Shutter Speed Testing Focal Plane Shutters

[Niglyn]

Hi all,
I have come over from my thread

Build a shutter tester for Focal Plane shutters - Cheap, Easy & it Works

Right now I'm relying on the onboard VR. But I'm still a bit struggling on the best way to connect the battery. I have connected it to the GND and VIN port so to use the onboard VR, but I'm using 3 AA batteries, which results in 3x 1.5V = 4.5V. From what I understand, the input power on the VIN...

www.photrio.com

As it seems everybody has the same issue with sensors having width, thus as the ratio of curtain slit & sensor width reduces, measured exposure becomes under-read.

It seems the issue is well known & it is a case of living with it, there is no (easy) solution & providing the measured exposure is within ISO tolerance, that is good enough.

As ic-racer posted in the other thread, calibration is easily done with a known slot width, but as the two curtains are not indexed, again, we are back to the same issue.

Also, whilst a camera with flash sync at 1/30s would have a slot width of half at 1/60, quarter at 1/120 etc, a camera with different flash sync speed would have different slot widths for each 'speed' so one could not simply apply a correction based on measured exposure.

My thoughts are the following
a) it is a known issue with all testers, so accept the faster shutter 'speeds' will always read low, don't worry about it.
b) use the smallest hole possible to mask the sensor (we are now banging lasers at the sensor, so hole could be less than 0.5mm possibly.
c) use the curtain speed, which should not be affected by sensor width, to add a calibration value as the two sensors 'seen' time is used for this measurement, thus does not have the same measuring issue.

Is it just as simple as curtain travel speed over 32mm took 16mS.
so curtain travels 1mm (width of sensor) in 16/32 = 0.5mS

Apply the correction as follows:-
Laser blocked time = Laser blocked time - 0.5mS

Of course the shutter curtains will not run at a linear speed, so there will always be errors, but it might give a more accurate result?

Have I solved the Holly Grail?

 

[snusmumriken]

My thoughts are the following
a) it is a known issue with all testers, so accept the faster shutter 'speeds' will always read low, don't worry about it.
b) use the smallest hole possible to mask the sensor (we are now banging lasers at the sensor, so hole could be less than 0.5mm possibly.
c) use the curtain speed, which should not be affected by sensor width, to add a calibration value as the two sensors 'seen' time is used for this measurement, thus does not have the same measuring issue.

Is it just as simple as curtain travel speed over 32mm took 16mS.
so curtain travels 1mm (width of sensor) in 16/32 = 0.5mS

Apply the correction as follows:-
Laser blocked time = Laser blocked time - 0.5mS

Of course the shutter curtains will not run at a linear speed, so there will always be errors, but it might give a more accurate result?

Have I solved the Holly Grail?

That may well improve the estimate, but I can't believe it is as simple as that. Surely there will be diffraction of the beam as it passes through the hole in the mask? The amount of correction would then depend on the size of the hole, the amount of diffraction, the effective width of the sensor, and the distance of the sensor from the mask.

 

[ic-racer]

Is it just as simple as curtain travel speed over 32mm took 16mS.
so curtain travels 1mm (width of sensor) in 16/32 = 0.5mS

Apply the correction as follows:-
Laser blocked time = Laser blocked time - 0.5mS



Have I solved the Holly Grail?

See post #2 here: https://www.photrio.com/forum/threa...speed-tester-calibration.196226/#post-2627677

Essentially the same conclusion (0.5 vs 0.4ms based on measuring the speed across 35mm or 32mm).

So, I did actually test that hypothesis...

Rotating FP shutter slit width = 2.25mm , fractional speed 1/960 = 1.0417 ms at 0.1652 seconds/rotation
1mm sensor error = 0.4ms
Sensor Latency Error = 0.113ms (BPW76A sensor datasheet)

Mathematically Corrected time (0.4 + 0.113 + 1.0417) = 1.5547 ms

Measured time (Oscilloscope) = 1.6553 ms

Difference between mathematically corrected and measured about 5%
Difference between measured and actual speed about 37%

 

[vandergus]

Nikon V1 I have achieves real 1200fps but resolution is very low at 320x120. It's still plenty to evaluate shutters. I got the whole kit in local classifieds for less than $200 few years back. They do appear for sale relatively often. It needs a lot of light at 1200fps to produce bit cleaner video. It will still be grainy.
There should be other cheap cameras with high fps.

I have a Nikon J1 and attempted some shutter speed testing with it. I printed a paper ruler and taped it onto the film plane to make measuring slit width and curtain travel time a little easier. Here's a video of a Pentax K1000 shutter firing at 1/500.



The main issue is that it doesn't record a true 1200 fps. It samples at 600 fps then duplicates each frame. I was counting frames to calculate curtain travel time and everything was off by a factor of two. Then I noticed, as I was advancing frames, that they were duplicated and I needed to count two frames for each unique image. 600 fps is still fast enough to sample slower horizontal curtain shutters as you get 6 or 7 frames over the entire travel time. But it's too slow for faster vertical traveling shutters. A Nikon FE2, for example, has a curtain travel time of around 3 ms and you're only going to get 2ish frames during an exposure. You could get a rough idea of slit width but it going to be hard to reliably measure curtain travel times.

But for $100-$150, there are worse shutter speed testers. It gives you a lot of information about how the shutter is behaving and can even help you judge curtain bounce and other odd behaviors.

 

[Light Capture]

I have a Nikon J1 and attempted some shutter speed testing with it. I printed a paper ruler and taped it onto the film plane to make measuring slit width and curtain travel time a little easier. Here's a video of a Pentax K1000 shutter firing at 1/500.



The main issue is that it doesn't record a true 1200 fps. It samples at 600 fps then duplicates each frame. I was counting frames to calculate curtain travel time and everything was off by a factor of two. Then I noticed, as I was advancing frames, that they were duplicated and I needed to count two frames for each unique image. 600 fps is still fast enough to sample slower horizontal curtain shutters as you get 6 or 7 frames over the entire travel time. But it's too slow for faster vertical traveling shutters. A Nikon FE2, for example, has a curtain travel time of around 3 ms and you're only going to get 2ish frames during an exposure. You could get a rough idea of slit width but it going to be hard to reliably measure curtain travel times.

But for $100-$150, there are worse shutter speed testers. It gives you a lot of information about how the shutter is behaving and can even help you judge curtain bounce and other odd behaviors.

I think there was a firmware glitch for 1200 vs. 600fps with duplicated frames. Update was released but not sure if it works on all models.

 

[vandergus]

I think there was a firmware glitch for 1200 vs. 600fps with duplicated frames. Update was released but not sure if it works on all models.

Oh wow. Thanks for the heads up. I'll try to install that and do some more experimenting.

 

[Joe Edwards]

View attachment 314941

I built this shutter tester for working on Ektra bodies. Lasers are used for as close to a point light source as possible and positioned near each side of the curtain aperture.

Phototransistors with short switching times are used for sensors, they are installed inside the black box to shield them from light and small apertures were poked in the plastic housing using a hot needle, allowing a small effective aperture of approximately 0.010" which reduces the effective sensor size, as does positioning the sensor further from the aperture.

Correction factor at 1/1000 is about 1.3, and I consider within 1/3 stop to be good enough for my older mechanical cameras, so this was acceptable to me. Trying smaller diameter apertures did not reliably trigger the phototransistors.

Slight difficulty was encountered, as for the Ektra in particular correction factor needs to be calculated differently for each sensor, since the curtain gap does not remain constant across the film plane. At the start of 1/1000, it's about 1.3, but by the end it is closer to 1.08.

This type of tester is best connected to a 2 channel oscilloscope for measurement.

Hunter, would it be possible to start a conversation/PM with you ref the Ektra? Thanks, Joe E.

 

[MattKing]

Hunter, would it be possible to start a conversation/PM with you ref the Ektra? Thanks, Joe E.

Joe Edwards:
We have turned on the Conversation function for you.

 

[Joe Edwards]

Thanks, Matt!