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750Z Housing

Underwater Camera Housing for the Pentax 750Z

Why make one when you can buy one?  Because, for the Pentax 750Z digital camera, you can’t buy one.

I have an old and unreliable Pentax 330 digital so I thought I’d start with a housing for that – call it a prototype – and then knock up something for the 750Z.  If the 330 got wet it was not going to be a big deal.

After looking at some commercial designs and some of the home made housings on the web I thought I’d start with a Perspex box with spring catches and a gasket on the back.  For a design specification all materials were to be immersed in 40 m of ocean which meant stuff on the outside of the housing needed to be plastic, rubber, brass or stainless steel.  All of the critical camera controls (on/off, focus & shutter, flash, zoom, and macro) needed to be accessible.

I tested the Pentax 330 camera by taking some photos through a piece of Perspex.  It worked fine without the flash, but with the flash on, the images were washed out by back-reflection from the Perspex.  This is the same effect that you observe with flash photography through a window.  I decide to use a block of Perspex as a light-pipe between the flash and the front of the housing to eliminate back reflection.

 

Figure 1.  That Darn Flash

 

The biggest difficulty that I envisaged was going to be the control seals.  The commercial housings use an O ring seal on a shaft through the case for rotary and press control.  I made two trial press control button in brass, one in a brass sleeve with two O ring seals, and one with a single O ring seal acting directly on the Perspex.  They both seemed to work okay when subject to the ‘suck’ test (putting one side in my mouth and sucking like crazy to detect leaks).  Given that the single seal design was easier to machine I decide to use this for the controls.

 

Figure 2.  Trial Control Seals

 

The housing was made from 10 mm thick Perspex with 5 mm internal clearance between the top, sides and back of the camera, and 3 mm clearance between the front of the lens and inside of the housing. The clearances were considered to be essential to ease getting the camera into and out of the housing, and to allow the housing to compress a little at depth without crushing the camera.  The housing was glued together using Acrifix 192.  The laminated back was glued together using Araldite Ultra-Clear epoxy.  The housing construction is detailed later.  I used Perspex blocks attached to the front and left-hand side of the housing to position the camera.  The blocks were cut to size and glued in place with epoxy.  I used trimmed down self-adhesive rubber feet (purchased from Jaycar Electronics) on the top, back and right-hand side of the camera housing to ensure that the camera was held snugly in the closed housing.  The gasket between the removable back and the front of the housing was cut from 1 mm thick EDM rubber sheet.  I made a Perspex block as a light pipe between the flash and the front of the housing.

I tested mounting of the catches on some scraps of Perspex.  They would certainly hold the back plate in place but the closing tension was variable and not well distributed around the gasket.

With the housing case made, but with without the controls or catches, it was into the kitchen sink for a water tightness test.  At 200 mm deep the darn housing kept on leaking at the corners of the gasket, even though I was pressing hard on the back of the housing where the catches were to be fitted.

I tried a range of different types (softness) and thickness of rubber but they all leaked.  The bloke at Para Rubber (the rubber shop) suggested that I used closed cell foam, but it was only available in sheets 3 mm thick and that would not fit with the case that I had made.

No matter how carefully I finished and polished the sealing edges, the catches were not going to provide a reliable seal when diving shallow (which tends to happen on the way down and on the way up).  At greater depth the gasket, like properly made O ring seals, should become self-sealing because the increased pressure acts to squish everything together.

Hmm...  I needed even pressure all around the gasket.  I decided it was less important to be able to quickly whip the back off the housing with spring catches than achieve a decent seal.  Lets face it, how quickly do you need to access the camera when you’ve got decompression time to elapse between dives.  Plan B was to use wing nuts on studs around the perimeter of the back of the camera.  I fitted eight 3 mm diameter machine screws with nuts and washers around the edge of the housing and retested in the sink. Success!  The box was watertight.

 

Figure 3.  The Pentax 330 Prototype Housing

 

The next test was with the camera mounted in the front of the housing (with the back off and high and dry) and manipulation of the controls with a piece of bent wire.  The camera worked fine without the flash.  But with the flash turned on back-reflection was a real problem.  I decide to mask everything but the lens opening with cardboard, and paint the outside of the flash light-pipe to limit light transmission to other parts of the housing.  Alas, before I could complete this experiment, the Pentax 330 suddenly decided to stop working again.  I did not want to be faced with the underwater photo opportunity of a lifetime and have my camera clap-out on me.  It was time to start on the 750Z housing.

The 750Z is a larger camera than the 330 and has a longer lens.  A bigger housing was made out of 10 mm Perspex with 5 mm clearance between the top, the back and the right-hand side of the camera and the inside of the housing, and 3 mm clearance between the lens, at full extension, and the inside of the front of the housing.  In measuring the camera I failed to allow for the strap attachment point which meant that the camera would not have fitted in the finished case!   Fortunately I noticed this before the case was glued together and was able to mill a slot in the inside of the right-hand side of the housing to accommodate the offending strap attachment.

 

Figure 4.  The First Pentax 750Z Housing Complete

 

The only bothers with the case construction were laminating the Perspex on the back of the housing, getting perfect edge joins, excess glue, and scratches on the Perspex. I shall discuss each of these issues in more detail.

The back of the camera is made from a piece of 3 mm thick Perspex laminated to a piece of 10 mm thick Perspex to form a shoulder to hold the gasket in place under compression, and ensure that the back plate  locates centrally.  I decided to use Acrifix 192 adhesive for the laminating because it produces a stronger bond than epoxy (which I had used on the 330 case).  If air bubbles are carefully excluded when the sheets are laid together the resulting laminate is completely transparent.  The laminated backing looked great until I machined it to fit the front of the case.  Due to stress imparted in the machining (milling the edges to size) the laminate developed stress cracking in the adhesive layer.  Although these cracks did not assign the backing to the scrap bin, they detracted from an effort that I would be proud of (pride - one of the seven deadly sins) and also risked interfering with the clarity of the LCD camera display.

Gluing Perspex sheets together to form perfectly transparent (and hopefully strong) joints is still hit-and-miss with me.  I have tried numerous solvents and glues, including chloroform, Perspex dissolved in chloroform, acetic acid, cyanoacrylate adhesives, epoxies and proprietary Perspex adhesives, with varying results.  I truly wish that the people that work at the museums and art galleries of this world would share the secret of making display cases with us mere mortals.  After numerous experiments I have settled on using Acrifix 192 which is a proprietary Perspex adhesive.  The edge joints that I am getting are still not always perfect.  Air bubbles, voids, and stress cracks still occur from time to time.

 

Figure 5.  Acrifix 192 Perspex Adhesive

 

Excess glue is a hassle no matter what sort of adhesive you are using.  My best advice is to mask everything in sight (and particularly everything in the vicinity of the joint that you are working on), practice on some scrap before hand, keep everything clean - oil and dust free, have everything conceivably useful on hand, and pray a lot.  Excessive chloroform adhesives cloud the surface of Perspex.  Excessive Acrifix 192 forms uneven, high-build surfaces that are difficult to remove, and excessive epoxies form high build surfaces that turn into greasy smears when you try and remove them.  Excess glue is just an unsightly mess if it is not in the optical path of the housing.  It will ruin the work if it is between the camera optics (lens, focus sensor, reduced light illumination, etc.) and the object of interest.  You can polish defects out of Perspex using increasingly finer grades of abrasive and polish, but at tremendous effort and seldom resulting in the clarity of a new piece of Perspex.

Unlike glass, Perspex is susceptible to scratching.  Provided that scratches are outside of the optical path they can be ignored at the expense of a tacky looking finish, or they can be remove using fine grades of abrasive paper, metal polish (I use Brasso, Silvo and jeweller’s’ rouge), a clean cloth, a flat surface, elbow grease and time.  Scratches in the optical path will ruin the housing.

A word of warning about flame-polishing of Perspex.  Flame polishing using a non-carbonizing (neutral or oxidizing) flame at the correct temperature and for the right time is an excellent technique for removing very fine scratches on Perspex.  However it can also go terribly wrong and by the time you notice, it is to too late.

Back to the construction.  The camera mounts and a flash light-pipe were made from Perspex and glued in place in the housing using epoxy.  I glued some rubber strips on the camera mount surfaces to protect the camera.  I used trimmed rubber feet to make the camera secure in the housing as I had done with the 330 housing.  This time I added a layer of double-sided adhesive foam tape between the rubber feet and the case.  This made the mounts a little more resilient than rubber alone.

With the case made it was back to the sink for another trial.  Success, no leaks, but I needed a deeper test tank for a longer immersion.  Our turtle tank is 750 mm deep and a flower pot in the tank provided a handy way of keeping the somewhat buoyant case submerged.  After a few hours with the turtles the case was retrieved.  Still no leaks, but another problem had arisen.  I could not get the back off the housing!  The gasket had worked too well and the reduced pressure inside the case could only be relieved by drilling a hole through the case.  A 6 mm diameter pressure relief screw was machined out of brass and fitted with an O ring seal to plug the hole.

 

Figure 6.  Pressure Relief Vent Plug

 

The next job was to machine the controls.  When discussing the merits of my prototype with a mechanically minded friend, he questioned the reliability of a single O ring seal.  After some thought I decided to change my original design and use a double O ring seal as the extra machining was minimal and the cost of seal failure was potentially high.  Ideally the controls should be as thin as possible to minimize the pressure of ocean trying to get inside the case.  The smallest O rings that I could find locally made me settle on a 4.5 mm diameter design.  As it happened this was convenient because I could use springs from a cheap ball-point pen, and retaining circlips from an old printer mechanism that I’d kept to salvage the motors, gears and guide rods.

The seals were machined from brass rod.  Although I had suck-tested the prototype I hadn’t water tested it so, rather than make all of the controls at once, I started with just two for the critical on/off button and focus/shutter.  When marking out the location for the 4.5 mm holes through the case I put the camera on the inside and used a scriber to mark the control centres.  This is not a good technique because of the refractive index of Perspex.  One hole ended up slightly off centre.  I promised myself that all future control holes would be measured and marked.  The short-term fix was to reduce the diameter of the end of the offending control rod.  To ensure a free running surface inside the holes I polished these with metal polish on the shaft of the drill.  A 45 degree chamfer was cut on the edge of the hole to prevent damage to the O rings when fitting the controls.

 

Figure 7.  The Control Double O Ring Seal

 

With the controls made and fitted it was back to the turtle tank for leak testing.  The controls worked fine – no leaks.

With the control buttons installed I could now test the case with the camera fitted (but not yet underwater).  The photos were fine except when the flash was used.  The flash kept reflecting back from the Perspex housing in front of the lens.  This was a problem because the sun doesn’t shine too brightly at 40 m and the colour spectrum changes as you go deeper.  I was certain that I would need the flash at these depths.  I tried masking everything with black cardboard but I could not get rid of the flash reflection.  The field of view reduces when you go under water (which is why everything looks magnified through a mask).  Maybe this would help?  With my precious 750Z camera fitted in the housing it was time for the turtle tank.  No leaks, and the camera worked fine at a depth of 750 mm, but the flash reflection was still a problem.

 

Figure 8.  Turtle Annoyed by the Flash (and so was I)

 

Another issue arose at this point.  The camera and housing were buoyant.  With the case dimensions and weight noted, I calculated that I needed to add 200 g to achieve neutral buoyancy in sea water. 

How did the commercially made camera housings avoid the flash reflection?  All of the cases that I had looked at had the lens housing proud of the camera body.  I set about making a Perspex cylindrical lens housing on a piece of Perspex sheet to experiment further.

 

Figure 9.  Testing a Lens Housing Concept

 

Darn.  The flash reflection kept getting to lens.  It turns out that the Perspex case acts as a light pipe, transmitting some of the flash light through the case walls to the front of the lens.  This required a redesign.  I need to use something opaque to separate the Perspex in front of the camera lens from the rest of the housing.  I wanted to stay with plastic to avoid the complication of metal-to-Perspex seals.  I got a number of different opaque plastics and experimented with adhesives to find one that I could reliably glue to the Perspex.  PVC worked best using AcriFix 192 or cyanoacrylate.

I made a new cylindrical lens housing from a piece of 60 mm diameter solid PVC rod and fitted a 6 mm thick Perspex lens.  A word of warning – use negative rake on machine tools with PVC otherwise the tools are likely to catch and dig in, ruining the work.

 

Figure 10.  PVC Lens Housing Prototype

 

The test was partially successful.  The flash problem had been fixed, but the corners of the PVC housing kept getting in the picture (except when the camera was zoomed in).  When calculating the diameter of the housing I had failed to appreciate that the maximum field of view with a 2 x 3 aspect ratio was across the diagonal of the picture.  I needed to increase the diameter of the PVC, but this was going to obscure the sensing and low-light focus illumination on the front of the camera.

 

Figure 11.  Lens Housing Obscures the Corners

 

A new stepped PVC lens housing was made and I reduced the clearance between the front of the lens at maximum zoom to just 1 mm.  I used 6 mm thick Perspex for the lens in order to reduce the lens housing length.  This fixed the problem of the PVC appearing in the corners of the photo, but gluing the lens in place and cleaning it were a hassle to say the least.  I decided to make the Perspex lens removable to aid cleaning/replacement and remove the glue problems.  The design used a simple O ring seal held under light compression by six 2 mm diameter machine screws spaced around the perimeter of the lens.  I ordered some 2 mm diameter stainless steel machine screws and a suitable thread cutting tap from RS Electronics, and went back to the O ring shop for a 40 mm diameter, 1.7 mm thick O ring for the lens.

The new lens housing was made and tested.  Success.  No flash reflection, no PVC in the photos, and minimal interference with the sensors and low light illumination on the camera.  But was it going to be strong enough?  I test its compressive strength by standing on it (equivalent to just over 500 kPa).  At a depth of 40 m the pressure differential on the camera housing is a little over 400 kPa. The lens housing should be up to the task. 

Now all I needed was another Perspex case to mount the lens housing on!  The new design had the added advantage that the reduced case volume should largely overcome the positive buoyancy issue identified previously.  Initially I thought about cutting the back of the old 750Z housing and mounting a new front on it.  After some consideration I rejected this because the control buttons were not perfectly aligned, the case was too small (remember the slot that I had to mill to get the camera to fit), and removing the camera mounts and flash light-pipe were going to make a mess of the useful bit of the existing case.

The new case was made over a weekend from 10 mm Perspex.  I’m starting to get good at cases now.  I started by cutting an oversized rectangle, slightly larger than the face of the housing body, and machining the lens housing step-recess.  Then I glued the sides, top and bottom the camera together.  The front of the case body was then cut to size to ensure that the camera lens would accurately align with the lens housing with the camera in position.  The sides of the housing were then glued to the front.  The back of the housing was made along with a new gasket.  rubber feet, double sided foam tape and a strip of EDM rubber were used to make a comfortable fitting and secure camera mount.  Gasket studs were fitted.  The existing control buttons and pressure relief screw were transferred from the old to the new housing.   The PVC lens housing was finally glued into a step recess on the front of the housing body.

 

Figure 12.  The 750Z Housing

 

Another thought.  Would the control return springs be strong enough to prevent being pressed in at 40 m?  Some maths was required.  The 4.5 mm diameter controls would need to return against a mass of about 650 g.  A test with the current springs showed that the return action would be marginal.  I replaced the ball point pen springs with stronger 12.5 mm lengths of  0.25” x 0.02” utility compression springs purchased from the local Mitre 10 hardware store.  These did not deflect appreciably under a 1 kg weight, yet the controls worked smoothly without undue force.  The only problem with these springs was that they were zinc plated steel which would rust in due course.  I decide to live with this because the springs were cheap and readily replaced.

With the new housing made it was time to consider how to attach the camera housing to a strap.  I decided on a simple anchor point made of brass and screwed and glued into the body of the camera housing.  I tested the attachment method by gluing a 6 mm diameter machine bolt into a threaded hole in the edge of a piece of 10 mm Perspex.  Provide that the hole is central in the sheet I could not break, pull out or unscrew the bolt.  The strap mount was made and fitted.

 

Figure 13.  The Strap Toggle Glued and Screwed in Position

 

Test Time.  Straight to the turtle tank (without the camera).  Two hours immersion with frequent control manipulation and water tightness was confirmed.

 

Figure 14.  Yurtle and Louise Witness a Water Tightness Test

 

With the camera mounted in position the test was repeated.  That intermittent fault with the 330 had gone again so I had a camera to take photos of the camera.  The buoyancy looked good – slightly negatively buoyant in fresh water.

 

Figure 15.  Louise Examines the Housing

 

The photos were examined.  No problems with or without the flash, and no problems with lens obscuration.  It was now time for an open water trial.  The next dive date was a whole week away.

Off to Leigh for two dives at Matheson’s Bay with my old buddies at Dive HQ who were running an open water scuba course.

 

Figure 16.  Matheson’s Bay, Leigh

 

Rather than risk soaking my Pentax 750Z immediately, I decided to use a collection of rocks of the same weight as the camera, wrapped in a paper towel, as ballast for the housing.

 

Figure 17.  Dummy Camera - 256 grams of Rock and Paper Towel

 

Buddy restrictions on the dive limited my depth to 15 m.  The visibility on the day was approximately 5 m and the water temperature was about 16C.  The dive was great fun.  I tried to stay last man and was fully entertained stopping runaway assents and trying to keep the students together (one buddy pair was single-mindedly hell-bent on chasing a crayfish no larger than a shrimp).  I exercised the controls on the camera housing every five minutes or so over a 55 minute downtime while closely examining the seals.  There were no problems with control use, even with gloves on. The buoyancy was neutral - perfect.  On return to terra-firma after the second dive the mock camera (rocks and paper towel) were perfectly dry.  Excellent!

After some humorous discussions with the dive instructor, Sharkbait, I was unsure if the flash going to be so critical after all.  His take on the lighting issue was if you were diving deep then a flash would only enhance the visibility a metre or so at most.  Rather than be stuck with the flash on or off for the duration of the next dive, that night I set about machining another control in the back plate of the housing for the flash.  Then it was off to the turtle tank for another test.  The control, which was based on the previous designs for on/off and focus/shutter, did not leak.

Back to Matheson’s Bay the following day with the camera safe and secure in the housing.  The day was overcast.  A nor-westerly wind was building but the sea was sheltered near the coast with no chop and hardly any swell.  The camera was attached by a split-ring through the strap toggle, attached by a retractable clip to a D ring on my BCD,   The water temperature was 15C and the visibility was about 6 m, not ideal.  The maximum dive depth was 14 m, with a light current running from the north.  Not a great deal of action down-under.  A few small fish (goatfish, cod, leatherjackets) and a starfish or two.  The bottom was kelp on sand with a few rocky outcrops, and extremely susceptible to muck-stir which aggravated the already-limited visibility.

 

Figure 18.  First Shots (the decompression bar is at 6 m)

 

The camera operated perfectly.  On loading my BCD back onto the boat the camera got hooked on the gunnels and the retaining split ring was pulled straight, almost detaching the camera.  In future I’ll pass the camera up by hand.  Once back to the beach I decided to complete the second dive, rather than open the housing and preview the photos, then take the housing home, rinse it off with clean, fresh water and look at the photos at my leisure on the ‘big screen’.  I took the split ring off my car keys and replaced the damaged one.

 

Figure 19.  Mask Replacement Exercise

 

The second dive was on a reef a few hundred metres off shore.  The wind had picked up and the sea had built to a 1 m swell with a moderate chop.  The sky had darkened appreciably and rain was imminent.  The visibility remained about 6 m but there was less particulate matter in the water out from the coast.  The current had increased.  My maximum dive depth was 24 m.  There was a bit more going on below.  The terrain was kelp forest on rock, with patches of sand and some good crags and canyons.  There were a few schools of Sweep and Yellow Fin, some leatherjackets, goatfish, an eel (which I didn’t get to see), and Sharkbait found a small (but legal sized) cray.  Photo opportunities were limited during this dive as my student buddy took my secondary regulator after about 5 minutes in a pre-dive planned manoeuvre to extend his air.

 

Figure 20.  Goatfish at 14 m

 

Home, case open, camera and housing inspected and images downloaded.  The gasket had sealed tightly and it took some effort, even with the air relief screw removed, to get the back off the camera.  I may try a new gasket material from silicon or natural rubber for the next outing.  The camera and housing were both fine.

 

Figure 21.  Kelp Forest

 

The photos were not postcard material but indicative of the day.  There are a number of reasons for this.  The low light meant that the camera shutter speed was between 1/15th and 1/50th of a second.  Holding things still this long was difficult in the conditions, and the sea life refused to stay put.  I was also experiencing some difficulty with the first-press focus action on the shutter, tending to take the photo without focusing.  The flash was not helpful in the conditions because the light bounced off all the particles in the water, leaving lots of speckle in the flash photos.  These problems were nothing to do with the camera or the housing - clearer water and better operator technique are required.  Maybe I should have re-read the section on underwater photography in the Padi Adventures in Diving Manual?

That said, the project has been a success.  I have an underwater housing for my Pentax 750Z and it’s good for at least 36 m (Great Barrier Dive on 4 Nov 06) and I’m confident it will do 60.

 

Figure 22.  Sweep Schooling at 16 m

 

Easter 2007- the Death and Resurrection of the Pentax Optio 750Z

I guess it had to happen.  On a dive at Middle Arch on 7 April 2007 at the Poor Knights my camera became detached from its retractor clip while I was working through some kelp.  By the time I had noticed it wasn’t there it was too late.  I was ascending at the end of the dive and my decompression limits meant there was no way I could return to 24 m to search.

I was pretty confident of where the camera had become detached as I had been searching a small area of wall between when I last used it and when I had noticed it was missing.  I set about planning a return dive.  Martin and Jill from PK Dive put the word out that it had been lost and I sketched up a map showing the probable location.

 

Figure 23.  Lost Camera Map

 

The Poor Knights is a popular dive site and the following day the camera was found by one of the staff from Tutukaka Dive.  My sincere thanks to the Tutukaka dive community for finding the camera and handing it in.  After more than 24 hours of immersion some water had got into the housing so they opened it, removed the battery and the memory card from the camera and dried it off.

Later in the week I returned to Tutukaka to pick up the errant camera and squeezed in a midweek dive.  Alas all was not well – the controls had gone intermittent and there was signs of corrosion along the camera case seams, through the battery compartment, and at the on/off and shutter control.  With nothing left to loose I took the camera out on the first dive.  Despite the flaky control operation I managed to get a couple of movie sequences and a few stills before it packed up entirely.  At the end of the dive there was more water in the case, the inside of the housing was fogged, and the camera was dead.

That night I disassembled the camera, cleaned everything as best I could with isopropyl alcohol and left it dry.  The printed circuit boards and lens assembly looked in good order but the salt water had contaminated the on/off switch.  I set about some kitchen table surgery and replaced the on/off switch.  The camera sprang back to life.  All functions checked out okay.  The Pentax 750Z will dive some more!

 

Figure 24.  New On/Off Control

 

How had water got into the case?  Due to the location of the damage it’s likely that either the shutter button or the on/off button had leaked.  Prior to the ‘lost camera dive’ I had added a new rotary function control and replaced the control springs with weaker, slightly smaller diameter stainless steel ones.  On inspection there were no signs of leakage (surface corrosion) on the brass between the O ring seals on the new control.  The lens housing and back gasket were fine, as were the rear function buttons.  But both O rings on the on/off control were damaged, as was one of the pair on the shutter control.  I suspect that these had been damaged when I fitted the new springs.  I’ll revert to the stronger steel springs, clean the controls and replace all of the O rings before the next dive.

Alas the salt water ingress has finally killed my 750Z.  On disassembly it is clear that the salt has penetrated the switch membranes making the camera unserviceable.  Time for a new camera and housing.  See the A20 project:  A20 Housing.

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