Lecture Exposure/histograms

Exposure - Four Factors A camera is just a box with a hole in it. The correct exposure is determined by four factors: 1. How large this hole is (the aperture) 2. How long it stays open for (the shutter speed) 3. How quickly the camera reacts (the ISO) 4. How much light is reflected off the subject (the light level) Exposure - Four Factors A camera is just a box with a hole in it. The correct exposure is determined by four factors: 1. How large this hole is (the aperture) 2. How long it stays open for (the shutter speed) 3. How quickly the camera reacts (the ISO) 4. How much light is reflected off the subject (the light level)

Aperture Inside the lens is an adjustable device, the diaphragm, which alters the size of the opening - the aperture. Changing the aperture is useful as it has a handy side-effect. A very small aperture makes everything (background and foreground) in focus. A large aperture makes only the subject you're focused on in focus. Try squinting your eyes (everything is in focus) and then opening them wide (some things are blurry).

F-stop

F8, F5.6, F2.8

Which has F22?

Shutter Inside the camera is a mechanism (a shutter) which controls how long light is allowed to act on the film. When you take a photograph (by pressing the shutter-release), the shutter opens and then closes a fraction of a second later. How quickly this is done is called the shutter speed and is measured in seconds (shortened to s). A fast shutter speed (say 1/500th of a second) is good for action shots as it freezes movement. A slow shutter speed (say 1/60s) blurs moving objects, which is useful when you want to emphasize movement and speed.

Fast shutter Capture details, the motion of objects.

Slow shutter Freezes a moment, blurs objects, need a tripod. (Six hours lens was open in sierras) Freezes a moment, blurs objects, need a tripod. (Six hours lens was open in sierras)

Depth of field Depth of field is the range of distance around the focal plane which is acceptably sharp. The depth of field varies depending on camera type, aperture and focusing distance, although print size and viewing distance can influence our perception of it.

Low, medium, high depth of field Shows everything in sharp focus (high) Shows some parts in focus, some blurry (low)

Which is which? low depth of field High depth of field

Three things that control good exposure: 1. Aperture 2. Shutter 3. Light 1. Aperture 2. Shutter 3. Light

What is noise? Noise is the digital equivalent of film grain. It can even look like grain, though more often it looks more like ugly speckles or color artifacts. It results from a variety of sources, including sampling errors in pixels, temperature-induced "dark current" in sensor elements, and signal amplification circuits.

Histogram Possibly the most useful tool available in digital photography is the histogram. It could also well be the least understood. Virtually every digital camera, from the simplest point-and-shoot to the most sophisticated digital SLR has the ability to display a histogram directly, or more usually superimposed upon the image just taken. On most cameras though the histogram display takes place on the rear LCD screen, and most cameras can be programmed to do this both on the image that is displayed immediately after a shot is taken, or later when frames are being reviewed. Possibly the most useful tool available in digital photography is the histogram. It could also well be the least understood. Virtually every digital camera, from the simplest point-and-shoot to the most sophisticated digital SLR has the ability to display a histogram directly, or more usually superimposed upon the image just taken. On most cameras though the histogram display takes place on the rear LCD screen, and most cameras can be programmed to do this both on the image that is displayed immediately after a shot is taken, or later when frames are being reviewed.

For more information : ncolour.com/tutorials/ histograms1.htm

Histogram This is where the histogram comes in. It is a simple graph that displays where all of the brightness levels contained in the scene are found, from the darkest to the brightest. These values are arrayed across the bottom of the graph from left (darkest) to right (brightest). The vertical axis (the height of points on the graph) shows how much of the image is found at any particular brightness level.

Low key image with the majority of pixels to the left of center of the graph

High key image with the majority of the pixels to the right of center of the graph

Examples

Tonal Range

A broad range of values without any clipping on either end.

This histogram is spiked at both ends. It should be obvious on the white end -- the sun is burned out, but the foreground black is clipped as well.

The falcon histogram is shifted way to the right

You will see a histogram in photoshop too