Shutter Speed Testing

[j.eff]

Even though I know they already exist, I'm designing a shutter speed tester. My tester design is still in flux at this point, but it will likely use an infrared emitter with a phototransistor as a sensor. At this point, I'm measuring the time with an oscilloscope, but I'll eventually run it from a microcontroller and have the time output displayed with an LED or LCD display.

My problem now is trying to understand the relation between the physical position of the shutter and the shutter speed. In other words, the shutter speed is based on what positions of the shutter?

For example, with a simple blade shutter like a Holga or Diana, at what points would the exposure time be considered to begin and end? First crack of light to last? Shutter halfway open to halfway closed? Fully open to fully closed? In the case of the Holga, from the time I begin to see light at the sensor to no light at the sensor, I've got a time measurement of approximately 14.6 milliseconds, which would translate to a shutter speed of roughly 1/70. Checking my three Dianas, I get roughly 1/50, 1/70, and 1/88. These all seem likely shutter speeds for these cameras, so I'm inclined to think the exposure would be considered as the time from first to last light.

Is it reasonable to think the shutter speed would be based on that time span?

Would it be the same for a leaf shutter?

Would it be the same for a focal plane shutter, or should it be considered the amount of time a specific point on the film sees the open shutter in front of it?

I know these answers are probably pretty straightforward for anyone with a camera design background, but unfortunately, that's not me. I haven't been able to find the answer on the web either. Part of the problem is the 42x10^9 search results explaining shutter speed and how it relates to exposure when you search for anything about camera shutters.

Any help is appreciated!

 

[480sparky]

Shutter speed should be based on how long a given point on the film plane is exposed to light.

 

[Jim Jones]

The best method of measuring shutter time may be to integrate the light passing through the lens/shutter combination. This is easy to approximate with your oscilloscope system. Lacking that, I'd use the points where the transmitted light is half the brightness of the fully wide open shutter. If you measure from the first crack of light to the last with a leaf shutter, the measured value will be longer than the effective value at high shutter speeds. This will also vary with the lens aperture, with a smaller diameter aperture indicating a higher shutter speed than a larger diameter. For the top speeds, leaf shutters should be tested at about the apertures commonly used.

 

[Gerald C Koch]

I don't know just how complex you want your device to be. But there are six types of shutters. Each has its own calculus. The math may be more complicated than the electronics.

1) Packard shutter (roller blind)
2) Simple single blade as in box brownies
3) Leaf shutter
4) Focal plane shutter
5) Rotating shutter as in cine cameras.
6) SLR's like the Exa which use the mirror assembly as the shutter.

Probably the leaf shitter is the most complex.

The following may be helpful.

https://en.wikipedia.org/wiki/Shutter_(photography)

 

[SalveSlog]

Regarding aperture, maybe you'll find (there was a url link here which no longer exists) interesting.

 

[Bill Burk]

The best method of measuring shutter time may be to integrate the light passing through the lens/shutter combination.

Literal times are good for long shutter speeds, such as 1 second.

But once you get to the higher shutter speeds, things change and now it's not the real time that is important, but the effective exposure delivered to the film expressed as a measure of time.

It would be nice to have a focal plane shutter speed analyzer that would tell the effective shutter speed at several points along the path of the curtains. Then you might be able to tell if the camera needs adjustment (If the shutter is "capping" where the curtains don't follow each other at the right speed).

 

[bernard_L]

Exposure time is driven by two goals:
1. Film exposure (in conjunction with aperture setting)
2. Freeze motion (or blur it, if such is the intent).
Regarding goal #2, what you call "the time from first to last light" would be the proper criterion.
Regarding goal #1, what must be considered is the "equivalent time"? Look at your oscilloscope trace. I assume that your measurement circuit properly preserves the DC component, so the trace has a rising part, a flat top, and a decrease to zero (and does not undershoot to negative values, as when there is a series capacitor in the circuit). Ask yourself: what is the width (duration) of a rectangular trace (zero rise and fall time) that has the same height as the actual trace? The duration of that fictitious trace is the correct measure of the exposure time. A pretty good approximation, involving no calculation, is the duration between the half-light points.

 

[Xmas]

There may not be a market.

On the web there should be a DIY design for a photo transistor circuit that you make up two off and wire up to a stereo jack.
On your PC you load up a stereo sound analysis programme and measure the two times off the graph of intensity v time. You need the two times to set up focal plane shutters.
You don't need a special light source.

If you have a raster scan TV it is even better.

There should be proprietary kit off the shelf but expensive.

Google says everyone is doing this...

http://photo.net/classic-cameras-forum/00bdOI

Note I have no vesting interest in this ad and it is not what I use...

 

[wombat2go]

(there was a url link here which no longer exists)

I think it is important to arrange the photodiode to only see a narrow beam of almost axial light.
My set up is similar to that described by jeff, and here is a typical trace showing the penumbra.
(I hope the link works)
I use it for both focal plane and leaf shutter. The ripple on top is due to the use of an incandescant lamp in front of the lens as a light source. the 120Hz ripple is useful as a cross check of the time.

 

[Dan Fromm]

I don't know just how complex you want your device to be. But there are six types of shutters. Each has its own calculus. The math may be more complicated than the electronics.

1) Packard shutter (roller blind)

Packard shutters are leaf shutters. See http://www.packardshutter.com/

You may be thinking of Thornton-Pickard shutters, one of the many makes of roller blind shutters.

 

[bdial]

Timely topic (no pun intended), I've just started learning to work with microcontrollers, and a shutter tester is one project I have in mind.

 

[Steve Smith]

I built a speed tester a few years ago. It's just an oscillator driving a digital counter with LED display with the input gated by the operation of an IR phototransistor. No microprocessor required.

Set it so light only shines on the sensor with the shutter open, reset counter and fire the shutter.

It runs at 10kHz so a count of 10 = 1mS = 1/1000 second, 20 = 1/500, 40 = 1/250, etc.

EDIT: Or it might be 100kHz - I can't remember!


Steve.

 

[Xmas]

(there was a url link here which no longer exists)

I think it is important to arrange the photodiode to only see a narrow beam of almost axial light.
My set up is similar to that described by jeff, and here is a typical trace showing the penumbra.
(I hope the link works)
I use it for both focal plane and leaf shutter. The ripple on top is due to the use of an incandescant lamp in front of the lens as a light source. the 120Hz ripple is useful as a cross check of the time.

No the sensor needs to see more than the lens exit pupil.

 

[Gerald C Koch]

Packard shutters are leaf shutters. See http://www.packardshutter.com/

You may be thinking of Thornton-Pickard shutters, one of the many makes of roller blind shutters.

You may be right. The type is was thinking about work something like a guillotine. A board with a circular aperture drops past the lens. There are so many styles of shutters each with their own geometries.

 

[wombat2go]

No the sensor needs to see more than the lens exit pupil.

Ahh, sorry, I should have explained that my photodiode assembly is pressed against the ground glass which is on the film plane.
When I was testing the Speed Graphic focal plane shutter, I could see by moving the blind slowly that the photodiode was coming off the dark current before and after the slit was aligned with the photodiode. I presumed by scatter through the ground glass, which would not happen on the emulsion..
So I put the photodiode at the rear of a drilled block with a small long hole so the photo diode angle of view onto the ground glass is restricted to only about 3 degrees, or a spot about 1mm dia. ( typical clear body pd acceptance angle is around 40 degrees i think.)

I also mention that it is necessary to bias the photodiode and adjust the light intensity to keep the pd in its active region, ie it should not get near saturation. Operation from dark current to about half saturation is good.

 

[Dan Fromm]

You may be right. The type is was thinking about work something like a guillotine. A board with a circular aperture drops past the lens. There are so many styles of shutters each with their own geometries.

See http://www.suaudeau.eu/memo/Les_obturateurs/Les_obturateurs_centraux/Histoire/Les_obturateurs.html

It misses some, for example the venetian blind types used in Omera and S.F.O.M. 670 and 671 aerial cameras.

 

[j.eff]

Wow. Thanks for all the great responses. I've got a lot to read through and consider now, and some new ideas and experiments to dabble with.

Just a couple of extra points I should have put in my first post... I don't expect to end up with a marketable product. There's a very limited need, and already plenty of professional and hobbyist solutions. I'm doing this more for the experimentation and learning, and to maybe end up with a functional instrument I've designed and built. If anything, I might throw it out in a blog post or elsewhere, but anything I do will probably just be open-source.

And just for kicks, here's a screen capture from the oscilloscope of what I'm seeing with the Holga. This is using an IR emitter in front of the lens with the backless camera on top of the phototransistor. From beginning of fall to end of rise, the time is 12.7 ms, which works out to just under 1/80 sec.

 

[DWThomas]

Mayhaps it's my ancient electronics experience burping up, but I would take the width at the 50% mark as a shutter speed which, assuming those horizontal divisions are 2 mS, makes that more like 10mS or 1/100. That slowly starting downward exponential curve on the left isn't really contributing much to an exposure for the first 1.5 or more mS. But that's me -- anyway, it can be a fun project. I acquired a few parts to do one using the computer audio inputs a couple of years ago but never got around to it. (Either my shutters are working fairly well or I've magically adapted to their flaws! )

 

[j.eff]

I would take the width at the 50% mark as a shutter speed which, assuming those horizontal divisions are 2 mS, makes that more like 10mS or 1/100.

After my last post, I working with exactly that, based on input from bernard_L...

A pretty good approximation, involving no calculation, is the duration between the half-light points.

So I did some additional testing, and used a different phototransistor that didn't have such a steep fall and rise. I was getting widths at the 50% point around 9.4 ms with a peak-peak voltage drop of 3.6 V, which would put the shutter speed around 1/106. That's starting to seem like a more valid number, and I think using that strategy probably yields a more reasonable estimation than the first-last light. Now I just need to see if it yields similar success with a focal plane and leaf shutter, but unfortunately, the cameras with shutters I have the most faith in are both loaded right now. Guess maybe I need to have some film shooting experiments this weekend to empty some cameras.

Edits to fix quote links.

 

[Xmas]

___/\___

The typical between the lens shutter does not really have a 'linear' scale and you need to know this for shots with fast colour transparency film in bright light alas gone now. Only a problem in four corners, we don't get sun here.

The 1/500 (or 1/400) is the snap open and close as above diagram where if the iris is wide open you are not getting max aperture other than instantenously as the blades stop to reverse motion for a very short time.

At 1/500 & /16 the top is cut off the top so you get a flat plateau and pro rata more exposure than you might have thought. The lens is at the selected aperture for longer as opposed to 'never'.

If you are testing between the lens shutters you need to know this.

Alas a similar effect occurs with focal plane shutters especially when they are stood away from the film plane for film carriers.

For people taking photos I keep an avery label on the back of hoods as a 2 dimensional table in abstract it says between 1/125 at /8 compared with 1/500 at /16 is not four stops... Cause of the diagram above... It is three stops...

The exception between the lens shutters are eg the Werra from EGermany.

The 35mm Barnack clone shutter has other 'features' which I got flamed last time I mentioned the design is very clever indeed.

Ive never taken a larger than 35mm focal plane shutter apart.

 

[wombat2go]

I recently meaured one of the lenses in my RB67 collection using the photodiode/oscilloscope method.

 

[Xmas]

I recently meaured one of the lenses in my RB67 collection using the photodiode/oscilloscope method.

Hi Wombat

You have omitted

The aperture the lens was set to
The criteria you used for beginning and closing

Noel

 

[wombat2go]

Hi Wombat

You have omitted

The aperture the lens was set to
The criteria you used for beginning and closing

Noel

OK, I still have to test the Sekor C 150mm and 50mm in my "new" RB67 kit
The RB67 ProS body is all back together after new bellows and mirror foam, ready for a first test this weekend with the 90mm at the Henry Ford Motor Muster. I will load New Portra 160.
I will take the 2x3 Baby Graflex , just in case.

So next week I will take some time to set up to test the next Sekor lens a bit more rigorously.. I will take photos of my test setup and waveforms, check variation over aperture, and with jeff's permission, post the results in this thread.

Edit: Can anybody advise if the old Graflex RH10 backs work OK on the RB67ProS.?
They seem to clamp on OK and the shutter fires OK.
Thanks

 

[Xmas]

Try a film at varying aperture the focus might be off. There are adapters for the Mamiya press backs.

 

[Xmas]

check variation over aperture,

It is desirable to record the aperture used.