Photosensitive substance in the photo

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Lecture notes on biological photography (lecture by Dr. Tian Rungang)

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Section 1 Black and white photosensitive materials

First, black and white photographic film

(1) type

1. According to the photosensitive speed.

Sensitivity refers to the sensitivity of film to light. The GB system in China is the same as the DIN system in Germany.

It is a logarithmic system, and the photosensitive speed is twice as fast for every difference of 3 levels. For example, GB24 is twice as fast as GB 2 1. It was adopted by the United States.

Use the arithmetic system. ASA 100 is equivalent to GB 2 1, and ASA 200 is equivalent to GB24. ISO system.

It is a sensitivity sign published by ISO 1979, which has been widely accepted and expressed by ASA system.

The sensitivity values are the same.

Low-speed film: GB 18 or less. Fine lines, slow photosensitive speed, suitable for shooting under strong light.

Medium speed film: GB 2 1, fine texture, moderate speed, suitable for shooting in natural light.

High-speed film: GB24, with fine grains and fast photosensitive speed, suitable for low-light shooting.

Lecture notes on biological photography Shaoxing University of Arts and Sciences Department of Biological Sciences

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(4) Ultra-high speed film: GB27 or above, with fast photosensitive speed but poor imaging quality.

2. According to color sensitivity.

Color sensitivity refers to the ability to feel various colors of light, which can be divided into three categories: color blind film, dichroic film and panchromatic film.

Color-blind film: it only feels blue and purple light, with fine particles and large contrast, and is used to remake charts and texts.

Color separation: you can't feel red, and you can operate under the red safety light for printing and plate making.

(3) Full-color film: it can feel various components of visible light, but its ability to feel green is weak.

Common black-and-white films developed under safety lights belong to this category, and can be used for people, landscapes and micrographs.

Divide according to size

Include 120,135,620,127,220, etc. , of which 120 and 135 films are the most commonly used. 36438.68666666667

The film is 82cm long and 6. 1cm wide, and each film can shoot 16 4.5×6cm or 12 6×6cm negatives.

135 film length 160cm, width 3.5cm, each film can shoot 36 24×36mm negatives.

(2) Structure

Black and white film is composed of film base, emulsion layer, adhesive layer, protective layer and anti-blooming layer.

1. basis

There are two kinds: dry plate and film. Dry plate is glass with stable size, which is used for printing and scientific research. Movies in movies

The base materials mainly include polyester base materials, polyvinyl acetate base materials and polystyrene base materials.

2 emulsion layer

The main components are silver halide and gelatin. Silver halide includes silver chloride, silver bromide and silver iodide, among which bromide.

Silver is the most sensitive to light, and silver iodide is the weakest. Zoroastrianism is to disperse silver halide and prevent it from reuniting.

Silver halide is only sensitive to blue-violet light. In order to make the film feel all visible light, it needs to be added to latex.

Vector of infection.

3. Adhesive layer

It plays an adhesive role and consists of gelatin and a small amount of tablet solvent.

4. Protective layer

It is made of gelatin to prevent the film from being damaged during use and development.

5. Anti-halo, anti-static and anti-curl layer

Pigment can absorb the light passing through the film base, avoid halo and reduce the imaging quality. At the same time, it also

Materials that prevent static electricity and film curling are taught to make it flat.

(C) the characteristics of black and white photographic film

We have introduced the sensitivity and color sensitivity of black and white film in the film type. In addition,

Black and white film also has some other characteristics, such as contrast, resolution, latitude, density and so on.

1. Comparison

That is to say, the ability of the film to reflect the brightness (contrast) of the subject is determined by the emulsion.

Formulation and process determine that the development process will also affect the contrast of the film. Generally speaking, the higher the film speed, the better the contrast.

The smaller, the stronger the contrast.

Step 2 solve

Also known as analytical ability, it is usually expressed by the number of lines that can be resolved within one millimeter. The higher the resolution, the more images.

Safe. Generally, the resolution of low-speed film is higher than that of high-speed film, and the resolution of medium-speed film is 70- 100 line/

Mm. Since each inch is equal to 25.3995 Mm, the resolution of medium-speed film is equivalent to 800 of that of digital camera.

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More than ten thousand pixels.

Step 3 tolerate

Refers to the film's ability to reflect the contrast between light and dark in proportion and express the image level. The wider the film, the better.

The greater the safety factor of correct exposure, the higher or lower the speed or aperture, and the film will not be exposed.

Insufficient light or overexposure.

4. Density

Refers to the amount of silver particles deposited per unit area after exposure and development of photosensitive materials. The overexposure of film is intensive.

The density is too high (too thick) and too low (too thin) under exposure.

5. Gray mist

That is, the density of fog, in the development process, a small amount of unexposed silver salt is reduced due to instability, and the shape is

The density of particles evenly distributed on the negative film is the haze.

Second, black and white photosensitive paper

Photosensitive paper, also known as photographic paper, is used to copy the negative film to the positive film.

(A) the type of photosensitive paper

1. According to the purpose

① Printing paper: The emulsion is composed of silver chloride, which has slow photosensitive speed and can be operated under the red safety lamp.

Copy the negative phase of the negative directly, such as transmission electron microscope photos.

② Photographic paper: The emulsion is made of silver bromide, which has fast photosensitive speed and can be operated under dark red safety lamp.

Used to enlarge 135 negative.

③ Printing paper and playback paper: The emulsion consists of silver chloride and silver bromide, and its sensitivity is between that of printing paper and playback paper.

Between.

2. Contrast and division

Photographic paper in China is divided into 1, No.2, No.3 and No.4. The bigger the paper number, the greater the contrast. 1 soft, 2 soft.

Neutral, No.3 hard, No.4 hard. General 1, No.2 photographic paper is used for portraits and natural scenery, 3,

No.4 photographic paper is used for scientific research photos.

3. Divide by paper

Including: glossy, matte, textured, textured, suede, in which glossy contrast is large, and it is used a lot.

In scientific research, portrait photography uses other images.

(B) the structure of photosensitive paper

From the surface, there are protective layer, latex layer, white powder layer and paper base layer in turn. The protective layer is gelatin, which is anti-aging.

Stop scratching; The emulsion layer is silver bromide; The function of white powder layer is whitening. Some photographic papers use barium sulfate, which

When the photographic paper is dried after printing, it should be dried by glazing machine. Some photographic papers use polyethylene coating and titanium dioxide structure.

It is called plastic coated paper or unpolished paper, and it can be dried directly after development. The paper base is made of high-grade pulp.

Soaking resistance, acid and alkali resistance and deformation resistance.

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Third, the preservation of photosensitive materials.

1. Low temperature environment (4-10℃);

2. The relative humidity does not exceed 50%, so as to avoid mildew, it is best to keep it sealed;

3. The film taken out of the refrigerator should be placed for a period of time to avoid the formation of water vapor and affect the shooting effect;

4. After the film is loaded, it should be shot as soon as possible, not for a long time.

5. Photosensitive materials have a certain period of validity (usually about 1 year). In order to ensure the shooting quality, please

Use within the validity period.

Section 2 Color photographic materials

First, the type of color photographic film.

1. According to the purpose

① Color negative: The formed image is opposite to the main color, which is what we often use.

Color film.

(2) Color reversal film: also known as color copy film, used for film copy or slide production and exposure.

It's too low to shoot.

(3) Color positive film: used for printing plate making or making slides, which can be obtained and photographed by reverse printing technology.

The color of the subject is a transparent positive film with the same light and shade.

2. Divide by color temperature

The color temperature represents the spectral components of the light source and is represented by the absolute temperature scale K. In a light source with a high color temperature

Blue light has more components than red light; In the light source with low color temperature, the red light component is more than the blue light.

The color temperature values of different light sources are different. The color temperature of sunlight at noon is about 5400K, which is higher than that of sunlight in the morning and evening.

The color temperature is about1900 k ~ 2800 K.

Daylight type: suitable for color temperature of 5400K, and can be shot in natural light.

(2) Lighting type: suitable for 3,200-3,400 K color temperature, and can be photographed under indoor lighting or used for exhibition.

Microphotography.

③ Dual-purpose type of daylight illumination: the suitable color temperature is 3300-5400 K.

④ Color temperature latitude: the color temperature latitude of sunlight color film is 1000 K, and that of lighting color film is 1000 K.

The tolerance is 700K K K.

Second, the structure of color negative

There are many kinds of color negative films, but they all contain three basic photosensitive layers: blue photosensitive layer, green photosensitive layer and red photosensitive layer.

They form three monochromatic images of yellow, magenta and cyan respectively, and form different colors according to the principle of three primary colors.

Each photosensitive layer contains a silver halide emulsion and a corresponding color former. When the developer reacts with photosensitive silver halide,

The silver salt is reduced, the developer is oxidized, and the oxidized developer can be combined with the color former to form and photographed.

Then, after bleaching and fixing, silver particles and unexposed silver salts are removed, so that

Form a stable image.

Lecture notes on biological photography Shaoxing University of Arts and Sciences Department of Biological Sciences

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Third, colored photosensitive paper.

Also known as colored paper, regardless of printing paper and photographic paper, there is only one specification of neutral contrast in structure and color.

Color negative is similar, with three basic photosensitive layers, except that the film base is paper, and the photosensitive speed is low.

Chapter III Irradiation and Use of Light

Section 1 exposure

First, what is exposure?

Exposure, also known as photosensitivity, means that the reflected light of the subject falls on the film through the lens, which is caused by the film.

Photochemical reaction, the process of forming latent image.

There are two standards for correct exposure: ① technically correct exposure, development and fixing solution.

After processing, we can get an image that reflects the objective reality; ② Correct artistic exposure and film processing.

Only then can we show our creative intention and emotion, infect and attract the audience. In biological photography, we emphasize that

the former

Second, the reciprocity rate.

The exposure (e) is equal to the illumination (i) of the image surface multiplied by the exposure time (t), that is:

E=I×T

From the formula, when the exposure is constant, I is inversely proportional to T, and the greater the illumination, the more time it takes.

The shorter the interval, and vice versa, that is to say, the values of illumination and time are interchangeable, so it is also called reciprocity rate.

This is the so-called reciprocity law. For example, f/8,1125 and f/ 1 1, 1/60 have the same shooting effect.

Reciprocity is correct in most cases, but when the light is too dim, the photosensitive properties of photosensitive materials are bright.

Significantly reduced, that is, insensitive to weak light; Or the action time is too short for light to penetrate.

Through the film to the latex layer, and then the reciprocity rate fails.

Reciprocal failure principle is very important in biological photography, and sometimes we try to add it.

This principle can be used to compare images, eliminate interference factors and highlight key objects. For example, in

When developing chromosome photos, we don't want the "cytoplasm" component to affect the pictures, so we can

Using the method of low speed and weak light, the light passing through the "cytoplasm" can not expose the film; And in

You can use strong light and fast speed when taking pictures, so that the corresponding part of the chromosome can't be exposed.

Light, and the cytoplasmic part is completely illuminated.

Thirdly, photometric method.

1. Appearance inspection

The combination of aperture speed is determined according to the photographer's experience. Novice use, without metering function.

Lecture notes on biological photography Shaoxing University of Arts and Sciences Department of Biological Sciences

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When using a camera or a photomicrograph camera, you can take several shots of the same scene in succession, such as "positive exposure"

Really ",one is to raise one level, the other is to lower one level, and then choose a suitable photo.

You can also refer to the exposure reference table provided on the film box.

Table 1 ISO 100 Reference Table for Outdoor Exposure of Film

The weather is strong, the sun is thin, the clouds are thick and cloudy.

Aperture f16 f11f8f5.6

Shutter speed (seconds)112511251/25.

Note: the above table applies to two hours after sunrise to two hours before sunset in spring and autumn. In summer, the aperture should be reduced by one notch.

In winter, you should speed up.

2. Automatic measurement

The light intensity is measured by the internal metering system on the camera. Nowadays, many single lens reflex cameras use TTL.

The English full text of "TTL photometry" is through the lens, which means a way to measure light through the camera lens.

Line method, referred to as "TTL photometry". TTL metering mainly includes full-field average metering, central key metering and spot metering.

Light, regional comprehensive photometry, etc.

① Average metering: 135 Both cameras and miniature cameras have this metering mode to measure the average brightness of the whole picture.

Value. This metering method is suitable for occasions where the contrast of scenery light is not very large and the distribution of light and shade is extremely uneven, such as snow scene.

And the negatively stained slide is easy to deviate.

② Central key metering: The metering element mainly measures the brightness and exposure readings in the center and lower part of the picture.

70% come from this area. This metering method is suitable for the situation that the contrast between the subject and the background light is large, and there is deviation at this time.

The subject photometry is not affected by background light and shadow.

③ Spot metering: Only the brightness of the scene in the center of the picture is measured at1100. This metering method is not bright by other scenes in the picture.

As long as a very small photometric area is aimed at the medium gray tone in the scene, accurate exposure data can usually be obtained.

(4) Comprehensive metering by region: This is an advanced metering system, which uses multiple metering elements, each of which is oriented independently.

Photometry is performed in different areas of the whole picture, and then the respective photometric data are processed and input into the computer in the machine for calculation.

Calculate and get accurate automatic exposure data. For example, Canon's EOS camera has this metering mode.

3. Exposure compensation of automatic camera

Advanced automatic cameras and automatic microphotograph cameras have automatic exposure compensation devices and adopt average measurement.

In bright mode, by adjusting the exposure compensation device, the picture with uneven light and dark distribution can be accurately exposed.

There are usually five grades, namely: +2,+1, 0,-1, -2, which can increase or decrease the exposure of 1-2.

You can increase the exposure of 1-2 when shooting snow scenes, and decrease the exposure of 1-2 when shooting sunset to emphasize dusk.

Atmosphere.

In an aperture priority camera, an automatic exposure compensation device acts on the shutter to raise or lower the shutter.

Speed; In a camera with speed priority, the device acts on the aperture to open or narrow the aperture.

4. Independent exposure meter

It is divided into incident exposure meter (illuminance exposure meter) and reflection exposure meter (brightness exposure meter), and single lens.

In-camera photometry is also reflective photometry, and most independent exposers have two photometry modes.

The use method of the incident exposure meter is to place the exposure meter under the same lighting conditions as the shooting object,

The measurement of the illumination of the photographed object does not consider the reflection ability of the photographed object to the incident light.

Volume and dark objects represent the same exposure, which can correctly reflect the light-dark relationship of the scene.

The use method of reflective exposure meter is to aim the exposure meter at the photographed object and measure the brightness of its reflected light.

It is also possible to measure the brightness of reflected light with a gray reflector with a reflectivity of 18% instead of the subject.

The exposure meter measures the average brightness and processes the brightness value into an intermediate tone (neutral gray on the negative).

Color), so it is not easy to get accurate exposure when shooting unbalanced pictures.

Lecture notes on biological photography Shaoxing University of Arts and Sciences Department of Biological Sciences

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Fourth, the factors that affect the exposure.

There are four main factors affecting exposure: ① light intensity, ② film sensitivity, ③ aperture size,

Shutter speed. In addition, the brightness and color filter factor of the scene also affect the exposure.

1. light intensity

Different seasons, different weather, different time (morning, noon and evening), different geographical latitude and different altitude are all different.

The illuminance of the light should be adjusted according to the actual situation.

2. Brightness of the scene

① Lighting direction: it can be divided into front light, back light and side light. Generally, the side light is 1-2 level higher than the front light.

The backlight is 2-3 levels higher than the previous one.

② Reflectivity (reflection coefficient) of scenery: snow 96%, white wall 96%, white paper 80-95%, green grass.

Land 7%, red brick wall 10- 15, sandy land 30-50%, yellow land 12-20%, asphalt road 7- 10%, yellow bark.

Leather 16-25%, kraft paper 16%.

3 distance

Generally speaking, the farther away from the scene, the higher its brightness, and the closer it is to the scene, the lower its relative brightness.

Therefore, the exposure can be moderately reduced when taking a long shot, and it can be moderately increased when taking a close shot.

4. Color filter factor

The color filter absorbs light to varying degrees, so the exposure should be increased according to the instructions of the color filter.