Phochron XA shutter tester

[shawn]

Just received a Phochron XA shutter tester. I ordered the deluxe model with the test fixture. It all arrived today and is a nice setup.

The fixture makes getting everything lined up easier.

Seems to be very consistent with a leaf shutter, in this case a Copal 0 through a Rodenstock Grandagon-N 65mm.

Shawn

 

[Sarcophilus Harrisii]

I have an older Kyoritsu myself, as trustworthy a brand of camera tester as one can ever hope to own—but I kind of wish I'd ordered a Phocron to complement it, now. I had good intentions of doing so, but didn't get around to it. I particularly like the Phocron visual display, simply because it's one of the few affordable units which enable you to quickly and reliably spot curtain bounce, which can still be a problem, even when the accuracy of the actual exposure times are bang on.

Unfortunately, Eric Bergstrom has announced (via the Phocron XA Facebook group) that he's ceased—or is ceasing, perhaps, after completing pending orders—production of the Phocron for various reasons. So take good care of yours, it might appreciate in coming years possibly.

 

[shawn]

I had meant to pick one of these up for a long time and when I checked his site in Jan/Feb it mentioned production was stopping. I emailed him then about getting one and lucked out to get one of the last units. Didn't realize he had a facebook page, from a post about a week ago it looks like he might sell the PCB in the future for those that want to assemble their own Phochron. Initial impressions are very good.

Shawn

 

[GMOG]

I bought one last year, with the accessory sensor for focal plane shutters (35mm). It's easy to use, and provides information useful for setting up curtain tensions from scratch, including speeds for each curtain, and effective exposures at the beginning and the end of the run (slit width).

 

[rolfe]

I have one as well and it's a shame he has stopped making them. I found it worked flawlessly on everything from 35mm leaf shutters up to my Speed Graphic focal plane.

There is one from Romania on eBay that seems very similar.

Rolfe

 

[shawn]

I have the focal plane shutter accessory too but haven't tried it yet. Glad to hear it works well!

Shawn

 

[shawn]

Had a chance to spend more time with the tester. It is usable with a Barnack through the front of camera but positioning the sensor and light source is fairly critical. I put a piece of glossy paper covering the pressure plate.

Turning off the lights in the room helped too. It read consistently from 1sec up to 1/500.

At 1/1000 of a second it didn't always catch the shutter firing this way. Was a decent way of just getting a baseline on the shutter without having to remove the body shell.

Also tried the focal plane sensor. This was interesting to see how the relative curtain timings changed with shutter speed.

Shawn

 

[Sarcophilus Harrisii]

So sensor 1 is the start of the gate, 2 is the end?

Given the start time is so close to the nominal 1/1000, this informs that the 1/1000 curtain release timing adjustment is very good; but the first curtain is not staying ahead of the second enough, the exposure has decreased slightly by the end of the gate.

 

[shawn]

Yes, sensor 1 is start, sensor 2 is end.

I'm still learning how to interpret this. I *think* it is saying that the second curtain is traveling slightly faster than the first, at least at the 1/1000 speed and 1/500. At slower speeds I think it is traveling a little lower. But I might be misreading this. Either way, very cool tool. The Tower 45 was CLA'd several years ago by YYE.

I'll have to do the same with my F2/F3, CLE and M3 to see what they look like and to learn more on how to interpret this.

Shawn

 

[Sarcophilus Harrisii]

Yes, sensor 1 is start, sensor 2 is end.

I'm still learning how to interpret this. I *think* it is saying that the second curtain is traveling slightly faster than the first, at least at the 1/1000 speed and 1/500. At slower speeds I think it is traveling a little lower. But I might be misreading this. Either way, very cool tool. The Tower 45 was CLA'd several years ago by YYE.

I'll have to do the same with my F2/F3, CLE and M3 to see what they look like and to learn more on how to interpret this.

Shawn

Yes, that's correct. It's not a terrible result, above (referring to post 7, the Tower, and its 1/1000 result). The first curtain is however a little lazy. This is typical.

Speaking generally, this is usually the case, and it's something I used to wonder about. Tapering exposure is not uncommon in older cameras which have not been serviced, but, it's less usual for exposure to increase during the curtain run. Why is it nearly always the first curtain that tends to slow?

I better understand the reasons now, obvious, in hindsight, as things usually are. In almost all designs (there are a few exceptions, Eg. pre-war Contax) the first curtain has to do all the work of running the mechanism. Its spring(s) has to power the curtain timing gear and trip the release latch for the second curtain. The second curtain just runs itself (for clarity, at slower speeds in Eg a Leica, it may have to defeat an escapement before it can cross the gate, but we're talking about fast times here, the slit speeds).

So, the first curtain will generally slow down a bit if service is needed or wear has occurred. It's doing all the heavy lifting, and this is why it typically runs a bit more spring tension than the first, it has more work to do.

With the time of the Tower at the beginning of the gate being so close to nominal the timing system must be running well. The second curtain delay is spot on. There's probably a slight amount of friction in play that's slowing the first curtain as it runs. After some cleaning and lubrication I would expect such a shutter to be bang on again without any adjustment.

I can't explain why the curtain times are somewhat different at the slow speeds: I don't know enough about the design specifics of the Tower to comment on that, sorry. But the times look OK, and, generally speaking, the fastest exposure times running narrow slits (notably, the 1/1000, which is very tight) are usually the most problematic to get right, because the tolerances are small.
Cheers
Brett

 

[shawn]

Thanks, good info. I ended up testing a F2, SRT-201, F3 and a Bessamatic CS. As expected some variability between them all.

The SRT-201 did well and was very close for all speeds except at 1/1000 where it was slow. Only a little faster than 1/500th.

The F3 did well (electronic shutter) but I saw a lot of variability at 1/1000 (several stops) so I want to look into that more. The rest of the speeds were consistent.

The F2 was pretty good overall but fast at 1/1000 and I think I forgot to record the 1/2000 shutter speed.

The Bessamatic CS was slow at 1/500 and 1/250 and the rest of the speeds probably were too. Interestingly however the rest of the speeds matched up pretty close to the old shutter speed scale, but the camera is marked with the new speeds.

Pictures here

Shawn

 

[Sarcophilus Harrisii]

Thanks, good info. I ended up testing a F2, SRT-201, F3 and a Bessamatic CS. As expected some variability between them all.

The SRT-201 did well and was very close for all speeds except at 1/1000 where it was slow. Only a little faster than 1/500th.

The F3 did well (electronic shutter) but I saw a lot of variability at 1/1000 (several stops) so I want to look into that more. The rest of the speeds were consistent.

The F2 was pretty good overall but fast at 1/1000 and I think I forgot to record the 1/2000 shutter speed.

The Bessamatic CS was slow at 1/500 and 1/250 and the rest of the speeds probably were too. Interestingly however the rest of the speeds matched up pretty close to the old shutter speed scale, but the camera is marked with the new speeds.

Pictures here

Shawn

I looked at a few of the read outs for the 201—it is in a similar situation to the Tower. The first curtain is a little lazy. If you look at all the readings down to 1/125—they are all tapering across the gate.

The 1/125 is a good case study. The reduction in exposure is so negligible, who cares? And, absolutely, in itself it is of no consequence in use.

What it does however illustrate is the decreasing tolerance for variations in curtain running speeds as the exposure times shorten. 1/125, 1/250, 1/500 and 1/1000 all manifest tapering exposure. And yet, an alteration in the speed of the first curtain which will have an effect on consistency of the 1/125 exposure across the gate that's undetectable, on film—will be problematic on Eg 1/1000. It well serves to demonstrate how little scope for maladjustment there is with that tiny 1/1000 slit and (to a lesser extent) 1/500.

The slit on the start side of the gate is clearly wide. For ease of reference your test gave 1/577 start; 1/778 end. So—balancing the curtains again will get that exposure even, but probably at an even 1/600–1/700. The slit might tighten itself marginally with the first curtain running at the correct speed (the first curtain powers the timing gears and trips the second curtain release, remember?). If it's a bit lazy this might lengthen the slit slightly by itself. But it's not slowed that much, really—yes it's out of spec, but not ruinously so.

Hence, curtain balance won't get you a good 1/1000 by itself. The slit needs to be tightened.

There is an easy to reach adjustment for the 1/1000 release beneath the speed dial after removing the ASA setting parts from the dial centre. But it's probably not needed. If the mechanism is cleaned and lubricated, not only will the first curtain probably get back to its correct travel time, but the release will likely trip more quickly, and tighten the slit up. If that's not the case only slight adjustments to the curtain tension and/or release timing would be called for.

I hope the above all makes sense.

The take away is that balanced curtains make the exposure consistent. But the timing system controls the interval before the second curtain moves, and this interval (which determines the slit width), at the 1/60 to 1/1000 times, particularly, is what determines how much exposure will occur.

It might be interesting to re-test the Bessamatic with a 50mm lens attached. My Kyoritsu, for instance, is meant to be used to test leaf shutters with a lens mounted. Many leaf shutter cameras are impossible to test with a lens removed (short of dismantling them, anyway). By all means, follow Eric's instructions. But in the absence of advice to the contrary—if the faster times are improved with a (wide open) lens attached—I would probably accept these as being a more accurate test result. In my experience, a serviced Contaflex (which uses a substantially identical Synchro-Compur shutter) will return surprisingly accurate top speeds (with a lens of course). I'm quite partial to Bessamatics, myself, so I should run one or two over my own tester. Don't overlook the operation of the capping plate, either. If it's not lifting completely, this will cause its own problems.

Apropos the Facebook resource —the group for Phocron XA users is here, Eric monitors it and provides support:
https://www.facebook.com/groups/413149092376839/

Also the Learn Camera Repair group has a lot of relevant SRT related service information:
https://www.facebook.com/groups/360490091319202/

 

[SyPat]

The main problem when you adjust speeds is to maintain the slit equal for a given speed from the beginning to the end of the exposure. For each speed, you have to check the real time on the right AND on the left. The is always a difference, but it must not be superior to 18%. This is the real average speed.

Incidentally, in a constant progression of speeds, the correct shutter duration of "1/30" is not "30" but 32. For an "average speed of 475 at 1/500, we are below the expected value of 512 by 7% (which is acceptable if you have something like 500

and 450

, but very bad if you have 639 [r] and 310 [l)).​

 

[Sarcophilus Harrisii]

The main problem when you adjust speeds is to maintain the slit equal for a given speed from the beginning to the end of the exposure. For each speed, you have to check the real time on the right AND on the left. The is always a difference, but it must not be superior to 18%. This is the real average speed.

Incidentally, in a constant progression of speeds, the correct shutter duration of "1/30" is not "30" but 32. For an "average speed of 475 at 1/500, we are below the expected value of 512 by 7% (which is acceptable if you have something like 500

and 450

, but very bad if you have 639 [r] and 310 [l)).​

One often reads comments to this effect (bold, above). But in reality it's not actually correct.

If a mechanism preserves a slit which is truly constant across the entire film gate—the result will be some tapering of exposure. This is because shutter curtains accelerate as they traverse the film gate. If their velocity was truly constant, your assertion above would then be correct. Since curtain velocity increases as they run, to achieve an equal amount of exposure, in reality, what needs to happen is the slit width must increase slightly during the run: this counterbalances the increasing curtain speed.​

 

[SyPat]

(...) shutter curtains accelerate as they traverse the film gate. If their velocity was truly constant, your assertion above would then be correct. Since curtain velocity increases as they run, to achieve an equal amount of exposure, in reality, what needs to happen is the slit width must increase slightly during the run: this counterbalances the increasing curtain speed.

in other words, you are saying the exact same thing: the goal is to make the duration roughly the same at the beginning and at the end.

 

[Sarcophilus Harrisii]

in other words, you are saying the exact same thing: the goal is to make the duration roughly the same at the beginning and at the end.

You've claimed the width of the slit should remain constant.

I've informed that this will cause tapering exposure, and have explained why it's not ideal.

One scenario will provide consistent exposure. The other, will not.

These are not the exact same thing, far from it.

 

[BWF]

I am not tech savvy but managed to build the Cameradactyl/Ethan Moses version and it works great too. It doesn’t have the nice interface as shown above. I highly recommend it.

https://emulsive.org/articles/guides/how-to-build-a-simple-arduino-shutter-speed-tester-by-ethan-moses