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Is there any disadvantage using the highest possible flash sync speed all the time?


Here are my concerns: I have D700 (Nikon) and it's default value for flash sync is 1/250s, while there's an option 1/320s (Auto FP).

The manual says:

1/320s (Auto FP)
Use auto FP high-speed sync with SB-900, SB-800, SB-600, and SB-R200 flash units. If other flash units are used, shutter speed is set to 1/320 s.

I have SB-700 and it seems to work fine with this option. Also, this seems to work fine with the built-in flash of D700.

One more interesting quote is:

When 1/320 s (Auto FP) is selected for Custom Setting e1 (Flash sync speed, pg. 305), the built-in flash and optional SB-900, SB-800, SB-600, and SB-R200 flash units can be used at shutter speeds as fast as 1/320 s; at faster speeds, Auto FP High-Speed Sync is available with optional SB-900, SB-800, SB-600, and SB-R200 flash units.

(I think SB-700 is not mentioned, because it's a newer model from D700, if I'm not wrong).

So, why would I choose 1/250 (fixed; the default one) or less, instead of always using 1/320 (Auto FP)? Or maybe this Auto FP is something that I don't understand?


P.S. I'm talking about standard usage of the flash, using TTL, no front/read curtain, no more external flashes, etc.


EDIT just to be clear - I don't mean, that when I choose 1/250 (Auto FP) (or similar), I'll choose the same shutter speed. For example, if I may use 1/60 as shutter speed, combined with 1/250 (Auto FP) sync speed. And this should be fine, right?

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I think this article: "Understanding Nikon's Auto FP High-Speed Flash Sync Mode" can give some information to this question –  Yao Bo Lu Jan 21 at 9:09
    
@YaoBoLu - this source awesome! So, correct me if I'm wrong, but as far as I understand, it's perfectly fine to use 1/320 (Auto FP) all the time, just to be careful with the shutter speed. Because using <1/320s shutter speed, the flash works normally and using >1/320s the flash uses this "Auto FP" mode, which produces hundreds/thousands pulses in a sec, to make the exposure correct, but the "downside" is - less power produced. Am I correct? If so and if you write some summarizing answer + quoting that link in your comment, I'll accept it. Thanks! Really useful article. –  Kiril Kirov Jan 21 at 9:27
    
From what I understood from the article, shutter speed slower than normal max sync speed (on the camera 1/250) the flash will not have any problem, but everything between 1/250 and 1/320, and the flash support FP, the "downside" will occur. Everything faster than 1/320 you may get a problem where part of the image is dark. –  Yao Bo Lu Jan 21 at 9:44
2  
@YaoBoLu - not exactly. I've posted a summary by myself, you can see it and comment, if you think something is wrong with my statements. Thanks again for the provided source. –  Kiril Kirov Jan 21 at 10:18
    
just went through the article again, it seems like you are right there –  Yao Bo Lu Jan 21 at 10:39

3 Answers 3

Thanks to @YaoBoLu and the quoted article: Understanding Nikon's Auto FP High-Speed Flash Sync Mode, I think I get it.

Here's a summary of what I understood.

The key appeared to be in what Auto FP means.

For example, let's say 1/320 (Auto FP) is used. This would mean:

  • for each shutter speed lower than or equal to 1/320s, the flash will work "normally", as expected.
  • for shutter speed faster than 1/320, the Auto FP mode of the flash is activated.
    And this is the key part here - this means, the flash will fire hundreds or even thousands of pulses of light in a second (will look like a single flash for the human eye). This is necessary to make the correct exposure possible to achieve. But this has its downside - to be able to do this, the flash reduces its power. The faster the shutter speed is -> the more pulses are generated -> the less power per pulse. Which means - the power of the flash is "reduced", meaning - the subject should be closer or will not be lit enough.

What I also understood is, that 1/320s is a bit risky (as it kinda pushes the flash to its limits) and sometimes (often the bottom) parts of the photo may be underexposed.


This made me think the following - 1/250s (Auto FP) is the optimal choice. Using shutter speeds lower than or equal to 1/250s will allow the flash to work "normally", without the risks of getting underexposed parts of the photo. Using shutter speeds faster than 1/250s (like 1/320, 1/400, etc.) activates the Auto FP mode of the flash.

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1  
Since you've decided not to use the FP there's a good argument for turning it off and use it deliberately as you need it instead. Also remember that the reason for controlling shutter and using flash is to balance the ambient/background light with your lit subject, not to freeze action since under normal mode the flash will do that anyway. Always going at full-tilt is not appropriate for all situations... –  James Snell Jan 21 at 10:54
    
@JamesSnell - I haven't given up using FP, it will still be activated with faster shutters than 1/320 (or 250), right? About the freezing and only balancing the light - isn't this achieved by flash compensation (sometimes combined with exposure compensation)? Which will still be available with 1/250 (Auto FP). Am I wrong? –  Kiril Kirov Jan 21 at 11:17
1  
The best way to do it is expose with flash at the normal level for the subject and adjust the shutter time for the background until it's right, once you go beyond 1/60 then you could start winding back the flash output until you've got a good balance, but generally speaking more light is good. Otherwise you have to start bumping up your sensitivity (ISO, which may also be on auto.) –  James Snell Jan 21 at 11:31
    
@JamesSnell - I get it, thanks. –  Kiril Kirov Jan 21 at 11:34

It sounds like a Auto FP mode will automatically turn on high speed sync when using a compatible flash and the shutter speed goes faster than the maximum sync speed of 1/320s. High speed sync allows you to use a faster shutter speed to reduce the impact of ambient light, but at a cost of effective flash power.

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Yeah, it seems like this, but I needed something a bit more to understand it completely. I posted an answer by myself to check if I've understood this correctly. +1 from me for your answer. –  Kiril Kirov Jan 21 at 10:19

I guess it depends on how you define disadvantage. There are times when you may want to balance dim ambient light in the background with your subject in the foreground that is illuminated by your flash. In order to do so without requiring an ISO value higher than you wish might require a shutter speed much slower than 1/320.

For more on using shutter speed to balance ambient and flash see:

What is "Dragging the Shutter"?
Why is my Metz 58 AF-2 using long shutter values when my Canon 60D is in Av mode?
Why is flash TTL metering independent from ambient light metering?

share|improve this answer
    
Sure, but with 1/320 (Auto FP) you can use slower shutter speeds, can't you? I mean, shooting at 1/60 for example would be well exposed with this option, as this does fix the fastest possible shutter speed, not the current one. Or I'm wrong? –  Kiril Kirov Jan 21 at 11:13
1  
If shooting with a shutter speed below 1/250 it doesn't matter if you've selected one of the Auto FP options or not. The flash will fire a single burst at the necessary power level. The only time the 1/250 s (Auto FP) and 1/320 s (Auto FP) settings apply is when the shutter speed is set at or above 1/250 or 1/320, respectively. Then the flash will pulse in a series of many short bursts instead of a single burst. So my assumption (perhaps incorrect) was that when your question asks is there any disadvantage to using the highest possible sync speed it meant you would also select that shutter. –  Michael Clark Jan 21 at 11:50
    
Aha, I see. No, sorry, I didn't mean that, I should've been more clear. But I get your point, thanks. –  Kiril Kirov Jan 21 at 12:07

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