# Lens Focal length - Technical

#### Flynnstone

##### macrumors 65816
Original poster
How do I figure out what focal length and f stop?

Things I know : (or will know)
- distance to object
- depth of focus
- angle of view ( is this diagonal? )
- CCD or CMOS sensor size
- underwater

How do I determine what lens I need?

I think if I know the focal length and the depth of focus, then I figure the f stop. Then I can determine how much light I need.
Can anyone point me in the right direction?

#### Father Jack

##### macrumors 68020
Are you really asking about "Depth of Focus" or do you mean "Depth of Field"?

The focal length you require depends on how much of the scene you want in your picture.

The f stop you need will depend on the depth of field you require.

#### Flynnstone

##### macrumors 65816
Original poster
What is the difference between "depth of field" and "depth of focus"?

#### Father Jack

##### macrumors 68020
What is the difference between "depth of field" and "depth of focus"?
Depth of field is the zone of sharpness in a photograph. It's controlled by four factors:
1. Focal length of the lens
2. The aperture in use.
3. The distance between the lens and the subject.
4. The maximum permissible size of the "circles of confusion"

Depth of focus is to do with the tolerance of placement of the image plane (the film plane in a camera) in relation to the lens. In other words how much the film (or sensor) can be out of alignment from the focal point while still able to produce an exceptably sharp image.

Many photographers seem to think the two are the same, but this is definitely not the case ..

#### ChrisA

##### macrumors G4
How do I figure out what focal length and f stop?

Things I know : (or will know)
......
- underwater

How do I determine what lens I need?
The under water part "trumps" everything. The above water rules still apply but the environment is so different. First off air is mostly clear. Water is not. Water selectively filters light, removing the reds first.

The #1 rule in UW photography is get close. The #2 rule is "get closer". #3 is "See rule #1" Now, what you need if you are to follow these rules is a very wide angle lens, the wider the better. In general being more then a few feet from the subject is to far, 18 to 24 inches is reasonable. THe exception to neededing a wide angle lens is macro. where you are four to five inches away

Light is the other issue. You really do need a light of some kind, UW strobes are expensive but on-camera flash is fare worse UW then on land. What happens is the stobe lights up all the little "floaties" in the water and the image is filled with white specks. Move the light over to the side and this s greatly reduced.

OK Direct answer. Here is how to compute what you need: Pretend the llens is a pin hole one focal length away from the sensor (say 100mm). Assume the sensor is 24mm wide. Make a diagram with a dot as the pin hole 100mm above a 24mm long line. Now connect the ends of the line to the dot to make a 24mm wide by 100mm tall triangle. The angle at the apex is the same as the field of view. You caqn measure the angle or use trig.

#### compuwar

##### macrumors 601
Depth of field is the zone of sharpness in a photograph. It's controlled by four factors:
If you're going to be nit-picky, it's "apparent zone of sharpness" as there's a plane of focus and then the zone around it which isn't exactly in focus but appears to be.

#### Flynnstone

##### macrumors 65816
Original poster
Light is the other issue. You really do need a light of some kind, UW strobes are expensive but on-camera flash is fare worse UW then on land. What happens is the stobe lights up all the little "floaties" in the water and the image is filled with white specks. Move the light over to the side and this s greatly reduced.

OK Direct answer. Here is how to compute what you need: Pretend the llens is a pin hole one focal length away from the sensor (say 100mm). Assume the sensor is 24mm wide. Make a diagram with a dot as the pin hole 100mm above a 24mm long line. Now connect the ends of the line to the dot to make a 24mm wide by 100mm tall triangle. The angle at the apex is the same as the field of view. You caqn measure the angle or use trig.
Distance somewhere between 2 to 6 feet.
Will light up as required.

#### sjl

##### macrumors 6502
Distance somewhere between 2 to 6 feet.
Will light up as required.
Nothing in this equation changes when you add water, but the equation itself changes subtly.

Point one: the refractive index of water is different to the refractive index of air. So you'll get refraction at the water-air interface, meaning that the object will appear to be closer than it actually is. It also means that, unless the lens port is carefully designed, you'll get artifacts in the shot - this is why you'll see a flat port when the photographer is doing macro shots, and a dome port (think of a sphere cut down the middle as a pretty close indicator) for wide angle shots (at least with SLRs; compact cameras are a different story, because the port is custom designed to suit the lens. In theory, anyway.)

Point two: as has been stated already in this thread, lighting becomes more of an issue. You lose the red, orange, yellow, and green part of the spectrum progressively as you get deeper, so you need flood fill to replace them in the shot ... but that introduces the issue of backscatter, which is only really solvable to any degree by using strobes located a fair distance away from the lens.

You already have the first three rules of underwater photography. The fourth: you'll get better shots if you're shooting up than shooting straight ahead or down. I'm not entirely sure why this is so, but it matches my own experience.

#### Father Jack

##### macrumors 68020
If you're going to be nit-picky, it's "apparent zone of sharpness" as there's a plane of focus and then the zone around it which isn't exactly in focus but appears to be.
Quite so ...