Diopters.........

[roflcopter]

Im going to be attempting macro for the first time. I will be using a canon 5d mark 3 and canon 100mm 2.8 L series lens. I've seen some people suggest a +2 or greater diopter at the end of the lens.

Pardon my ignorance, but does this have any benefit other than allowing me to get closer to my subject? The lens has a minimal focusing distance of 12 inches. Does this shorten that? Im slightly confused here.

Thanks for the help / advice!

 

[fmerkel]

That's basically what diopters do, allow you to get closer. That makes it work like a magnifier. They also cut out some light and decrease your depth of field.
At 100mm I'd say just start there and get used to the results. The diopter up if you want even more macro.

 

[KeithG]

it depends upon how macro you want to go. as per fmerkel i would suggest you start by trying to get "close ups" just using your current lens. maybe that is as macro as you want to go?

if you want to go super macro then you will need a diopter. i use a subsee +10 on my canon g16. very small depth of field. sometimes very frustrating to get a shot.

this artcle has some good background info (a google search will turn up a bunch more that are also good reads)

Super Macro Underwater Photography

 

[bvanant]

There are two ways to use diopters. Per the OP's original thread, you can put the diopter on the lens before it goes in the housing. This works well but you will not have infinity focus. Alternatively and far more widely practiced is to put the diopter on the outside of the housing (wet diopter) and then you can do both.
Bill

 

[Zac786]

I have a question on the subject of wet Diopter lenses. Why is wet diopter so much more expensive ($100+) than the ordinary use (dry) diopter ($!5+) of similar diameter? I have a housing with a thread of 67mm on front and I used my ordinary (dry) diopter lens of +8 on it underwater in a swimming pool and it worked fine. Am I missing something? If anything you would need more multi coating for the land use as air lens interface would reflect more light than the water lens interface. Also the lens metal attachment is aluminum and would not rust in water. I have used it a few time in the pool without any detrimental effect on the lens.

 

[fmerkel]

Glass underwater has almost the same refractive index as water. That means glass wet diopters pretty much don't work. You can use acrylic/plastic, but it's softer, scratches more easily and usually isn't of the same quality. Wet diopters are generally 2 surfaces with an air or oil center so the 'magnification' works.
Essentially, they are more complicated.

FWIW tried some cheap dry diopters on an Olympus XZ1. Lousy results.

 

[KeithG]

Wet diopters are more expensive because they have a sealed air space inside them. They need the air space in order to not have a magnification power loss underwater.

The following link has a great (simple) explanation about half way down - look for the Section called "Power Loss in Water"
Super Macro Underwater Photography - The Definitive Guide, Part 2B

Note that you can use simpler dry diopters underwater as long as they are mounted inside your housing. This gives them the needed lens to air interface.

 

[KeithG]

Glass underwater has almost the same refractive index as water. That means glass wet diopters pretty much don't work. You can use acrylic/plastic, but it's softer, scratches more easily and usually isn't of the same quality. Wet diopters are generally 2 surfaces with an air or oil center so the 'magnification' works.
Essentially, they are more complicated.

FWIW tried some cheap dry diopters on an Olympus XZ1. Lousy results.

I do not believe that swapping materials accomplishes anything as the refractive index of water, oil and acrylics are all about the same. I understand that the air gap is key.

List of refractive indices - Wikipedia

 

[bvanant]

True, the air gap is the key that is why they are expensive. you can of course put your diopter in the housing which can lead to some very nice shots but no infinity focus. I wonder what it would look like to fill the gap with some very high index materials like Ge. Do you get negative magnification?
Bill

 

[Zac786]

Thanks that explains a lot. I am going to try put a flat glass filter in front of the dry diopter lens to trap the air and experiment with it and see. Then it should work as a magnifier.