flash synchronization

High Speed Sync vs Neutral Density Filters

HSS vs Neutral Density

I was reading online forums again (yeah, I know...)... I saw a discussion where a photographer was trying to figure out exposure settings for outdoor flash vs ambient light using neutral density filters. He was adamant that he did NOT want to use high speed sync (HSS) because HSS robs the flash of a lot of its power.

This got me thinking... Doesn't using ND filters also rob the flash of power? If you put a 6-stop ND filter over the lens you are effectively lowering the power of the flash 6 stops and also lowering the amount of ambient light 6 stops. If you use high speed sync to raise the shutter speed by six stops you lower the ambient light 6 stops. You also lose power in the flash, about the same 6 stops. Seems pretty much equivalent.


NOTE: Some of the links in this post are affiliate links and I might be compensated if you purchase equipment using the links. But you will also get a discount by using the link or the code CORNICELLO10 on the Interfit Photographic web site. Win/Win situation!


Testing:
So, time to do some testing. Camera is a Canon 5D mkIII with an 85mm f/1.8 lens. All of the photos were made with the white balance set to Daylight. Flash is an Interfit Photographic S1 with high speed sync capabilities modified with a 24-inch collapsible beauty dish. Ambient light meter reading was 1/125 at f/9 and the flash was a bit brighter (f/13). I wanted to make the photographs at f/1.8. 

I needed about 5 and 2/3 stops of neutral density. I had two 0.9 (3-stop) neutral density filters handy, so I stacked them on top of each other and made the first series of photos below with normal sync. Then I removed the filters and switched the Interfit S1 to high speed sync mode and made a similar set of images above the normal sync speed of the camera. The third set as the Interfit S1 in HSS mode plus TTL metering. All the photos were imported into Adobe Lightroom Classic with no adjustments made to them and then I made the following groups via the Print module. The power level on the strobe was not changed between shots in the first two sets of photos. In the third set the strobe was in TTL mode, so it did vary the power (raising it as the shutter speed increased) to maintain the proper exposure.

While this test isn't super scientific, the things I notice are a definite color cast in the photos made with the ND filter, the ND photos do not appear to be quite as sharp as the HSS photos (a complaint I often hear about variable density ND filters, but these were two single density filters), and the ND filters I used required me to take off the lens shade (something I rarely if ever do) to attach them. The ND filters make it more difficult to see through the viewfinder (6 stops more difficult, I don't want to do the math to figure out how many times darker that is). I could/should have bumped the flash power up a third of a stop or so because of the extra density. The background would have remained the same with the subject being a little bit brighter. But it is what it is. All in all a bit of a pain to work with, but it gets the job done if you have a strobe unit that doesn't do HSS.

In the middle set, using the flash in manual exposure mode with high speed sync. Again, I should have/could have bumped up the power of the flash as I went above 1/3200 sec. on the shutter speed. But the last three are still OK and salvageable. 

The set with the S1 in high speed sync mode and TTL exposure seems to be the winner to me. I was able to adjust the shutter speed to make the trees in the background lighter and darker while at the same time keeping the exposure on the subject pretty consistent. I did not make any adjustments on the flash.

Until fairly recently, I hadn't used HSS or TTL all that much with my flashes. I've been a pretty strict manual mode and in the studio type of photographer. Working with the Interfit S1 strobes has changed my mind about this. What are your thoughts about using neutral density filters to balance flash and daylight versus using high speed sync? 

For those interested in how to figure out the exposure using ND filters, here is how I do it.

  • Take a normal meter reading at 1/125 sec. shutter speed (this gives me leeway to raise the shutter speed slightly to darken the background without going over the sync speed of the camera)
  • Example: ISO 100, 1/125 Sec. at f/11
  • Adjust the output of your flash to read the same f/stop (f/11 in this case) or a bit higher WITHOUT THE ND FILTER (I went for f/13 above)
  • Decide on the aperture you want to use for depth of field. In this case I wanted f/1.8
  • Figure that from f/13 to f/1.8 is 5 and 2/3 stops
  • Find a 5 and 2/3 stop neutral density filter or adjust your aperture to match the ND filter(s) you have. If you have a 3 stop filter you can go from f/11 to f/4, with a 4 stop filter you can go to f/2.8, with a 6 stop filter you can go to f/1.4 (I opted for stacking two 3 stop filters above)
  • Take the photo with the ND filter(s) in place
  • Adjust the shutter speed up/down to darken or lighten the background
  • Adjust the power of the flash up/down to get the proper exposure on the subject
  • Deal with focus and color issues

To figure out the exposure with high speed sync

  • Decide on the aperture you want to use
  • Set the camera to that aperture
  • Make sure your flash is in HSS mode
  • Adjust your shutter speed up/down to darken or lighten the background to how you want it to look
  • Adjust the power of the flash to give proper exposure on the subject or use TTL if available

Do note that HSS on speed lights will run through batteries quicker. The folks at Interfit, though, tell me that the S1 battery actually lasts longer in HSS mode. HSS may also shorten the life of the flash tube. But everything is a tradeoff in photography.

More shutter speed and speed lights

Speed lights

Thanks for all the feedback on yesterday's shutter speed post here and on social media. Some folks wondered about the action stopping capabilities of the speed light. I only have one speed light to test with, a Canon 430 EXii. So here are some more photos of the fan.

First, I will start out with this series of photos taken with no flash. Here only the shutter speed of the camera is affecting the amount of motion blur in the fan blades. Shutter speeds range from 1/8 second to 1/1000 second using window light.

No flash, just window light at the indicated shutter speeds

No flash, just window light at the indicated shutter speeds

The next series was taken with the flash on Manual power control and the camera shutter set to 1/15th second. The first image is without flash to show the ambient light at 1/15 second at f/8. The images then progress from 1/64 power to full power. The f/stop was adjusted for each frame to give the same exposure.

flash-15th-second

This next series is the same as the above, except that the shutter speed was increased to 1/125 second. I want you to notice that most of the images look exactly the same, despite the three step difference in shutter speed. Where you will see a difference is in the low powered shots where the aperture had to be opened up to compensate for the low power flash, causing more ambient light to be recorded at 1/15th than at 1/125th. But the edges of the blades are still about the same, despite the ghosting.

flash-125-second

Here are close-ups of the fan blades at 1/4 power on the flash. On the left is 1/15 second at f/11 and on the right is 1/125 second at f/11. The motion blur is the same. The flash duration is the effective shutter speed in the studio. The flash doesn't care how long the shutter is open, and the flash isn't affected by how long the shutter is open (as long as the shutter speed is at or below the sync speed). If the room was completely dark the camera shutter could be set to 30 seconds or more and the exposure would still be the same as at 1/125. The only light hitting the fan is the brief flash from the speed light.

15-vs-125.jpg

One more series of photo and then I hope we're done with the fan and can get back to more creative endeavors. This time the flash is in eTTL mode at a range of shutter speeds. The aperture is the same (f/5.6) in the 6 flash lit shots, the output of the speed light is controlled by the eTTL circuitry. The seventh image is the fan at 1/2000 second with no flash for comparison.

ettl-flash

One of the things to notice in this series is that at 1/15 and 1/125 (the first two images) the flash is in normal mode and the speed of the flash freezes the blades. Above 1/200 second the flash is in high-speed sync mode where the flash fires a series of flashes (too quick in succession for the eye to see) as the shutter curtains move across the sensor. Because of the rapid movement of the fan blades, they are in different positions for each of the flashes and the fan blades show more motion at the faster shutter speeds (1/250, 1/500, 1/1000 second). At 1/2000 second the shutter speed alone stops the blades, as you can see by comparing the last two images, both taken at 1/2000 second. One is with the and one is without the flash.

As pointed out by Paul in the comments on yesterday's post, most subjects probably won't be moving as fast as the fan is, so you might not get a lot of subject motion blur using high-speed sync. I am exaggerating the effect here to make the point that high-speed sync is a tool used to help balance outdoor exposures between flash and sunlight while maintaining large apertures for shallow depth of field. It isn't meant to help stop subject motion, and, as can be seen here, may actually show more motion in extreme conditions.

Shutter Speed with Flash or Strobes

Avoiding Motion Blur

In response to some questions in the comments section about power settings and freezing motion with a speed light I added a new post to the blog today.

Yesterday I got a question from a friend on Facebook asking how to get their flash to sync with their camera at a shutter speed higher than 1/125 so they could freeze the motion of their subject (children in this case). This question also comes up in almost every CreativeLive strobe lighting class that I've worked on. I hope that this post can help clear up the big misconceptions about shutter speed and flash photography.

Instead of using children, I decided on something that moves even faster--a box fan.

In the first series of images below we see 6 images of the fan. The first one (1) was  taken without a flash. The rest were taken with different brands and models of studio strobes. And the most important thing to recognize here is that all 6 images were taken at 1/15 of a second, and the fan was running at the same speed in all the images. What I hope to show you here is that in the studio with strobes the shutter speed is pretty much irrelevant as long as it isn't so slow that ambient light infiltrates the exposure or isn't too fast that the camera's shutter cannot synchronize with the flash, which will leave a black band across one side of the image (see my previous post on sync speed for more information on that).

shutter-speed-strobe-motion

As noted above, photo 1 was taken at 1/15th second with no flash
#2 was with a Speedotron 805 pack and a model 102 head (1/670 second flash duration)
#3 was with the same Speedotron 805 pack, but with a model 202 head (1/900 second flash duration)
#4 was taken with an SPS Systems Excalibur 1600 monolight (unknown flash duration)
#5 was taken with a Speedotron Force 5 monolight (1/1500 second flash duration)
#6 was taken with a Paul C. Buff Einstein monolight in Action mode (1/4826 second flash duration, and yes, the fan was spinning)

studio-shutter-speed-strobe

In the above series of images, the shutter speeds in both rows go from 1/15 to 1/30 to 1/60, and finally 1/125 (all at f/7.1). In the top row the studio strobe was not turned on, so you can see the base exposure. There is a slight rendering of the subject in the 1/15 and even the 1/30 second shots, but not enough to affect the exposure when the strobe is turned on (second row). At 1/60 and 1/125 there is no ambient light at all in the exposure. All of the images in the bottom row show the same exposure, controlled by the ISO, flash power, and aperture. The shutter speed is inconsequential to the overall exposure in a reasonably dark studio setting.

What about speedlights? What about high-speed sync and speed lights? Let's look at another set of images. As above, #1 is the same reference image as used above at 1/15 second with no flash.

flash-sync-speedlight-shutter-speed-blur

#2 was taken with a Canon 430EXii flash at 1/15 second
#3 was taken with a Canon 430EXii flash in high-speed sync mode at 1/500 second

Here you can see that engaging high-speed sync does not help stop subject motion. It actually led to more subject motion, even though the shutter speed was at 1/500 second. Additionally, high-speed sync robs the flash of power, leading to an underexposed image. This is because high-speed sync works by pulsing the flash multiple times at a lower power during the exposure. This makes the speed light less bright, helps burn through batteries, and does not help stop motion blur.

I hope that this helps clarify why shutter speed is usually not an important factor in determining exposure or in stopping motion when working in a studio with speedlights or strobes. The important factor here is the flash duration of your strobes or speed lights.


Taking it outside

Shutter speed with flash does play an important role in outdoor flash photography with a lot of ambient light. There your ISO and aperture control the exposure on the flash-lit subject and the shutter speed affects the brightness of the background. By slowing down your shutter speed in this situation you will make the background brighter. By speeding it up, you darken the background.

This situation (outdoor flash photography) is where high-speed sync (HSS) comes into play. Without HSS you are limited to shutter speeds at or below the camera's sync speed. Depending on the camera, this could typically be anywhere from 1/60 to 1/250. You will need to consult the owner's manual for your particular camera to find the sync speed.

The usual situation here is that you want to take a flash photo outdoors on a sunny day and want to use a large aperture (maybe 2.8 or faster) to blur out the background. The base exposure, without flash in this situation might be something like 1/5000 second at f/2.0. If you add a flash that doesn't have HSS your shutter speed is limited to, in this example, 1/160 second. This means that you cannot shoot at f/2.0, you need to stop down to f/11. But this gives you too much depth of field (the background objects are too in focus and distracting). By engaging HSS on your speed light you can now sync the camera at 1/2000, but your flash will need to be in close to your subject because of the reduction in output power that enables HSS.

fill-flash-no-high-speed-sync

The image on the left above is at 1/160 second at f/11 with no flash
The middle image is 1/160 at f/11 with flash
In both images, the background is too distracting. To fix that, we want to shoot at f/2.0
The image on the right is 1/5000 at f/2.0 with no flash to give shallow depth of field. But the eyes are in shadow. We would like to use a flash to fill in the shadows.

fill-flash-high-speed-sync

Here we start with the last image from the last series. 1/5000 at f/2.0 with no flash
In the middle image I turned on the flash without high-speed sync which dropped the shutter speed to 1/200 second (maximum sync speed on the Canon 5D mkIII). Because I am at f/2.0 for the background, the image is way overexposed.

On the right I turned on high-speed sync which let the camera and speedlight sync at 1/5000 second at f/2.0. Now there is some light in the eyes, shadows are opened up a bit, and the background is nicely blurred to help make the subject stand out.

Controlling the ambient exposure

Earlier I mentioned controlling the ambient exposure via shutter speed. Let's take a look at that. Here I have combined high speed sync, manual exposure, and manual flash settings.

The base setting for the sky exposure is 1/2000 at f/4. My on-camera flash is set to full power so that it has enough reach for high-speed sync. The images above are shot at 1/640, 1/1000, 1/2000, and 1/4000 with flash. Notice that the background is lighter in the first two images, normal in the third, and darker in the last image. The last image, at 1/4000 shows a darker subject because the flash didn't have enough power to handle high-speed sync even at full power at this distance and at such a high shutter speed.

Questions? Comments? Thanks for following along!