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Working With Photographic Film
Understanding Film Physics
Photographic film is made from a strip of clear plastic coated with a gelatin emulsion that holds a microscopic coating of silver halide crystals. The size and density of those crystals affect the light sensitivity, the graininess and contrast of the film. Black and white film only has that one layer while color has at least three layers containing color sensitive dyes. Some color films have twelve layers of color sensitive dyes.
Print film creates negative images where dark areas appear clear and light areas dark on the film. These negatives are used to produce positive prints on a special paper.
Orthochromatic black and white negative film: Used silver halide salts which are sensitive to blues and ultraviolet light and less sensitive to red and green. The film makes yellow appear black, skies appear overcast, blond hair washed out, blue eyes white and red lips look black. It is however it was great for dark skin tones and it will accurately render a tanned skin. Also it could be processed while under a red safelight.
If a cyanide definitive were to be added to the film the result would be more sensitivity across the color spectrum making it panchromatic. But then you are dealing with a deadly poison. The two types overlapped for years and many photographs taken in the 1940s used ortho film. The main advantage was that it cost less than panchromatic film.
Panchromatic black and white negative film: It cost two or three times what orthochromatic film cost and it had to be developed in total darkness. It is however sensitive across the full visual spectrum so it is much less complicated and time consuming to get accurate results.
What happens in black and white film is that the silver salts are exposed to light and are altered. When the film is developed the silver in the bright portions of the image remain behind as a metallic silver negative image, that is, what was originally dark in the image is now relatively clear in that portion of the film while the bright areas appear dark on the film. This silver blocks the light from the enlarger to the paper, which exposes the salts in the paper creating a positive image after it is developed. There is a considerable amount of latitude of exposure as the printing process can correct many of the exposure errors.
This gets divided into at least three layers of color sensitive chemicals plus black. This is known as CMYK. Cyan, Magenta, Yellow and K stands for black. It is developed the same way as black and white with additional processing for the color dyes. This type of film is the most tolerant of exposure variations, which can be corrected during printing.
Color Reversal Film creates color transparencies also known as “slides.” It is much more sensitive to exposure variations. A precise meter reading is required to get good results and there are no post-production corrections available.
Black and white reversal film to make slides exists, but is rare. We won't be talking about specialty films such as XRay and Infrared.
Film Speed and ASA/ISO
What the heck is that? It is a measure of a film’s sensitivity to light. Typically the higher the ISO number the “faster” the film.
A low sensitivity to light is known as a slow film, typically 25/15º ISO (the old Kodachrome 25 slide film.)
The advantage is that this slow film has very little grain and excellent color. It works well in large poster sized prints and murals.
The disadvantage is that the lighting has to be bright; also the image will lack the contrast of a faster film.
Moving up the scale are the 125/22º ISO, (Plus X film), the 400/27º ISO, (Tri-X film) and roughly doubling up to 3200/36º.
The advantages of these faster speeds are greater light sensitivity often eliminating the need for additional lighting and higher contrast for artist effects.
The disadvantage is that as the number goes up the grain or noise increases and in the extreme cases the image will look like it was made of sand.
Grain and Contrast in Film
There are more influences on grain and contrast than film speed alone. During development of the film over agitation of the developer and too high temperature of the chemistry can increase grain and contrast. Over agitating the developer increases the chemical reaction and can overdevelop the film resulting in unplanned grain and contrast. This effect is permanent.
High temperatures can cause reticulation where the gelatin layer holding the silver halide salts cracks. This results in an irreparable pattern of what looks like dry mud cracks. On occasion this can be used for artistic effect.
Normal temperature for development is listed as 68° F. That can be difficult to achieve in warm climates where the tap water is warmer. Also, be aware of the hardness of the water to be used. Hard water should be treated or avoided.
Normal agitation during developing should move the solution over the film just enough to keep a “fresh” mix in contact with the emulsion. So don’t stir it or worse shake it. Just gently turn the developing reel about one turn every few seconds.
Be patient when you develop film.
Shutters and Shutter Speeds
There are three types of shutters; the Leaf Shutter, the Curtain Shutter and no shutter.
In the case of no shutter this is only seen on very old view cameras from the 1800s and the photographer would remove the lens cap, count off the seconds of exposure and replace the lens cap.
Copal shutter or Leaf shutter – This type of shutter is built into the lens. Used on large format cameras and occasionally older 35mm cameras with fixed lenses. It is a series of metal “leaves” similar to a set of aperture leaves, (looks like a rosebud that is just opening), the difference being that the leaves start out fully closed to block out the light open to a F/Stop and then close fully after the preset exposure time. A leaf shutter can be sync’d to a flash at much higher speeds up to 1/600ths of a second where a curtain shutter is limited to 1/60th or sometimes a bit higher. It is claimed to provide more control over available light than a curtain shutter. The disadvantage is the cost and difficultly in obtaining one that fits a current 35mm camera.
Curtain shutter – This type of shutter is built into the body of the camera. It is a type of cloth or metal screen that blocks all light. It is traditionally a two-curtain shutter. It creates a different sized gap that is drawn across the film horizontally. With slow shutter speeds the curtains open completely, typically at 1/60th of a second or slower, and with faster speeds there is a variable width gap that sweeps across the film painting it with light. Occasionally an object will be moving across the image in the same direction and at the same rate of speed as the gap, which creates unnaturally stretched and distorted images. Also when using a flash the bright light will only expose part of the film leaving the rest underexposed unless the shutter speed is set to the flash sync speed.
Aperture – F/Stops
Aperture blades are fully open for maximum light during focusing. These are identical looking to the leaf shutter, but they operate differently. The aperture is often referred to as the F/Stop. The F/Stop is indexed in half click stops on the lens. It reduces the amount of light allowed to get to the film. Think of the effect as if you were squinting your eyes in bright sunlight and you will get the idea. The more closed down the aperture is the less light is allowed in, higher F/Stop, and the image is sharper. When the shutter is released the F/Stop during the exposure time is closed to the preset click stop. Then it opens back up when the exposure is complete and it is ready for the next shot.
Black and white film can be “pushed,” which means that the photographer sets the ISO number to a higher value and that causes the meter to give shutter speeds and F/Stops that will under expose the film. Then during development of the image(s), the film is over developed for a longer period of time to create the negative. The downside of this is that if the film is pushed more that two F/Stops the image can become very grainy and contrasty, which may not be desirable.
Putting It Together In General Terms
ISO rating sets the baseline for exposure; the lower the number the smoother and softer the image and the higher film speed numbers means increasing harshness and contrast, but more sensitivity to light.
Shutter Speeds; the lower speeds allow more time for light to hit the film, but runs the risk of camera shake or motion blurring. The higher speeds freeze motion while allowing the light to expose the film for a short period. Higher speeds generally cannot be used in low light conditions.
Aperture, F/Stop; controls the amount of light exposed to the film. In low light the F/Stop number will be smaller and allow more light in at the cost of less sharpness. In bright light the F/Stop can be raised allowing less light in while increasing the image sharpness.
There are many combinations of film speed, shutter speed and aperture to create pretty much any effect you would like.
There is no book, blog or website that can substitute for actual hands on experimentation with these basic guidelines. So get out with your camera and start shooting pictures. You will learn a lot.