Light reflected from a subject, through the camera apparatus to the light sensor, produces the photographic image. Each camera component along the light path introduces a change that will alter the resulting image. The simplistic representation below shows the spatial relationships among the components. A cursory understanding of the image transformations along the light path are described in this blog entry.
Photographers can optionally install filters. Multiple filters can be installed to produce composite effects. Although a plethora of filter types are available, I’m limiting the discussion here to four common types.
Ultraviolet Light Filters (UV)
Light entering the camera is composed of spectral frequencies that are influenced by the light source and reflective surface of the subject including those in the ultraviolet range. The light sensor is sensitive to frequencies that are similar, but not identical, to the naked eye. Although UV light is not visible to the eye, sensors are slightly sensitive to it. In the vast majority of photographic situations this is not disruptive to the final image. When taking a picture that contains a bright, clear sky, excess UV light can make the sky appear slightly hazy in the photograph. A UV filter defeats this by preventing the UV light from entering the camera and striking the light sensor.
Some photographers always use a UV filter to protect their equipment. It’s a physical barrier that prevents dust, dirt, and moisture from damaging the camera lens. It’s cheaper to replace a filter than a lens.
Circular Polarizing Filter (CPL)
We’re all familiar with polarized sun glasses and their ability to reduce glare. The same principle applies in photography. Light coming from the sun and most light sources is non-polarized. The light waves occur in many orientations. They are not aligned. When the light is reflected off some surfaces, the light waves become aligned or polarized. The CPL filters out light based on the orientation of the light waves eliminating the glare. CPL filters are not in a fixed position. The photographer can rotate the filter to adjust the direction of polarized light being removed.
CPL’s are used to eliminate glare. Among the situations where one should consider installing a CPL are photographs containing water, glass, snow, or other high-glare surfaces. CPLs are also valuable when taking pictures of vegetation. Plants have a waxy cuticle on the surface of their leaves that can create localized areas of glare. Eliminating this glare will produce a photograph with sharper contrast and better color.
Neutral Density Filter (ND)
Neutral Density filters are designed to reduce the quantity of light passing through them without altering its spectral or polar characteristics. Over exposure in bright conditions can limit a photographer’s creative options by forcing fast shutter speeds and small apertures. A light reducing ND filter provides additional creative freedom without undercutting image color.
ND filters come in a variety of densities and can be combined to create even more variety. Selection of the density is one of the many artistic choices facing a photographer.
Florescent Light Filters (FLD & FLW)
Florescent light spectra have a strong yellow component that causes an undesirable hue in photographs. The remedy is to filter out some of the yellow light using a florescent light filter.
The available florescent light filters can be distinguished by the desired end result. The first is the a florescent light to day light filter (FLD). As the name indicates the filter is designed so that the light exiting the filter has spectral characteristics similar to sun light. The other common choice is the florescent light to tungsten filter (FLW). It strives to convert the light to a spectral characteristic similar to one produced by an incandescent light that has a tungsten filament. This is, of course, common indoor lighting.
The lens brings the light beams into focus to form an image on the light sensor. Although drawn as a single lens, the lens configuration usually involves multiple lenses that collectively provides capabilities like zooming.
Lenses can be grouped into four categories based primarily on their focal length. A standard lens is one that has a focal length that’s equal to the diagonal of the camera’s light sensor. For a 35mm camera, the standard lens has a 35mm focal length. Advanced Photo System type C (APS-C) sensors, which are more common, have a 28mm focal length. When these measurements are equal an image will be produced that accurately reproduces the scene as viewed by the human eye. Relative object sizes are preserved avoiding aberrations like fish bowl effects.
Zoom lenses have a zoom ring that allows for focal lengths beyond that of a standard lens. This allows the photographer to get up close and personal with the subject without being physically close. While any focal length beyond standard constitutes zoom, these lenses frequently have lengths in the range of 100 – 300mm. It is not uncommon to find lenses with focal lengths as large as 500mm.
As the focal length increases the width of the field of view necessarily becomes narrower making zoom lenses a poor choice for landscape photography. The longer focal length also makes it more difficult to stabilize the camera. This often necessitates a tripod to avoid blurry images. Finally, the cost rises as the focal length increases.
Wide Angle Lenses
Wide angle lenses are exactly the opposite of zoom lenses. They have focal lengths that are shorter than the standard lens. Shortening the focal length allows a wider field of view making them especially well suited for landscape photography.
Macro lenses are a subset of zoom lenses that are specifically designed for close up photography. To be considered macro photography, the subject needs to be less than about 25 mm and the reproduction needs to be ten times that size. Flowers and bugs are typical examples of macro photography subjects.
Prime lenses are identifiable by their fixed focal length. Fixing the focal length simplifies the lens configuration reducing the number and severity of optical aberrations and provides superior support for large aperture photography. In keeping with the goal of creating a excellent optical experience, these lenses are usually manufactured using high quality materials.
While prime lenses provide this high quality experience, they suffer from lack of flexibility. The fixed focal length precludes zooming. They also tend to be more expensive than variable focal length lenses due to the superior materials. For these reasons many amateur photographers don’t own prime lenses.
The aperture is an adjustable opening that the light passes through in route to the sensor. It allows the photographer to control exposure by reducing the amount of light reaching the sensor. The aperture also controls the depth of field of the image that’s produced. Depth of field is the range of distances from the camera can be subjectively described as in focus. A blurry foreground and background creates a bokeh that attracts the viewer’s attention to the subject.
The shutter controls the period of time that the sensor records an image. There are two common types of shutters. Many digital cameras have both.
These shutters are distinguished by two curtains that work together to control the exposure. When the shutter release is pressed, the first curtain drops exposing the sensor to light. After a period time equal to the shutter speed, the second curtain drops ending the light sensor’s exposure.
The advent of digital light sensors created the opportunity to make shutters quieter by eliminating the physical movement of the curtains. Silent shutters electronically control image recording. The sensor begins recording when the shutter release is pressed and stops after the period defined by the shutter speed.
Silent shutters are especially nice when the photographer doesn’t want to alert the subject. In wildlife photography a shutter sound could scare away a deer depriving the photographer of additional opportunities.
Interestingly, many digital cameras that have silent shutters contain an option that allows a shutter noise to be created via the camera’s speaker. Some photographers like the auditory feedback when a photo is taken.
Light sensors capture the image. They are the most technically complex of the hardware components. Numerous types exist that are distinguished by size and technology. The technological distinctions are based on the electronic technologies used to fabricate them. The two most common types are charge coupled devices (CCD) and complementary metal oxide semiconductors (CMOS).
CCD sensors produce a high quality analog output that is superior to the digital output from a CMOS sensor. However, they consume more power and are more expensive than CMOS sensors. Recent advances in CMOS technology is improving their performance.
The most common sizes are 35mm and 28mm. The larger size enables higher resolution images and is especially relevant when creating a large picture to hang on a wall. Most cameras use the smaller size. They’re cheaper and good enough for most wall photos.
Also, in the path to creating a final photo is image processing software. This subject is beyond the scope of this blog entry. Nonetheless, it’s probably worth mentioning that some cameras contain built in image processing capabilities. These can be further supplemented with packages like Photoshop or Gimp that will run on a personal computer. This software gives the digital photographer the ability to edit the image after it’s taken.