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Compact Digital Cameras

SLR Lenses

Lenses capture the light. Camera bodies will be replaced with something better as technology improves, but a good quality lens will give you years of excellent service. One reason photographers build an allegiance to a particular brand is the investment in lenses. What gives an SLR an advantage in this area is the ability to change lenses. Lenses come in a variety of focal lengths and sizes, ranging from the ultra wide angle and fisheye to ultra telephoto.

Angle of View: The angle of view is the maximum angle of viewable area when the lens is focused on infinity. This describes when you look through the lens how wide or narrow of a view you get. Draw 2 lines starting from the Nodal Point (see below) extending to infinity. The angle that these lines orininate at is the angle of view. This is important mainly in wide angle lenses, because you get a good idea how much area will be viewable when using that lens. Fisheye lenses can have a 180 degree angle of view.

The front of a lens sees upside down and backwards, similar to a mirror. The rear elements of the lens flip the image around and make it correct. The Nodal Point is the point at which the image is flipped around correctly. In most lenses, this can be found near the center.

Lens Aperture

The aperture range of a lens is the another factor to consider. The aperture is the iris built in the lens, which controls the amount of light allowed to enter the lens. The aperture also controls depth of field. Depth of field is the term which describes how deep into the picture the focus goes. On SLR lenses, focus depth can range from a couple inches to infinity. When an aperture closes to a smaller hole you get a higher number (example: f/16, f/22). This creates deeper focus. This also decreases the amount of light able to enter the camera. If you want deep focus in an image, there must be enough light to support a slower shutter speed. In bright sunlight, this may not be as much of an issue, but in most shooting conditions a tripod would be helpful. The opposite is also true. When you have a bigger hole, meaning more light and a lower f/- number, you get shorter focus. More light and faster shutter speeds will give you the ability to photograph in more situations without having to use a flash.

Many SLRs have a depth of field preview button. This button stops down the aperture without taking a picture. This will give you an idea of how deep the focus looks without taking a picture. Since this button does stop down the aperture, your viewfinder will get dimmer.

Depth of field and using the aperture can really help your photography as well as helping you decide what lenses you may want with your camera. SLRs have an exposure mode, aperture priority, which will allow you to choose the aperture, and the camera will choose the shutter speed.

When you look at the label on a lens there are a few different numbers. First is the focal length. Example: 70-300mm. The next numbers are the aperture. If there are 2 numbers, the lens has a variable aperture. Example: f/ 3.5- f/ 4.5. This means that at 70mm the aperture is f/ 3.5 and that at 300mm the aperture is f/ 4.5. One stop of light was lost by zooming in. Every time a stop is decreased or increased, light is doubled of halved.

Other lenses have non-variable apertures. This is usually designated by only 1 set of numbers after the focal length. Example: 70mm-200mm f/2.8. At 70mm and 200mm the aperture is still f/2.8. Non-variable lenses allow in more light when zooming. This can help in just about every photographic situation. A few examples are: Telephoto lenses are susceptible to the natural vibrations from your hand and body. This translates through the lens and causes the lens to vibrate a little, which will render unsharp images. More light will allow for a faster shutter speed. Fast shutter speeds are also needed to freeze action for sports and wildlife photography. Non-variable aperture lenses typically have a low aperture (f/2.8 or f/ 4) and are available in most focal length varieties.

Many digital SLRs use an APS sized sensor which is smaller than a 35mm negative. This creates a crop factor. Lenses create an illuminated area larger than the sensor because most SLR lenses were built for a 35mm negative. The crop factor essentially makes every lens a little more telephoto. Multiply the focal length of the lens by the crop factor to convert it to the actual focal length the camera sees. Check your camera’s specs to see what crop factor your camera may have.