The Camera and Photography

Typical Imaging Chain for Photography source (sun) camera processing Displayed here is a typical imaging chain for photography. The source could be the sun, or a flash bulb. Some light from the source is reflected by the object, then collected by the lens. The exposure is controlled by the aperture and the shutter, and then captured by a piece of photographic film. The film is then processed, resulting in the final image. Now let’s look at each piece of the imaging chain in a little bit more detail. object collection (lens) exposure (aperture & shutter) detection (photographic film) image

Collection For a camera to be efficient, the pinhole is replaced by a lens. The lens redirects light rays emanating from the object. In a standard camera, to increase the amount of light collected compared to an ideal (infinitely small) pinhole, but defeat the problems we discussed earlier (blurring of image caused by confusion and interference of multiple rays), a lens is utilized. A lens is usually made of transparent material such as glass, quartz, or plastic, and is capable of redirecting the rays cast from the object by way of a phenomenon called refraction.

Depth of Focus: The range of film locations for which the photo of an object at a given distance comes out acceptably sharp. Depth of Field: The range of object distances that result in an acceptable photograph.

Exposure camera Lens Shutter Aperture One of the main functions of a camera is to form an image with an optical system using lenses. The other important function is to correctly expose the detector (in this case, film). This total amount of light that falls on the film is defined as exposure, where exposure is the product of illuminance and time. Exposure is defined as the total amount of light falling on the film. Exposure = Illuminance * Time

Illuminance Small aperture Large aperture Illuminance is the rate of light falling on a given area (i.e. energy per unit time). Illuminance is controlled by aperture: a larger aperture brings more light to the focus. Illuminance is defined as the rate of light falling on a given area. A bright scene would have more illuminance than a cloudy scene. Illuminance is controlled by an aperture, also known as a stop. An aperture is simply a light blocking device with an adjustable size hole. This limits the light collected by the lens by blocking the light that falls outside the hole. The larger the aperture, the more light will be collected, and more light will fall on the film. Then it is possible to make a bright scene have the same illuminance as a cloudy scene.

Aperture and “F number” F# (F number) is often used in photography to describe the aperture. F# = focal length of the system/diameter of aperture Aperture is measured using a scale known as F number in photography. F number is the focal length of the system divided by the diameter of the aperture (or the diameter of the hole). F number increases as the diameter of the hole decreases. In addition, if the F number is doubled, then the illuminance increases by the square of that value, or quadrupled. This is due to the fact that the area of the hole is increased by a square of the that factor. So increasing the F number effectively reduces the exposure, because increasing the F number decreases illuminance. POSSIBLE HOMEWORK: Find the relationship between F# and amount of illuminance as stated in paragraph two of these notes.

Exposure Time Shutter Closed Shutter Open Exposure time is controlled by the shutter: when closed, the film is not exposed to light. Exposure time is simply the time interval between opening and closing the shutter. The time of exposure is controlled by a light blocking device called a shutter. When the shutter is closed, the film is not exposed to the light since the shutter blocks the light completely. The shutter opens for a specified amount of time on command, exposing the film . Increasing the exposure time increases the exposure.

Types of Shutters Between the Lens (BTL) Focal Plane Shutter Or Simplified Camera There are two types of shutters. A between the lens (BTL) or a leaf shutter is located between the lenses of a system. The focal plane shutter is located right before the image plane where the film is. Between the Lens (BTL) Or Leaf Shutter Focal Plane Shutter

BTL or Leaf Shutter Made of overlapping “leaves” that slide out of the way when shutter opens. Located between the imaging lens elements. CLOSED Between the lens or a leaf shutter is made of overlapping leaves of metal or plastic. As the leaves contract to the side, the shutter opens in the middle. After some set time, the leaves overlap again, closing the shutter. The shutter mechanism is located between the lens element. Therefore, there must be a shutter mechanism for each lens element if the camera can interchange the lens. OPEN

BTL or Leaf Shutter Advantages Disadvantages Uniform illumination independent of film size. Entire film frame illuminated at once. Disadvantages Illumination of frame not constant over time. Limitations on shutter speed. The leaf shutter provides a uniform illumination independent of the film size since the light is controlled at a location far from the image plane. In addition, the entire film is exposed at once, so there is no spatial distortion present that appears in other shutter mechanisms. The disadvantage is that since the BTL shutter is located in the lens itself, there must be a shutter mechanism for each lens if a camera has an interchangeable lens. In addition, the lens takes some time to open and close, so the illumination is not constant over time. Therefore, the lens must be calibrated and offset slightly so that the effective exposure (the actual amount of exposure) is equal to the desired exposure time.

Focal Plane Shutter Metal or fabric with a narrow slit opening which traverses the area to be exposed. Located just before the detector (film) at the focal plane. The focal plane shutter is a metal or fabric sheet with a slit opening located just before the image plane (also known as “focal plane”). The slit traverses across the image plane, exposing the film at slit width over time. The speed of which the shutter traverses, and the width of the opening governs the exposure.

Focal Plane Shutter Advantages Disadvantages Cost effective (one shutter needed for all lenses - great for interchangeable lens systems) Can achieve very fast shutter speeds (~1/10000 sec) Disadvantages May cause time distortion if the film size is large (since the shutter slit must traverse the film) The advantage of the focal plane shutter is that it is cost effective. Since the shutter is located in the camera itself, there’s no need to have separate shutters for each lens. (Unlike the BTL shutters) For this reason,the focal plane shutter is used often in 35mm SLR (the “serious cameras”) with interchangeable lens system. In addition, it can achieve very fast shutter speeds (Approx. 1/10,000 sec) The disadvantage of the focal plane shutter is that it may cause spatial distortion of the image if the film size is large, since the shutter must traverse the entire film. Especially when the subject is moving at a high speed, the subject may have moved slightly from one end of the film to the other.

Why control exposure with aperture and shutter? Flexibility! Fast shutter speed for freezing action (e.g. sports photography). Slow shutter speed for low light levels (e.g. sunsets). Small aperture for bright scenes or to enable longer exposures. Large aperture for low light conditions (taking candle lit or moon lit pictures). So why control exposure with both aperture (which controls the illuminance) and shutter (which controls time of exposure)? The main reason is flexibility. Fast shutter speeds are needed for a subject moving at a fast speed, like sports photography. Slow shutter speeds can produce blurs which may be used to create a blur effect. A small aperture is used for bright scenes, and a large aperture is used for low light conditions. In addition, the aperture can effect the depth of focus. There are many different “correct” settings for exposure. In today’s camera, the exposure setting can be fully automated so that the user doesn’t have to adjust for the changing scenes. Many of the point-and-shoot cameras use a fixed aperture or shutter speed, and vary the other. Many advanced cameras can pick which one to make constant, which is known as a shutter priority or aperture priority exposures. Many of the professional photographers still want to control every aspect of this, so they may choose to use a fully manual system.

Capture The image can now be captured using a detection system, such as photographic film. Film must be processed to yield a permanent, visible image. The capture of an image formed by the pinhole can be performed by a light sensitive material called photographic film. Photographic film is made of material where the property of the material changes with exposure to light. This material change is then processed to yield a stable and visible image.

Image Capture - Photographic Film Film consists of a photosensitive compound known as Silver Halide (AgX), which reacts with light. The area exposed to light turns dark after processing. The total amount of light hitting a given area of film (known as exposure) determines how dark that area turns. This photosensitive material used in traditional photographic film and paper is known as silver halide or AgX for shorthand. Ag is the periodic table of elements chart shorthand for silver. In most emulsions, the exposed area turns dark after processing, making the captured image a “negative:” bright parts of the image are captured as dark spots on the film, and dark regions as light spots. There are special types of emulsions available such that the captured images reproduce the light/dark pattern of the real scene, such as slide films. The amount of light hitting the given area of the film (known as exposure) determines how dark that area turns. Of course, some film turns darker than others when exposed to the same amount of light, and that determines how sensitive the film is. The measure of how dark or light the film becomes when exposed to a given amount of light is called density. More on silver halide film and processes in the later sections.

What is “Silver Halide”? Silver Halide is a compound made of silver (Ag) atoms and atoms from the halide group (Cl,Br,I) of the periodic table. The material property of a silver halide crystal changes after absorbing photons. We keep referring to the photographic film system as silver halide system. Silver halide is a compound made of silver atoms (which has a periodic table symbol of “Ag”) and atoms from the halide group of the periodic table which includes Chlorine,Bromine, and Iodine. These compounds are light sensitive; The material property of the silver halide crystal changes when photons are absorbed.

Silver and halide atoms within the periodic table of the elements Silver (Ag) Halide group

Structure of a Typical B&W Film Film base Plastic Antihalation backing Prevents light from reflecting back. Emulsion A modern black and white film has several layers. The film base is made of plastic. [mouse click] The antihalation backing is coated on the underside of the film. The purpose of the antihalation backing is to absorb any light that was not absorbed by the film in order to reduce the back scatter of light. The light would otherwise be reflected back and re-expose the emulsion, causing a foggy halo around bright objects. [Mouse click] The emulsion is coated on the front side of the film and is made up of the light sensitive silver halide crystals, which are suspended in gelatin, like fruits in a pool of jell-o. The gelatin is an ideal substance for the emulsion because it absorbs liquid well during the film processing and remains stable. Silver Halide Crystals Suspended in gelatin, like fruits in Jell-O™!

Exposed AgX Crystals When a silver halide crystal is exposed to light, some of the AgX molecules break up into their constituents, one of which is metallic silver (“pure” Ag). hn Exposure When a silver halide crystal is exposed to light, some of the silver halide molecule breaks up as a result of photon absorption, leaving just the metallic silver. After Exposure

Processing Photographic Film Turns latent image into visible image. Basic steps: Developer Stop bath Fix bath All chemicals used in the process are in liquid form. The processing steps turn the latent image, which is invisible to human eye, to a visible image. The four basic steps are development, stop, fix, then wash. There are other minor intermediate steps in order to make the image better., but is not necessary for image formation. This processing is a wet process, meaning that all the chemicals are in liquid form.

Silver Halide Process Chain Exposure Processing Develop Stop Fix Visible (Stable) Image Latent Image Shown here is a simplified processing chain of silver halide film after exposure. During the exposure process, the film is exposed to light. This forms a latent image, which is invisible to the human eye. In addition, it is still sensitive to light, so it must be kept in the dark. After processing, this latent image is turned into a visible and stable image. A latent image is formed after exposure (invisible to human eye). After processing, the latent image is turned into a visible, stable image.

Processing Photographic Film Developer “amplifies” the atomic silver to visible silver strands. Stop Bath stops the development process. Fix dissolves the unexposed AgX crystals, making the film safe to expose to light. First, the developer amplifies the metallic silver to a visible silver strand. Development happens faster when there exists a silver nucleus in the crystal, so the exposed crystals develop much faster than the unexposed ones. It is possible to overdevelop by soaking in the developer for too long. [mouse click] Then the stop bath stops the development process. At this stage, there is a visible image; however since there still are unexposed silver halide crystals, it will turn black if the film is exposed to light. Soaking the film in fix dissolves the unexposed AgX crystals, making the film safe to view in light. And washing the film in water rinses the fix away, and you have a stable image made of silver strands. Wash with water to rinse fix chemicals away.

“Grain” of Film and Paper Electron Photomicrographs of Emulsion Grains (n.b. Measurement Bars indicate scale) These silver strands or filaments that were made during the development process are called a grain. They absorb the light that hits them so they look dark. There’s a notion of metallic silver being shiny; however, since these silver strands are not solid packed like a silver metal, it does not shine.

Silver Halide Grains

Why does processed film look “negative”? Silver strands formed by exposure of photographic film to light actually appear dark (they are NOT shiny). So, where light hits the film during exposure, it turns darker. The films look negative because of the sensitivity curve of the photographic film. Since silver is formed where light hits, the lightness is reversed.

What determines how dark film becomes? THE GRAINS! Size Shape Chemical composition Distribution The sensitivity curve mentioned before is called a D-log H curve. It is a scientific measurement of how the film responds to different amounts of exposure. D stands for Density, and it is the measurement of how dark that part of the film is. Log H is the the exposure (H) in logarithmic scale. Such a scale is used because the exposure range of film is very wide, so the scale compresses the data for easier referencing. As you can see here, as the exposure increases, the density gets higher and higher, until it rolls off. This is the maximum darkness that the film can achieve. Notice that the curve is fairly complicated with curves and straight sections.

D-Log H Curve and Contrast More contrast Less contrast Image Log H D Log H D Film response

Photographic Finishing In order to get a “positive” final print, the negative must be projected onto photographic paper. Negative * negative = positive! 2 basic “finishing” methods: CONTACT and ENLARGEMENT/REDUCTION In order to get a positive final output, the negative is then printed to a photographic paper. A negative image exposed to a negative paper results in a reversal of lightness, meaning a positive image!

Contact Printing Light Negative Photographic Paper A CONTACT print: There are several methods of making a final image. The simplest is called contact printing, where the negative is placed directly on top of the photographic paper. The main difference between photographic film and paper is the base- film uses plastic, and paper uses - paper. Then the paper is exposed with light, under proper exposure. The paper is then processed just like the film was. A CONTACT print: The negative is in direct contact with the photographic paper --essentially creating a shadow-gram.

Enlarger / Reducer Light Negative Photographic Paper The second method is utilizing an enlarger. Often, the film size is smaller than the desired print size (35mm film is little over 1.5 inches!) The enlarger images the negative onto photographic paper, and depending on the optic setup, the image can be enlarged, or reduced. The exposed paper is then processed to yield a final image. Optics are used to produce an image of the negative on photographic paper.

Contact Printing vs. Enlarger No loss of signal by the optics. No reduction in resolution. Simpler system. Fixed image size. Flexible image size. Some loss in resolution due to enlargement. Additional optics may degrade final image quality. Again, flexibility! There are several pro’s and con’s of contact printing and enlarging. Contact printing uses no optical system, so there is no loss of signal. It is a simpler system, but the image cannot be enlarged- this is not convenient if the negative is small to begin with. Contact printing is often used with large format films. The enlarger, on the other hand, offers flexible image size. However, with the addition of optics (magnifying lenses) and since the small negative is being enlarged, the image may lose some sharpness. The photographic grains may become visible if the negative is enlarged excessively. But the flexibility outweighs the tradeoff by tenfold.

Latent Image Formation (Ex.: shadowgram) Group of AgX Crystals Mask (object) prevents AgX crystals underneath to be exposed. “Unmasked” AgX is exposed to photons. In order to explain the latent image formation, we will first go over making a shadowgram, where the shadow of an object is imaged. [mouse click] The object prevents the silver halide crystals underneath to be exposed. Exposure by photons The exposed crystals have different material property than the unexposed crystals because of the metallic silver. Exposed crystals have different material property (I.e. some AgX bonds have been broken).

Latent Image Formation (with optics) Group of AgX Crystals Optics used to image object onto the film. Illuminated AgX is exposed to photons; but not all AgX is illuminated. Now using optics to image the object on the film . . . Exposed crystals have a different material property.