by Gerald Boerner
Today, we start a new series of postings on Photographic Concepts that will look at how to choose the right lens and/or lens setting for each situation. We have just finished our series on “Drawing with Light” in which we explored the working and settings of the camera. Today’s post will attend primarily to the reason we need to use a lens to guide the light into our camera to produce a GREAT image. We will look at how images vary according to the Focal Length of the Lens, whether it is in a SLR or a compact camera. We will be posting additional entries on this topic over the coming weekends. Enjoy it and we hope that it will help your improve your photo images. GLB
“I would love to have a photographic memory. It would come in handy with the rants I’m given on Scrubs… often on short notice!”
— John C. McGinley
“I don’t think I think when I play. I have a photographic memory for chords, and when I’m playing, the right chords appear in my mind like photographs long before I get to them.”
— Earl Hines
“I have this certain vision of the way I want my comics to look; this sort of photographic realism, but with a certain abstraction that comics can give. It’s kind of a fine line.”
— Daniel Clowes
“I was brought in, not in the photographic department at all, I was brought in on a thing called Special Skills. I was to do posters, pamphlets, murals, propaganda in general, you know.”
— Ben Shahn
“I was extravagant in the matter of cameras – anything photographic – I had to have the best. But that was to further my work. In most things I have gone along with the plainest – or without.”
— Edward Weston
“I was making $50 a week as a house model at Christian Dior for nine months before I learned that photographic models made $50 an hour!”
— Lauren Hutton
“Never have I found the limits of the photographic potential. Every horizon, upon being reached, reveals another beckoning in the distance. Always, I am on the threshold.”
— W. Eugene Smith
“The particular features of the photographic method of detecting atomic particles enabled us to establish the existence of transient forms of matter which had escaped recognition by other methods.”
— Cecil Frank Powell
Choosing Your Lens: Overview of Operation
A photographic lens (also known as objective lens or photographic objective) is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically.
While in principle a simple convex lens will suffice, in practice a compound lens made up of a number of optical lens elements is required to correct (as much as possible) the many optical aberrations that arise. Some aberrations will be present in any lens system. It is the job of the lens designer to balance these out and produce a design that is suitable for photographic use and possibly mass production.
There is no major difference in principle between a lens used for a camera, a telescope, a microscope, or other apparatus, but the detailed design and construction are different.
A lens may be permanently fixed to a camera, or it may be interchangeable with lenses of different focal lengths, apertures, and other properties.
Basic Theory of Operation
Most photographic lenses can be thought of as modified pinhole lenses. A pinhole lens would be excellent except for a few serious limitations. They are limited in their resolution because, while geometric optics says that making the pinhole smaller improves resolution, this also reduces light; furthermore, diffraction limits the effectiveness of shrinking the hole.
Most photographic lenses can be thought of as an answer to the question "how can we modify a pinhole lens to admit more light and give higher resolution?" A first step is to put a simple convex lens at the pinhole with a focal length equal to the distance to the film plane (assuming the camera will take pictures of distant objects). This allows us to open up the pinhole a bit. The geometry is almost the same as with a simple pinhole lens, but rather than being illuminated by single rays of light, each image point is illuminated by a focused "pencil" of light rays. Standing out in the world, you would see the small hole. This image is known as the entrance pupil: all rays of light leaving an object point that enters this pupil will be focused to the same point on the film. If one were inside the camera, one would see the lens acting as a projector. The image of aperture is the exit pupil.
With a large pinhole, the image spot is large,
resulting in a blurry image.
With a small pinhole, light is reduced and diffraction
prevents the image spot from getting arbitrarily small.
With a simple lens, much more light can be
brought into sharp focus.
Practical photographic lenses include more lens elements. The additional elements allow lens designers to reduce various aberrations, but the principle of operation remains the same: pencils of rays are collected at the entrance pupil and focused down from the exit pupil onto the image plane.
Aperture and Focal Length
How focal length affects
adjusting the camera’s
distance from the main
subject while changing focal
length, the main subject
can remain the same size,
while the other at a different
distance changes size.
The two fundamental parameters of an optical lens are the focal length and the maximum aperture. The lens’ focal length determines the magnification of the image projected onto the image plane, and the aperture the light intensity of that image. For a given photographic system the focal length determines the angle of view, short focal lengths giving a wider field of view than longer focal length lenses. The wider the aperture, identified by a smaller f-number, allows using a faster shutter speed for the same exposure.
The maximum usable aperture of a lens is specified as the focal ratio or f-number, defined as the lens’ focal length divided by the effective aperture (or entrance pupil), a dimensionless number. The lower the f-number, the higher light intensity at the focal plane. Larger apertures (smaller f-numbers) provide a much shallower depth of field than smaller apertures, other conditions being equal. Practical lens assemblies may also contain mechanisms to deal with measuring light, secondary apertures for flare reduction, and mechanisms to hold the aperture open until the instant of exposure to allow SLR cameras to focus with a brighter image with shallower depth of field, theoretically allowing better focus accuracy.
Focal lengths are usually specified in millimetres (mm), but older lenses might be marked in centimetres (cm) or inches. For a given film or sensor size, specified by the length of the diagonal, a lens may be classified as a:
angle of view of the diagonal about 50° and a focal length approximately equal to the image diagonal.
angle of view wider than 60° and focal length shorter than normal.
Telephoto lens or long-focus lens:
angle of view narrower and focal length longer than normal. A distinction is sometimes made between a long-focus lens and a true telephoto lens: the telephoto lens has a telephoto group to reduce the physically length of the objective.
special lens corrected optically for close-ups, e.g., for images to object ratios ranging from about 1:10 to 1:1. and having a particularly flat image plane suitable for flat images. A macro lens may be of any focal length, the actual focus length being determined by its practical use, considering magnification the required ratio, access to the subject and illumination considerations.
How Does Lens Choice Affect Angle of View?
An example of how lens choice affects angle of view. The photos below were taken by a 35 mm camera at a constant distance from the subject.
A side effect of using lenses of different focal lengths is the different distances from which a subject can be framed, resulting in a different perspective. Photographs can be taken of a person stretching out a hand with a wideangle, a normal lens, and a telephoto, which contain exactly the same image size by changing the distance from the subject. But the perspective will be different. With the wideangle, the hands will be exaggeratedly large relative to the head. As the focal length increases, the emphasis on the outstretched hand decreases. However, if pictures are taken from the same distance, and enlarged and cropped to contain the same view, the pictures will have identical perspective. A moderate long-focus (telephoto) lens is often recommended for portraiture because the perspective corresponding to the longer shooting distance is considered to look more flattering.
Apparent Influence of Lens Focal Length on Image perspective
Although in practice a photographer intuitively perceives a natural relation between perspective and the focal lengths he is using, no such relation actually exists. The immediate reason for this impression is that lenses of different angles of view may be used differently; wide-angle lenses tend to be used close-up and telephoto lenses for photographing distant motives. Cameras or lenses with tilt/swing facilities don’t really change perspective, but rather distort the image by oblique projection onto the image plane. The only factor controlling the perspective is the distance from the motive to the lens’ front nodal point. Anyone can check this fact by doing some fundamental geometric sketching on a piece of paper.
An example illustrating this is the "portrait" lens. It typically has a field of view that includes a person’s head and shoulder in the image at a favourable distance, e.g. ten feet or so. At such distance, the features in the face are rendered in reasonable proportions, without too much emphasis on any single feature, like the nose. The closer the camera gets the larger appears the closest features.
Photographing a distant scene with a wide-angle lens will yield the exact same "compressed" perspective associated with a telephoto lens; enlarging the same section of the wide-angle image as that photographed with the latter will confirm this fact. When it comes to looking at a picture, the distance from which it is viewed will also influence the way in which it is perceived. The perspective in the image will determine the most agreeable viewing distance.
Selecting a Lens
When one looks at the specifications for a “compact”, digital camera, one that has a non-interchangeable lens. A telephoto mechanism, such as the one to the right, will likely be included. One will likely encounter a description such as the Canon PowerShot G11 camera which list the following specs for the lens:
- Focal Length:
6.1 (W) – 30.5mm
(35mm film equivalent: 28 (W) – 140 (T) mm)
- Digital Zoom:
- Focusing Range:
Normal: 1.6 ft./50cm-infinity
Macro: 0.4 in.-1.6 ft./1-50cm
- Autofocus System:
So, what do these numbers mean? First of all, keep in mind that most digital cameras use a image sensor that is smaller than the typical 35mm film (or “full-frame”) camera. Therefore, the minimum (wide-angle) and maximum (telephoto) settings are given by two numbers each. That is, The Focal Length for the wide-angle is 6.1mm, which is equivalent to 28mm in a 35mm camera. Likewise, the Focal Length for the Telephoto is 30.5mm, which is equivalent to 140mm in a 35mm camera. This multiplication factor is 1.6, but that factor will vary from camera to camera and vendor to vendor.
The Focal Length settings from the wide-angle to the telephoto refer to the optical zoom properties. Many digital camera, especially on the lower end, price-wise, will include a digital zoom factor as well. In the case of the G11, this is 5x, or five times the image size. This may sound great, but beware: this artificially creates an image by taking the actual pixels of the image and create more “in between” to accomplish this greater zoom. Therefore, you are getting an image that will have a lot of digital “noise”, e.g., an image of relatively poor quality.
A Word to the Wise:
Do NOT Turn Digital Zoom On!
The Autofocus System refers to how the automatic focusing system of the camera works. In this case, it uses a “Through-the-Lens” (TTL) autofocus system. Other lenses use an infrared or some other beam to determine the focal distance. This information is used to adjust the lens’ focal length to obtain a sharp focus in automatic (autofocus) mode. It does not affect focusing in the manual (manual focus) mode.
That’s about it! I hope that the above explanation will help you understand a little better about the lens in your compact digital camera. Next time we will look at SLR and dSLR lenses in more detail. These are the lenses that can be changed out and they allow you to choose the lens most appropriate to your situation.
Barbara London, Jim Stone, & John Upton. (2008) Photography. Pearson, Prentice-Hall
Background and biographical information is from Wikipedia articles on:
Wikipedia: The Camera…
Wikipedia: Photographic Lens…
Web Sites and Blogs:
Camerapedia.org: Lens (Composition Details)…
Photo.Net: Lens Tutorial (Technical)…
BrainyQuote: Photographic Quotes…