Basic Photography concepts

Aperture and f-numbers

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. You can calculate an f-number, if you are keen or don't have much of a life, by dividing the lens focal length by the diameter of the aperture. 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. A typical aperture range may look like this:

f 1.4; f 2; f 2.8; f 4; f5.6; f 8; f 11; f 16; f 22; f 32

There are smaller and larger f-numbers but the actual numbers used are always the same and will maintain a constant value over different lens focal lengths. This just means that f-8, for instance , will always pass the same amount of light no matter what camera or lens you may be using. Similarly, f-16 will pass half as much light as f-11 and f-4 will pass twice as much as f-5.6. The difference in value between one full f-number and the next is known as a 'stop'. If you change aperture from f-8 to f-5.6 you will give your film one stop more exposure. The smaller the f-number is then the larger the aperture is and the more light it will pass. The f-number is also used as a guide to the light gathering abilities of a lens. Lenses with large maximum apertures ( small f-number ) are described as being 'fast'.

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. Like the aperture, shutter values or 'speeds' follow a standard sequence with each one being half that of the next, allowing half as much light to pass through. A typical shutter speed range may look like this;

1sec; 1/2sec; 1/4sec; 1/8th; 1/ 15th; 1/30th; 1/60th; 1/125th; 1/250th; 1/500th; 1/1000th; 1/2000th

Example

Lets say your camera is set to f-8 at 1/125th of a second. The picture looks good but you decide that you want to change the lens and have your choice of an f-11 or f-5.6 (fixed focal length).

You could reduce the aperture by one stop to f-11 (Stop down or close down). Now your film is receiving half as much light as it requires (underexposure). To compensate for this you select a slower shutter speed of 1/60th of a second so it now stays open twice as long as before and passes twice as much light as before. Or. You could increase the aperture by one stop to f-5.6 (Open up). Now your film is receiving twice as much light as it requires (overexposure ). To compensate for this you increase your shutter speed to 1/250th of a second so it now stays open for half as long as before and passes half as much light as before.

Strobes

Strobe Placement

Best pracitce to obtain the best image quality is in light placement. (Remember you can't take a picture of what you can't see.) We should not only light the subject, but the background as well; the background lighting gives depth to the subject.

The best results have come when using more than two strobes per subject. This allows us to aim two strobes

toward the subject, and use an additional strobe or two to light the area around/behind the subject to gain depth.

Strobe Adjustment

The strobes require careful ajdustment of the output power. We want to insure our subject is well-lit but without washing out detail.

The strobe software allows control of the two outputs, where each output has four channels. The output corresponds to a bank of strobes (there are two physical banks per strobe box). The channels correspond to (what do they correspond to?! There are two strobes per output, so 4 channels means that each strobe has 2 channels?)

ViDAQ Data Acquisition

Login to vidaq servers & Configure Acquistion Settings

Remote Desktop into the vidaq servers that are connected to your high speed cameras(the camera cables are labeled): - vidaq-m1.cgm.ucdavis.edu - vidaq-s1a.cgm.ucdavis.edu - vidaq-m2.cgm.ucdavis.edu - vidaq- s2b.cgm.ucdavis.edu

Launch the CaptureLR3 application on each server (should be located on the desktop)

Make sure the Live button is depressed

Set your desired frame rate (FPS) by clicking the numeric FPS indicator. From the pull down menu select 210 FPS 640x480 for maximum capture rate & resolution in "free-mode".

Set your shutter speed to increase the amount of time that light is exposed to each frame.

By default if setting to the maximum frame rate, the shutter will be 1/200fps.

Click on the Options button and select the recording during in seconds. Default should be 10 seconds. Select the following options: Live Display During Record Enabled and Local Time. Click the OK button when finished

On the main screen, select the desired trigger mode in percent. Default should be 10%.

Once you set your gamma/gains/etc for desired image, click the Record button.

Save Data

Click the Save button

Select OSM from the the Save as Type pull down menu

Format the file name with <vidaq-number>_<mmddyyyy>@<spin_start_time>@<event_time>@<# of rpm>rpm EX: m1_03112008@102000@113000@70.3rpm

Save the file in a local researchers project folder(create one if necessary under the D: drive EX: D:\LJD01