ShutterFreaks Lens Selection and Use

Lens Selection and Use

by Hugh Stockton

Before we can discuss selecting lenses, we should define some terms and decide on some conventions.

The following will discuss lens focal lengths referenced to focal length number stamped on the lens. It is the reference if the lens is used on a standard 35mm camera. To convert this number relative to a digital camera a multiplication factor must be used.

Multiplication Factor

So, what is the multiplication factor? The Sensor (CCD or C-MOS) on the current generation of digital cameras is smaller than the film area of a 35mm camera. The replaceable lens digital cameras use lenses designed for film cameras. The lens focal length cannot be changed. The image produced by the lens is larger than the sensor.

In these pictures assume the rectangle on the left is a piece of 35mm film, and the rectangle on the right represents the digital camera sensor. The gray circles, both the same size, represent the image produced by the lens. The red circles, both the same size, represent the object photographed. On the right, it can be seen that less of the image produced by the lens is captured by the sensor. If we project these two images to the same physical size, the digital image will appear to have been taken with a lens of narrower field of view, and higher magnification. The apparent difference is referred to as the “Multiplication Factor“. A typical multiplication factor is 1.5. Thus, a 50mm lens on a 35mm camera is roughly equivalent to a 75mm lens on a digital camera (50mm x 1.5 = 75mm).

Please note that these differences do not require any compensation when composing pictures through the viewfinder. The viewfinder optics provide the necessary compensation. What is seen in the viewfinder is what will be captured by the sensor.

Lens Characteristics

What do we mean when we speak of a lens? A camera lens is a set of glass elements designed to focus light onto the film plane/sensor. The characteristics of the elements determine how much of the scene is captured, and how the light is manipulated. The lens is arguably the most important component of the photographic process. The highest resolution Sensor can not make a sharp clear picture of an image distorted by a poor lens.

The essential characteristics of a camera lens are focal length and lens opening. Lens opening is often referred to as f/stop. F/stops will be discussed in some detail shortly. Focal length is the number stamped on the barrel of the lens. It is measured in millimeters (mm). It is the distance from the primary element of the lens to the film plane/sensor. It determines the magnification and view angle of the lens. For a given lens, a longer focal length will produce greater magnification and a smaller view angle. Lenses are generally available from 20mm to 400mm. Smaller and larger are available, but very expensive, and necessary only in special circumstances.

Magnification

Lens magnification is calculated relative to the “normal“ 50mm lens. With a 50mm lens, a 6’ man will fill the frame at 8’. A 20mm lens is about half the focal length of the 50mm lens, and therefore has a magnification of 1/2. A 6’man will fill the frame at about half the distance, or at about 4’. A 400mm lens is about 8 times the focal length of the 50mm lens, and therefore has a magnification of 8. A 6’ man will fill the frame at about 64’.

The standard or “normal“ field of view of the human eye is about 40 degrees. This is about the horizontal field of view of the “normal“ 50mm lens. A 20mm lens is about half the focal length of the 50mm lens and has a field of view about twice as wide, or about 80 degrees. A 400mm lens is about 8 times the focal length of the 50mm lens and has a field of view about 1/8th its width, or about 5 degrees.

Apertures and f/stops

Lens opening, or f/stop, determines how large an opening is available for the light to pass through onto the sensor. Lenses with large openings available are spoken of as “fast“. Those with only smaller openings are said to be “slow“. Fast and slow are relative to the focal length. For instance, f/2.8 is slow for a 50mm lens, but fast for a 150mm lens. Most lenses are adjustable within a range of f/stops. A typical range for a 50mm lens is f/1.4 to f/16. A typical range for a 150mm lens is f/2.8 to f/22. The f/stop scale is inverted, that is, the larger the f/stop number, the smaller the lens opening. The relationship between f/stops is 1.4. Starting with f/1, the next larger f/stop is f/1.4. Note that the f/stop number doubles for every two f/stops.

Stops	1	2	3	4	5	6	7	8	9	10	11
F/STOP	1	1.4	2	2.8	4	5.6	8	11	16	22	32

Lenses are usually adjustable in one-f/stop increments. With electronically controllable lenses it is typical to be able to adjust in 1/3 stop increments. Decreasing the lens opening by increasing the f/stop setting one f/stop is equivalent to doubling the shutter speed. Changing to a smaller lens opening is generally referred to as “stopping down“. Changing to a larger lens opening is “opening up“.

The most basic relationship in photography is between f/stop, shutter speed, and exposure. A given amount of light is required for proper exposure. Assume a proper exposure can be obtained at f/8 with a shutter speed of 1/60 of a second. If you want to stop down one stop, to f/11, it is necessary to double the shutter speed to 1/30 of a second. If you want to open up one stop to f/5.6, it is necessary to halve the shutter speed to 1/120 of a second.

Depth of Field (DOF)

Depth of Field (DOF) is a critical function of a lens. Lenses can be focused from some minimum distance to infinity. The focal distance is the distance from the camera that an object is in exact focus. It is the distance at which a very small point of light appears to be exactly a point. A point of light nearer to, or farther from the camera will appear not as a point of light, but as a circle of light, commonly called a “circle of confusion“. The farther a point of light is from the exact focal distance, the larger the circle of confusion becomes. The Depth of Field is the range of distances within which the circles of confusion are small enough to appear to be in focus. The DOF varies inversely with the lens opening, and therefore, directly with f/stop. Increasing the f/stop increases the DOF.

For example, if a lens is focused on an object 50’ from the camera, the focal distance is 50’. If objects at 40’ and 60’ appear to be in focus, the DOF is the 20’ starting at 40’ from the camera and continuing to 60’ from the camera. If a narrower DOF is required, it can be obtained by using a smaller f/stop. If a wider DOF is required, it can be obtained by using a larger f/stop. For a given exposure, a smaller f/stop will require less light (a faster shutter speed), and a larger f/stop will require more light (a slower shutter speed).

Note that the distance the DOF extends in front of and behind the focal distance is a characteristic of a given lens model. Typically, the DOF extends farther behind the focal distance then in front of it.

Types of Lenses and Their Uses

A useful segregation of lenses is into Primes and Zooms. Primes are single focal length lenses. Zooms are adjustable in a range of focal lengths. Primes are smaller, lighter, less expensive, sharper, brighter, and, except in rare cases produce higher quality images. Zooms are more convenient. It could take from two to four primes to cover the range of a particular zoom. In most cases, and for most people, the quality differences between a prime and a zoom will not be noticeable.

WIDE ANGLE – Less than 50mm (Less than 32mm for digital) Wide-angle lenses show more of a scene, but objects appear smaller/farther away. They are typically used for landscapes. They can also work very well indoors when the ability to move farther away is limited. They are very susceptible to angular distortion unless kept level, front to back, and side to side. That is, vertical and/or horizontal lines will appear to converge. They have very wide effective DOF. It is common to be able to have everything in a scene appear to be in focus.
NORMAL – 50mm (32mm for digital) The normal lens is the workhorse. It shows a normal field of view. It is, perhaps the easiest to manufacture. At least, a high quality, fast lens is usually much less expensive in this focal length than in shorter or longer focal lengths.
MEDIUM TELEPHOTO – Greater then 50mm to 150mm (32mm to 100mm for digital) Medium telephoto lenses are pretty much specialty lenses. Their most common use is for portraits. Their relatively short DOF makes it easy to have a main subject/person in focus while keeping the background blurry.
TELEPHOTO – Greater than 150mm (100mm for digital) Telephoto lenses are typically used for sports and wildlife. Those are situations where detail is needed, but the ability to get “closer“ is limited. They tend to be heavy, “slow“, and expensive. They can also be used very effectively to isolate small areas of a scene.
ZOOM – Various ranges Zoom lenses can be adjusted for a range of focal lengths. Typical ranges are 17mm to 35mm and 24mm to 80mm. They are very convenient for being able to capture a range of compositions without moving or changing lenses. They tend to be “slow“ and, unless very expensive, of medium quality.
MACRO/MICRO – various focal lengths A macro/micro lens is capable of producing a real size image on the film/sensor. That is, an object 10cm wide will produce an image on the sensor 10cm wide. In order to do so, they must be able to focus on objects very close to the lens. Macro lenses tend to have relatively short DOF. They are available in a range of focal lengths. Nikon has a 60mm, a 105mm, and a 200mm. Some standard lenses have macro settings. They are usually a compromise.

Choosing Lenses

And, finally, what lenses should you buy? As with most things in life, and all things in photography, the answer is, “It depends.“ It depends mostly on your style. Unfortunately, it takes some time and experience to develop a style, and by the time you have one, you will know what lenses you need/want. To avoid spending a lot of money and ending up with a large collection of glass that does not do what you want, I suggest a conservative approach.

Start with a “normal“ lens. The biggest bang for the buck today is the 50mm lens. A quality, fast, sharp 50mm lens is a fine first lens and will be sufficient for the majority of your pictures. If you can afford it, another option would be a medium wide to short telephoto zoom. The 24-70 comes to mind.

Take many pictures. Notice particularly when the lens limits your ability to get the picture you want. Decide what lens would have worked better. When your frustration level exceeds your discretionary income, buy the lens that solves most of your problems. Do not let lens envy push you into buying an expensive specialty lens that you will not use very often.

Cut to the chase. When you ask most people who are serious about photography what they carry, not what they have, you get just about the same answer:

50mm
Something in the range of 24mm to 32mm wide angle, or a wide angle zoom.
A medium long telephoto. 200mm to 300mm with a 2x teleconverter.

Good luck with your lens selection. Have a good time with your photography, and we hope to see you on the ShutterFreaks Forums.