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Pentax Digital Spotmeter - Illustration
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(Based on Farzad's 5-stop Film / Digital Zone System ©1987-2005)


4.1. What is the Multiple Tone Metering System all about?

So far, all of our techniques have involved light measurement and exposure determination of a simple tone. This tone was either the only tone of a simple subject or the Reference Tone of a complex subject.

The idea behind choosing a Reference Tone for a complex subject is that if one tone of a complex subject is properly exposed, then the rest of the tones in the subject also will be properly exposed. We used this technique without any consideration for the type of film that we used, the brightness range of a subject, or the amount of detail in the different tones of the image. To explain and illustrate this new technique, we will be using the Tone Train concept again. If you look back at the Tone Train illustrations of the previous chapter, every one of our subject tones (loaded tank cars) was eventually aligned with an image tone that was represented by an empty tank car. In this chapter, for subjects with a long tonal range, the loaded train (the subject) can have more than five tank cars. Here, since the subject train is longer than the film train, some end cars of the loaded train will not correspond to a tank car on the empty train. Here the cargo cannot be unloaded. In photographic terms, these tones will simply not be recorded on the film. Their image will be either washed out (too white) or will be totally black. The Multi Tone Metering System is a method that allows you to control your image tones in order to produce the best image. Before we start with a detailed explanation of this technique, here are a few terms that you need to understand. These are:

1) The angle of view and the angle of measure
2) Equivalent exposures
3) A spot meter
4) The Subject Brightness Range (SBR)
5) The film and its Contrast Range (FCR)
6) The image detail


4.2. The angle of view and the angle of measure (acceptance)

The angle of view is the angle with which an instrument such as a camera views the subject. Generally, if you are looking at a subject and the subject looks larger than what you see with your eyes, then the lens on the camera is considered to be a telephoto (telescopic) lens. Similarly, if the subject looks smaller than what your eye sees then, the lens is considered a wide angle lens. If the subject looks the same size as with the naked eye, the lens is considered "normal."

The viewing angle for a normal lens is about forty-six degrees (46) and its focal length is about 50mm. To test this with the 50mm lens of your camera, simply look through the viewfinder with one eye and keep the other eye open. The images from both eyes will coincide. Telephoto lenses have a smaller angle of view and wide angle lenses have a larger angle of view. Please see Diagram 4.2.2.

Why should we be interested in the angle of view? Well, this is the way the meter in your camera sees the subject and finds its normal exposure. In standard cameras without the spot metering feature, the angle of view and the angle of measure are generally the same. In a spot meter the angle of view could be about 26 degrees where the angle of measure can be as narrow as 1 degree (1° ). In Diagram 4.2.2, you can see the approximate angles of view for different lenses for your 35mm camera.


4.3. Equivalent Exposures and the Law of Reciprocity in slide films

Shutter speed/aperture pairs producing identical exposures

The following row illustrates eight consecutive exposure times in increasing order

















The above row illustrates eight consecutive aperture openings in decreasing order

When you find the exposure for a scene or a gray card, and your camera's meter shows a correct exposure of 1/1000 @ f-2.8, you must realize that this is not the only correct exposure. The following table shows other exposures that will produce the same correct exposure.

Equivalent exposures are possible since every full click on the aperture is equivalent to a full click of the shutter speed dial, and a full click on each dial is also equivalent to one stop. If a photographer is interested in freezing the action, perhaps he uses 1/1000 @ f-2.8, and if he requires the maximum depth of field, then he uses 1/30 @ f-16. The law that governs equivalent exposures is known as the law of reciprocity. Like anything else in life, there are limits to the practical applications of this law. For example, the following exposures 1/15 @ f-2, and 8 sec. @ f-22 theoretically produce equivalent exposures. In practice, this law does not hold if you are using Kodachrome 64, Ektachrome 64, or Agfachrome 50 RS slide films and the exposure is longer than 1/10 sec. or if you are using a Fujichrome 50 film and the exposure is longer than 1 second.

For these films, the longer the exposure, the less sensitive they become. For example, if an exposure of 1/15 sec. @ f-2 produces a correct image, then the equivalent exposure of 8 sec. @ f-22, using Kodachrome 64 film would result an underexposure of almost 2 stops. Another side effect of reciprocity failure in color films is the unwanted color shift. A serious photographer who is using long exposures with slide film must be aware of these drawbacks and should compensate for them. If you do not know what the reciprocity failure characteristics of a film are, you can contact the manufacturer or simply take a test roll and see how it behaves. At times, you may have to ignore the manufacturer's recommendations. For example, although Kodachrome 64 film is not recommended by Kodak for exposures of more than 1 sec., some successful photographers have used it for exposures of four to eight hours. These exposures are used to trace stars in a moonless night or for photographing a moonlit scene. For extremely long exposures like these, Kodachrome 64 acts like a film with an approximate ISO of 8.

 4.4. What is a spot meter?

A spot meter, as the name implies, is a long distance, narrow-angle, and usually off-camera light metering device. This characteristic makes it the ideal meter for finding the normal exposure of a simple subject and to determine the brightness range of a complex subject (we will be talking about this shortly).

The evolution of CDS (CaDmium Sulfide cell) technology gave birth to development of the spot meter in the late 50s and early 60s. This development revolutionized the light measurement and for the first time in the history of the Zone System enabled the photographer to make accurate readings from the distant scenic subject zones. The first Pentax analog (spring supported needle) was introduced at 1960 Photokina and it was mass produced for public consumption in 1961. Minolta’s version, Auto spot 1 was introduced into the market in 1968. Another spot meter that has been around for many years is made by Soligor.

My Pentax digital spot meter's viewing angle is about 26 degrees (26° ). Located in the center of this viewfinder is the 1° measuring circle. When you look through a spot meter, you must point this circle toward the area of the simple subject that you wish to measure.

When using a spot meter, Please note that the 1 degree circle in the viewfinder of the spot meter must fall within the boundaries of the simple subject that you are measuring; otherwise, your normal exposure reading will not be accurate and can not be interpreted correctly.

As I mentioned before, to illustrate the spot meter functions, I have used my Pentax digital spot meter. If you have a different brand of spot meter, read your meter's manual to see how you can obtain a normal exposure reading from a simple subject. Techniques that you learn in this chapter are independent of the brand of your spot meter. They will enable you to operate your meter skillfully to find the proper exposure for any complex subject.

The gray-making property of your spotmeter is very much like your camera. Therefore, when you obtain a normal exposure from a simple subject, this exposure will create an 18% gray image tone on the film. With the spot meter, the normal exposure is determined by pointing it at a simple tone. The spot meter then provides the photographer with a number that ranges from 1 to 20 (your meter's range may be different). Using the dials on your meter, this number can then be converted to a series of shutter speeds and aperture settings (Equivalent Exposures) for a given film. The unit for this readout is EV, which stands for "Exposure Value." The brighter the light, the higher is this number. We will learn more about the EV scale in the next section.

Like any off-camera light measuring device, when using a spotmeter in conjunction with a camera that has filters, extension tubes or teleconverters, you must remember to compensate for the loss of light through such devices. To simplify things, in most of our examples, we will not be using these attachments.

Currently there are two types of spotmeters on the market.

1) The standard (analog) spot meters use a moving needle to show the EV number.

2) The digital spot meters use digital readouts to show the EV number. The question is, which one of these would serve you best?

Here are the advantages of a digital spot meter over the analog:

l A digital spotmeter has no moving components, it is smaller and is more resistant to impacts.

l A digital spotmeter can be used even at night to find the normal exposure of a very dark subject. The digital readout from my Pentax spotmeter simply shines in the viewfinder. Under these circumstances, if you use a standard spotmeter, chances are that you will not be able to see the needle indicating the EV reading.

Here is the only advantage of the standard (analog) spotmeter:

l The only advantage of this meter is its price. It is usually 30 to 40% cheaper than the digital spotmeter that is produced by the same manufacturer.

Spotmeters currently on the market include: Soligor, Sekonic, Gossen, Pentax and Minolta.


4.5. What is the EV numbering system used in spotmeters?

All the spotmeters use some type of EV numbering system. EV is the abbreviation for Exposure Value. The brighter the light, the higher the EV is. To understand EVs, simply relate it to the amount of money that a person has. If a person's wealth is only a few dollars, the person is considered to be poor (low-light, low EV) and if a person's wealth is measured in millions of dollars, then he or she is considered to be rich (bright surfaces, high EV).

Exposure Value (EV) as it relates to the amount of light has an exact scientific definition. We leave this definition for the scientists who created it and do not worry about it! As far as we are concerned, if we place a white piece of paper about 3 feet away from a household candle, its brightness would be about 1 EV.

Another example? Set the ISO of your camera to 800, your shutter speed to 1 sec. and the aperture to f-4 and do not touch these settings for the next experiment! Now get a piece of white paper or mat board and place it beside a candle or a household night lamp. Move the white mat board away from light until your camera shows a normal exposure, i.e., the needle is in the middle with the exposure setting of 1 sec. at f-4. The intensity of the piece of mat board is approximately 1 EV.

If we place the same white piece of paper in the bright sunlight, its brightness reading would be about 17 EVs (equivalent to a normal exposure of 1/500 sec. @ f-16 for ISO 100 film). If you measure the EV of an 18% gray card, in the noon sun, it would be about 15 EVs (equivalent to a normal exposure of 1/125 sec. @ f-16 for ISO 100 film).

For example, an EV of 15, for ISO 100 film will produce a series of equivalent exposures (please refer to sections 1.15 and 4.3). These include 1/250 @ f-11, 1/125 @ f-16, 1/60 @ f-22 or 1/30 @ f-32 and so on. As you notice, every one of the above settings exposes the film with the same amount of light. These equivalent exposures are considered to have the same Exposure Value. Increasing or decreasing the light by 1 EV is identical with increasing or decreasing the light entering your camera by 1 STOP.

For example, a surface with a brightness of 3 EVs is twice as bright as a surface with brightness of 2 EVs. The same surface is 1/2 as bright as a surface with brightness of 4 EVs (please see Diagram 4.5.0 with electric lamps). By using the sliding scale on the spot meter, any EV reading can be converted to a series of equivalent exposures that can be set on your camera. 

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