jeffmcbride.net

I’ve been working on some electronics and software to do precisely timed photography setups. One of the issues I encountered was that I seemed to have a lot of unpredictability in the delay between when I fired the shutter signal and when the shutter opened. I took some measurements to find out what range of shutter lag I needed to account for when calculating trigger times, and once I got setup it quickly became clear that there were two delay modes I was seeing: a short, very predictable delay, and a much longer and much more varied delay. It didn’t take much longer to realize what the difference was between these mode, and that this was what was causing my problems:

The D90 has a very predictable shutter lag; When it is ready to go. When the display is still active showing the image from the previous shot, there is a much longer shutter lag, and it varies a lot. So, when precisely controlled timing is required, you need to make sure the display is off before triggering. This can be done by either waiting for the shooting info display to turn off, or half-pressing the shutter button. In my setup, I wired the half-press and full-press together, because I didn’t think I’d have any reason to command a half-press. In the future, I think I will wire them both to be controlled separately, so that part of the sequence can be to send the half-press command early enough to ready the camera before sending the full-press shutter command.

The Setup

I controlled the camera via the remote shutter cable input. I had both the half-press and full-press wires connected together, so that both were shorted at the same time to the common wire with a FET. I used a microphone to detect the shutter, and connected both the trigger signal and the audio signal from the microphone to an oscilloscope to measure the delay.

The Results

When the display was still on, the shutter delay was around 210 ms. It varied a lot. At least 20 ms, probably more, but to be honest once I figured out what was going on I didn’t really record measurements for this case.

When the display had turned off, the delay was 70.2ms, +/- 0.5ms.

The capture below shows the sequence. The camera was set for 1/5 shutter time, or 20ms. I’m fairly sure that the first long sound is the mirror opening, and then the is a quick sharp sound which is the first shutter blade opening, followed 20ms later by the second shutter blade following it to block the sensor. The time between these two pulses varies as it should with exposure time.

As you can see here, the bulk of the shutter appears to be taken by the mirror movement, which takes around 40ms. It is possible that there is something else going on that the camera would have to wait for anyway, but I have to wonder why they don’t have a mode to hold the mirror open to begin with. Even in live-view, where the mirror is normally up, the D90 closes the mirror when you press the shutter button, then opens it again to take the shot. I’ve no idea why they would do that.

I’ve jumped on the e-reader bandwagon. I bought a kindle 3 a few months ago, and it was totally worth the $140. Compared to the traditional paperback, it has some downsides, but it also has some serious upsides. I guess I’ll start with the negatives:

Cons

1) Cost. First you have to buy the kindle, obviously. Then, sometimes you have to pay more for the books. There are often books which can be had for $4 shipping from third parties on amazon, for which you must pay $10 (or sometimes a bit more) to get the kindle version. On the flip side, if you do have to buy it at the new price, sometimes the kindle version is a bit cheaper, but not much.

2) Tactile navigation. Sometimes you just want to flip through a book: jump back 20 pages, read a second, then jump back 10 more, etc. Or, you just know by feel about where in the book something is, and you open and start searching. You lose this completely with the kindle. You can go the the table of contents and jump to a chapter, and you have the added ability to do a text search instead of random flipping, but sometimes you just want what I call tactile navigation.

3) Resale/lending. I can’t sell a book after I’m done with it, and I can’t really loan it to a friend. Amazon has just started a new loaner feature, but it is pretty restrictive, and only available on some books (as the publisher wishes). Basically, you can’t do it. Honestly, the only book I’m going to want to sell is the rare high value one.  Loaning is a bigger deal – I’ve already wished I could do this – but still not something I would do enough to really deter me.

Pros

1) Size. Some books are OK to carry around, some are big. More than one is almost always too much. With the kindle, any and all books fit into one slim package, which fits nicely into a cargo pocket, or is light in a backpack.

2) Convience. I never appreciated until I had it how nice it is not to have pages. You can set the reader down in front of you, without having to hold it open to your page. Outdoors when it is windy, there are no pages to get blown around. Page turns are just a button press (on either side of the device). There is never any excessive force required to open the book flat enough to comfortably read near the inner margins. I was surprised just how much nicer the kindle is to read.

3) Display. The e-ink display on this thing is awesome. I wouldn’t even consider an LCD display over this, color be damned. Easy on the eyes, comfortable in any lighting you could read a book in, never have to adjust screen brightness; it is also fine in sunlight. Not to mention the power savings. Interestingly, the display is never off. When you turn “off” the kindle, it just puts up a screensaver image until you turn it back on. You could remove the battery, and that screen will holds its display indefinitely with zero power. The only thing it really needs is color…but most books aren’t printed in color anyway.

4) Battery life. If you leave the wifi off, you can charge it every 3 or 4 weeks. You can travel with it, forget all about charging it, whatever. It is hard to run the battery out.

5) Dictionary. I’ll admit it: My vocabulary is so-so, and I don’t like having to find a computer to look up a word. With the kindle, I just put the cursor on a word I want to look up, and it searches for it in the installed dictionary, giving me quick access to the definition. I use this a lot. It’s handy.

6) Highlighting and search. Also handy is the ability to go back to interesting sections quickly. There are two ways I do this: a) Search the text or b) Save highlight sections. While reading, you can highlight a section, or add a note, and then recall those your notes/highlight sections later. It is kind of nice for someone with a bad memory, like me, to be able to finish a book, then look back at highlights to remind yourself of the interesting bits.

7) Preview.  It’s easy to download the first part of a book, and read that before buying it. This is probably as good a marketing technique for amazon as it is a convenience for me.

Overall, it has been great to have. I’ve played with the sony e-readers a bit, and they just don’t seem as polished. The browsing is not too hot, but it can work in a pinch, and it can even be useful for reading heavily text based sites..news feed, etc. You could also use it to look up a wikipedia page…if for some reason you don’t have your phone.

Hacks

If you do have a kindle, and are feeling a bit adventurous: Install this jailbreak for custom screensavers, fonts, etc.:

http://www.mobileread.com/forums/showthread.php?t=88004

If you know me at all (or have read this blog much), you probably realize that I’ve developed a bit of an interest in photography. I don’t call myself a photographer, because I think that either means “person who takes photos” or, perhaps, “Person who makes a living taking photos”. In the case of the former, the term is pointless; do YOU know anybody who doesn’t? In the case of latter, well, I don’t.  But I like having good pictures to document my life, I like learning the technology, and I like to play with new gear. Anyway, to get back on topic, I’m also an engineer, a good part of my job involves digital imaging, and I am never satisfied with the technical info I can glean from product manuals. They just don’t tell you, even remotely, how they work. So sometimes I just have to do a bit of reverse engineering to figure out how things really work.

One of the really cool things about today’s photo gear is the ease with which you can control remote flashes. Nikon calls their flash system CLS, for Creative Lighting System. Basically, you can put remote flashes “out there” somewhere, and control them using the pop-up flash on the camera body. They do the same type of “through-the-lens” (TTL) metering that is done with an on-camera flash, and they’re power levels can be set from the camera. It’s pretty cool, but I found myself asking, “how does it work?” There are at least two things, besides curiosity, which prompted me to ask this: a) I was looking for remote triggers (optical slaves) for non-nikon flashes, but could not make sense out of what would work with CLS and what might be falsely triggered by the “pre-flashes” involved, and b) I could not find any consensus in internet searches as to whether the commanding flash fires during exposure or not. Furthermore, if I could understand the system enough, maybe it would be worth just building my own optical slaves.

There is a surprising lack of solid technical info on the internet about these things (but a good supply of well-intended, but ultimately misinformed, pseudo-technical explanations). The one good source I did find (only after running my own tests, incidentally) is here.  Fortunately, I have ready access to the tools needed to measure this sort of thing, so I took some time and collected some data, so I’m going to present that data here so it is available to whoever else may find it useful.

My test setup consisted of two IR phototransistors, connected to an oscilloscope. Because I wanted each sensor to be isolated and read only the power output of one of the flashes while ignoring the other, the phototransistors I had were far too sensitive. To solve this, I ended up wrapping two layers of copper tape around them (I was surprised to find just how transmissive copper tape is in the near IR band). I then positioned the sensors about an inch in front of each flash.

To summarize my findings:

• The communication is all one way. The signal is sent from the commander flash by pulsing the flash with ~50-100 microsecond firings.
• There are always at least two command sequences. The first I’m calling the “channel sequence”, and the second the “firing sequence”. The first one seems to be a function of the channel (1, 2 or 3) and the groups being commanded (A,  B, C). The second sequence contains the firing commands (e.g. output level) for all groups being commanded (CLS can command up to 3 groups).
• When using only manual mode (where the output power for each group is set ahead of time on the camera) these are the only two data sequences. When using TTL metering mode, the channel sequence is sent, and then each group is commanded to fire a monitor pre-flash by another pulse sequence (The camera measures the amount of return light it sees from this monitor pre-flash and uses that information to determine the proper flash power level). If the return measured from the first monitor pre-flash is not strong enough, the camera will fire another burst to request a second, higher power, monitor pre-flash.
• After the firing sequence is sent, which tells each flash group what power level they should fire at, there is a delay (around 50ms), and then the commander fires one quick flash to trigger the slaves to fire. This means that the on-camera flash ALWAYS fires at a low level while the shutter is open, even if it is commanded to be off. In most cases, the power is low enough to not affect the photo. If the camera is close to the subject, or if the subject is very reflective (try shooting at a piece of glass, for example), you will see the commander flash. For this situation, nikon sells an IR pass filter which will block the visible light from the flash, while allowing the infrared light to pass through to trigger the flashes. Your lens has an IR blocking filter, so it will not affect the exposure
• For Manual mode, the firing command is very straightforward to interpret. Each firing level is assigned a binary value, starting at 0 for 1/1, 1 for 1/1.3, etc. I had hoped that the firing sequence would be the same for TTL mode; but it is not. The commands are completely different when in TTL, and I’m not sure how to interpret them.

In the following scope captures, the green trace is the slave flash, and the yellow trace is the pop-up commander flash.

This first captures shows the most basic sequence, with one flash group set to manual mode. You can see the channel sequence, followed by the firing sequence which says, basically, Group A, fire at power level 1/1, and then after a 55ms delay, the commander flash goes off (followed only a microsecond or two later by the remote flash).

The next to captures show the flash being fired in TTL mode. In the first case, I put a target up close to the camera and flash, so it got a strong return. In the second, I removed the close target so that the camera was shooting more like 15 ft away. In this case, it requested the second higher power monitor pre-flash.

The next capture shows the channel sequence, which is the first burst in the above captures. The first three pulses designate which channel is being commanded. I believe the rest of the pulses say which groups will be commanded. What is interesting here is that, for the first three pulses at least, it is not as simple as, “if a pulse is fired during this bit time, it is a one, if not it is a zero”. Instead, the time between pulses is varied. In one case, the time between pulses is about 140us, in the other it is about 200us. Channel 1 is short-long, Channel 2 is long-short, and channel 3 is long-long.

Finally, this capture shows the “firing sequence”, from several man mode test cases, to illustrate the binary counting. Man mode commands are fairly easy to figure out. When multiple groups are used, the firing sequence is extended, with each additional group firing level specified in the same way. Those with a bit of binary counting experience will see that at the end of each of these sequences is a counter, increasing by one each power level.

I guess that is enough for now. I have a bit more data, and can capture more as necessary, so feel free to drop me an e-mail if you want to discuss any of this further. My main take-away here is that implementing an optical slave that is “CLS tolerant” should be trivial: Wait for a pulse, then wait for 40ms or so without a pulse, restarting the wait counter anytime another light pulse is detected. After 40ms of “dead time”, the next pulse will be the fire command. But more interestingly, implementing one that will take its power level commands in manual mode from the camera should not be difficult, which means I could buy an $85 chinese flash and control it as a CLS manual slave, if I could just figure out how to control the power output of the Yongnuo flash electrically (I have reason to believe this can be done using a pulse width on the shoe connection). The value of this may be questionable since I could A) just buy the cheap flash and set its power manually using the buttons on the flash or B) buy a $200 nikon SB-600 and get full CLS compatibility without the hassle, but it would certainly be more fun to do it myself.

Last week, I got a Nikon 70-300mm VR lens. Tonight when I got home, I found my new Manfrotto 190XPROB tripod and a Manfrotto 222 joystick head waiting at the doorstep. I put the new lens on the tripod, and love how stable and easy to adjust this thing is, it is FAR superior to the cheapo tripod I picked up at REI last year. On the down-side though, the weight is more than doubled (at a little over 5lbs, I think), and I’m not sure I REALLY want to add this thing to an already heavy backpack. In any case, once I got bored of the novelty of slewing the joystick around, the search began for a subject to photograph. After scouring the apartment for random objects, I ended up with these two:

As for the lens: So far I’m giving it two thumbs-up. The VR is noisy. It turns on when you half depress the shutter button, and turns off a while after (to save power, presumably), and it makes a hell of a lot of noise when it “clicks” on. However, I really don’t care, because it does a hell of a job. I can take 1/30th exposure shots at 300mm with reasonable success. It is sharp. Sharper, I think, than the 18-105mm VR kit lens that I got the with the D90. And finally, I’m really happy with the depth-of-field you can get at the long focal length. I guess if you can’t afford an f2.8, just back up and go longer.

I got a Camelbak fourteener backpack about a year ago, and have taken it all over since then.  It is probably the best backpack I’ve owned. While climbing Mt. Baldy a month or so back, one of the plastic clips that holds the sternum strap onto the shoulders of my pack broke off. I sent an e-mail to Camelbak telling them it broke, and asking if I could get just a replacement part.  I was more than happy to pay a few bucks for the part, I just really didn’t want to buy another $120 backpack. But, they did even better:

Hello,

Thank you for contacting CamelBak.  The defect you have encountered should not be expected from any of our products.  We stand firmly behind the quality of all the products we make, and I apologize for this inconvenience. Just let me know your U.S. address and I’ll be happy to send a replacement sternum strap for you.

I got the new strap last week, and it’s back to being good as new (just dirtier). Just wanted to share the pleasant response, and say that I can strongly recommend camelbak gear, even if the clip did break.