Disadvantages of always setting flash sync to 1/320s (Auto FP)?

Question

Is there any disadvantage using the highest possible flash sync speed all the time? That is, leaving the flash sync setting at 1/320s (AutoFP) even when using a slower shutter speed?

---

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?

Answer 1

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.

Answer 2

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.

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?

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.

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?

Answer 3

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.

Answer 4

FWIW, Auto FP HSS is not about sync at all. The only point of HSS is that it eliminates sync concerns. HSS is a radically different deal. HSS becomes "continuous light" (for duration of shutter speed), same as daylight and incandescent are continuous light (meaning, there before and after the shutter duration - hardly any before or after, but the shutter has no concern about that part). HSS acts like continuous light, and does Not act like flash.

The principle idea about using HSS is that now all of these below are Equivalent Exposures for BOTH sunshine and flash. (see http://www.scantips.com/lights/flashbasics2b.html )

1/400 second at f/11
1/800 second at f/8
1/1600 second at f/5.6
1/3200 second at f/4
1/6400 second at f/2.8

Any of these "Equivalent" combinations will NOT change exposure in the HSS picture exposure.
Good to know, but regular speedlight flash mode ain't that way. :)

But 1/250 second is a sync for the regular speedlight flash mode. And some Nikons can do 1/320 sync.

The disadvantage of maximum sync shutter speed indoors is that it tends to keep out all of the ambient. It is dim indoors, and if you want some ambient to register in your indoor flash pictures, use slower shutter speeds to expose the ambient a little. A studio situation probably always uses Maximum sync, because they are controlling the light with flash. But if you want the room to show, you better use a slower shutter.

The Nikons with 1/320 second sync still show 1/250 maximum sync in the specification chart. They also say 1/320 works, and it definitely does, but Nikon says it might decrease the range of the flash. My own notion is that this can only mean if at speedlight maximum flash power level. Maximum speedlight power is slow (long duration), and 1/320 probably might slightly chop off the trailing tail of it. Should not be any issue at shorter faster powers of a speedlight.