Question on High Speed Sync (for Nikonians)

[dje]

The definitive answer must come from Nikon.

George

Yes indeed.

[William W]

The definitive answer must come from Nikon.

OK. That's probably a good position to take with the conversation between you and me too.

I'll leave it there, then. And we can simply agree to disagree.

***

BTW- I thought of commenting on this matter earlier, but I chose not to, for fear of clouding the main discussion.

This is an important point to mention to you, because it touches on a matter which is often addressed very seriously by many people, including Teachers and Photographers

I hope he doesn't mind I borrowed his pcture.

I am not speaking for Neil. I have worked with Neil on a Judging Panel and I found him a most professional and pleasant person. I am not implying any opinion that he would have on this matter.

I am simply pointing out to you, that the images that you published on this thread, are copyrighted material.

Also at their origin they are designated: "all rights reserved" (This additional point is only mentioned, because you appear to be posting from within The Netherlands).

WW

[george013]

I know it was tricky to show the pictures. They had their function and I removed them. The link is still there for those who want to see them.
George

[pnodrog]

Following on from comments from LPaul and John, I've tried to put in some numbers for the D810.

My assumptions are :

• The first shutter curtain takes about 2.5 ms to traverse the sensor (based on some figures measured by Jim Kasson).
• The effective shutter speed (or time) for a rolling shutter is determined by the time the first curtain takes to traverse the sensor plus the time the sensor is fully exposed before the second curtain starts to close. (providing the shutter speed is slower than the curtain speed, 1/400s or 2.5 ms in this case)

Based on these assumptions, a shutter speed of 1/320s (3.1 ms) will have the sensor fully exposed for about 0.6 ms (ie 3.1 -2.5). A shutter speed of 1/250s (4ms) will have the sensor fully exposed for 1.5 ms (4-2.5). As the flash is fired at the start of the sensor fully exposed time, this means that for 1/320 SS, there is 0.6ms for the flash to fire and decay whereas for 1/250 SS, there is 1.5ms for the flash to fire and decay.

The figure of 1.5ms should be adequate for full illumination by the flash but with a figure of 0.6 ms, you will probably miss some of the significant output from the flash as it decays - hence a small drop in exposure. This could also involve some dark banding at one edge as shown in the reference I linked to.

Make sense ?

Dave

Hi Dave, due to my traveling a bit of a delayed response.

The measurements by Jim Kasson were for 1/000 of a sec and both curtain need to be traveling in unison. The curtains maybe able to travel a bit faster when only travelling one at a time - they may not but it is a possibility although high speed photography of an older Canon Rebel indicated a travel time of about 2.7 microseconds for a longer exposure where the curtains moved one at a time.


Your 600 microseconds (0.6ms) fully open at 1/320th is probably about right but Nikon maybe able to make it a fraction longer.

The first options they have with their CLS compatible units is to pre trigger the flash to allow for switching delays or even having the flash just starting to fire before the first curtain is fully open. It may even be possible to gain 60 microseconds which would a 10% gain at 600 microseconds. (N.B. modern engine management advance and retard ignition timing in response to all sorts of factors)

The second option with CLS is to reduce the flash's power and therefor shorten it's duration to avoid any significant shadow from the second curtain starting close.

A very unlikely option is that they strobe the flash 2-10 times rather than at 20-50khz frequency used in full FP (HSS) mode.

I would not call any of these options "tricks" but they would certainly be interesting refinements


We know who knows but they have not told me....

[William W]

L.Paul,

Nice thinking.

. . .The first options they have with their CLS compatible units is to pre trigger the flash to allow for switching delays or even having the flash just starting to fire before the first curtain is fully open. It may even be possible to gain 60 microseconds which would a 10% gain at 600 microseconds.
I would not call any of these options "tricks" but they would certainly be interesting refinements.

I've been cogitating on both of those thoughts. I didn’t think of the others that you mentioned - again nice thinking on your part.

I’ve been stuck for free time to research this other point that I thought of: a list of the compatible Nikon Units that allow the option of the two shutter speeds would be useful, then to compare those release dates to the release dates of the cameras which have the optional shutter speeds so as to compare any trends, could give a framework of the TR&D that might have happened.

I asked earlier if anyone was a member of NPS – is anyone? Does anyone have any contacts in the technical department there?

WW

[dje]

Thanks LPaul and Bill for your further comments. It does seem that timing optimisation and flash pulse shaping might be the key to getting that extra bit out of flash sync. I'm afraid I don't qualify for membership of NPS (and never will) but it would be nice to see what information is available to members of that group.

Anyway thanks to everybody for their contributions to this thread. I may not have received a definitive answer to my question but I sure have gained a much better understanding of the process of flash synchronisation.

Dave

[dje]

OK so curiosity got the better of me and I decided to do some tests on my D610 with an SB-500 flash (which is fully compatible with the Nikon CLS). I built up a light detector as per this article, borrowed a mate's storage oscilloscope and set up the flash on manual control and with Flash sync set to 1/250 (Auto FP).

Just to re-cap, the D610, like many of the Nikon cameras, has "two maximum sync speeds", 1/200 and 1/250 in the case of the D610.

These are some waveforms which I think illustrate what happens quite well.

SS 1/100, half flash power : flash duration about 1.5ms



SS 1/200, half flash power : flash duration about 1.3ms



SS 1/250, half flash power : flash duration about 0.4ms


The first thing to notice is that that tail of the flash is truncated, obviously a feature of the CLS.

Secondly for the SS of 1/250, the flash has a much shorter duration which is consistent with my comments in post 31. ie with the higher of the two "max sync speeds", the shutter fully open time is shorter and the flash duration is shortened to fit this time period. This would cause a drop in exposure.

And just for interest here is what HSS looks like (SS 1/320)



Hope this is of some interest.

Dave

[pnodrog]

Dave I see you triggered the scope using CH1 from the light detectors response. It would be interesting to see what it looked liked triggering off CH2 when connected to the flash sync output of the camera. That way we should get more idea of the overall timing. You will probably require a pull up resistor (100k?) to a isolated low voltage power source (1.5v battery?)

It is not my gear so I am not worried about you blowing anything up...

[dje]

Dave I see you triggered the scope using CH1 from the light detectors response. It would be interesting to see what it looked liked triggering off CH2 when connected to the flash sync output of the camera. That way we should get more idea of the overall timing. You will probably require a pull up resistor (100k?) to a isolated low voltage power source (1.5v battery?)

It is not my gear so I am not worried about you blowing anything up...

I'll need to have a closer look at that LPaul, I may need a written guarantee on that suggestion.

Dave

[dje]

when connected to the flash sync output of the camera.

Actually my camera doesn't have a flash sync terminal, I'd have to buy a hot shoe adapter for that.

Dave

[george013]

Dave,

How can you have half-power and CLS. As far as I know by head it's either manual, and then you can set the output, or it's CLS and the flash determines the output, with a EV correction if wanted.

George

[dje]

Dave,

How can you have half-power and CLS. As far as I know by head it's either manual, and then you can set the output, or it's CLS and the flash determines the output, with a EV correction if wanted.

George

Hi George

As far as I'm aware, Nikon's term CLS is a general term referring to the system which controls how their cameras and flash units work together. It sounds like a bit of a marketing term to me.

Within that system, you can control the flash power either manually or automatically using through the lens metering (iTTL/TTL). I chose manual to give me full control over power and remove any uncertainty in the measurements.

Dave

[William W]

. . . Nikon's term CLS is a general term referring to the system which controls how their cameras and flash units work together. It sounds like a bit of a marketing term to me. . . .

It is.

CLS is an acronym - Creative Lighting System


WW

[george013]

Hi George

As far as I'm aware, Nikon's term CLS is a general term referring to the system which controls how their cameras and flash units work together. It sounds like a bit of a marketing term to me.

Within that system, you can control the flash power either manually or automatically using through the lens metering (iTTL/TTL). I chose manual to give me full control over power and remove any uncertainty in the measurements.

Dave

You're right. I don't know how to call it.

If you had the flashpower set manual, then the output must stay the same. And then also the flash duration. As far as I know is the total output a result of the maximum lightintensity and the duration in time.
When I understood your graph, then the vertical ax is the lightintensity and the horizontal the time. The lightintensity stays the same, time is changing.

If you look again at the photo's of Van Niekerk, http://neilvn.com/tangents/auto-fp-f...on-d300s-d700/ , and examine the 1/250 photo's, the "exact" sync time, you will see the curtains allready. You can compare the D700 and the D300, the curtains have an opposite direction. With the non-cls even at 1/200. I believe it will be the same with the D610 and its sync time.

Then there is still the question why in Van Niekerks images the non-CLS flash has a sharp black size. It looks to me that the moment the flash starts is different.

George

[Manfred M]

As far as I'm aware, Nikon's term CLS is a general term referring to the system which controls how their cameras and flash units work together. It sounds like a bit of a marketing term to me.

Companies (especially marketing departments) love acronyms. Us end users don't really care.

What is important is that we get TTL metering functionality in two modes; regular i-TTL and the TTL-BL (Balanced Light), which is really their fill light mode (lots more acronyms too). If I am winging it, then I will tend to choose one of the iTTL modes. If I want repeatability, I'll switch to manual mode (on the flash and camera).