Q - Of what use are the depth-of-field scale markings on the lens of my 35mm camera?
A - It depends on the focal length of the lens. On a wide-angle lens it is a very useful focusing device; on a telephoto it's only an ornament.
You can test this for yourself by comparing a wide-angle, like a 24mm, with a telephoto, such as a 105mm.
Set a 15-foot distance on both lenses and use the 18th scale. You'll see right away that on the 24mm lens you're focus from five feet to infinity, while on the 105mm you're sharp on the near side somewhere between 12 and 15 feet and on the far side somewhere between 15 and 20 feet.
You can use the depth-of-field scale on your wide-angle lens as an aid in focusing.
FFor scenics, where you don't have a definite focus point, set the infinity loop within the far distance marking of the f-stop you're using and let the near side come where it may. You'll notice that the close distance will fall well within your close-focus needs.
This technique will make sure that your horizon is always sharp, as well as your foreground, leaving you time to concentrate on composition instead of trying to focus on a mass of foliage waving in the wind.
Snapshooting falls within the depth-of-field scale. Again, use a wide-angle lens and set your distance from the subject. For example, suppose you're taking pictures of people walking. Choose a distance that will give you the size image you want and set your depth-of-field scale so that this distance falls in the middle of the marks for the f-stop you're using.
When your subject comes within range, shoot. The depth of field will take care of distance errors. This pre-set way of focusing is much more accurate than trying to focus on a moving subject.
The depth-of-field marks can also be relied on for those in-focus-from-here-to forever pictures:
Suppose you want to take a picture of a bowl of fruit in the foreground and the orchard in the back, both equally sharp.
First focus on the fruit and see how close you are on the scale. Then set this close distance and the infinity mark within the depth-of-field scale is a way to focus by the numbers when you can't by eye.
Q - What do I do about that X-ray film surveillance at the airports?
A - There is certainly a lot more heat than light on this X-ray controversy.
I haven't yet heard of an authenticated case of X-ray damage to film while passing it through the devices used in the United States.
However, there is such a thing as a cumulative build-up over a number of passes. So, in any case, I would advise requesting a hand inspection.
The FAA says that this is your right and you can do so even if at times the surveillance teams try to convince you that no fogging danger exists.