Wetmate Question

I have a wetmate lens that fits on the front of my Ikelite ports for the 60 mm and 105 lens.

I realize the Wetmate decreases working distance.

I was pondering the Depth of Field chart that comes with the 105VR and it dawned on my that perhaps the real reason so many of you like the Wetmate is DOF and not working distance (as I never saw the grand advantage of that).

At 1 foot distance one gets a 1/1.0 reproduction ratio (forgetting about the crop factor, does that give a > 1/1.0 ratio?). At f/32 the depth of field is from

1 ft 5/16th inch to 1 ft 8/16 inch (only 3/16th of inch in focus).

At 1.5 foot distance one gets a 1/2.6 reproduction ratio and the DOF is from 1 foot 5 10/16th inch to 1 foot 6 6/16 inch (12/16 inch DOF).

Sooo, if one backs away from 1 to 1.5 feet the reproduction ration falls from 1/1.0 to 1/2.6 but the DOF increases from 3/16th to 12/16th on an inch).

Now here is my question. Does the wetmate allow one to close increase the magnification yet maintain the increased depth of field?

For example, if one set the lens for a 1.5ft focus (12/16th depth of field) and put on the wetmate, does one get improved magnification AND maintain the improved DOF?

Sorry if the question is confusing, but suddenly I think I see the advantage of the Wetmate but can't do the math in my head and must ask ye gurus here.

Steve Dingeldein

Well I've studied optics and still can't answer your question. The advantage of the wetmate is that you can double your magnification. Both the 60 and 105 Nikons will magnify to 1:1. By halving the working distance you go to 2:1 magnification.

My brain can't think through the optics to tell you if you maintain your depth of field. Just as a guess I would say if you double the magnification you would decrease the depth of field by half. Meaning that you would have to use a microscope type optics to get magnification and maintain a certain depth of field.

Thanks Dave. I'm an ophthalmologist but I studied optics too long ago to remember.

SD

Actually I forgot one critical thing. The wetmate is not a simple diopter. It is a sealed multi-optics system. Meaning it has more than one lens in it. I have no idea how that affects your question. Probably makes is necessary to have the optical diagram of the wetmate and a sophisticated computer optical program. The optics of the wet mate may be indeed technically those of a microscope.

Unless Backscatter has calculated out the answer to the question of depth of field with the wetmate I suspect you will have to do some practical tests to determine that to your satisfaction.

Hmmm. I'm all for practical testing. Now where shall I go? Bonaire, Little Cayman or the Y Pool.

Thanks for your thoughts. Perhaps the only advantage is that you get increased magnification.

My simplistic thinking is that it would (as a plus diopter) pull the near focus and far focus point forward leaving the spread the same or with an effect on DOF not in proportion to the magnification, though I know it isn't close to that simple.

So most photographers use the Wetmate to get better than 1:1 reproduction.

SD

Well I'm leaving for Sipidan on Sat. I will bring my wetmate with me.

Actually I forgot one critical thing. The wetmate is not a simple diopter. It is a sealed multi-optics system. Meaning it has more than one lens in it. I have no idea how that affects your question.

I don't think it does affect the question; the multi optics are just to make it achromatic. It can be treated as a simple diopter for simple calculations.

When you work at great magnifications (around lifesize) the lens

don't determine DOF.

It's the same with the 60mm, with the 105mm

and with or without dioptre lenses.

Only the bokeh it's affected by this.

BTW,

the magnification of a dioptre lens is different

depending the prime lense that you are using.

Magnification is higher on longer focals.

With a +3lens you obtain 1,3x on a 105mm

and 1,75x on a 200mm lens.