Correct Exposure Measurement

From a photography perspective, correct exposure depends on many different factors. Some of them are what we usually know and can be confirmed in advance. However, if there are other factors, you must use an appropriate metering table.

For example, the film speed, the maximum aperture of the lens, the available shutter speed, and the aperture range are all known factors.

Unknown factors are the illumination of light and reflection of objects. Therefore, correct measurement and exposure are actually the determination of these unknown variables, and the measured numbers, such as the existing information in the metering table, are combined for processing, so that a set of optimal speed and aperture can be directly used on the camera. Next we will give a detailed description of the incident type and the reflection type respectively.

Incident Light Measurement

The incident metering indicator is to direct the metering table to the camera from the main position. This method is mainly used to measure the intensity of the optical line, and does not take the reflection of the subject into account. Obviously, it is necessary to measure the light from the main position only when the light conditions change greatly within the shooting range. For example, when artificial lighting is used at a close distance, the light will obviously weaken as the distance increases. Therefore, the correct intensity of the light line must be determined from the position of the main body. When shooting in the Outdoor sunlight, the light is always measured close to the camera.

As the incident light measure measures the light projected on the plane, the light receiving angle of the metering table must be large enough to include a range of 180 degrees. To achieve this, you can add a spherical bulb mask to the metering port of the metering table. The measured number is based on the average reflectivity of the subject of 17%. For objects with high contrast, a certain degree of correction is required: the exposure of the negative part should be slightly more than that shown in the metering table, while that of the reverse part should be slightly less. Because this method does not support selective side light and point light measurement, we have to make some estimation work and have some experience to achieve good results. If the light is difficult to measure, another test of reflected light is usually required to compare it with the number measured by the incident light. If the two are too different, you should find out the cause of the difference and correct the exposure accordingly.

Reflective light measurement

In this way, the metering table is always directed to the subject from the camera position. To obtain accurate results, the angle of measurement of the metering table is similar to that of the lens used. If the measurement angle is much larger than the lens angle, you should put the measurement table closer to the main body, so that the measurement table's field of view is roughly the same as the lens.

In the reflective light measurement, the metering table displays the average reflected light of the subject. At the same time, it also measures all other reflected light in the metering angle.

As long as the measured values of the highest highlight and the lowest Shadow are not much different from the average measured values of the whole scene, this method can always provide accurate exposure values. If a scene contains a small bright or dark part, the total average value must be corrected. The degree of calibration depends on the type of film used and the importance of these bright or dark parts on the screen.

If the negative part is used, it is best to apply a selective close-range light measurement for the shadow part of the performance level, and reduce the exposure value by 1 (that is, enable a large aperture ).

If you use a reverse slice, the opposite is true. The bright part of a scene (not necessarily the brightest highlight part) still shows a certain level. Therefore, the exposure value should be measured and the measured result should be opened in the aperture of 1/2-1. This method is the best for selective light measurement, because the metering table only measures the highlights of the small area of the object.

You can use a metering table that can change the metering angle as needed, or use a calibrated metering table to be close to the subject, which is easy to choose from.

If we understand and master the principles of these two optical measurement methods, both of them can achieve good and consistent results. We know there are two ways: 1. Average (or overall), 2. Selective (or point ). There is a fundamental difference between the two.

As the name suggests, the average light measure (overall light measure) measures the total brightness of an object. It combines the brightest and least-dark parts for simultaneous measurement to provide an average brightness value, it is displayed in combination with appropriate speed/aperture. For objects with low contrast or even area distribution (such as banma), this method can be used to measure the light, which is not only reliable, but also the most suitable result.

If a certain part of an object is absolutely exposed correctly, for example, the skin color in the image is accurately reproduced, the selective light measurement is applied, the final determination of the exposure is based on the obtained measurement value. The remaining less important parts can be slightly overexposed or underexposed. If there is a bit of optical measuring accessories, the selective optical testing will be much smoother.

Measure object contrast

Some new optical meters are widely used: they can be used to determine not only the appropriate speed/Aperture combination, but also the contrast range of objects. The scope of this contrast must be limited to a certain extent, otherwise the subject cannot be reproduced in the field.

The word "contrast" can be used in various occasions. The following is an explanation:

Image contrast: it is the range of accurate reproduction of the film emulsion.

The contrast of an object. It is the difference between the brightest part and the dark part of an object. It can also be called the total brightness range.

Light contrast: The relative relationship between brightness of different light sources, such as the relationship between primary light and secondary light.

To correctly estimate the effect of contrast in photography, you need to understand the interdependence between the negative differences and consider the contrast range that can be achieved by the film.

1. image contrast is the product of object contrast and light contrast:

Image Contrast = Object contrast x light contrast

2. The contrast scope of a certain film photosensitive emulsion depends largely on its purpose: it is mainly used as a reverse film for screening. Its contrast scope is far from the negative film used for photo production.

People's eyes have an extreme sense of color and contrast, and they are also extremely unreliable. To accurately measure the contrast and obtain the optimum number of exposures, you must have a high quality hand-held optical indicator table to determine the incident light and the reflected light.

First, we need to determine the contrast (or the total brightness range) of an object, which is determined by the light transmittance and color value of each part of the object. Therefore, the contrast between objects can be obtained by measuring a few local values using the reflection method. For example, when the aperture is f5.6, the shutter speed of the brightest part is 1/500 seconds, and the lowest part is 1/30 seconds. The shutter speed is the brightness range of the object. That is to say, the contrast of an object depends on the ratio of the two shutter speeds, that is

500/30 = 1/60 = (Object contrast)

It is also easy to determine the light contrast, because we can measure the brightness of the incident light from each light source. For example, in the same aperture, If the measured shutter speed of the main light is 1/1000 seconds and the secondary light is 1/30 seconds, the light contrast is: 30/100 = 1/3)

In this way, we can calculate the image contrast based on the obtained inverse difference:

Object contrast x light contrast = image contrast

1/16 × 1/3 = 1/48 =

Assuming that the photographer must use an image contrast smaller than to make the color negative film have a good reproduction ability, then he can only change the light contrast from to reduce the scene contrast from. This means that the light contrast of an object is smaller and lighter. In this way, you can get a negative image with a contrast of and use it to print the photo to show the perfect color range.

After determining the inverse value of the object based on the measured values of the highlight and shadow, the next step is to calculate the appropriate exposure. We can set the speed between 1/30 seconds, that is, 1/125 seconds and f5.6.

Generally, the light contrast of an object is. In this contrast range, the relationship and color of the light and dark parts of an object can be truly reproduced. In other words, as long as the difference between the brightest light and the darkest light does not exceed the Factor 3, there will be no exposure problems due to the excessive contrast of the lighting. The lighting contrast that the negative slice may accept is large, which can reach. However, the image contrast between the two types of films must be compressed to so that transparent or negative films can be used as the original base for printing and copying. However, to achieve the best effect, it is best to use the image contrast as low.

On the other hand, if the transparent film is only intended to show the slides, the image contrast can reach. In such a large range of contrast, the contrast between objects and the contrast between light can naturally provide greater flexibility. Light contrast not only works outdoors in the photo studio, but also plays a major role. In a moderate latitude area, when the sun is clear, the light contrast is, and when the mist is. Dense fog will change the nature of normal outdoor light and completely eliminate the contrast under normal sunlight, reducing the light ratio. In general, there are many ways for the photographer to control the contrast of light. He can add some light to the shadow to achieve the balance of light ratio, or use the reflector to achieve the same purpose.

(Germany) K * E * Get Carter for Li Liang