Vibration and Exposure

[rjlittlefield]

Unfortunately second-curtain flash is disabled on Canon Speedlites when used wirelessly (master or slave)

Seems like it's disabled most of the time I'd like to use it. That's really annoying because I can't think of any technical reason why the feature is not available all the time for all types of flashes. After all, it's just a sync pulse and surely the camera knows when the shutter is about to close.

Is this the case even if the flashes are in manual mode? And to be clear, it is the speedlites on which second-curtain flash is disabled, not the camera body?

The combination of my T1i and Speedlite 580 EX II will provide second-curtain sync, even in E-TTL mode. The option can be set either by menu on the camera or by button on the flash. The deficiency at the root of my badly phrased grousing shows up when a non-dedicated flash is attached. In that case the camera's external flash menu is disabled, and I can't find any other way to turn on second curtain sync. The camera will happily drive a basic center-pin-trigger device, but first curtain only. I'd like to say that I have no idea why Canon does it that way, but really I suspect it's marketing.

Steve's complaint is different.

--Rik

[BugEZ]

The posts to this thread are quite interesting. Thanks for the interesting ideas and comments. I had suspected that there would be replies about the resemblance of the character in the schematic to cookie monster but you all refrained.

Chris S. and Rik suggested that I “gate” the lighting (gate in this context means to use electronic logic to turn the light on and off) turning the light on approximately .3 seconds or so after the shutter opens. Rik observed that my home built and programmed actuator controller allows this. Rik is correct. I am able to switch my LED lighting on and off using the controller. This is a good idea and I will add a transistor and jack to the bucket light’s regulation circuit to interface with my controller. I have such a gate circuit for the low power supply that I use with my less powerful lights (~ 30 LEDs) and the interface cable between the controller and the regulator box is ready to go. I discontinued the use of this capability as it did not appear to make much difference in sharpness. I now understand why. The 1.5 -2 second exposures I needed with the dimmer lights were long enough to provide low mean displacement.

I certainly appreciate the comments from naturephoto1 and Harold Gough regarding their experiences with film cameras where they learned to avoid short exposures in the .1 to .5 second range.

There are also many interesting discussions on flash exposure. I must defer to those who use flash in the studio. I only own one strobe and it was purchased in 1976. I did pull it out about 3 weeks ago and it still works. It allows some automatic compensation for distance (a light sensor that turns the strobe off after the integrated returned light reaches a certain threshold) and presumably could be doctored to make a very brief pulse. (Charles Krebs has described this very well). I did a quick experiment where I fooled the strobe to provide the minimal pulse using a reflector. I took some photos at 10X and compared the sharpness of the individual frames to 2 second exposures. They were very similar in sharpness. They could only be distinguished by the large difference in color balance as the strobe light had stronger “red” content. I did not wire the strobe to my rig and attempt a stack as I feared the old strobe’s interface voltage might fry my DSLR’s electronics.

It is possible to use the equations from my original post to predict the mean displacement amplitude for flash exposures. I don’t know how long a flash actually flashes. A quick web search turned up multiple values ranging from 1/200th to 1/5000th. If one assumes that the flash pulse is approximately 1/1000 long (.001 seconds), and the camera is set at 1/60th for sync and plug this into the equations, the average displacement magnitude will be .0000614 inches during a first curtain sync flash and .0000238 inches during a second curtain flash. For this example, a 2 second time exposure would be superior in sharpness to a flash exposure (lower average displacement) by a factor of 3X.

If the second curtain flash is after a 1 sec exposure, the rig should be essentially stationary and the displacement magnitude will be zero. This is obviously satisfactory.

Enough for now, time to go wire a gate into the bucket light!


Keith

[Chris S.]

Keith, the character in your drawing looked to me like something out of Gary Larson—which gets an automatic thumbs-up.

Gating your bucket light—the ability to do that strikes me as one of LEDs’ most attractive characteristics for macro lighting, so it will be interesting to watch your results. I use both flash and halogen continuous lighting (not at the same time), and for some time have been thinking of adding electronic shutters to my halogen lights in order to have darkness for the early portion of the exposure, when shutter movement is dissipating. With halogen lights, of course, I don’t want to cycle the power, as this would kill bulbs quickly and give me weird color problems. My current approach is to use long exposures—maybe 8 or 16 seconds—so that the settling period represents a small fraction of the total exposure; but in deep stacks, this costs a lot of time.

FYI, the flash durations on my old Nikon SB-26 speedlights range from 1/1000 second at full power to 1/23,000 second at 1/64 power. These durations were fairly typical, though they may not be similar to your flash. It does seem slightly unusual that your flash has an automatic setting that seems to reduce duration, but does not permit manually choosing a lower power?

I’m curious as to how, in your laser tests, you measured displacement so finely. And to be clear, do your numbers describe displacement of the reflective foil, or of the projected laser beam? If of the reflective foil, I take you are using angles, distance, and math to work back from displacement as projected to calculated displacement at the reflector? Also, do I understand correctly that your settling times are calculated, as opposed to measured? How did you determine values for f and k?

Very interesting project! Thanks for sharing it with us.

Cheers,

--Chris

[BugEZ]

Chris S. wrote:

FYI, the flash durations on my old Nikon SB-26 speedlights range from 1/1000 second at full power to 1/23,000 second at 1/64 power.

If the flash duration is 1/23,000 sec then the mean displacement would be very low, definitely lower than with a 2 sec exposure per my equations. The cross-over point where a 2 sec exposure is equal to a first curtain flash is if the flash duration is about 1/8000 sec.

Chris S. asked:

I’m curious as to how, in your laser tests, you measured displacement so finely. And to be clear, do your numbers describe displacement of the reflective foil, or of the projected laser beam?

Hmmm... I am starting to get a bit nervous here. I feel a bit like the wizard in the Wizard of Oz movie and you are peeking behind the curtain... The distances I estimated (.001 in for the shutter bounce and .003 for the mirror bounce) are based on the shift of the reflected pattern on the wall. I did not accurately measure this with a ruler but approximated. The wall where the pattern was projected was approximately 10x farther from the rig than the laser so I divided the patch motion by 10 then picked the "Disp0" term in the equation to be half of that since I observed a "total amplitude" and the Disp0 them is a "half amplitude".

The absolute magnitude does not matter as much as the relative magnitude, and I definitely got the proportions right.

Chris S. also asked :

Also, do I understand correctly that your settling times are calculated, as opposed to measured? How did you determine values for f and k?

Hmmm... more behind the curtain stuff... The k term is the decay constant and was chosen so that the observed decay approximately matched the output of equation. Things were definitely steady after about a half to a quarter of a second, even with the large mirror snap, so I picked a k term that matches that. I would add that this is a rather subjective observation. The f (frequency) term is also a bit subjective. The rig (or any structure for that matter) will have many vibration modes and the damped decay of each mode will vary a bit. It appeared to me that the motion in the oscillatory "blur" of the patch as it settled was nearly discernable, but not quite. I take that to be something a bit faster than 15 Hz so I chose 20. Again a more subjective choice than a measured value.

Hope this helps. These are good questions. My posting appears a bit more "studied" thatn it actually is. If I can locate an inexpensive phonograph cartridge, I will fashion a crude accelerometer and make some proper measurements of settling time and vibration frequency and relative amplitude on my rig. Perhaps Charles Krebbs comment about vibrations:

...it seems to me that the key thing is to be aware they exist and tackle them appropriately for your set-up.

is most appropriate.


I have figured out my "gate" scheme to allow me to switch on the bucket light and made a schematic. I have to buy a switch to allow me to choose between an "automatic" and "manual" mode on the power regulator, but the rest of the parts are in-hand. I should have it wired up in a few days.

Kind regards,

Keith

[DQE]

Unfortunately second-curtain flash is disabled on Canon Speedlites when used wirelessly (master or slave)

Seems like it's disabled most of the time I'd like to use it. That's really annoying because I can't think of any technical reason why the feature is not available all the time for all types of flashes. After all, it's just a sync pulse and surely the camera knows when the shutter is about to close.

Is this the case even if the flashes are in manual mode? And to be clear, it is the speedlites on which second-curtain flash is disabled, not the camera body?

The combination of my T1i and Speedlite 580 EX II will provide second-curtain sync, even in E-TTL mode. The option can be set either by menu on the camera or by button on the flash. The deficiency at the root of my badly phrased grousing shows up when a non-dedicated flash is attached. In that case the camera's external flash menu is disabled, and I can't find any other way to turn on second curtain sync. The camera will happily drive a basic center-pin-trigger device, but first curtain only. I'd like to say that I have no idea why Canon does it that way, but really I suspect it's marketing.

Steve's complaint is different.

--Rik

I am not sure the link below is relevant, but perhaps something in the details of this fellow conquered his Canon camera's refusal to allow him to use second-curtain flash freely will be helpful to others:

http://www.rodandcone.com/2008/09/how-t ... -rear.html

(Please excuse this post if it's accidentally off-topic and otherwise irrelevant.)

[BugEZ]

I have modified the power supply of the Bucket Light to allow me to switch it on and off under the control of my stacking controller. To facilitate this I completely rewired the light’s power supply. The "baseline" supply was quite compact in that I had the regulating circuits packed on a circuit board that was about 1.5" square (4cm X 4cm). The modified circuit board has about twice the footprint and incorporates larger resistors (higher power dissipation rating) and some space for future modifications if required.

I still need to tweak the remote switching circuit in the power supply to improve the immunity to electrical noise. The tiny bit of electrical hum picked up by the cable between the controller and the supply causes it to illuminate very slightly when it should be completely off. I will bias the switch input so that it is fully "dark" when it is supposed to be.

I have reprogrammed my stacking controller to allow approximately 1 second of settling time after shutter actuation before turning on the bucket light. The light then cycles off after a delay that is adjustable based upon the position of the “delay” pot on the controller chassis. I need to adjust the controller software to allow the “delay” pot setting to represent the illumination time and still provide time for the camera to finish buffering the shot to memory, even if the “delay” (now exposure) is brief. The original intent of the “delay” pot was to set the rate of photos in the stack.

Last night and this AM I made a few test exposures. The results are interesting. The "gated" photos are better (less blur) than the 1/4 sec exposures. Surprisingly, it was difficult to distinguish a gated exposure from a non gated 1/2 sec exposure.

I purchased an inexpensive used phonograph cartridge at a Hi-Fi repair shop. I have plans to make it into a vibration monitoring device. I think I can amplify the signal a bit and feed it into my PC’s microphone port. The signal can then be recorded and displayed. That should be rather fun.

Kind regards,

Keith

[ChrisR]

Using a lot of diffusion for a 100x objective I once needed full flash from a Nikon SB900. Mirror lockup and first curtain sync certainly showed vibration, which disappeared when I switched to second curtain sync and a 2 second delay. (It was a Nikon, so less contrary than my Rebel). That was the only interesting result of the shoot, apart from my learning to be less ambitious.

[BugEZ]

I have finished the mods to the bucket light power supply and my stacking actuator/controller to allow the controller to make shots with the controller turning on the bucket light after the shutter has opened and the rig has had some settling time. This included fixing the electrical noise issue so that the bucket light goes completely dark when switched off by the controller. It also involved some software mods to the controller to allow the “delay” pot to become the “exposure” pot.

I did some test shots with the rig operated in various modes to compare sharpness. The figure below describes of the three stacks made for the comparison. One stack was shot with the no switching of the bucket light and the camera’s shutter set for a 2 sec exposure. For this stack the bucket light reduced in intensity to allow proper exposure. The second stack was shot no switching of the light and the camera’s shutter set to 1/6 second. The light was set to maximum brightness. The final stack was shot with the bucket light switched on and off by the controller. The camera’s shutter was set to 2-seconds, and the controller cycled on the light 1 second after the shutter had opened, then off 1/6 second later. The shutter then closed.



The comparisons are shown in the figures below. The “normal” 1/6th second exposure shows some blur due to rig vibration. The 2 second exposure and the and the pulsed exposure both show similar sharpness.





Keith

[ChrisR]

Well done that's nailed it then Keith! It looks like you only need to wait about a third of a second before turning your bucket on?

[Peter De Smidt]

This was a very well done test.

[BugEZ]

Thanks Peter and Chris for the kind comments.

There appears to be a color shift between the photos exposed with different bucket light brightness settings. I had not noticed this previously. I'll have to check that out. My color balance may need to be adjusted when I fiddle with the intensity... I will have to explore this on the web to see what the spec sheets for LEDs say on this topic.

I have sketched a schematic for a pre-amp to allow me to use a phonograph cartridge for direct vibration measurement. Parts are in hand. That will keep me busy for a few days...


Keith

[rjlittlefield]

Excellent test and documentation. I'm looking forward to seeing what your instrumentation says about the vibration.

--Rik

[ChrisR]

pre-amp to allow me to use a phonograph cartridge

Hmm good luck. Did some work with accelerometry years ago, which makes me think you'll have to be a bit lucky at least.
There are accelerometer modules for things like Arduinos - may be worth a look if you have trouble with the your cartridge's range/frequency/sensitivity/resonances...

[Chris S.]

Keith, a very nice test! And well presented.

My tired brain was momentarily tripped by the use of "exposure" for what I think of as "shutter speed." For scenarios in your test, I'd suggest that "exposure time" is the intersection of "shutter open period" and "LED on period." I've occasionally kept shutters open for several minutes in very dark rooms, with the exposure created by a brief burst of flash somewhere during those minutes. Since my exposure was gated by the flash duration, rather than the shutter-open time, I reported exposure time as something like "1/60,000 second, by high-speed flash."

It will be interesting to hear what you find out about color shifts with short-duration LED pulses. If you're using phosphor-based white LEDs, my gut feel is that what you're seeing here is not too surprising.

Regarding the use of a phonograph cartridge--do you see any benefit to this vs. an inexpensive three-axis accelerometer, other than eliminating the need for a tightly regulated current supply? I imagine you're going to run the output into an Oscilloscope? I've been thinking a lot about experimenting along these lines.

Keep up the great work!

Cheers,

--Chris

PS--Thanks for your informative reply to my earlier questions.

[BugEZ]

Chris S. wrote:

My tired brain was momentarily tripped by the use of "exposure" for what I think of as "shutter speed." For scenarios in your test, I'd suggest that "exposure time" is the intersection of "shutter open period" and "LED on period."

I completely agree that my interchanging exposure and shutter speed was confusing. I very much like your suggested nomenclature. I will adopt it!

ChrisR wrote:

Did some work with accelerometry years ago, which makes me think you'll have to be a bit lucky at least.
There are accelerometer modules for things like Arduinos - may be worth a look if you have trouble with the your cartridge's range/frequency/sensitivity/resonances...

This concerned me. I am a mechanical engineer, and only do electronics as a hobby. 30 years ago my boss was fond of saying "better lucky than smart" usually about me... I managed to get the phonograph cartridge sensor to work reasonably well, so I guess my luck is holding.

I am spawning a new thread to describe the phono-cartridge sensor I used for vibration measurements. While not perfect, I am satisfied with the result and others may wish to do something similar.

I also think items related to color shifts when white LEDs are dimmed belong in a different thread. So if I find out something I will share it elsewhere. If someone else discovers some info on this first, it would be fine for them to post it.

Kind regards

Keith