Basic Photography Terms
Single Lens Reflex Camera (SLR)


Before going into the working of a SLR camera, let me answer one of the most common question. How is SLR camera different from a point and shoot camera? In an SLR camera, the image you see in the viewfinder is exactly the image that will be recorded (after the shutter time delay of course!) on the recording media. Whereas in a point and shoot camera, the image seen in the view finder is an approximation of the the actual image that will be recorded in the media (film or ccd) Now that we have cleared that, let me give one line explanation of the important parts listed in the schematic above. More details about the elements that affect the picture are discussed in detail in latter sections. Lens Aperture: This is equivalent to the iris of human eye. This variable opening controls the amount of light allowed by the lens.This is not a SLR body component, but is a lens component. So when u change the lens, the aperture specification changes. Focusing Aiding Screen: SLR cameras have special screens like the split prism, ground glass screens that aid accurate manual focusing . Pentaprism: This prism corrects the inverted image from lens to upright position suitable for viewing and directs it to the viewfinder. Mirror: This element reflects the light from the lens to the pentaprism. This will swing out of the way of light path when the shutter release button is pressed so as to allow the recording medium to be exposed. Shutter: This equivalent to the eyelid of a human eye. When the shutter release button is pressed to take a photograph, this opens (or moves out) to allow the recording medium to be exposed to light from the subject. Recording Media: This is a film or CCD chip depending on whether the camera is film or digital SLR. Light Path in SLR camera when the shutter is not released Light from the subject is inverted by the lens and the amount of light is controlled by the aperture area opening. This light is reflected to the pentaprism by a mirror. The pentaprism, directs it to the viewfinder and in the process corrects the inverted image. The photographer sees the image sees through the lens (known as TTL viewing) and can now correct the focus with the help of the focusing screen. Once s/he is satisfied with the composition, exposure setting etc, can now press the shutter release button to capture the image to the recording media. Light Path in SLR camera after the shutter is released When the shutter release button is pressed, the mirror swings upwards and immediately the shutter contracts to allow the light from the subject to strike the recording media. During this time, there will be no image visible in the view finder. After the selected shutter time elapses, the shutter expands to block light to the recording media.
Focal Length:
Simply put, Focal length is the distance from the lens to the point of focus. In the following schematic, the lens used to illustrate is a biconvex lens. Understanding focal length is very important as it influences the following Field of view Field of view is the portion of the subject that will be recorded in the frame. As the the focal length decreases, the field of view that will be recorded increases. This is shown inthe following schematic. Depth of field Depth of field for a given aperture increases as the focal length decreases Refer to the Depth of Field section further down for more info on this. Shutter Speed Focal length has no direct bearing on the shutter speed. But, in hand held conditions, to prevent camera shake, the shutter speed is recommended to be atleast 1/(focal length). This limits the usablity of a particular focal length depending on the lighting condition. Chromatic Abberation: Now, The focal length can vary for different wavelength of light (or colors) referred to as chromatic aberrations. This due to the fact that focal length depends on the refraction property of glass and the refractive index varies With the wavelength of light used. Therefore, there is small change in the focal length between say, blue light and the red light. There is always a small amount of chromatic aberration associated with lens made of refractive elements (most common type). However, a lens made of reflective elements (like a Schmidt cassegrain or Newtonian reflector lenses) does not suffer from chromatic aberration. Chromatic aberrations can be corrected using low dispersion elements (Nikon designation is ED). Many high-end photographic lens have this correction incorporated.

Lens Types:
Classification based on Focal Length Normal Lens For a camera using 35 mm film, a 50 mm (well 45-55 mm) lens closely matches the vision of a human eye and is referred to as the Normal Lens. This type of lens used to be a part of a standard SLR packages in earlier days. This has been replaced by consumer level zoom lens (like 35-80 mm) in recent times. A 50 mm lens is usually very cheap and is one the sharpest prime lens you can find. Wide Angle Lens A wide-angle lens refers to lens whose focal length is smaller than 50 mm and therefore will have wider field of vision than that of a 50 mm lens. Wide-angle lenses are very popular with landscape photography. As the focal length becomes smaller, there will be barrel distortions introduced in the image. Vertical lines will start looking curved and this can be rectified using special corrective elements. Wide-angle lenses with corrections incorporated for barrel distortions are known as Rectilinear wide-angle lenses. These lenses tend to be quite expensive. Telephoto Lens A lens is referred to as a telephoto lens when its focal length is greater than that of a normal lens (50 mm). The field of vision in a telephoto lens is much smaller than that of the normal lens and the objects will appear bigger in the telephoto lens than that of a normal or wide-angle lens for the same subject to camera distance. A telephoto lens is basically a telescope attached to the camera body. Usually, lenses with focal length over 200 mm are referred to as super telephoto lens. Super telephoto lenses are extremely popular with nature photographers. Though prospect of filling the film frame with the subject is very tempting, telephoto lenses amplify any camera movement making the use of tripod almost mandatory. Also, to have a reasonable shutter speed, telephoto lenses require wider aperture for more light gathering. Unfortunately, the increase in size and the high cost associated with the bigger the aperture makes buying super telephoto lenses well beyond amateur photographers reach. Some companies add some active elements in their lens to counter small movements induced during handheld photography, commonly referred as Image Stabilization (canon) or Vibration Reduction (Nikon), that will reduce or prevent blur even at low shutter speeds. As you can guess, these lens are quite expensive. “Fish eye lens” Fish eye lens are extreme wide-angle lenses with uncorrected barrel distortions. Any straight line other than the radial ones will appear curved outwards (barrel distortion). Macro Lens Macro or Micro lenses are special purpose lenses that will allow the life-size or greater than life size of reproduction of subject on the recording media (ccd or film). They allow the subject the lens distance to be smaller than the lens to film distance. One thing to keep in mind is that the working distance varies with the focal length of the macro lens. A 50 mm macrolens and a 200 mm macro lens can produce 1:1 image, but the 200 mm lens will have much higher working distance than the 50 mm macro lens which can be crucia l when photographing live subjects like insects. Also the depth of field in the macro range is extremely limited. Classification based on lens element. Refractive Lens or Reflective/Reflex/Mirror/Catadiaptric Lens Lenses can be either made up of refractive (lens) elements or reflective (mirror type) elements to focus an image. The most common type is of the refractive type because of their high image quality. Refractive lenses also tend to have very good sharpness and contrast. The refractive lenses tend to be very expensive as the focal length increases due to the high manufacturing cost. Also, the refractive lenses become bulkier as the aperture size increases. If cost is not a concern (!), then for the same focal length a refractive lens will outperform any reflective lens types. Reflective lenses on the other hand are very popular in astronomical telescopes due to their compact size even for very high focal length. Since the lens is made up of mirrors to gather and direct light, they are inherently immune to chromatic aberration. Reflective lenses also tend to be very cheap!/ The disadvantages of the reflective lens is their poor image quality, very small maximum aperture (photographic reflective lenses tend to have fixed aperture of f/8 or f/5.4).They are also referred to as mirror/reflex/catadiaptric lenses. The most common design of reflex lens is of Maksutov-Cassegrain or Schmidt-Cassegrain type. The difference between the aforementioned lenses is the corrector mirror design
Aperture:
Aperture is the opening of a lens. Bigger the aperture, more the light gathering capability of the lens. If you consider the camera as an equivalent to the human eye, then the aperture equates to the iris. Therefore, the more open the aperture diaphragm is, the more light it allows. Aperture has a direct effect on the depth of field and shutter speed, which will be discussed at some detail in further sections. One of the important concepts to learn is the interpretation of the F Numbers written on the lens. The F-numbers are listed in a sequence like 2.8, 4.0, 5.6, 8.0, 11, 16, 22.	Aperture and F-stops
The Diameter of opening of a lens is the ratio of the focal length of the lens and the F-number. The progression is like this because of the diameter is proportional to the square of area of opening! So for a lens as the F-number increases, the diameter of the opening decreases and light that is allowed in is reduced. The light allowed in at a F-Num of 2.8 is double that of the light at F-Num of 4.0 and is four times that of light in F-Num of 5.6 and so forth. So, the F-number has definite value only in conjunction with a focal length and cannot be used as an independent measure to compare lenses of different focal lengths. For example, a 50 mm f/1.4 lens has an opening diameter of 50/1.4 = 35.7mm And a 300 mm f/4 has an opening of 300/4 = 75 mm! F-Stops and Stopping Down At Wide apertures (Smaller F-Num), Lenses usually have light fall off at the corner portions. To avoid it photographers usually recommend selecting smaller F-Stop refers to the next (before or after) F-number. For ex, a polarizer results in a loss of about two f-stops” Stopping down refers to selecting a smaller aperture (bigger F-Num). For ex, To get good performance you should stop down to about f/8” Fast and Slow Lenses We now know that smaller the F-number, More the light that is allowed by a lens. This would mean that the photographer could select a faster shutter speed. For example, if the shutter speed is 1/30 of a second at an F Number of 4, then for an F-number of 2.8 (double light) it is sufficient to expose the film for 1/60th of a second. Lenses that have smaller F-number (less than are equal to 2.8) are generally referred as fast lens. This has meaning only when used during a comparison with another lens with same focal length but with smaller aperture. For example, one can say that a 300 mm f/2.8 lens is faster than a 300 mm f/5.6 lens.
Shutter speed:
Shutter speed is the duration of exposure of the film to light (image). Faster the shutter speed, less is the amount of light exposed to the film. Shutter speed coupled with the aperture is the primary controller of the final image. The shutter speeds range from 1/4000th second to 30 seconds. For the same aperture, at every decrease in the shutter speed, the light is doubled. For example a shutter speed 1/60th of second exposes the film for twice the duration as that of 1/120th of second for a given aperture. It is photographer’s decision to select the correct shutter speed. If the objective is to freeze the motion, then a faster shutter speed is preferred and on the other hand if the idea is to blur motion (like milky looking water falls) then a much slower shutter speed should be selected.
High Shutter Speed (Freeze)	Low Shutter Speed (Blur)
Metering:
Now that we have read about the important concepts of aperture and shutter speed, we need to know the criteria of selection. First and formats is the quality of light. All light are not created equal; Light at noon is different from that of the Light available during sunset. Metering is process of measuring the amount of light available to record an image on the media. This is done by either handheld meters or meters built into the camera. It is very important to understand which portion of the image needs to be recorded in detail. For example, imagine taking portrait of a person against bright sky as background. Here, metering the light available on the person’s face is the key, else the image will be recorded as a silhouette!. Most of the modern cameras have built in meter that offers different programs of metering. Depending on the conditions, the correct metering program should be selected. Most commonly available metering methods are the Center Weighted, Matrix and the Spot metering. Center Weighted In this metering technique, importance is given to the light available in the center portion of the frame than that of the edges of the frame. This is based on the assumption that the object of interest is closer to the center than the edges of the viewfinder. Most commonly available are the 60/40 or 80/20 center weighted metering. Matrix Metering This kind of metering is used for complex lighting conditions and the lighting calculation is done by sophisticated microcomputer in the camera body. Spot Metering This metering takes into account of the light in a very small portion of a frame (like 2 degrees). Imagine a nighttime shot of full moon, any of the above metering will result in a very slow shutter speed (meaning long exposure) since both center and matrix metering will result in very small overall light values requiring long exposures to properly (from it s point) expose the image. But, this will result in an overexposed (burnt out) and a streaked picture of the moon! For this situation spot metering is ideal where the light available in a very small portion is taken into account. When used properly, spot meter is an extremely valuable tool. Light meters are instruments used to measure light. Light meters can be either incident light meters or reflected light meters. Incident light meter calculates the light that is falling on a subject. So the photographer has to check the amount of light at various points of the subject to deciding on the aperture and shutter speed. Reflected light meter on the other hand measures light coming into the camera lens. This would mean one single reading that will be used to select the aperture, shutter speed combination.
Depth of field:
Depth of field refers to the portion that is in acceptable focus before and after the main point of focus. Note the emphasis on acceptable because what is acceptable at a smaller image size will not be acceptable at a larger size. Take a look at the following two pictures. The first one has the subject and only the subject in focus whereas the second image has everything from in focus. We can say that that first image has shallow depth of field and the second image has an infinite depth of field. Depth of field is a function of the aperture opening. More the opening, shallower is the depth of field. Less the aperture opening, deeper is the depth of field. Therefore for a given focal length (lets say 50mm) The depth of field decreases as we move from F Number 22 to F-number 1.8 because the diameter increases from 2.2mm to 27.7mm. On the other hand, for the same aperture (lets say F Num of 2.8) The depth of field increases as we move from a lens with focal length 300mm to a focal length of 50mm because the diameter decreases from 107.1mm to 17.8mm. An image with shallow depth of field will draw ones attention to the main object of focus (like portrait photo) and an image with infinite depth of field will be used in a story telling setup (like landscape or cityscape) Many cameras have a depth of field preview button to check the depth of field at a given aperture. Also, in manual focus lenses there are marking to calculate the depth of field at various apertures.	Shallow DOF High DOF
Film Speed (ISO):
The film speed is designated by its ISO number. The ISO number corresponds to the sensitivity of the emulsion (film ) or ccd. The ISO numbers are designated as 50, 100, 200, 400, 800 etc. Basically, a Film of ISO 200 is twice as sensitive to light as a film of ISO 100 and four times as sensitive to light as film of ISO 50. Then you might ask, why would any one use the film with lower ISO. The answer lies in the fact that the size of light sensitive grain increases as the ISO increases and the resulting image has a grainier (or coarser) look. The grain size will limit the size to which the image can be printed. So, for large printings, a film with small grain size is preferred. The idea is to select the smallest grain sized film for a given lighting condition. The ISO numbers also apply to digital camera as the sensitivity of the CCD is varied with the ISO number selected by the user. Films such as fuji velvia are rated at ISO 50 and have very fine grains resulting in very sharp images.
Exposure:
Exposure refers to a photograph! Yup. That is it! This term is used in a very loose sense. Exposure is the also the process of judging the combination of aperture, shutter speed and film speed to get the desired effect. This is the control that the photographer can use creatively to produce strikingly different results of a same subject. Think about a condition of photographing a waterfall using ISO 100 Film using in-camera meter in center weighted mode Aperture F/2.8, Shutter speed of 1/1000 sec for correct exposure is equivalent to using F/32 at a speed of 1/8 sec, but the effect will be completely different due to slow shutter speed.
Distortion:
Barrel Distortion Barrel distortion is when horizontal or vertical straight lines bulge outwards like a barrel. This type of distortion is usually found in wide-angle focal lengths. Pincushion Pincushion is a type of distortion where horizontal or vertical straight lines bend inwards. This type of distortion is usually found in telephoto focal lengths.	Barrel Distortion Pincushion Distortion
Bokeh:
Bokeh is the term that is used to refer to the out of focus areas of an image. If the out of focus portion is well blended so as to not take attention away from the main object of focus, then it is called a smooth bokeh. If the out of focus portion has distinct pattern so as to irritate (subjectively) the viewer, then it is called a harsh bokeh. Bokeh is very subjective and there are always debater on the quality of bokeh of a given lens (when there is nothing better to talk about other characteristics! :-p )	Bokeh
Vignette:
Vignette refers to the gradual light fall off around edges of a photograph. Vignetting can occur due to Excessive light fall off at wide apertures and due to stacking of multiple filters on a lens. The reason for this effect is that at wide aperture, the opening at the other end of the lens blocks a part of the incident light. This problem can usually be mitigated By stopping down couple of apertures.	Vignette

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