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Archive 2008 · Flash Duration Analisys
  
 
Dalantech
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p.1 #1 · p.1 #1 · Flash Duration Analisys


There's a really interesting thread going on over at DP Review (no kidding -DP Review!) on flash speed and the minimum speeds required for sharp images at macro magnifications. It's not news to me -I've known, and have said, for quite some time that you have to get the flash duration insanely short if you want sharp images (even with everything on a tripod) but it's cool to see math back me up

http://forums.dpreview.com/forums/read.asp?forum=1029&message=29244641



Sep 09, 2008 at 08:32 AM
LordV
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p.1 #2 · p.1 #2 · Flash Duration Analisys


Interersting stuff - especially the maths stuff on shutter speed needed for different magnifications. Don't think anyone is denying flash helps a lot with stopping motion blur in macro- we just disagree on the relative importance of flash duration effects vs diffraction effects . But as has been said before - it tend to result in the same practical solution- open up the aperture although you could obviously go to higher ISO.
If I'm getting the maths right for the MPE-65 at 4:1 allowing for 1.6crop then it seems to indicate a required flash duration of 1/1040th sec ? or does **2 mean squared ? ie then would be 1/2600th - both would not be not unreasonable flash durations.

As a side issue- the maths does indicate that focal length does change the required shutter speeds for different focal length macro lenses even if they are all at 1:1 - remember having a lively discussion with someone about that.

Brian V.

Edited on Sep 09, 2008 at 09:19 AM



Sep 09, 2008 at 09:08 AM
Eyvind Ness
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p.1 #3 · p.1 #3 · Flash Duration Analisys


John, can you quantify "insanely short"? Is it 1/2500s, 1/10000s, or higher still? And is it also not a function of focal length, as Brian points out, and of course, magnification. I'd like to see numbers and hard facts, not yet another qualitative observations and theory involving terms like "insanely short"




Sep 09, 2008 at 10:24 AM
Dalantech
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p.1 #4 · p.1 #4 · Flash Duration Analisys


LordV wrote:
As a side issue- the maths does indicate that focal length does change the required shutter speeds for different focal length macro lenses even if they are all at 1:1 - remember having a lively discussion with someone about that.

Brian V.


I also remember someone saying that macro doesn't change the 1/focal length rule -and getting into a lively discussion with said individual about that



Sep 09, 2008 at 10:55 AM
Dalantech
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p.1 #5 · p.1 #5 · Flash Duration Analisys


Eyvind Ness wrote:
John, can you quantify "insanely short"?


As short as possible -or "the shorter the better". Even if you increase the 1 / focal length rule by a factor of four for life size shooting the 1 / FL rule is a best case calculation -I would never shoot a natural light closeup at 1/200 of a second with a 200mm lens no matter how well I'm bracing it cause I can't keep the critter perfectly motionless. In my experience I get more keepers at double 1 / FL when shooting with natural light, and I'd lean more toward doing the same for macro (so that would be eight times 1 / FL at 1x if I'm not getting confused on the numbrs).

To keep my flash duration as short as possible I'm currently using a diffuser plastic that only blocks 2% of the light that's hitting it -hard to put that into stops but if I'm doing the math right it's somewhere around 1/10 of a stop. By comparison most commercial diffusers block 1 to 2 stops.

Sorry if I'm not answering your question. I'm in a discussion with JimH over at DPR about his tests. The problem with his measurements is that it gives people a false sense of how well a flash can freeze motion because he measure a flash hitting a photo diode and not how fast the flash fired at a 13% gray card, and it doesn't take into account the light lost from diffusing the flash...



Edited by Dalantech on Sep 09, 2008 at 12:27 PM GMT


Edited on Sep 09, 2008 at 11:27 AM



Sep 09, 2008 at 11:09 AM
melsen
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p.1 #6 · p.1 #6 · Flash Duration Analisys


Thats a very very interesting read actually....

Thanks for sharing that John.



Sep 09, 2008 at 11:16 AM
Tim Dollear
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p.1 #7 · p.1 #7 · Flash Duration Analisys


Agreed. Thanks a lot for sharing this.


Sep 09, 2008 at 12:44 PM
lextalionis
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p.1 #8 · p.1 #8 · Flash Duration Analisys


Very interesting read...it's really nice to see oscilloscope pulses and talks about photodiodes etc. Mathematically I think I followed his reasoning through his first post, but after that I got "frizzel-frazzeled"

I would like to ask if I should consider these "tid-bits" when shooting slightly less than 1:1 with the objective to, of course, obtain sharp subjects AND pleasant BG exposures. Looking at my past two months worth of shots I've found that I shoot really close to the 1/F rule (1/125 on average) with my 100mm at slightly less than 1:1. I'm pretty confident too that my ring-flash with diffuser is dropping at least 2 stops of light output.

I just find more often when I reach into the 1/200th barrier that my BG gets darker and darker.

-Roy



Sep 09, 2008 at 01:19 PM
Dalantech
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p.1 #9 · p.1 #9 · Flash Duration Analisys


It really depends on what "look" you are trying to achieve Roy. Sometimes a black background helps to isolate the subject and really keep the viewer's attention right where you want it. But it can also look too much like a "studio" shot. IMHO the quality of the light and the composition are far more important than getting color in the background...

IMHO a photo looks best when the background is properly exposed or flat black -but anything in between looks "off".

It's possible that you are exposing the background with natural light and the subject with the flash, and in that case the shutter speed isn't relevant. You don't need detail in the background so any motion won't be noticed, and the flash duration is freezing the motion so you can get a sharp subject...



Sep 09, 2008 at 01:31 PM
lextalionis
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p.1 #10 · p.1 #10 · Flash Duration Analisys


@ Dalantech: Yes, that's exactly what I'm doing...lowering shutter speed enough to have ambient expose the BG. Yep, agree sometimes clear/sharp/good lighted subject on black looks best and sometimes the same with an exposed BG looks best. I get the feeling (assumption since I've never shot greater than 1:1) that the lower the magnification the more probable the BG should be exposed, unless you are looking for a lot of black in the frame.

-Roy



Sep 09, 2008 at 01:56 PM
 

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Dalantech
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p.1 #11 · p.1 #11 · Flash Duration Analisys


It depends on how close the background is Roy -the higher the magnification the shorter the flash fall of distance becomes so the background has to be close to get exposed. One work around is to setup a second flash just for the background and slave it off of the subject flash. I've got a bracket for my 580EX II and one of these days I'm gonna take a hand held shot with both it and the MT-24EX just so I can say that I did it


Sep 09, 2008 at 02:01 PM
lextalionis
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p.1 #12 · p.1 #12 · Flash Duration Analisys


Dalantech wrote:
One work around is to setup a second flash just for the background and slave it off of the subject flash. I've got a bracket for my 580EX II and one of these days I'm gonna take a hand held shot with both it and the MT-24EX just so I can say that I did it


Ah yea, that would do the trick (in my case my 580EX on ring-flash and 430EX as slave)...bracket OR possibly an attractive assistant...now if I could only get the wifey to help me out

BTW, I'm glad you posted this and the link to the "heavier" talk. It goes to show that there is really more than meets the eye with flash-photography.

-Roy



Sep 09, 2008 at 05:47 PM
Dalantech
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p.1 #13 · p.1 #13 · Flash Duration Analisys


lextalionis wrote:
BTW, I'm glad you posted this and the link to the "heavier" talk. It goes to show that there is really more than meets the eye with flash-photography.

-Roy


If it were easy then everyone would be doin' it



Sep 09, 2008 at 05:50 PM
JimH.
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p.1 #14 · p.1 #14 · Flash Duration Analisys


Dalantech wrote:
Sorry if I'm not answering your question. I'm in a discussion with JimH over at DPR about his tests. The problem with his measurements is that it gives people a false sense of how well a flash can freeze motion because he measure a flash hitting a photo diode and not how fast the flash fired at a 13% gray card, and it doesn't take into account the light lost from diffusing the flash...

Edited by Dalantech on Sep 09, 2008 at 12:27 PM GMT



Glad to see this thread started here, too.

You make a good point about the conclusions that people should draw from looking at my waveform tests.

Regardless of the diffusion or the scene set-up, the flash pulse widths that I've measured would be the same *in my tests* because I set the flashes to "manual" mode and directly entered the flash power settings. Doing that eliminates any consideration of the scene reflectivity, diffuser losses, etc.

It also spared me from the requirement to trigger on the pre-flash and then use delayed sweep to try to "find" the real flash pulses. I have thought about building a high-speed logic circuit to place between the photodetector and the oscilloscope's external trigger input so that I could set it to ignore the first pulse, but pass the second pulse through.

But in any case, I really only wanted to establish what the pulse widths were for any given flash power output level. And thus, using the manual mode was actually more suitable because it's repeatable and gives me clearly defined flash power levels that we can all compare.

So what you see presented in those oscillograms are simply the flash pulse shapes and timing for a selected range of absolute flash power levels.

The thing that is important to keep in mind (and this is your point, of course) is that the use of a "lossy" diffuser will require you to set the flash's power level higher than it would need to be without that diffuser, and that, in turn, will result in longer flash pulses.

I'm very interested to find out more about your new diffuser material. I'd prefer to get the least loss possible from any diffuser that I build or use, so when you can, I'd love to hear about what you're testing

If anyone is interested in the flash pulse widths they might actually be getting in their "real world" shots, you can determine the power level that you're using by setting the flash to manual and then seeing what setting it takes to get the exposure you want for that particular scene and lens settings.

I often shoot in manual mode anymore for macros because the 40D seems to expose erratically in ETTL mode with the MT-24 anyhow. Manual is more predictable and faster to use for me with my 40D. With my 20D, I often use ETTL for flash macro shooting.

But you must also take into account the two heads of the MT-24EX.

In most of my tests, I enabled only one head to keep things simple. But normally, you'd be using both heads. Since using both heads at once increases the brightness, it follows that you'll get faster pulses for any given exposure when you do use them both. But again, you can determine the actual power levels you're using by playing with the manual mode and adjusting until you get the same thing you've been getting in ETTL mode.

I thought one of the interesting things was the look of the waveforms that I captured when using both heads. I purposely set one head to produce a longer pulse than the other for those tests, and you can see how the intensity rises to a high level while both heads are firing, and then suddenly drops to about 1/2 that level as the head set to the lower level suddenly "switches off" in mid-pulse, leaving only the light from the head that was set to the higher power level.



Edited on Sep 09, 2008 at 07:55 PM



Sep 09, 2008 at 07:52 PM
Dalantech
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p.1 #15 · p.1 #15 · Flash Duration Analisys


JimH. wrote:
Regardless of the diffusion or the scene set-up, the flash pulse widths that I've measured would be the same *in my tests* because I set the flashes to "manual" mode and directly entered the flash power settings. Doing that eliminates any consideration of the scene reflectivity, diffuser losses, etc.


I never questioned your analysis, just that it doesn't map to the real world very well and people look at the shortest flash duration and think that it can freeze any motion -and it probably can. But you'll never get that pulse at the short end of the scale

JimH. wrote:
The thing that is important to keep in mind (and this is your point, of course) is that the use of a "lossy" diffuser will require you to set the flash's power level higher than it would need to be without that diffuser, and that, in turn, will result in longer flash pulses.


Add a diffuser and lose at least one stop -add a bounce card and lose about two...

JimH. wrote:
I'm very interested to find out more about your new diffuser material. I'd prefer to get the least loss possible from any diffuser that I build or use, so when you can, I'd love to hear about what you're testing


I'll contact the company again and see if they will sell in smaller quantities than the minimum $100 USD they quoted me the last time I talked to them...

JimH. wrote:
I often shoot in manual mode anymore for macros because the 40D seems to expose erratically in ETTL mode with the MT-24 anyhow. Manual is more predictable and faster to use for me with my 40D. With my 20D, I often use ETTL for flash macro shooting.


I should use manual mode as well, but I find myself shooting at odd angles and I'm not so sure that I could get close to the right exposure on the first shot -might take too much chimping to be practicle...



Sep 09, 2008 at 08:18 PM
Dalantech
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p.1 #16 · p.1 #16 · Flash Duration Analisys


Something else I brought up at DPR -and maybe this will drag Frans into the discussion: He uses four flash heads in his stop motion rig (so he can set them to very low power to freeze motion and still get a good exposure), and two standard camera flashes in his laser triggered setup for shooting insects in flight and he doesn't always manage to freeze the motion in their wings. IMHO Frans's work proves that a single flash can't freeze all of the motion in a scene...

FWIW: I wish I knew half of what Frans has forgotten about stop motion flash photoraphy...



Sep 09, 2008 at 08:59 PM
MichAg92
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p.1 #17 · p.1 #17 · Flash Duration Analisys


someone help me out here. I understand the dpreview math about calculating ss for freezing macro motion, but I want to make sure I understand the basic argument you are advancing for the effect of flash duration. Is the basic idea that the flash is the limiting factor for "exposure speed" , therefore the shorter your flash duration, the sharper the shot becuase your effectve "exposure speed" is then shorter. If my summary above is correct, that mostly makes sense to me, except that this must assume that there is no/little exposure happening from the ambient light, right? Any ambient light exposure shouldn't have its exposure (and potential motion blur) limited by the flash, but instead by the ss? Is this correct?


Sep 11, 2008 at 01:05 AM
DamonBarnett
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p.1 #18 · p.1 #18 · Flash Duration Analisys


how does this relate to the popup flash on my XSI. I'm shooting at 1/200. I was under the impression that's as high as I could set the flash. Is there something else I could be doing flash wise? I don't have an add on flash yet but I plan on getting one.


Sep 11, 2008 at 03:18 AM
pwnell
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p.1 #19 · p.1 #19 · Flash Duration Analisys


Normally at typical aperture values for macro photography - especially at higher than life size magnification, you will always be more than 3 stops above ambient so the only exposure is from the flash. Hence ambient does not play a role.

If however you are at lower magnification, high ISO and bright daylight then ambient will start having an influence. And start having an effect on motion blur.



Sep 11, 2008 at 04:32 AM
pwnell
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p.1 #20 · p.1 #20 · Flash Duration Analisys


1/200 refers to the shutter speed for the flash - called the x-sync speed. If you are using flash to expose your subject and are 3 or more stops above ambient then shutter speed plays no role.

Try it - set your camera to some aperture value at 1/200s. Turn flash off and take a photo. Provided the conditions are right (such as indoors, low light level, large aperture low ISO etc) the photo will be black. In this case the flash will determine the exposure, and the flash duration will be your effective shutter speed as far as stopping motion is concerned.



Sep 11, 2008 at 04:35 AM
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