Nikon D90 - Digital Camera SLR Manuale operativo - Pagina 17

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For shallow camera pointing angles (see the 15 degree entry) the dimensions of each

photograph for a 50 mm lens focal length become very large (approximately 2240 feet

horizontal by 8900 feet perpendicular to the aircraft). Resolution is lower at the center of

the camera axis; it becomes significantly degraded near the edges of the photo.

Telephoto focal length settings (greater than 50 mm) provide increased resolution with

smaller ground-level coverage. See the 100 mm data entries. Where maximum detail is

required, use of a telephoto lens setting adds value.

Maintaining a Focal Length Setting

During route photography and aerial mapping it is important

that the lens focal length setting not change. There is no built-

in mechanism to lock the focal length setting at any value

other than 18 mm. The following photo shows a simple

solution.

Wide rubber band

Where overlapping photographs are required, the time between shutter-release must be

determined. A 25% photo overlap is a good choice and will be used as an example.

Horizontal picture dimension should have already been determined in the steps above. If

shooting parameters result in each photo nominally covering 800 feet at ground level,

25% overlap means that a photo should be taken every time the aircraft moves (100%-

25%) x 800 feet = 600 feet. At 90 knots, the aircraft moves 150 feet in one second.

Therefore, a photo must be taken every four seconds. Program the Multi Function Timer

Remote Control for four seconds.