max it out!

December 13, 2008 @ 11:09 pm · Filed under exposure metering, flash photography, lighting, technique

settings:  1/250th @ f2.8 @ 400 ISO, with +1.0 flash exp comp

In previous postings I have mentioned that the specific settings for a photo aren’t often of direct value to us in figuring out the method of exposure. But the one setting that is of huge importance when using flash, is your camera’s maximum flash sync speed.

When the ambient light levels are low, then your shutter speed can vary appropriately, depending on what you want to achieve. But once you work in bright conditions, or have your subject against a bright background, then most often it just makes the best sense to work at maximum flash sync speed, as in that image above.

There is something very sweet happening at that point, and I believe it is imperative for every photographer that use flash, to know what is happening, and why.  I would venture as far as saying that this blog posting contains some of the most essential information I can give you about flash photography.

So let’s work through some of the basics …

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Let’s work through a hypothetical scenario  where our subject is in shade, and our background is much brighter.

For the following tutorial, get your camera and lens out, with a speedlight so that we can go through some of the settings.

Let’s say our background exposure is 1/60th @ f11 @ 200 ISO
Now, it should be obvious that the following are all the same exposures:

  • 1/60th @ f11 @ 200 ISO
  • 1/125th @ f8 @ 200 ISO
  • 1/250th @ f5.6 @ 200 ISO
  • 1/500th @ f4.0 @ 200 ISO
  • 1/1000th @ f2.8 @ 200 ISO

If this isn’t immediately clear, then  check out this posting on Things You Need To Know.

Let’s go ahead:

Step 1. 
Switch your Speedlight on, and take it out of bounce position.  The reason for this is that I want you to be able to see the distance scale on the LCD display. Put your flash-head directly forward.  The distance scale disappears in the bounce position, since your camera and flash then have no idea of the distance you’re going to bounce your flash off.

The Nikon SB-800 scale looks like this:

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.. and the Canon 580EX scale looks like this:

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The distance scale shown on the back of the speedlight, shows the range in which the Speedlight can give correct exposure.  The maximum distance is how far your camera has calculated you will still get correct exposure for the ISO and aperture settings you’ve chosen.

Some speedlights like the Nikon Sb-600 and Canon 430EX don’t show the distance scale.  Sorry, you’re out of luck there, but please follow the rest of the explanation anyway.  There is a certain logic and consistency here, and hopefully it will all make sense in the end.

Step 2:
Switch on your camera and set your camera to manual exposure mode.
If you are using a Nikon D-SLR, set your exposure compensation on your camera body to zero.

Step 3:
Set your flash to TTL exposure, or whatever flavour of TTL your camera spouts, whether E-TTL or E-TTL2 or i-TTL or D-TTL.  It’s all pretty much the same.

Step4:
Set your flash exposure compensation on the camera body and on the back of the speedlight to zero.

Step 5:
For the purposes of this explanation, disable high-speed sync on your camera / flash.
If you don’t know what is meant by this step, then good.  Hang in there.  This will make sense in the end. :)

For Canon users, High-Speed Sync (HSS) is set on the back of the Speedlight.  It is that little H button with the lightning symbol. Push it so that HSS is disabled, ie no H and arrow shows on the LCD display.

For Nikon users, this is usually custom function E1.  Set it the default, ie, not on Auto FP.

Step 6:
Not so much a step, but a reminder that shutter speed has no (direct) effect on flash exposure.   This little snippet of info is hugely important.  If you’re not sure of this, or haven’t seen this in practice yet, just make a mental bookmark .. we’ll come back to it again.  But for now, just remember, shutter speed has NO effect on FLASH exposure.

Step 7:
Let’s see what the distance scale says on the back of our speedlight for that  series of settings mentioned earlier:

  • 1/60th @ f11 @ 200 ISO
  • 1/125th @ f8 @ 200 ISO
  • 1/250th @ f5.6 @ 200 ISO
  • 1/500th @ f4.0 @ 200 ISO
  • 1/1000th @ f2.8 @ 200 ISO

Now, as much as we had said that shutter speed has no effect on flash exposure, for this example, let’s just change the shutter speed settings as well.  For ambient light, there is a direct correlation between shutter speed and aperture.  They work in tandem.  In that sense, shutter speed has an indirect effect on flash exposure, since it affects our aperture .. and aperture (and ISO) has direct effect on flash exposure.  (Check this post if you’re unsure about this – TTL flash vs manual flash.)

Now set your camera to 1/60th @ f11 @ 200 ISO with your speedlight on, and pointing directly forward.  Your distance scale should read something along the lines of having a maximum distance of 20 ft. Within this range, your speedlight should give you correct exposure for your subject.  (And here we are working with the hypothetical scenario where our subject is in deep shade against a background that is 1/60th @ f11 bright.

Step 8:
Now change your camera settings to 1/125th @ f8 and notice how your speedlight’s range increases as you open your aperture from f11 to f8

Step 9:
For the next step, set 1/250th @ f5.6 .. but note that some cameras will be limited to 1/200th or 1/180th.  The Canon 5D for example won’t go higher than 1/200th now with the speedlight on.  (Hang on there, we’ll get to the explanation of why.)

Step 10:
Set your camera to 1/500th @ f4 … and note that the moment you touch your shutter button and activate your camera’s meter, the shutter speed limits again to 1/250th (or 1/200th or such)

Here’s why you can’t set your camera’s shutter speed over a certain value when you have your speedlight switched on (and have HSS disabled):

As can be seen in this explanatory diagram, the light from your flashgun  is dissipated as an instantaneous burst of light.  It is somewhere in the order of 1/2000th of a second. Fast! It is hugely important to realise here that the light from your speedlight  isn’t continuous light, but instantaneous.  (Or rather, near-instantaneous, for the pedantic ones reading this.)

Your shutter consists of two curtains that travel across your sensor (or film gate). So for flash to be exposed over the entire frame, your shutter speed needs to be low enough that the first curtain has completely cleared the sensor area (ie, the sensor is fully open), BEFORE the second curtain starts moving. In that moment, the flash is discharged, and the entire frame is exposed.

If we work in a dark enough area such as a studio that is completely dark, then we can get correct flash exposure at shutter speeds like 1/8th or 1/15th or 1/60th … the shutter speed has no effect on flash exposure … while we remain below maximum sync speed.

So maximum sync speed, is the highest shutter speed at which the entire frame is still open for flash to expose for the entire frame (whether a digital sensor or piece of film).

If we go over maximum sync speed, we’ll get one of the shutter curtains blocking the flash exposure, such as in this example:

 

The image on the left had the shutter speed at 1/60th, and the image on the right had the shutter speed at 1/320th. That dark area on the right is the shadow of the one shutter curtain obscuring the light from the flash.

Step 11:
So we’ve now hit a ceiling – the highest shutter speed we can set on our cameras with the speedlights switched on.  (This is also the highest shutter speed at which you could work with studio lights.  But for studio work, you’d use a shutter speed lower than maximum sync speed.  There is quite often propagation delay, causing the flash to be triggered a touch slower and you still seeing the edge of the shutter curtain.  So most wise studio photographers will work at a shutter speed like 1/125th which is well below maximum sync speed, and will avoid propagation delay.)

However, we’re working with on-camera speedlights, and can comfortably work at maximum sync speed.  But … we’ve hit that ceiling.  We can’t use flash at shutter speeds higher than max sync speed.  However, that is only true for ’old school technology’.  

High-Speed Sync Flash

So with the older flash technology, flash is dissipated as that high-energy burst of light .. but camera manufacturers came up with the stunning adaptation of that technology, where they dissipate the energy from the flash as rapidly pulsed light.  In effect, the flash now becomes continuous light over a very short period.  The light from the flash is now dissipated even as the shutter curtains move across the frame.  As that window between the two curtains move across the frame, the light from the camera’s speedlight is dissipated … exposing correctly for the entire frame.  Remarkable technology!

But .. and yes, there is always a but … this comes at a price.

Instead of the energy from the flash being dissipated now as a high-energy burst, the light from the flash is now dissipated over a longer period.  This means the effective power from our speedlights is reduced when we switch to we switch to high-speed sync mode, instead of the old-school way of triggering our flash as that high-energy burst of light.

Let’s confirm this:

Step 12:  (This will vary for the different makes and models of cameras)

all Canon users (except for the 5D):
Select High-Speed Sync (HSS) on your speedlight by pressing that button on the back of your speedlight.  Now set your camera to 1/250th @ f5.6 (as we had it in this example).  Now lightly touch your shutter button to activate your camera’s meter … and while watching the distance display on the back of your speedlight, change your shutter speed from 1/250th to 1/320th.  You should see an immediate drop in the effective range from your speedlight as your move from the high-energy mode of flash dissipation, over to HSS.

for Canon 5D users:
Switch HSS off.  Now set your camera to 1/200th @ f5.6 (as we had it in this example).  Now lightly touch your shutter button to activate your camera’s meter … and while watching the distance display on the back of your speedlight, hit the HSS button on the back of your speedlight.  You should see an immediate drop in the effective range from your speedlight as your move from the high-energy mode of flash dissipation, over to HSS.

for all Nikon users
Enable Auto-FP on your camera.  For most Nikon D-SLRs this is custom function E1.  Auto FP is Nikon’s version of High-Speed Sync (HSS).  For the purposes of this explanation, switch it to 1/250th Auto FP.   Now set your camera to 1/250th @ f5.6 (as we had it in this example).  Now lightly touch your shutter button to activate your camera’s meter … and while watching the distance display on the back of your speedlight, change your shutter speed from 1/250th to 1/320th.  You should see an immediate drop in the effective range from your speedlight as your move from the high-energy mode of flash dissipation, over to HSS.

** the D700 with the SB-900 doesn’t quite follow the linear change in settings as explained here.  I’m not sure why yet at the moment, but will try and figure it out.

So everyone should be able to see how switching to the High-Speed Sync mode, reduces our effective range by about half, or even more.

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So where do we have the most range / effective power from our speedlights ?
At maximum sync speed! 

For Canon 5D users, this would be 1/200th with HSS set off.  For all other cameras that have HSS as a feature, maximum sync speed will be 1/250th or perhaps 1/180th or such. 

For non-5D users .. the moment you go over the maximum sync speed, your camera will go into HSS mode, and your flash range will be reduced.  For 5D users, the moment you set your camera to max sync speed (1/200th with the H on), your flash range will be reduced.

It becomes apparent that maximum sync speed is indeed a very sweet spot to work at when we work in bright conditions with flash.  Or if we want the most range from our flash.

So let’s state the same thing in a few different ways, and see how it makes sense:

  • At maximum sync speed I have my widest aperture (which is in fact what gives us the most range / effective power) from our flashguns … while still remaining in the non-HSS (ie, old school) way of dissipating flash.
  • If I work at maximum sync speed, I have my widest aperture while remaining in the high-energy mode of dissipating flash, and hence my speedlight will recycle faster, and my battery pack work less hard. 
  • I will have the most effective power from my speedlight at maximum sync speed, because my aperture is wider.

Let’s look at the opening image again:

I bounced my flash behind me into the open church, in order to get soft light on my bride.  Since I knew I would by dumping a lot of juice from my flash in the hope that enough light reflects back to expose correctly for her .. I immediately went to maximum sync speed.

If I can’t do it at 1/250th @ f2.8 @ 400 ISO .. then I won’t be able to do it at 1/125th @ f4 .. and especially not 1/60th @ f5.6 … and indeed, in this instance, I was actually at the very edge of what my speedlight could deliver.  Hence the stained glass windows blowing out a touch.  I would’ve preferred 1/250th @ f4 to get more saturation in the windows, but my flash couldn’t deliver that with the way I was bouncing flash.  So I accepted the compromise of the stained glass windows being a little over-bright.

If I had used 1/250th @ f4 @ 800 ISO, then the balance between my ambient light (the stained glass windows behind her), my flash (the light on my bride), would’ve been the same.  Bumping up my ISO affects both my flash exposure (or range), and my ambient light.

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Another example:

settings:  1/250th @ f4.5 @ 400 ISO, with +1.0 flash exp comp

Once again, with this scenario – the model against a very bright background – I immediately went to maximum sync speed.  Whatever my aperture and ISO settings are, they will see-saw around my choice of max sync speed.

Similar to our previous reasoning: 1/125th @ f6.3 would’ve given us the same ambient exposure, but my flash would’ve had to work twice as hard at 1 stop down.  My speedlight would be slower to recycle, and I would’ve had to shoot slower.  Using this smaller f-stop mean I would use up the speedlight’s batteries faster.  My speedlight would also start overheating faster.  And .. I might not even be able to push that much light from my speedlight if I am bouncing into a large room.

So it just makes sense in this kind of scenario, to immediately default to maximum sync speed.

Photographers using off-camera lighting in bright ambient light, will, for very similar reasons, work at (or very close to) maximum sync speed.

As I had mentioned earlier, there is something special happening at max sync speed, and we need to be aware of it … and use it.

30 Comments »

  1. planet neil - tangents » yours, factually .. Said,

    December 14, 2008 @ 3:14 am

    [...] the only setting I can give that you absolutely have to be aware of, is your camera’s maximum flash sync speed.  There is something very sweet happening there.  But that is the topic of a seperate [...]

  2. Stephen Said,

    December 14, 2008 @ 10:43 am

    Hello Neil,
    If I chose 1/250 as the shutter speed for the max sync speed, and I am in manual mode, how would I calculate the aperture and ISO? In manual mode, I can set the aperture, ISO, and shutter speed to whatever settings I want (even though the camera will indicate if the exposure is correct or not). I presume the settings you come up with in these photos are based on the histogram method, light meter method, experience, or another technique you have previously mentioned in the blog or workshop.

    For example, if I know I want to shoot at 1/250 (max flash sync speed) and at 400 ISO, then I would use the histogram method or light meter to inform me about the aperture that would give me the correct exposure.

  3. Gene Said,

    December 14, 2008 @ 7:46 pm

    This is great if you are using a Canon or a camera other than a Nikon ( or if you want to use your Nikon in TTL mode ). This is basically the algorithms that come in to play when setting a Nikon flash to TTL-BL ( eg; Bright background vs darker lit subject ).

    Could you not just switch your camera mode to “P” mode and your flash to TTL_BL , if you were pressed for time, and capture this image in the same way as long as the bride is in the centre of the frame? Would the fact that the flash was not direct and bounced instead affect the BL mode?

    Thank you for blogging this great information. You have taught me much about my flash that i would never have known otherwise.

  4. Neil Said,

    December 14, 2008 @ 8:05 pm

    Gene .. thanks for the compliments!

    About whether shooting in Program mode (or any other automatic mode for that matter), would’ve given you the same result … and the answer would have to be an emphatic .. probably not.

    I want to control how bright my background is, and I don’t want to have my exposure change as I change my composition.
    For all this, I need to set my camera to specific settings .. ie, use manual exposure mode.
    There is no other way.

    Looking at that last image again, I used 1/250th @ f4.5
    I can’t take the chance that the camera will set something like 1/60th @ f9 (which is the same ambient exposure). But program mode could’ve chosen an entirely different exposure, which isn’t something I want. *I* want to choose.

    But the real problem is that since I am bouncing my flash behind me off some wall, I was able to get correct exposure at f4.5 … but who knows at f9.0 ? f9 is a relatively small aperture, especially for bounce flash.

    Which is exactly why I went to max sync speed. It gave me my widest aperture (which gives me the best chance of successfully getting correct exposure with bounce flash). It gives me the widest aperture while still remaining in the high-energy mode of flash dissipation. Meaning … I could’ve gone to 1/500th @ f3.2 .. which is a wider aperture, but then I am in high-speed sync mode, and my effective flash power is reduced by half.

    So my best option in these situations, is to just immediately go to max sync speed.

    Neil vN

  5. Keshia Said,

    December 14, 2008 @ 9:25 pm

    Amazing post….thanks again, Neil!

  6. Nick Said,

    December 15, 2008 @ 3:57 pm

    Hi Neil,
    Just when I think I’m getting this flash synch speed lark, my d300 throws up flash shutter speed! What’s the difference!?!
    Nick
    Great site by the way. Very infromative.

  7. Neil Said,

    December 15, 2008 @ 4:02 pm

    Nick,

    Are you referring to custom function e2 of the Nikon D300 ?

    If so, that refers to the slowest shutter speed that you’re willing to allow your camera to go to when using flash while shooting in one of the automatic exposure modes. But this shouldn’t affect you at all .. since you are shooting in manual exposure mode on your camera. ; )

    Neil vN

  8. Nick Said,

    December 15, 2008 @ 4:18 pm

    Neil,
    I’ve just tried it on the camera. You’re right. Varying the aperature in aperature priority the shutter speed remained the same. What would be the point in that? This means that you are always better off shooting in manual mode.
    Nick

  9. Neil Said,

    December 15, 2008 @ 4:22 pm

    Well, it means that as you move into brighter areas, the shutter speed will go higher .. but as you start shooting in darker areas, your shutter speed won’t go lower than your preset. So it has its uses for those who insist on shooting in an auto mode.

    But you’re better off shooting in manual exposure mode anyway.

    Neil vN

  10. Nick Said,

    December 15, 2008 @ 4:27 pm

    Oh of course, it’s the minimum shutter speed. I see now. It makes sense now when you read articles on pros who always use manual with flash. Thanks again and thanks for your patience.
    Nick

  11. Harry Simpson Said,

    December 17, 2008 @ 11:58 am

    1/250th @ f4 @ 800 ISO would not produce the same result (as 1/250th@f2.8 400 ISO) – it would be great to see a side by side comparison of the difference in the future sometime. Obviously the bokeh would be different due to aperture but was wondering what other things would be different…..Great post!! Best darn explanation of sync I’ve seen….i’m very visual so the diagrams really helped. Thanks!!

  12. Neil Said,

    December 17, 2008 @ 1:02 pm

    Hi there Harry ..

    In terms of exposure (and the balance between flash and available light), the results would be the same. Of course, depth-of-field and noise would differ between the two images. But this explanation is primarily concerned with exposure and balancing flash.

    Neil vN

  13. Neil Said,

    December 17, 2008 @ 2:04 pm

    I just received a question via email, whether (with Nikon cameras) it is best to leave Auto FP “off” and set max sync speed in bright light.

    My suggestion though would be to leave auto FP “on” as a default … but just be very aware that your flash output will fall considerably when you go over 1/250th.

    But going over max sync speed is usually not a problem if you are using your flash as fill. Where it could have impact though, is when you need a LOT of juice from your speedlight .. then it makes sense to remain at max sync speed.

    Neil vN

  14. Dwight M. Creech Said,

    December 18, 2008 @ 9:21 am

    Excellent explanation! Keep this up. Dwight

  15. Lindsay Said,

    December 20, 2008 @ 11:00 am

    Hello Neil,
    Thanks for the great information on this blog. In Custom Function E1, my D300 allows a max sync speed of “1/320 (Auto FP),” but it also gives a choice of using “1/250 (Auto FP).” Is there any reason I would use 1/250th instead of 1/320th as the max sync speed? Thanks.

  16. Sheri Johnson Said,

    December 24, 2008 @ 9:32 am

    You have shared so much very helpful information here. The great part is that it reinforces so much of how I shoot. I tend to do a lot of it just by instinct, however it is great to read why it works so well and how to grasp the concepts to intentionally get consistent results.

  17. Niels Thijssen Said,

    December 29, 2008 @ 8:36 am

    Hi Neil,
    I just love your site, it’s great to see how’s all the magic is explained!
    But in this chapter the situation is explained for a subject is in shade, and background is much brighter.

    But what changes in the explanation above, when you use flash in a situation where background is darker (like a gala, or concert (with band on stage at the background) and subject is /has to be lit?
    Can you tell us about this situation?

  18. Neil Said,

    December 29, 2008 @ 8:32 pm

    Niels ..

    With that kind of scenario, there’s nothing you can do with just on-camera flash to get the background brighter than it is in relation to your subject.

    Neil vN

  19. Mark C. Said,

    January 6, 2009 @ 5:12 pm

    Hi Neil,

    Thank you so much for all your posts on flash photography, most definitely the best I’ve found. I’m an enthusiastic amature, new to non-point-and-click photography, and have found your insights on Flash photography truly enlightening (pun most definitely intended).

    This post on Max Sync Speed has been very insightful, and I ‘think’ I understand. If working against a brighter background keep the largest aperture needed to achieve your aim (max range/least drain), only going smaller if the required shutter speed for the background goes beyond your max sync speed. If you have to go smaller, your flash will have to work harder and your range will decrease.

    I’m using a Vivitar 283 mounted on the hot-shoe of a Fuji Finepix S9600, and have no Max-Sync Button to press. To work out my max sync speed, would I simply keep taking pictures at faster and faster shutter speeds until there was a noticeable drop in exposure?

    Thanks once again

    Mark C.

  20. Neil Said,

    January 9, 2009 @ 10:23 pm

    Hi there Mark …

    I’m glad the webpages have been of such help.

    To answer your question:

    If working against a brighter background keep the largest aperture needed to achieve your aim (max range/least drain), only going smaller if the required shutter speed for the background goes beyond your max sync speed.

    Not quite the way you state it.
    The reason why you would want as large an aperture as possible (while still retaining enough depth-of-field), is that your speedlight’s range decreases as your aperture becomes smaller. ie, the larger your aperture, the more reach your speedlight will have.

    Looking at it from another approach. We want the most juice / reach / effective power, from our flash. For this, we need to be at our widest feasible aperture .. since the wider our aperture, the more range we get from our speedlight. But we want to do this while not going into HSS (high-speed sync), because then we will lose effective power.

    So it stands to reason that in this context, the widest aperture we can get will be at maximum sync speed. This then becomes a very quick short-cut decision every time we use flash in bright ambient conditions – go to max sync speed.

    Neil vN

  21. planet neil - tangents » camera settings: Nikon D700 Said,

    February 1, 2009 @ 7:32 pm

    [...] I do know that the flash’s range is halved by going into Auto FP mode, and therefore stay at 1…. But I also like the option of not hitting that ceiling of 1/250th if for some reason I need to go to a higher shutter speed while using an on-camera speedlight. [...]

  22. Bruce Morgan Said,

    March 10, 2009 @ 3:34 pm

    Hi Neil. Can’t begin to tell you how much I am learning from your site. I am trying to replicate your range/max sync results with a D3 and SB900 but I start to have a falloff of range as soon as I exceed 1/125th. Is that what you were referring to when you mentioned the non-linear results on a D700? If so, have you come across an explanation? If this is not the same as the D700, any thoughts on what I may be doing wrong?

    Thanks for inspiring all of us! When is the new book out?

  23. Neil Said,

    March 10, 2009 @ 3:41 pm

    Bruce, yup, that would be the same effect that you’re seeing there with the D3.

    I don’t have an explanation yet as to why the range should appear to fall off (according to the display on the LCD), well before we get to the max sync speed.

    The book will apparently be published in August ‘09.

    Neil vN

  24. Bruce Morgan Said,

    March 12, 2009 @ 4:30 pm

    Hi Neil. Just following up on the range falloff … You said it happened on the D700. On mine, it was the D3 with an SB900. My D3 with an SB800 does NOT have the problem. I’m curious – was your problem with the D700 associated with an SB900?

    I sent a tech request to Nikon USA – they are perplexed and have asked Japan for help. I’ll keep you posted.

    BTW your last comment left open the possibility that the LCD may have a reporting problem. I ran an exposure test and found actual power loss of about 1 stop when going from 1/125th to 1/250th. – Thats exactly what the LCD predicted.

  25. Neil Said,

    April 17, 2009 @ 3:56 pm

    Bruce .. I experience the same now with my D3 and SB-900, but not the SB-800.

    Why this happens, I have no idea.
    It certainly doesn’t follow a rational real-world explanation like I am used to.

    If I find something out or figure it out, I will definitely post about it.

    Neil vN

  26. Vishi Anand Said,

    April 22, 2009 @ 3:26 pm

    Reading this blog for the first time. Thanks for your tips. But I am confused. To control the background exposure where bg is bright I have to :

    1. Keep my shutter speed at the max sync speed of my camera

    but I would

    2. Also need to select appropriate aperture, & ISO so that my bg doesn’t blow out, isn’t it.

    For example, let us say you have someone under shade on a tree with bright sunny situation behind the person. So if I need to properly expose the bg, I would use my max sync speed, i.e. 1/250 but I would also have to use something like ISO100 and f11 so that I don’t blow out my bg. I would prefer to shoot at wider apertures but restrcited by the lower max sync speed of cameras. In those situations, high speed sync can be nice option assuming you have enough flash power for your case.

  27. Neil Said,

    April 22, 2009 @ 9:36 pm

    Vishi .. you’re entirely correct there.

    In bright conditions, a shutter speed like 1/250th will imply an aperture and ISO combination in the order of f11 and 100 ISO.

    If you wanted shallow depth-of-field, you’d have to go to high-speed sync. But that reduces your flash’s effective power by half or more.

    So this is where you need to find your balance ..
    .. if your flash is close enough to your subject, or
    .. you have a strong enough flashgun, (although I think the SB-900 and 580EX II are as strong as HSS-capable speedlights come),
    then use high-speed sync flash.

    But when you find that your speedlight can’t cope in high-speed-sync mode, then you will have to revert to the usual way of flash dissipation, and then you have your maximum efficiency from your flash at maximum sync speed.

    But it is all about what you want, and what your speedlight is capable of delivering.

    Neil vN

  28. Dave Graham Said,

    June 15, 2009 @ 12:09 am

    Lindsay,

    Not an expert. Own no Nikon gear to test with. Stuff below is conjecture.

    Nikon advertises a t.5 time for the sb-900 of 1/880 sec, which is not the entire flash pulse. This corresponds roughly to a 3.33 ms (1/300) t.1 duration, which is (almost) the entire flash pulse.

    Guesswork here : it takes perhaps 2 ms (1/500 sec) to move a shutter curtain across the sensor, so a 1/250 exposure goes something like this : at 0.0 ms start opening, at 2.0 ms fully open, at 4.0 ms start closing, at 6.0 ms done closing. This means the shutter may be fully open only 2 ms (1/500 sec) during a 4 ms (1/250 sec) exposure, which actually takes 6 ms to complete.

    A 1/320 exposure would be more like 0.0 start opening, 2.0 fully open, 3.2 start closing, 5.2 closed, leaving 1.2 ms (= 3.2 – 2.0) fully open time.

    The flash pulse must fit entirely within the time the shutter is fully open. So a full 3.3ms sb-900 discharge may or may not fit in a 1/250 exposure, but it does not fit in a 1/320 exposure. But a half-power pulse will fit in a 1/320 exposure. So setting your D300 (or D700) to sync at 1/320 instead of 1/250 is a tradeoff.

    If you are photographing at a short flash-subject distance, at half flash power or less, then setting the custom function to give 1/320 flash sync makes sense, because the shorter sync will not only lower the ambient, but also reduce the ambient blur distance.

    If your flash-subject distance is greater, or your ambient stronger, so that you need greater than 1/2 flash power, 1/250 sync will give you a stronger flash-to-ambient ratio, because it will allow (almost?) the full pulse in.

    It may be difficult to judge 1/2 power during TTL use. You could shoot at 1/320 sync, and if you can’t get enough flash in there, switch to 1/250 sync.

    Thank you Neil for giving so much,

    David Graham

  29. Neil Said,

    June 17, 2009 @ 3:31 am

    David .. thank you for the explanation there, and taking the time to post.

    Neil vN

  30. Amanda Tang Said,

    July 3, 2009 @ 11:11 pm

    Awesome! This explains it so well… and I really like the additions of Vishi’s post, and your follow-up, which helped bring it all together. It’s really clear now, thank you (again)!

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