Amphibico Phenom flip down issues

For those of use who use the Amphibico Phenom housing, some of us shoot macro and wide angle.

The Phenom can be equipped with up to two flip down lens holders. One for a magnification diopter, one for a color correction or other filter.

If you white balance properly against a blue/green slate, you shouldn't need the color correction arm.

As to the magnification diopter on the 2nd arm, there are issues with that. A +2 diopter seems to be a popular diopter. Many including myself have used a 67mm set Hoya glass + 2 diopter. These come in a set of 3 at +1, +2 & +4.

Take the +2 or whatever you decide and remove the locking retaining ring from the larger ring and drop the glass element out. You will have to either freehand diamond grind the diameter slightly to allow it to fit into the Phenom filter ring, or take it to an optician whose lab guy is not lazy and they can set a stock circle hoop that is 2.500" on their internal frame wire tracer and it will transfer the dimensions to the correlator and grind your lens to that size, which is the size you want. You an go up to .010 smaller, but not larger than 2.500"

Now one of the issues with the Phenom arms is the detent system designed to hold the diopter arm at either end of its travel is not designed to handle the weight of a thick glass diopter. There have been times when I wasn't paying attention and my +2 diopter has sagged and I blew the footage when I didn't catch it. Swimming and generally lightly jostling the housing along in the current can cause the diopter arm to sag. DO NOT try to increase the spring tension via further compressing the detent ball tension spring-it just flexes the arm.

To date my habit is when I am doing macro with the diopter in place, when I need the shot, at the beginning I reach forward and give a light push to make sure it is at the end of its upwards travel.

Now for those that want the versatility of moving a diopter arm in and out of the field, there are other solutions better than the above.

As mentioned, part of the problem of using diopter is the weight of the glass on the limited diopter arm retention mechanism. There are two things you can do.

On the Phenom, there is an allen head small screw in the 2 o' position clock upper right position above the diopter flip down arm. The position of this screw is such that if you bisected it and drew a straight line down past the pivot point of the flip down diopter and to the bottom of the two arm yokes that attach to the plastic ring that holds the diopter, you will see that that it is close to centerline. Now move the flip down arm all the way the other direction, and redraw your imaginary line from that upper right 2 o'clock screw and you'll see that the imaginary straight line from that screw to the lower yoke of the diopter arm is now above the flip arm pivot point.

What this all does for you is gives you the ability to take a very fine stainless steel spring and attach it to a small peice of sheet metal angled downwards secured behind that allen screw in the 2 o'clock position and attach it to the bottom of the two yoke arms that form the flip arm. Now move the flip arm in both directions, all the way up, and all the way down, and you'll notice that in both positions, the spring is past the centerline of the flip arm pivot. In engineering we call this an over-center lock.

But there is an easier way if you don't want to spring for a $2 spring. The issue with a +2 diopter is really the weight of the glass diopter. Now go back to that optician, tell his lab guy that he is not really lazy after all, that he is a real lady's man and a gentleman's gentleman and Sean Connery, Mel Gibson and Brad Pitt had better watch out, and ask him to grab the blank magnification diopters that they use for reading eyeglasses. These are optically excellent plastic, and they weigh half what the glass ones do. Now have him place the stock 2.500" circle on there tracer, and you now have a lightweight diopter that won't sag. Another advantage of using these blank eyeglass diopter discs is they are available in .25 increments. So if you want to go +2.5, +2.75, +3.25, no problem.

Now since you DID read my other post on how to white balance and throw away the red color correction filter arm, you now just freed up space so you can go to a diopter thicker than +2 by getting rid of the 2nd arm. Since the arm is part of the pivot shaft retention system that prevents the pivot shaft from egressing out of the housing and letting salt water come in & soothingly bath your $4k HD cam electronics in a delicious salt water medley, MAKE SURE, you either be el cheapo and make a small rigid plastic disc with a hole in it and put a screw threw the plastic disc and into the now unused flip arm pivot shaft, or just break down and splurge for the $5 or so for a stop kit from Amphibico. Or if you are a thrill seeker and have always wondered what it would be like to potentially give your videocam a salt water jacuzzi, then go ahead and see if maybe that shaft will get pulled out unwittingly in that jostling small skiff or rib and forget to visually check it!

Now the plastic diopters won't have the scratch resistance that glass will, but why would anything abrasive come in contact with mechanisms inside your housing?

But if you are the type who insists on glass electronic tubes rather than transistors and let's his wine breathe for 30 minutes, then go ahead and get a Schneider + 2 or +3 glass diopter for the $80 or so. These are pre-ground to exactly 2.500" and fit the Amphibico flip arms perfect, just remove the glass from the threaded black diopter carrier.

Lastly, before you choose any diopter, take the blank and look at a piece of newspaper print with it. Now angle your view around the periphery of the lens. Some poor quality magnification diopters have significant spherical aberration issues about the perimeter. I watch others' HD videos on video file sharing sites and I notice it right away. The Hoya or the Schneider don't have noticeable spherical perimeter aberration.

But if you are still that purist, remove all flips arms, disc retain them both or Amphibico sells a really inexpensive flip arm lever retention kit, decide above water that on the next dive you'll shoot macro only, and get a screw in Schneider aspherical diopter that uses two glass elements bonded. It is about 1/4" thick and has absolutely no visible spherical aberration.They are about $400 USD. A popular macro diopter is the 3.5, which will probably be my next aquisition.

For those who don't want to have to go take your Hoya diopter and have the o.d. ground, you can get a 67mm Schneider diopter that is a perfect fit with no grinding for about $80.