Read Part 5 here.

A part serialisation of a book by Jeff Goodman

PART 6: Lenses, Depth of Field, Angle of Coverage

I guess one could write a whole book on lenses, their applications and relationship with exposure i.e. shutter speeds and aperture (f stops) and depth of field, but for the sake of this guide I will just cover the basics. And I do mean the basics.

A rule of thumb with taking a lens underwater is that because of the different refractive index of water to air, the angle of coverage of the lens will decrease by 1/3rd as soon as it is submerged, as the diagram shows.

B

A

A 90 degree lens (A) on land is only going to cover 60 degrees (B) underwater. So what are the implications of this?

Because of the nature of filming underwater and the probability of there being some sort of debris affecting visibility, then it is generally preferable to get as close to the subject as possible to get a good crisp, clear picture. Hence the general use of wide angle lenses rather than standard or telephoto.

What has this to do with focus and depth of field? First of all what is depth of field?  Looking at the diagram we see a telephoto lens focused on an object at a distance of say 3 metres. Taking an average exposure and thinking purely in generic terms, an object (A) placed say 3 metres away when focused upon will of course be sharp. The area marked in front and behind the object will also be sharp. So the depth of field is taken as the distance between points B and C. Everything outside of that area will be out of focus.                     

Telephoto lens

With a standard lens you can see that the depth of field, with all other factors being equal, will be far greater.

Standard Lens

Wide Angle lens

With a wide angle lens the depth of field is greater still and will probably be from the lens front to infinity. This does vary depending where the focal point A is in the first place and the f stop of the lens, but generally the wider the lens the greater the angle of coverage and the greater the depth of field.

So for underwater work a wide angle lens is greatly preferable to any other as it allows you the possibility of getting close to your subject and maintaining a good depth of field.

Please do look up ‘depth of field’ if you want more information on how this all works, but generally, depth of field is governed by the aperture (or f-stop) of the lens and this aperture is directly affected by the shutter speed, which in turn is affected by the gain. The slower the shutter speed the smaller the aperture of the lens can be and the smaller the aperture the greater the depth of field. But, if your shutter speed becomes too slow, say less than 1/50th of a second then your video images may start to take on a blurry look. Ideally a 1/50th to 1/60th is good. Auto exposure will balance these three items against each other to give you the best overall exposure.

Higher shutter speeds are good for capturing fast action with nice sharp results, but the penalty you pay is the overall loss of light through the lens. This means that the aperture will have to be larger to let in more light to give a good exposure and that in turn will reduce the depth of field.

Starting to get a little cross-eyed? Don’t worry about it unless you are going to take up videography in some professional way or you really want to know what is going on in your camera. Otherwise be content with simply adjusting the ‘exposure’ dial until the picture on the monitor looks good.

Next time we look at Flat and Dome Ports.