vibrio
Banned
Registered: 28th Feb 01
Location: POAH
User status: Offline
|
NOTE - I'll put in my own pictures for the public version as I have used other peoples photos without asking 
Cameras share some common features:
a lens,
a light tight body,
a light sensitive medium (film or sensor) to capture the image.
a lens aperture which controls the amount of light reaching the medium
a shutter which opens and closes to allow light to act for a specified time
a viewfinder or screen on which you can see the picture you will take
there are 2 functions on the camera that you use to take a photograph
Aperture
The aperture is just a hole whose size can be varied to allow more or less light to pass through it. The size of apertures are expressed in f-numbers.
The range of f-numbers follows a standard sequence with each f-number being half as bright, passing half as much light, as the previous one.
f 1.4; f 2; f 2.8; f 4; f5.6; f 8; f 11; f 16; f 22; f 32
The difference in value between one full f-number and the next is known as a 'stop'
Shutter and Shutter Speeds
The shutter prevents light from reaching the film until the moment of exposure, when it opens for a predetermined time allowing light passing through the lens aperture to reach the film. Unlike the aperture, which is always in an open position the shutter is always closed. Shutter speeds are expressed in seconds or fractions of a second
1sec; 1/2sec; 1/4sec; 1/8th; 1/ 15th; 1/30th; 1/60th; 1/125th;
1/250th; 1/500th; 1/1000th; 1/2000th
As you may have worked out, changing from one shutter speed to the next changes the exposure by one 'stop' in much the same way as changing the aperture.
F2.8 @ 1/500s
f4 @ 1/250s
f5.6 @ 1/125s
Most camera's have Shooting modes which allow you to change the aperture (Aperture priority) or shutter speed (shutter priority) and the camera will meter the light and set the shutter speed (if in AV) or aperture (if in TV). There will also be a full manual control on most too along with a full auto or Program AE mode. Auto is a purely point and click mode which allows no adjusment with Program AE usually allowing small adjusment. Most of the time I use AV or TV depending on the type of photo I am taking (more of This later).
ISO speed
On digital camera's this ISO speed is the sensitivity of the sensor.
100 200 400 800 1600 3200 for my 20D
When in AV mode increasing the ISO from 100 to 200 increases the shutter speed by 1 stop
ISO 100 = f2.8 @ 1/100s
ISO 200 = f2.8 @ 1/200s
when in TV mode you increase the aperture by one stop when you up from 100 to 200 iso
ISO 100 = 1/100s @f2.8
ISO 200 = 1/100s @ f4
if you increase from 100 to 400 you increase by 2 stops.
The down side to this is noise. Noise is like snow on a TV screen.
ISO 100
ISO 400
ISO 1600
Pictures courtesy of http://www.dpreview.com
Using these 3 functions allow the photographer to get the photograph he wants.
Control of aperture
To control aperture is to control depth of field (DOF). DOF is the area in focus. In this picture you can clearly see the DOF (the bt of carpet in focus) for the given aperture (f2.8). the blur in the background is called Bokeh
However this Picture was taken at a relatively close shooting distance. This close proximity to the target gives a shalow DOF. Taking a picture of an object further away increases the DOF at the given aperture and focal length (focal length is decribed in mm). The smaller the aperture (higher f number) the larger the DOF you'll get.
Small DOF is good as it makes your subject stand out
however particularly with closer (macro)shots you need a greater DOF to maintain the whole subject in focus. This particular shot was at f11 and you can see some of it is still not in focus
It all depends on where the subject is in relation to the plane of focus. a macro shot of a bug longways like above requires more DOF compared to a side ways macro of a bug.
Control of shutter
Shutter speed is important for blur and camera shake. if you are using a lens at 200mm but are using a low shutter speed (1/60s) your more than likely to get camera shake and a bluring image.
The rule of thumb is to use at least 1/focal length as a shutter speed. e.g 200mm focal length min shutter speed 1/200s.
Shutter speed is used to control movement. A high shutter speed will capture a fast moving object and make it look sationary.
while slow shutter speed will induce background blur while hopefully capturing the target.
Slow shutter speed (TV mode 1/20s)and moving at same speed as target.
slow shutter speed (TV mode 1/50s target moving at 20mph) and panning (follwing target with camera)
Slow shutter speed can also be used to make light trails
or make water look sliky
Expsure compensation
Most people do not have hand held light meter to take a reading (meter) of the subject and set the camera manually to get the correct exposure they want. Most camera have various types of metering modes which allow you to meter the light to give different exposures.
Evaluative - This mode takes the average from several different points ( my 20D has 35).
Partial - on most camera this mode takes the meter reading from the center of the view screen (on my 20D it is 9%)
Center-weighted average - this averages out the light reading from around the center of the view screen. I reads more than partial.
Spot - this takes a reading from a particular spot which is usualy around 2-4% in size compared to the view screen.
However the camera can be fooled when if there is a particular bright or dark portion of the composition. This can cause either overexposure (bright) or underexposed (dark) image. This can be used in either TV or AV mode. In TV mode the shutter speed stays the same with the aperture increasing or decreasing and in AV mode the aperture remains constant with shutter speed increasing or decreasing. On my 20D I have 2 stops of compension +/- from the meter reading and can be adjusted in 1/2 or 1/3 stops.
Exposure compensation is required when shooting snow.
No compensation 
+2/3 stop 
+1 1/3 stop 
White balance
White balance [WB] is the colour temperature of the image and is measured in kelvin (k). The adjustment of white balance is required depending on the light source which is illuminating your subject. the sun has a different value compared to a flash or indoor strip lighting. Most cameras will have basic settings (photo shop CS RAW settings)
• Auto (3000 - 7000 K) (4900 for my pic)
• Daylight (5500 K)
• Shade (7500 K)
• Cloudy (6500 K)
• Tungsten (2850 K)
• Fluorescent (3800 K)
• Flash (5500 K)
some may have the ability to set a custom WB or input the kelvin value itself ( my 20D allows this).
[Edited on 18-05-2005 by vibrio]
|
