Art 50/150
Black&White Photography

Technical Camera Issues

The Lens

The focal length of a simple lens is the distance from the lens plane to the plane where infinitely distant objects focus. Closer objects will focus behind this plane (farther away). In an SLR camera with interchangeable lenses, the film plane is at one location relative to the back of the lens; hence, a compound lens is required. In a compound lens, the position of one or more simple lenses at the front can be changed relative to others in the composite construction; this enables the lens to change the plane of focus. Most camera lenses are compounds of five or more simple lenses. Adjustment of these lenses must allow us to focus objects from many distances in front of the camera onto the film plane. [Compounding also achieves solutions to a number of very important problems with physical lenses. For example, a simple physical lens (as opposed to an "ideal" lens) bends light differently depending on its color. This produces "chromatic distortion" in the focused image. Chromatic distortion is a problem for black&white photographers as much as it is for color photographers because the different intensities of the distorted image will look like blurring in the black&white image.]

Draw a sketch with a lens at the left side of a page. Draw two vertical lines on the right side of the page, one 28 mm to the right of the lens and one 80 mm to the right. Draw light rays from the top and bottom of the lens converging on the 28 mm line and another set converging on the 80 mm line. Now draw a vertical line between the lens and the 28 mm plane. Note the height of the line segments along this line for the 28 mm lens and for the 80 mm lens. This is one way of understanding the difference in image size produced by lenses of different focal lengths. When we want to magnify objects in the distance, we choose longer focal-length lenses; similarly, when we want to cover a wider scene with smaller objects, we choose a shorter focal-length (wide angle) lens.

The Aperture

The aperture of a simple lens is the diameter of the lens expressed as a fraction of the lens focal length. Thus, an 80 mm lens has an aperture of f/4 if the diameter of the lens opening is one fourth of the focal length or 20 mm. [20mm/80mm = 1/4] Notice that the descriptive number for aperture is in the denominator of a fraction so that a larger number means a smaller diameter of lens opening and less transmitted light. Notice also that in order to build an 80 mm lens that allows more light into the camera, say an f/2 lens, you would have to increase the diameter to 40 mm. This is called a "faster" lens because it admits more light, but it requires significant increases in size, weight and expense.

Drawing another simple diagram will demonstrate an interesting attribute of aperture. Imagine that you have two lenses, one with a focal length of 40 mm and one with a focal length of 80 mm. Further, imagine that both lenses have the same aperture, say, f/2. This means the diameter of the former lens is 20 mm and of the latter is 40 mm. Draw the two lenses on top of each other on the left side of the page with the smaller lens in the middle of the larger lens. Draw rays of light from top and bottom of the smaller lens to its focal plane 40 mm to the right. Then draw rays of light from top and bottom of the larger lens to its focal length 80 mm to the right. Note that the rays are parallel. This is an attribute of aperture and it means that neither lens, so long as they have the same aperture, intensifies the light image more than the other. No matter what the focal length of a lens, it will admit the same light intensity as any other lens when set at the same aperture.

Modern compound lenses are built so that the effective diameter of the lens can be made smaller than the inherent aperture. This is achieved by placing an iris in the rear nodal plane of the compound lens. The iris size is controlled with another ring on the lens. As this ring is rotated, the iris continuously closes to smaller diameters, restricting light passage. On standard lenses, the ring is set up in increments or "stops." Each stop cuts the amount of transmitted light in half. The stops are designated by the aperture for the effective diameter. Thus, for an f/4 lens the first stop is at f/5.6 which means that the effective diameter has been reduced enough to cut the transmitted light in half. [As above, if the lens has a focal length of 80 mm, its wide open diameter is 20 mm. At f/5.6 its effective diameter is 14.3 mm. Since the area of a circle is proportional to the square of the diameter, we can note that the square of 20 is 400 and that the square of 14.3 is approximately 200, being half as great.] You must keep in mind that aperture is a fraction so that larger numbers (denominators) mean smaller actual size. When we talk about closing a lens, we are making the effective lens diameter smaller, allowing less light through, and this means moving to nominally larger lens stops.

The Camera

There are two important features inside of the typical contemporary SLR camera. One of these is the shutter. Shutter designs and mechanisms vary considerably and there is little to be understood here. Essentially, the problem with shutter design is creation of a smoothly functioning mechanism that opens the film plane to light for a well controlled increment of time. Shutters are often made from interleaved thin screens that move against each other so as to dynamically expose a narrow band of the film plane homogeneously across the entire image surface within the narrow timeframe set as the shutter speed. There is little to be adjusted here; the more expensive the camera, the better the shutter.

One obscure factor can be considered in certain situations. The SLR camera requires a mirror to flip upward and closed against the top of the camera body to clear the path to the film plane. This can actually cause vibration of a camera that is mounted on a tripod! Hence, there is a window of shutter speeds between long openings and relatively short openings where the vibration caused by the mirror can have an impact on the image sharpness. In this situation, it is sometimes possible to lock the mirror up after setting the exposure but before releasing the shutter.

The second important feature of the camera body itself is the light metering system. This is housed in the view plane located on the top of the camera. During normal operation, the image is reflected upward onto the view screen by the mirror, and you inspect this image through the viewfinder eyepiece. Elements of the view screen are connected to the camera's electronics so that you can read the amount of light striking those elements. Contemporary SLR cameras use matrix screens, center-weighted screens, and spots. In automatic operation, this data is utilized internally by the camera electronics and aperture and shutter speed are set automatically. In manual operation, the data is displayed in some way. You need to know what the display means and what metering system the camera is using, and you can find this information in your manual.

Suppose you have the camera set on spot metering and the display shows a pointer on a graduated scale somewhere between "+" and "-" with an obvious middle point. Changing either the aperture or the shutter speed will move the pointer toward "+" or "-" to indicate more light intensity or less light intensity. The camera's metering system is calibrated with respect to the film sensitivity (ISO rating) so that any setting that centers this meter will render an exposure of the film at that part of the image that prints middle gray (Zone V).

The lesson here is to always set the camera's exposure when pointing at an object that you actually want to turn out middle gray in the final print. [Some people carry "gray cards" to use for this purpose, setting the exposure with the gray card and then withdrawing it from the scene.] Normally, looking at a color image, it is difficult to know what parts of the image ought to be middle gray. When this is the case, the next best approach is to use a bracketing technique. Point the camera so that the viewfinder screen measures the light from different parts of the image, especially the darkest (shadows) and the brightest (sky). Observe the meter readings for these objects. The "best exposure" for this image is probably at that setting of shutter speed and aperture that produces an equal balance of low and high meter readings as you survey the total scene. [However, you should note that the effective range of black&white film and papers is only around 5 stops (at very most 7 stops). Thus, if a shadow indicates underexposure by more than 2 stops, you will lose texture in it and it will appear simply black. If a bright object indicates overexposure by more than 2 stops, you will lose any detail in it and it will appear simply pure white.]

If your camera's meter display does not show the number of stops overexposed or underexposed, you may have to experiment with it. Point at an object that deflects the meter toward underexposure ("-") and then change the aperture one stop open. Note how far the deflection changes. Open the aperture another stop, until the ideal exposure is indicated. You should be able to estimate how many stops are indicated by a given deflection of the meter.

Remember that the critical exposure setting for film is in the blacks. Zone II just begins to show texture while Zone III shows texture. Aiming the camera at shadows that you want to show texture, you should set the exposure so that these shadows fall at Zone III, two stops underexposed from the camera's middle gray setting.

The final issue to remember is that, while different combinations of aperture and shutter speed can produce the same exposure, there are some important effects on the image. Faster shutter speeds freeze moving objects and prevent blurring because of hand movement. Smaller apertures produce greater depth-of-field, meaning that a larger number of objects will be in focus. This example can be extended. The combination of f/22 and 1/30 will produce the same exposure of the film but it will offer a larger depth-of-field. Unfortunately, at this speed you will probably show blurring because of hand movement. If you want that much depth-of-field, you will have to put the camera on a tripod.

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