Instruction manual
40
Three factors contribute to the depth of field available to you for any picture:
• The aperture of your lens is the first major factor affecting depth of field.
The smaller the aperture (higher the f-number), the greater the depth of
field will be.
• Focal length is a measure of your lens’s ability to magnify a scene. The
more you magnify your subject, the less depth of field you have available.
If you zoom in with a telephoto lens the depth of field drops dramatically.
• The distance from the subject determines how much depth of field you
can get in your scene. If you shoot a subject that is far away, the depth of
field will be much greater than it is for a subject close to the camera. In
aerial photography, where you are always far away from your subject, a
large region in front and behind the target will be in sharp focus.
Using depth of field you can isolate your subject by making it the only sharply
focused object in the frame. In aerial photography the only practical means you
have to accomplish this is by using optical zoom, but depth of field is almost a
moot point from > 1000' AGL.
3.5 Panoramas
There will be times when you want to shoot a target (e.g., a dam) that just
can’t be captured with a single photo. By shooting a series of overlapping photos,
you can use special stitching software to create panoramic photographs.
It is best to shoot with shorter focal lengths (e.g., “landscape” mode) as this
wider angle lets you shoot fewer frames, which is necessary when shooting from
a moving aircraft. Fewer frames also means you have less chance of making a
mistake, and fewer frames means less “seams” in your panorama (and more
seams increase the chance of more artifacts in your photo).
Fortunately for CAP purposes, you don’t have to worry about panning the
camera to get your shots – the movement of the aircraft does the work for you.
You just hold the camera still while the target passes before your lens, taking
photos with the proper overlap. Most stitching software recommends a 15-30%
overlap between photos. [Some cameras have a panoramic assist mode that will
give you on-screen cues as to how much you need to overlap.]
There are many variables to consider when shooting photos to stitch into a
panoramic image, so practice is essential. One task you must master is how to
take a shot while looking ahead to where you’ll take your next shot, in order to
obtain the proper overlap between photos. This usually requires a planning pass
by the target, which allows you to determine how many shots you’ll need and look
for markers that will assist in obtaining the proper overlap between shots.
Another task to practice is controlling exposure if the scene isn’t evenly lit
(i.e., there are fairly dramatic changes in lighting between one frame and the
next). If your camera has an exposure lock feature, you can lock your exposure
after the first frame to ensure subsequent shots are exposed with the same values
(a panoramic assist mode does this for you).