motor integrated in the lens-assembly. Autofocus has been around since the late 1970's when Leica produced the first camera with a working autofocus. The photographer can control the autofocus by selecting focus points. A focus point is a point in the picture where the camera will focus. This is necessary to get the main subject and not the background in focus. The photographer can also override the autofocus by using the manual focus controls.

Image stabilizer

Most modern cameras and/or lenses include some form of image stabilization. That means that there are g-meters inside the lens tube that measure the movement of the camera and one lens element with actuators that moves it to compenasate for the movement of the camera. This makes it possible to use up to four times longer exposure time without suffering from motion-blur. However, image stabilization only compensates for the movement of the camera. If the subject moves, the image stabilization will have no effect on the motion-blur.

Controlling the light

This photo was taken with 60 s exposure time. The bridge is very sharp, but the car that drove over the bridge is only seen as a light-trail. Also, the stars leave light-trails due to the rotation of the earth.

Given the factors above, the sharpest possible phicture is taken with the smallest possible aperture to get a long depth of focus, the fastest possible shutter speed to avoid motion blur, and with lowest possible ISO to avoid noise. However, this picture will probably be all black. Very little light is let in under very short time, and the light that actually get in will be mostly ignored by the low sensitivity of the sensor. So, in order to get a good picture the photographer must make a trade-off to get a picture with an appropriate exposure, not too dark (underexposed) or too bright (overexposed).

Long exposure

If the motif is static, the given trade-off is the shutter speed. If neither the object nor the camera moves there will be no motion-blur no matter how long the exposure. Some very fine night-time and twilight photos have been taken this way. To keep the camera perfectly static during the entire exposure it is best placed on a tripod or other static support and triggered either with a time-delay trigger or remote trigger.

Wide aperture

The main motif is the girl to the left. The girls in the backgrund are out of focus and blured.

If the main motif of a picture is a single object like a person, an animal etc., they are not likely to stand perfectly still even if they try. It is also desirable to make the motif stand out from the background. In this case, a wide aperture can shorten the exposure time to reasonable lengths, while at the same time the short depth of focus highlight the motif which stand out from a blurred background. Desirable blur of the background is called bokeh from the japanese word ボケ which means "blur".

Bokeh

Note that the odd shape of the (fixed) aprerture of a mirror lens (due to the secondary mirror) create disturbing patterns of out of focus objects. The bokeh will not be smooth but filled with doughnought-shaped patterns (the shape of the aperture). This is particurlary disturbing when there are lightsources in the picture that are out of focus.

In some cases, particularly in macro photography where depth of focus is very short, this effect can be used in a creative way with special lenses with custom-shaped aperture. In such lenses a thin sheet of plastic or metal is inserted into the lens to act as fixed aperture. The opening in the sheet is not round but shaped as a heart, an arrow, a star or a droplet etc. The shape of lightsources out of focus then take the shape of the aperture.

Rising the ISO

This very noise photo was taken with a compact camera set on ISO 1600.

When photographing things in motion, like sports, running or flying animals etc., not even the widest possible or desirable aperture is enough to bring the exposure time to reasonable values. Then you have to bite the bullet and accept some noise in the picture in order to avoid blur. An unsharp and blured picture is often beyond rescue, but a noisy picture can be improved with postprocessing.

The size and technology of the image sensor is a crucial factor for noise. A cheap compact camera with a small sensor are more sensitive to noise than a full-format DSLR. Also a CCD is more sensitive to noise than a CMOS-based sensor.

Flash

The so called “red-eye effect” is particularly noticeable on animals with good night vision because their retina is more reflective.

The light from a flash can be harsh and cause overexposure.

Another way to deal with poor light conditions is to add some light by yourself. A photographic flash can temporarily add huge amounts of light, which can be very useful, but it has some issues. The intensity of the light decreases with the square of the distance, meaning that flashes are only effective on short range. This can cause sharp contrasts between a sharply lit motif and a dark, unlit background. These contrasts are often beyond the dynamic range of the camera. The cure to this problem is to spread more light around the surroundings and less light on the motif. A well used trick is to not aim the flash directly at the motif, but to aim it at a large and light areas such as a wall, the ceiling or specially made reflectors that look like umbrellas. This spreads more light to the surroundings and less light directly on the motif. This is called to bounce the light. Another effect of this is that the reflected light illuminates the subject from different angles resulting in a more even exposure. This can also be done by letting the light pass through a diffuser. This is called to diffuse or to soften the flash.

The flash is often placed close to the lens to minimize the visible shadows. This can cause a problem known as the red-eye effect. That means that the light from a flash that enters the eye of the subject strikes the retina at the back of the eye and is reflected back again. Since the retina is full of blood vessels, the reflected light is red, giving the subject an almost demonic appearance.

Flash is also convenient to use when there is plenty of light, but in the wrong place, for example when the motif is backlit or in shadow and surrounded by brightly lit background. This usage of flash to even out the light is called fill flash.

Filters

A filter is device that is attached to the front lens in order to alter the incoming light in some way. Filters are usually relatively inexpensive and provides a priceworthy way to improve photography. There is an abundance of filters available on the market, but a handful filters are often sufficient for the ordinary photographer. Filters are either square or round with a threaded rim. Round filters are screwed on to the accessory thread of the front lens. Square filters are slided into an adapter attached to the front lens.

Polarizing filters

The effect of polarizing filters on both the lens and the flash.

A polarizing filter contains a lattice that blocks out light with a certain polarization. Light reflected from a glossy surface is often polarized. A polarized filter can hence be used to reduce reflections from windows, water surface etc. A polarizing filter should be able to rotate so its polarization can be tuned.

A polarizing filter is particularly useful when combined with a sheet of polarizing film in front of the flash. The polarizing sheet polarizes the light from the flash and the polarizing filter block away directly reflected light leaving only diffuse reflections. With such arrangement it is possible to photograph objects behind glass such as fish in an aquarium, objects in glass cases or oil paintings which often have a glossy surface.

Neutral density filters

Neutral density filters or ND-filters attenuates all light regardless of colour, polarization or other properties. Its usage is limited to photo of extremely bright objects such as the sun or the flame of a welding torch or to deliberately extend the exposure time to accentuate the movement of flowing water.

A more useful variant is the gradual ND-filter. This filter attenuates light most at one edge, not at all at the opposite edge and with a gradual decrease in attenuation in between. It is very useful to reduce the contrast between a bright sky and a dark ground.

A gradual ND-filter.

Colour filters

Coloured filters have been used since the childhood of photography. Especially in the era of black-and-white photography it was common to use yellow, orange or red filters to enhance contrast. With the advent of colour-film, coloured filters were used to control the white-balance; Blueish filters were used indoors in the light of tungsten lamps and amber filters were used outdoors to soften the harsh sunlight.

Today most cameras have built-in white-balance controls, but coloured filters can still be useful in some situations. For example; A blue filter can be used to avoid red-channel overexposure when photographing a bonfire. A gradual blue filter is often better than a gradual ND-filter on a clear day with no clouds. Gradual orange, purple or pink filters can make a sunset even more spectacular.

Ultraviolet filters

Ultraviolet filters or UV-filters are used on analogue cameras to reduce the bluish haze caused by ultraviolet light outdoors. Digital cameras are much less sensitive to ultraviolet light, but UV-filters are still popular as purely protective filters to protect the front lens from scratches and other damage.

A variant of UV-filters are called skyfilters or 1A-filter (1A is the index in the Kodak-Wratten filter series). They block light of slightly longer wavelength than pure UV-filters and gives the picture a faint pink tone. Skyfilters are used for a variety of purposes such as to reduce the bluishness of snow landscapes, reduce blue haze in mountain photos or to enhance warm colour skin-tones in portraits or autumn colors of foliage.

Star filters

Part of a christmas-tree photographed with a 6x star filter.

A star filter is a sheet of glass with a set of grooves etched into the surface. The grooves are usually angled 90°, 60°, 45° or 30° to each other. This causes light from bright areas in the photo to be smeared out in 4, 6, 8 or 12 lines. It is used to accentuate the light sources in otherwise relatively dark pictures.

Close-up filters

Close-up filters or magnifying filters are simply a magnifying glass attached to the front lens. It decreases the focus distance and makes it possible to take photos at closer range than otherwise possible. Close-up filters are sometimes called “poor mans macro lens” because it enables a standard lens to focus on distances otherwise only possible with a macro lens. Close-up filters come in a variety of strengths measured in dioptres.

Accessories

Apart from a flash and the filters already mentioned, there are some accessories that are more or less invaluable.

Bag

A camera bag with internal compartments.

Many people prefer to simply have their camera hanging around the neck. This way the camera is most accessible, but also most exposed to damage. Slung over the shoulder is slightly better because the arm gives the camera some protection and it is less likely to dangle. The best solution though is to have a camera bag. Most modern bags are padded, which gives the camera good protection, and has a snaplock or velcro that can be opened quickly to give fast access to the camera. Camera bags are available in a wide range of sizes, from the slimmest ones that fit the camera like a glove to rucksacks that can accommodate the equipment of a smaller photo studio. What size is most suitable is determined by the needs.

Tripod

A tripod with a camera mounted.

As mentioned under Long exposure above, a tripod or other solid support is cricual when taking photos with long exposure time. All photographic tripods have three legs and a mounting head to couple with a camera. The mounting head usually includes a thumbscrew that mates to a female threaded receptacle on the camera, as well as a mechanism to be able to rotate and tilt the camera when it is mounted on the tripod. Tripod legs are usually made to telescope, in order to save space when not in use. Tripods are usually made from aluminum, carbon fiber, steel, wood or plastic.

A monopod is a single pole that mounts under the camera. While a monopod relieves the weight of the camera from the photographer, it can not stand by itself and does not provide the steady mount required for long time exposure photos.

Remotes

Remote trigger

A RC-6 remote trigger from Cannon.

Even when the camera is mounted on a tripod, pressing the trigger or at all touching the camera is going to cause vibrations. For very short exposure times this is usually no problem, but then the tripod itself is also superfluous. For very long exposure times this not a problem either; If the shutter speed is up to a minute the effect of the vibrations during the first second are negligible. The problem is most noticable for exposures around one second and with long focal lengths. The time-delay trigger function included in most cameras delay the exposure until the vibrations has died out. A remote trigger also sovles this problem, but without a frustrating delay. Remote triggers can be corded or cordless. Cordless triggers can use either infrared or radio to trigger the camera. Infrared triggers have shorter range and may have a limited arc from wich it can be detected.

Remote flash

An Cannon OC-E3.

A flash can also be triggerd remotely. If the camera and the flash is triggerd with the same remote it is important that the flash receiver is configured with a slight delay. This delay is called sync speed and is usually around 1/250 seconds. This is the time it takes from when the camera is triggerd to when the shutter actually opens. If the flash is fired without the same delay it will burn out before the shutter opens and the picture will be very dark. If the transmitter is mounted in the flash socket on tha camera and triggerd by the flash signal from the camera the delay is handled automatically by the camera.

Lens hood

The lens hood is probably the most underestimated accessory to a camera. The most expensive part of a lens assemby is the lens coating. The coating is designed to eliminate unwanted reflections and the thickness of the coating is matched to the wavelengths of visible light. Light incident from an acute angle have a longer path through the coating and the wavelength does though not match. Also, ligth from an acute angle does not project onto the film or sensor, instead it projects onto the inside of the lens tube and the lens holders. These usually have a matte black finish to reduce reflections, but stray light inside the lens tube is bad and can cause shadows and ghost glare. The best way to avoid these effects is to use the lens hood.

Cleaning kit

When a camera is used outdoors it is often exposed to rain, sprey, dust, sand, mud etc, even if the photographer does the best to protect it. Most hign-end DSLR have at least some degree of weather-proofing for its internal parts, but the front lens will always be exposed when the lens-cap is not on. Stains and dust on the front lens also give visible degeneration of image quality. The camera will be even more vulnerable to foreign particles when switching between multiple lenses. When the lens is removed, the internal parts of the camera are left unprotected.

A basic cleaning kit should include

• A micro-fibre cloth for cleaning filters and the front lens.
• A small flask of lens-cleaning liquid.
• A blower for removing dust from visible parts in the lens socket.
• A fine brush for removing dust from the lens.