Luminous Landscape Forum

Equipment & Techniques => Medium Format / Film / Digital Backs and Large Sensor Photography => Topic started by: Alan Klein on November 04, 2019, 10:27:25 am

Title: Mamiya RB67 Lens Shutter tests
Post by: Alan Klein on November 04, 2019, 10:27:25 am

I decided to test my shutters to see how off they might be. They're around 35 years old. The 90mm is an original lens and the 50, 180 and 360mm are "C" type bought new back then. I used my Samsung S7 Galaxy to record the sound. Then analyzed it using Audacity, a free audio app for my desktop. I watched this guy do it although he apparently used a dedicated microphone.

Over the years, especially during about a 15 year or longer period when I didn't use the camera, the lenses stayed cocked I believe. I've also had them uncocked for long periods as well. It doesn;t seem to have much of an effect as you can see from the results. The faster speeds were sometimes hard to see. The sound seem to get really condensed so it was hard to tell when the shutter opened and closed. The shutters seem pretty good at maintaining their settings and operation.Please add any comments or testing you've done.

The first column in each lens groups is the shutter speed set on the lens. The second column is the length of the lens opened measured in Audacity. The 3rd column is the equivalent shutter speed fraction measure. It is calculated by dividing 1 by the time measure in column 2. Or = 1/time measured in the second column. The 4th column is the time of the shutter if it was equal to the shutter setting.

All speeds were within 1/3 of a stop from 1 second to 1/60. Above that, a couple seemed to be more. But due to the difficulty in determining where open and close were, the results of these may be questioned. I used this chart to check. It's very impressive that these lenses hold their shutter speeds for 35 years especially since I left them cocked for years at a time. This may confort many who were concerned about this issue as was I. Note that these tests were for RB67 regular and "C" lenses, not the RL lenses or the RZ lenses for the RZ67 camera.
Title: Re: Mamiya RB67 Lens Shutter tests
Post by: Alan Klein on November 04, 2019, 10:34:50 am
Here are samples at 1/4, 1/30, 1/60 and 1/250 left to right, top to bottom. You can see it gets difficult to determined just where to manually highlight (that's the white fill) when you think the shutter is actually open. Easy at slow shutter. Very difficult at fast shutters. The numbers at the bottom define the length of the white selection. For example, it reads 00.279 in the 1/4 sec test upper left. But even at 1/30th on the upper right, it's hard to determine where to put it. The bottom two even are worse. It's probably due to the bounce and the sound being picked up by the poor microphone in the cell phone.
Title: Re: Mamiya RB67 Lens Shutter tests
Post by: Alan Klein on November 04, 2019, 10:37:23 am
This was my testing procedure.

The lenses were tested on their own disconnected from the camera body since the shutter is built into Mamiya lenses. So I didn't have to deal with the reflex mirror and its noises. The camera wasn't needed for the test. Only the 4 lenses.

The tests were done with the mirror up knob on the lenses set to Manual. I cocked the shutter manually on the lens for each shutter speed test. Then I manually released it back to the shooting position. I then set the shutter to the setting I was going to test. Then I turned on the recorder and placed the camera adjacent to the cell phone. Then I released the shutter with a cable release to avoid extraneous sounds and vibrations. I also left the aperture to its widest.

I then emailed the recordings to my desktop and loaded each one into the Audacity program for analysis. As you can see in the above 4 samples, the slowest shutter speeds were the easiest to mark their length. The faster shutter tests became more difficult due to the "hash" and overlapping noise . So it was hard to tell when the shutter opened and closed on fast shutters. I guessed a lot on those. These lenses have T, 1 sec, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250 and 1/400. YOu can see the results on the handwritten charts above.
Title: Re: Mamiya RB67 Lens Shutter tests
Post by: Alan Klein on November 04, 2019, 10:42:15 am
Only a couple were off over 1/3 of a stop, an amazing reliability for 30 years old lenses.
To calculate stop differences, use this procedure on Windows scientific calculator. .012 and .008 are samples of shutter speed time  and actual time measured.

Enter 0.012, divide sign, 0.008, =
Now you see 1.5 which is the ratio of times.
Press the log button. Now it shows 0.176, which is the log10 of 1.5.
Press divide
Now enter 2, press the log button to take log10 of 2, you'll see 0.3.
Now press = (to complete dividing 0.176 by 0.3).
You should see 0.584...

It works this way because the log function has a higher precedence than divide, so the log button takes the log of whatever is showing immediately, while divide waits for you to enter the second number and press =.
Title: Re: Mamiya RB67 Lens Shutter tests
Post by: Alan Klein on November 04, 2019, 10:43:01 am
For those who'd just look at a chart, here's one showing 1/3 shutter stops.