Smart Phone or Tablet for testing a leaf shutter?

[drew bedo]

<p><strong><em>IPhone or Android Shutter Tester?</em></strong></p>

<p>I am an e-idiot (and maybe an i-idiot too). With that said: Is there a way to use an iphone or ipad to test the accuracy of a leaf shutter lens?</p>

<p>I would hope that there is some way that reads out directly in fractional shutter speeds, or even milliseconds.</p>

<p>Anybody know how or have any ideas on this?</p>

[joseph_wisniewski]

<p>Is $20 hardware acceptable?</p>

<p>There's nothing on the typical smart phone that's optically fast enough to do the job, but a photodiode on the audio port can give you 50 microsecond resolution.</p>

[richardsperry]

Is the PC flash connection the output?

 

Does it stay shorted the duration of the shutter?

[drew bedo]

<p>Joseph: Thanks for the suggestion. Please re read the first line of my post and explain your idea. I have a glimmer but not the whole picture.</p>

[sarah_fox]

<p>Drew, put the camera on a tripod, and photograph something moving at a known rate of speed. Or if we're talking about just a shutter mechanism, hold the shutter over the front of the lens on a digital camera, and do the same thing. Use the "bulb" setting on the camera. Determine how far the moving object has physically moved during the exposure, can calculate from there to determine the shutter speed. For instance, let's say a car is traveling 30 mph down the road. That's 44 feet per second. (Google "30 mph in feet per second" for the conversion.) Let's say the car moves a half a foot during the exposure. You would calculate 0.5 / 44 = 1/88 of a second (.011 sec).</p>

<p>When doing this, remember that it's a leaf shutter, and not a focal plane shutter. There will be a ramp-up and ramp-down time, during which the leaves are opening and closing. Perhaps you could measure movement from the middle of each ramp period. Same issue with a photodiode or PC flash connection.</p>

<p>PS You can do this same test with the camera held stationary inside a moving car. Photograph something of known dimensions along the side of the road (e.g. a mailbox) while moving at a known speed. Your distance from that object doesn't matter, as the measurement would be with respect to the object itself.</p>

[drew bedo]

<p>Thanks Sarah. Your suggestions are appreciated.</p>

<p>These methods are known to me. Aditionally, I have access to several quality repair shops here in Houston Texas. I am interested in hearing from someone who understands photography AND smart phones/tablets about how to work with this type of gear to easly test lenses in the field. While I don't know how myself, I will be surprised if it is not possible.</p>

<p>Is Joseph's idea workable?</p>

[sarah_fox]

<p>Sure, Joseph's idea would work. All you'd need is a small voltage source (battery) and an oscilloscope. There are numerous very affordable oscilloscope modules you can interface via USB with a laptop computer. Look on Ebay for the DSO-2090. I have that one. Would that do the job for you? Better still, check out the "SUPER MINI DSO203," which is a handheld stand-alone device with LCD display. Very slick!</p>

<p>Edit: Oh, a photodiode on the audio input. Well, I suppose that would work, too. You'd just need the software to look at the waveform.</p>

[drew bedo]

<p><strong><em>" Oh, a photodiode on the audio input. Well, I suppose that would work, too. You'd just need the software to look at the waveform."</em></strong><br>

<strong><em></em></strong> <br>

<strong><em>Right: Thats the kind of thing I'm interested in. How can this be done with a smart phone? What applications would work . . .elaswhere I have read about using audio software to determine the duration of the sound of the shutter. Will the photodiod plugged into the phone make an audio signal .? </em></strong></p>

[sarah_fox]

<p>Actually, the photodiode (or photovoltaic cell) would generate a low voltage pulse as light hits it. That should create a good enough audio signal to see/analyze with the right application. You'd only need to know the duration of the pulse. Whether any of the tablets or smart phones you're considering have a microphone input is another question. Still another question is whether you can find an app that will allow you to measure the length of the pulse.</p>

[tom_mann1]

<p>Sarah, Joe - There's going to be a problem in trying to use the audio inputs to measure slower shutter speeds because the audio inputs are almost always rolled off with a 20 Hz (or higher) high pass filter . The corresponding 1/e time constant for 20 Hz is around 8 msec. So, the pulse shape won't be too distorted for shutter durations significantly shorter than about 8 msec (ie, 1/125th of a second). As the shutter speed slows down, the waveform will become progressively more distorted and begin to look more like the derivative of the input signal. Most folks won't know how to interpret a waveform that looks like this.</p>

<p>Tom M</p>

<p>PS - One could think about just measuring the overall time duration and ignoring the shape of the waveform, but I would hate to do this since one can glean much info from the shape of the waveform. </p>

[sarah_fox]

<p>Tom, I was thinking just what's in your PS. You might not get much of the fundamental, but you'd at least get some of the harmonic content. Besides that, it's hard to know where a rolloff is placed. I've seen much lower input rolloff frequencies than 20 Hz. Of course you're correct that there's potentially a lot of useful info in the waveform, not the least of which is the duration of the ramp-up and ramp-down in brightness.</p>

<p>Drew, here's another idea: How about using a flash meter? Compare an "ambient" reading with the shutter on "bulb" to a "flash" reading with the shutter timed. The flash meter will think the pulse of light is a flash and will summate the total amount of light passed through the shutter. Then pick an aperture, any aperture. The "flash" at f/whetever will give you the same exposure as a certain shutter speed at that same f/whatever. That's your effective shutter speed. :-)</p>

[tom_mann1]

<p>Sarah, I really like your integrated energy approach. While it should be very accurate for short exposures, some flash meters may have an upper limit on on integration time. The OP would have to check that. Unfortunately, since the OP asked for a smartphone or tablet based system, I'm not sure how to integrate that with the flashmeter.</p>

<p>Tom M</p>

[drew bedo]

<p>Sarah, Joe and Tom:<br>

Thanks for making this an informative and productive discussion.</p>

<p>I was hoping that someone would know of one or a suit of "magic-apps" that would let me test shutter speeds on the fly as I set up the gear in the field, or at least from day-to-day. Surely a shutter changes behavior at extremes of temperature.</p>

<p>I am going to experiment with the flash meter technique. My main interest is in simply finding the shutter speed. I am guessing that the ramp portions of the open-close cycle are similar at all speeds for practical purposes. a read-out in fractional shutter speeds would be great but milliseconds would work. As I am working with view camera lenses and large-ish leaf shutters, shutter speeds will be 1/400th or below.</p>

<p>In any case, I am still wide open to ideas from anyone—using a flash meter and/or a smart phone.</p>

<p>I remaine open to other inputs concerning smart phones, light meters and tablet computers. Thee has to be an O-scope app out there—right?</p>

[sarah_fox]

<p>Well, just a thought... You ask in your original post whether there's an iPhone or Droid app. I presume that means you have a budget in mind for the solution. Why not simply buy the handheld O-scope for approx $150? It has its own display and appears to be about the size of a smart phone. It looks like a slick little device. It's made by the same people who made my USB O-scope module, and I have absolutely no complaints about that device. You'd be amazed what analytical capabilities it has. I swear it packs twice the capabilities as the old Analogic DATA 6600 I used back in the day (a very expensive and sophisticated waveform analyzer).</p>

<p>Anyway, I don't know whether there's an O-scope app for a smart phone. I can only say that it wouldn't give you DC -- only AC. All you really need, considering the hardware limitations, is an accurate time scale to measure the length of the pulse. Perhaps audio editing software would give you that.</p>

[drew bedo]

<p>Sarah: Thanks for all your great input. You are probably right about the oscilloscope. Using the right equipment for the job is always the best option in the long run. I do not have a smart phone at this time (yet). If I bought the o-scope, I would still not have a smart phone. Part of the attraction of the phone-as-shutter tester is having something that the wonkiest of my other camera collector pals don't have. </p>

<p>From another forum, it seems that there may be an audio progeram that will work with the photodiod idea. We'll see if I can justify a new phone here at home ("But its a phone , not a camera.")</p>

[sarah_fox]

<p>The hopeless wonk in me actually understands. Good luck making it all work. :-)</p>