August 4, 2012

maximum flash sync speed, and the Nikon SB-900 / SB-910 speedlight

Because of the way the focal plane shutter works in DSLRs, shutter speed doesn’t affect our flash exposure … while we don’t go over maximum flash sync speed. When we go into high-speed flash sync, our flash output drops. (The linked article there explains it thoroughly.)

However, when we work with ambient light (and intend to add flash to our subject), then a change in shutter speed has an indirect effect on our flash exposure. A change in shutter speed will mean a change in aperture, and this is what affects our flash exposure (with manual flash), or our flash output (with TTL flash.) Check this article for how manual flash and TTL flash differ.

Therefore when we add flash to ambient light, then our shutter speed choice becomes important.

If you feel a little lost with this concise summary, check these linked articles:
maximum flash sync speed
tutorial: high-speed flash sync
manual flash vs TTL flash
or check out my books:  on-camera flash / off-camera flash

There’s a linear correlation between aperture and flash range. You can see this with your speedlight on your camera, if your speedlight shows the distance scale. Place your flash on your camera, and choose 100 ISO, (or 200 ISO). Set a shutter speed of 1/125 and an aperture of f16 … then, as you change your aperture slowly up to f/2.8 you will see the distance scale move. This distance is the range your flash will give correct exposure for your chosen aperture and ISO. This works whether you set our speedlight to Manual or TTL.

related articles:
- getting the most out of your flash / speedlite / speedlight
- tutorial: how to use the guide number of your flash

Let’s work with a scenario where our background exposure is determined to be: 1/30 @ f/11
This would be the same ambient exposure as:
1/60 @ f/8  1/125 @ f/5.6  1/250 @ f/4

Run through those settings and see how your distance scale changes. With flashguns other than the Nikon SB-900 and SB-910, there is a linear change as you change the shutter speed / aperture combination. For every two stops change in aperture, the distance will double.

But with the SB-900 and SB-910, the distance shown start to “flatten out” as you approach maximum flash sync speed. Russ McDonald explains this on his Nikon CLS Practical Guide: When is Full-Power Flash Not Full Power?

I was curious though to see what the actual effect would be in this change in output as we get closer to maximum flash sync speed.

 

the quick explanation

To save those not interested in scrutinizing the histograms and explanations, here’s the quick explanation:

Even though the distance scale shows we’re losing some power when we shoot at maximum flash sync speed with the Nikon SB-900 and SB-910, in my view this doesn’t have as much impact  as one would expect, and I’ll continue choosing maximum flash sync speed as a good starting point when using flash in bright ambient light.

As we change from 1/125 shutter speed to 1/250 there is about 1/3rd stop change in output from the speedlight. The higher shutter speed will help freeze movement, and help eliminate camera shake. It will also give us a wider aperture and the subsequent shallower depth-of-field. So maximum flash sync speed still is the optimal point when balancing flash with bright ambient light. Even for the SB-900 and SB-910.

 

the longer explanation

I photographed a white backdrop in the studio, using the speedlight at full manual output. The change in the histogram shows us how our flash exposure changes.

I purposely shot at full power, so that there wasn’t any wriggle room with our flash output. It gave us everything it had with every exposure. Simple.

Not that it is terribly important, but here’s the setup. I used Nikon CLS to fire the slave, with the Master’s output disabled.  (The master flash was pointing upwards, and not directly at the white backdrop.) I used a battery pack to make sure the flash properly recycled for every shot.

There are two sequences shown here. The first is for the Nikon D4, and just for good measure, the Nikon D700. The results remain consistent between the two cameras.

I started off with a medium shutter speed, 1/100 to get a baseline of where the histogram would be, with the white exposed as white on the histogram. (More on using the histogram to determine exposure.)

Going to 1/125 has no effect on exposure.

1/160 looks about the same.

At 1/200 we’re seeing a slight drop compared to 1/100 but not significant. Less than 1/3rd of a stop.

At 1/250 there is another slight drop in exposure. To my eye, not significant yet either.

Taking the aperture to f/5 to give a third stop more exposure, to bring it in line with the exposure we had for 1/100 it would appear that the loss in power for 1/100 to 1/250 is around 1/3rd of a stop, or even less.

This for me, implies that I might as well keep to the simpler algorithm when I work in bright light with flash:
- lowest ISO
- maximum flash sync speed
- find my appropriate aperture.

Just for good measure, I decided to go a 1/3rd stop over into high-speed flash sync territory, and the decrease in power is dramatic. Again, this is explained in the tutorial on high-speed flash sync.

The sequence for the Nikon D700 follows the same pattern. I did use 200 ISO instead of 100 ISO, and therefore bumped my aperture down to f/8

 

summary

I wanted to demonstrate this via the histogram, since it is visual. I found the same results with a lightmeter though – a drop of 1/3rd of a stop over the range of shutter speeds.

The results here are easily repeatable by anyone who wants to double-check.

And to re-iterate:
Even though the distance scale shows we’re losing some power when we shoot at maximum flash sync speed with the Nikon SB-900 and SB-910, in my view this doesn’t have as much impact  as one would expect, and I’ll continue choosing maximum flash sync speed as a good starting point when using flash in bright ambient light.

As we change from 1/125 shutter speed to 1/250 there is about 1/3rd stop change in output from the speedlight. The higher shutter speed will help freeze movement, and help eliminate camera shake. It will also give us a wider aperture and the subsequent shallower depth-of-field. So maximum flash sync speed still is the optimal point when balancing flash with bright ambient light. Even for the SB-900 and SB-910.

 

{ 5 comments… read them below or add one }

1 Joe Leong August 5, 2012 at 8:09 pm

Neil, I noticed that your D4 is on sRGB. Any reason?

Reply

2 Neil vN August 5, 2012 at 9:32 pm

Joe .. I shoot in RAW, so my choice of color mode in my camera doesn’t matter.

My workflow is (now) Adobe RGB, so once I ingest the images (ie, RAW files) into my workflow, they are Adobe RGB.

I prefer shooting in sRGB because the display on the back of my camera looks more saturated and punchy than when I set my camera to Adobe RGB.

Reply

3 Jason August 6, 2012 at 10:34 am

I know I have mentioned this before, but if you use an sb-900 with the pw flex system, you can sync at 1/320 with no apparent loss of flash power. At least this is true on my D700. Thanks for posting this kind of stuff Neil, I appreciate it greatly!

Reply

4 davis August 6, 2012 at 10:35 pm

I wonder if this behavior has to do with the t.1/t.5 time of the flash when it’s at full power.

Reply

5 Neil vN August 6, 2012 at 11:43 pm

Davis … it appears to be because of that. A slow enough pulse of light from the flash.

Reply

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