Converting an iris Petzval to a Waterhouse Stop version!

[Steven Tribe]

This is the tale of a large Suter Portrait Petzval (no.4, 24cm and F.3.2). Absolutely in perfect condition except for the missing iris blades - all 24 of them. I have done iris blade replacement before and it is not really a quick fix. Just cutting out 24 blades and keeping the blades with zero distortion is very difficult. Then you have to drill 48 holes in the perfect position, prepare 48 pins of the right length and diameter and then fit these to the holes permanently, without adding to the thickness of the blades at the join.

Many of these big Petzvals have this many blades - we think of this as ensuring complete roundness of the aperture. But I think the main reason is that makers did not want to mess with their models with known speed. Suter, for example, had the WHS version of the no.4 as F3.2. For 10% extra you could get the iris version - with the same speed. To be able to do this, you would have use a lot of iris blades that were narrow rather than fewer, which being a good deal wider, would reduce the maximum speed.

So the many narrow blades are a problem and with makers like B & L, the composite material used for blades adds to the difficulties. I have had the misfortune to work on one of B&L's , otherwise superb, Petzvals. I got the mechanics to work, but replacement with bronze instead of the original blade material, meant the friction in the extreme positions was just too much. A WHS conversion would have been simpler and with a garanteed better result in function - if not in cosmetic appearance!

A repair was not possible with this Suter 🇨🇭 as they had introduced a separate iris section in the middle of the barrel which bulged out giving diameter for the two iris rings and giving extra space for the iris blades. Even when the front and rear parts of the barrel are removed, there is no clearance for removing the holed/slotted iris rings. They are going to be in there forever! So the idea was to use these two iris plates as a basis for guiding/holding/light trapping for the Waterhouse stops. The two plates could be pushed to one side allowing entry of slots through the cut-out through which the long gone iris lever used to travel.

[Greg]

Also have a brass Pertzval, a Darlot. Has an iris, but most of its blades unusable. Have been considering getting a Silhouette CAMEO 3 DIY Cutting Machine to cut the blades.
https://www.micromark.com/Silhouette...c-cutting-tool
Catalog says it can cut Sihouette aluminum sheets. A local Model Railroad club might be getting one, so have put off purchasing a CAMEO 3 till hopefully I can see one work in person. Anyone have any experience with this machine? Or thoughts on it?

[Steven Tribe]

My first thoughts are that replacing brass blades is not such a big problem - I think that fixing the pins is probably the key problem. Of course, there be a more complex mechanical wear problem in the iris itself which has caused the blade damage, rather than previous owner misuse!

The first step was to measure the exact internal diameter of the two loose iris plates and add a couple of mms. both sides. This would be the width of necessary waterhouse stops. If everything was made perfectly and there was no play in the stops, then this would be light tight. So a couple of mms. extra. The early WHS had a extra brass piece, so the stops ran in a guide track. Later Petzvals have a pair of discs pressure mounted and the WHS slides between them. In this case, the old iris rings were solid enough to function as a building block for a set of edge guides and a set of track guides.

I made a brass strip matching the width I needed to make a mock up to design the internal brass pieces. This strip will become future individual Waterhouse stops. I have found that working brass (edging, finishing and cutting holes) is much easier (and safer) if as much as possible can be done before the brass is cut into individual stops!
In this case, the width of the iris lever slot was perfect for brass WHS and I only needed to extend the slot by about a centimeter. The bottom of the new brass strip is rounded to match the curvature of the inside of the barrel. The two loose matching iris plates have been temporally glued together and stuck to the side of the "bulge" in the barrel so that there is daylight down through the slot. This was done before making the extension to the lever slot.

The brass plate, to be used for later WHS manufacture is inserted through the old lever slot. It should cover the iris ring hole perfectly. The next thing to do is to fill out the "quarter moon" gaps halfway across the barrel with the same brass material you have used for the Waterhouse plate section" This is necessary to provide a base for the track control rail and to allow fixture to the pair of iris ring plates (still temporary stuck in place!). These "quarter moons" have the same curve on one side as the WHS and are straight on the other side. It is easiest to do the curve first (on a larger piece of brass, again) and make the straight cut later. It is pretty impossible to measure accurately, so there is a lot of trial and error. Eventually, I reached a stage where both sides fitted and the was no play. In fact, I could insert and remove the WHS without the distance pieces moving at all.

The next to last job is to repeat with two new "quarter moons" which overlap the WHS stops. In my case, these are now in the barrel proper, rather than the "bulge". The overlap is just a few mms. More and you will be reducing the maximum F!

Finally, the 2 old iris ring and the 2 new guides have to attached together permanently. There is not a lot of room for mistakes. I considered using the existing pin holes in the iris plate but made new holes nearer the barrel. The assembly is held together by 2 M2 brass screws.

In theory, there is no permanent attachment to the brass barrel "bulge", but in practice it is very solid.

[Steven Tribe]

Obviously, it is not possible to work on the inside of the barrel with disturbing the mat black so this was sprayed, along with the new brass pieces.

I realise that this is a special iris design with quite a neat solution available. But there were many unique iris designs - many of them not very reliable anymore - which may have a similarly WHS solution available. And WHS apertures last forever.

[RedGreenBlue]

Slick and well thought out. I enjoyed reading how the solution evolved.

[Steven Tribe]

I have thinking about other Petzval models that might benefit from the same treatment.

The most obvious candidates are the iris "turn the barrel" patent Petzvals from Dallmeyer made in the 20th Century. These have the same number of iris blades as the Suter - 24. The design looks exactly like a modern type iris and definitely well made. Mounted directly in the barrel, not countersunk as in the Suter.

But what to do if the iris gets (is) damaged or becomes unreliable? Repairs or replacement? There is also the question of retaining "value" of these series -A, B and D. What might be a good idea for better functionality now may not be considered a "good idea" by the next prospective owner.

Damaged iris blades could be quite easily removed from the patent Dallmeyer and the iris lever could be unscrewed. The would be no other modification other than a slight lengthening of the iris lever travel slot. The fixed and turnable iris plates form a perfect mount and light shield for the WHS, without the additions that were necessary for the Suter design. Just remember to store the iris lever for the next owner who might be interested in "investing" in a custom made 24 blade iris!

"Turn the barrel" variant of these Dallmeyer patent Petzvals does mean it is turned. The WHS stop could fall out. This is solved by giving the individual stop plates a slight bend or inventing a catch system if you are nervous.

Another candidate (without knowing very much about them apart from their problems) could be the studio shutters set in the barrel. If the shutter function was given up, then a more traditional WHS modification could be made?