Focal lengths of macro wet lenses

[ChemProf]

I'm thinking about grabbing a macro lens for my housing and noted that the Inon UCL-165 (a +6 lens) gives a focal distance of 165mm from the lens to the object.

This kind of number isn't easy to find for other comparable lenses (e.g. the Fantasea UCL-05, and others).

I'm not up on the optical properties of lens, but wondered if that number would be the same for all +6 lenses?

Is there a general formula/idea which can be used to know these numbers, if the above is true?

Thanks!

 

[bvanant]

I think you are probably thinking about this somewhat incorrectly.
The focal length of a diopter is determined by its power (or vice versa) A lens of 4 diopters has a optical power of

.
So it has a focal length of

. The power of a combination of two or more thin lenses in contact is equal to the sum of the powers of the individual lenses.

What I THINK you want to know is about working distance and that depends on a bunch of other things.

What is it you explicitly want to know?
Bill

 

[ChemProf]

Ok, got what you have. From Inon's site:

"The UCL-165M67 is high magnification close-up lens providing 165mm focal length (+6 diopter) where 165mm shooting distance from lens surface when camera’s focus position is set to infinity. Using minimum focusing distance will shorten minimum focus distance allowing you to get closer to a subject enabling high magnification close-up photography."

Perhaps what they are saying is more clear to others than what I have initially interpreted it as being. Is the +6 saying that it is the equivalent of being 6 times closer (or bigger at the same distance)?

The working distance is indeed what I am trying to get at, so I have a feeling how far in front of the case+lens my subject will be. Additionally, I had the feeling that the above was suggesting that focus must be on infinity, but I'm not sure that makes sense...

 

[bvanant]

No a plus 6 diopter on a 100 mm lens will give you about a 1.5 increase in magnification with some significant decrease in working distance. There is no simple (that I know of) formula for minimum working distance of a lens diopter pair (the thin lens approximations all go away) but for some details look at this chart from Nauticam (The CMC Has Arrived) and you will see that the working distance is not in any way simply related to the CMC lens.

Bill

 

[Mellisa]

I think bvanant gave you excellent advice, however I can also advise to study free materials underwater photography tips on fixthephoto.com
Hope this helps you!