If I use flash, Is everything perfect?

jotafoto · Post by **jotafoto** » Tue Jul 12, 2011 8:34 pm

It is clear that if we do photos with flash, and fired the first curtain at higher magnification to 30X, you get an image with a slight shake. I did not know is that with the second curtain flash with an exposure time of 5 seconds ... You can get a small or large jitter, depending on a couple of factors. Is the flash freezes it all?,

I made this picture with times of 15 seconds, illuminating, with three LEDs 5 Wats.
It is a 40X with BD Plan 40 / 0.65 increments of 0.0015 mm. with stack shot. I know it's an outrage, and the stack and can not be perfect, but needed to do the experiment unless I was in the room. Program the controller and went outside to not interfere in the results with my breathing or my movimiento.El end is not photo quality, but note a fact that has disturbed me.

And this photo I made with the second curtain flash with 5 seconds of exposure. Everything was under total control, the flash was independent subject of the camera at a different table than the optical equipment and camera attached to an articulated arm. The time of 5 seconds is more than enough for all is still. Why has this happened?

I've done several tests and have come to the conclusion that if the flash is too close to the sample, 2-3 cm and fire with enough power (this photo was a 1 / 4 power of a canon 580 EX) and sample is very fragile and lightweight, heat stress produced by the flash, instantly creates pressure on the air molecules acting against the sample while it is emitting light. The result is a picture vibrated like this. If anyone thinks I'm wrong and has a better explanation, please say so, because I do not see another. Thanks

rjlittlefield · Post by **rjlittlefield** » Tue Jul 12, 2011 11:47 pm

Javier,

I agree with your explanation.

But I notice your test cannot distinguish between two cases:

1. Pressure originates at the wing when it absorbs light and heats up.

2. Pressure is generated at the flash and propagates as a sound wave that moves the wing when it arrives.

It would be helpful to know which is causing the problem because one is easy to fix, the other is not.

These cases can be distinguished (and case #2 fixed) by moving your flash back some distance. The speed of sound is about 350 meters per second. At 1/4 power, the duration of a 580 EX II flash is about 0.3 ms (personal measurement). So while the flash is going on, there is time for a pressure wave to travel about 10 cm. At 1/2 power, the duration is about 0.6 ms, so about 20 cm. Move back farther than those distances, and if there is any blur it will have to be due to pressure generated at the subject. If you set the flash at its longest telephoto setting and point it accurately at the subject, I think you will still have plenty of power even well beyond 20 cm.

By the way, the 580 EX II (at least my unit) has a strange feature that at full power the flash lasts for over 5 ms and has an odd piecewise curve. At full power minus 1/3 stop, it suddenly drops to only 0.8 ms with the normal rapid rise, flat top, and rapid falloff, and below that it scales nicely down to 1/128 power where I'm showing a rise and fall (no flat top) with an effective duration below 0.08 ms. (It's probably well below. I'm measuring 0.08, but I suspect my instrumentation is too sluggish down there.)

If you are still set up and can run a third test with a larger distance between flash and subject, I would be very interested in the results.

--Rik

Charles Krebs · Post by **Charles Krebs** » Wed Jul 13, 2011 11:22 am

No question about it... electronic flash can produce a pressure wave that can move a light-weight subject.
(Mentioned a few times on the forum... two are here, and here.)

This pressure wave is obviously miniscule compared to those made in arc flash accidents sometimes encountered by electrical workers (literally blow a man across a room). But if the flash is placed very close, and directly at a subject such as a butterfly wing, midge antenna, or other low mass, unsecured subject then you can get the results shown here.

Harold Gough · Post by **Harold Gough** » Wed Jul 13, 2011 11:37 am

Charles,

Your link doesn't work for me.

I raised this subject:

http://www.photomacrography.net/forum/v ... ssure+wave

Harold

Charles Krebs · Post by **Charles Krebs** » Wed Jul 13, 2011 11:41 am

Harold,

Fixed the link and added yours as well. Thanks for the heads-up.

jotafoto · Post by **jotafoto** » Wed Jul 13, 2011 7:35 pm

rjlittlefield wrote:Javier,

I agree with your explanation.

But I notice your test cannot distinguish between two cases:

1. Pressure originates at the wing when it absorbs light and heats up.

2. Pressure is generated at the flash and propagates as a sound wave that moves the wing when it arrives.

It would be helpful to know which is causing the problem because one is easy to fix, the other is not.

These cases can be distinguished (and case #2 fixed) by moving your flash back some distance. The speed of sound is about 350 meters per second. At 1/4 power, the duration of a 580 EX II flash is about 0.3 ms (personal measurement). So while the flash is going on, there is time for a pressure wave to travel about 10 cm. At 1/2 power, the duration is about 0.6 ms, so about 20 cm. Move back farther than those distances, and if there is any blur it will have to be due to pressure generated at the subject. If you set the flash at its longest telephoto setting and point it accurately at the subject, I think you will still have plenty of power even well beyond 20 cm.

By the way, the 580 EX II (at least my unit) has a strange feature that at full power the flash lasts for over 5 ms and has an odd piecewise curve. At full power minus 1/3 stop, it suddenly drops to only 0.8 ms with the normal rapid rise, flat top, and rapid falloff, and below that it scales nicely down to 1/128 power where I'm showing a rise and fall (no flat top) with an effective duration below 0.08 ms. (It's probably well below. I'm measuring 0.08, but I suspect my instrumentation is too sluggish down there.)

If you are still set up and can run a third test with a larger distance between flash and subject, I would be very interested in the results.

--Rik

Yes, Rik, I have tried with less power, 1 / 128, a short distance 3 cm, and I also tested with the same power, 1 / 4 but further, to 15 cm, and none of praise both cases produced the trepidation. I have not done more tests. I guess the position of the sample, and how to hold, 100% influence on the result. I guess a fly on a skewer subject not have a significant charge against these ... say impacts of heat. In my case I think the effect was increased, because the butterfly wing, takes effect and is easy ensures that whatever move it. The reflectors also help to curb the wave of heat before reaching the sample, so I will see that the wave can not reach, below the sample. Thanks.

Charles, It's something that I could not believe it could happen!. Light can not move anything but his energy released in a very short time in the presence of a medium like air, yes. I am glad to think that my guess was in a good way. From now on will be another factor to consider for the perfect stacking, when we got a lot of magnification.

Harold Gough · Post by **Harold Gough** » Thu Jul 14, 2011 12:00 am

A suggestion: try a transparent filter in front of the flash tube, angled at, say, 45 degrees upwards. This will divert and eliminate any pressure wave from the light path without confining it against the front of the tube (as my closely-attached filter method did) and possibly introducing other effects (which I did not observe but did not look for).

Hrold

Guido · Post by **Guido** » Thu Jul 14, 2011 12:06 am

Looks like a ghost image to me, one from the flash and one from the existing light.

mgoodm3 · Post by **mgoodm3** » Thu Jul 14, 2011 8:10 am

I have always wondered about flash with a long exposure. Seems like the only advantage is that you can fill in your exposure with ambient light once the flash is off and the shutter still open. If you are shooting in a dark room, I don't see a lot of point in it.

Big disadvantage in that you have light leaking into the image that likely has a different color temperature from the flash.

I use continuous lighting almost all the time for my subjects, so flash is a bit of a mystery to me.

Charles Krebs · Post by **Charles Krebs** » Thu Jul 14, 2011 10:33 am

mgoodm3,

I have always wondered about flash with a long exposure

A reason for using it is to entirely eliminate any vibration effects from the shutter (and of course mirror, if it is cycled). It seems like it would be overkill in most cases (and it might be). But I know from working with wet mounts on a microscope that it can be very obvious through the eyepieces when a mechanical first shutter curtain stops its travel. It is observed as a high frequency "jiggle" in the water and subject. The question is...is the flash duration fast enough to negate that? In some cases it surely is, but in others, when the flash is going off at close to full power, it may not be.

mgoodm3 · Post by **mgoodm3** » Thu Jul 14, 2011 10:52 am

So basically you can fire the shutter, wait for the vibration to die down, fire the flash, wait a second or two and close the shutter. Helping to decrease the shutter vibration in the image...

I can see where that would be helpful at very high mag.

Guido · Post by **Guido** » Fri Jul 15, 2011 4:05 am

Long exposure and flash means that your light comes from two sources with a different wavelenght. This means a difference in focus for each type of light. The extreme small DOF in this size is a possible cause for this result.

Chris S. · Post by **Chris S.** » Fri Jul 15, 2011 3:35 pm

I've found subject movement due to flash to be a really big problem, and have experimented with it quite a bit. Two conclusions:

1) The most important (and difficult to deal with) source of movement from flash appears to be Rik's case #1--the subject absorbs light and heats up quickly, exciting nearby air molecules which push on the subject and move it. It's exactly like a Crookes' Radiometer. (Note that many incorrect explanations of the Crookes' Radiometer exist, and including some that came packaged with retail versions of the radiometer. Contrary to some sources, the light photons themselves are not materially pushing the subject.)

2) Due to this movement, for many subjects flash is simply not a usable light source. I came to this conclusion with real reluctance. I've long preferred flash for macro, but some subjects will hold still only in continous light.

I could describe quite a few frustrating trials that support those conclusions, but experience with just one subject demonstrates a lot. This was a glass fiber filter on which some very small carbon formations had precipitated. Such carbon is very black--and so absorbs a lot of light; the fibers mostly appear reflective, depending on orientation, and so absorb very little light. With a 50x or 100x Mitutoyo objective and polarizing filter, a lot of light is needed to reveal any detail in the carbon. With every flash configuration I've tried, the fibers without carbon deposits remain still during exposure, but fibers with carbon on them move wildly--so wildly that they at best appear as a gray cloud, and at worst seem to disappear entirely. Under continuous light, they can be easily imaged.

Lots of photons are hitting the glass fibers--both those with carbon deposits and those without. Only the fibers with carbon--which absorbs visible light and surely releases it as heat--jump during exposure. With continuous light, I'm of course hitting the subject with just as many photons, but am spreading this energy across perhaps 5 seconds, instead of something between 1/1000 and 1/30,000 second. This makes the difference between jumping and non-jumping carbon.

(I'm traveling, and so may be very slow about keeping up with this interesting thread.)

Cheers,

--Chris

Charles Krebs · Post by **Charles Krebs** » Fri Jul 15, 2011 6:02 pm

Guido,

Long exposure and flash means that your light comes from two sources with a different wavelenght. This means a difference in focus for each type of light

If set up properly the ambient light should be low enough that no image at all is recorded other than that produced by the flash. This may make it an impractical method for some uses. But at very high magnifications it is generally possible to turn the room lights down to a very subdued level (after everything has been set up) and run the stack.

It is easy to do a test shot or two without firing the flash to be sure that the room lights are low enough.

Guido · Post by **Guido** » Fri Jul 15, 2011 11:23 pm

I presumed the ambient light was wanted in this picture, otherwise is see no use for the long shutter speed.

Shot at 1/180 sec in stead of 5 sec would rule out most of the influence of the ambient light.