cdj@mh3bs.UUCP (09/16/83)
On a recent scuba trip, I was loaned a Nikonos camera. Can anyone tell me whether the distance markings on the lens focus adjustment are for above surface or underwater measured camera-to-subject distance? (The owner didn't know and there was no manual.) Cris Johnson 3C412B Bell Labs Murray Hill, NJ
stevek@umcp-cs.UUCP (09/17/83)
Unless I am misreading it the NIKONOS lenses are marked for measured underwater distances. For example setting the lens at 4ft is the same as 3 apparent feet underwater. It makes little difference since you will almost always use a strobe and rely on the depth of field to be sure the subject is in this depth of field range. My reference is "The Nikonos Book" by Jim and Cathy Church. Stephen Kogge Dept. of Comp. Sci. Univ. of Md.
gee@ihuxa.UUCP (09/19/83)
The lenses for the Nikonos camera are all marked in what should be considered apparent distances. All one needs to do is set the focus at the distance the object appears to be at. Things do appear closer underwater than they really are, but remember that if you see it closer, then the camera will see it as being closer (the camera lense woks the same way as your eye) (oops that is 'works' not 'woks'). The reason for the object appearing closer is that the light reflected off the object has to travel through 3 mediums (water,glass,air) and due to the differences of refraction there is magnification of the object, which in turn not only makes the object seem closer but much larger. As a guide the following may help: an object that appears 4 ft away is actually 6 ft away -> a 2/3 change in distance, also an object that is 4 ft long appears to be 5 ft long -> a 25% magnification. As stated by someone else if you use a strobe you can be helped out a little by the depth of field of the lense, but another problem to think about is: if the object appears to be 4 feet away you focus at 4 feet but you MUST aim the strobe at the actual position of the object. Most strobes when used underwater do not cover a wide enough angle to illuminate the entire area being photographed (this is a greater problem with the 28 mm and the 15 mm lenses than with the 35 mm). Also a strobe is used mainly to add to the color spectrum rather than just providing more light (the color red is absorbed by the water, so the red part of the spectrum is missing at depths greater than 10 feet) this fact also makes strobes non-effective at camera to object distances greater than 6 feet (the light from the strobe travels 6 ft to the object and then 6 ft back to the camera for a total of 12 ft - in this 12 feet the red spectrum again is filtered out). To get your best pictures you need to get as close to the object as possible. The above measurements for change in distance and measurement are approximations (I do not remember the actual ratios, but its close), I have found that most people have a hard time estimating distance underwater, just remember to focus at the distance the object appears to be.