flash syncronization

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.

Hummingbirds in Sequim

Let's try something new!

competing hummingbirds

As previously mentioned, I was out in Sequim, WA last week to teach a lighting workshop. I stayed overnight at a friend’s place and about 15 minutes before we had to drive over to the workshop I decided to try to make a photo of some hummingbirds flying around the deck. I grabbed one of my Interfit Photographic S1 battery operated strobes out of its case along with its standard 7” reflector and a light stand. I set it up about 3 feet from the bird feeder and then popped a Canon EOS M5 camera onto a tripod with the 70-200mm f/4 IS L lens and a cable release. In just a few of minutes I captured some photos and then had to quickly pack up and head to the workshop (more on the workshop coming soon). For these the flash was in manual mode with the power dialed down to its lowest setting (2.0). The S1 uses IGBT technology* to control the flash, so dialing down gives the shortest flash duration to help avoid motion blur. The camera was also in manual mode and the settings were ISO 100, Shutter speed 1/160, and aperture f/6.3

No, these aren’t the best hummingbird photos. If I were to do this again I would give myself more than 15 minutes and would use at least 2, possibly three, lights for more dimensionality. I would also set up a backdrop behind the birds (where the third light might be used). Here the backdrop was a stand of dark trees about 60 yards away that went completely black. For this morning I was mostly interested in seeing if the flash could capture the wings of the hummingbirds and I think they did well. 

In lieu of the mediocre photograph, let’s make this a learning experience.

High Speed Sync?
Some might ask why I used normal sync at 1/160 shutter speed instead of putting the flash into High Speed Sync (HSS) mode. In my experience, HSS is great for matching the flash to the ambient light to photograph at larger apertures (f/4 to f/1.2) to get limited depth of field in an outdoor flash photograph (I didn't need that here, I actually wanted a smaller aperture for more depth of field). But it doesn’t help with freezing motion. Here is a series of photos made with a Canon 430EX II speedlite in normal and HSS modes where you can see what happens to the motion using HSS to photograph a spinning fan.

Notice that at normal sync speeds, even as slow as 1/15 second, the flash duration is fast enough to freeze the motion of the spinning fan blades. As long as the room is dark enough, the ambient light won't affect the photo and the short flash duration becomes your effective "shutter speed" even if the shutter is open for a full second or more. When you get into the HSS zone there is a lot of blur, even though the shutter speeds are faster.

How Flash and Shutter Speed Sync

If you are not familiar with High Speed Sync, what it does is pulses the flash rapidly during the exposure to enable it to synchronize with the moving blades of your camera’s shutter. In normal sync mode you press the shutter button, the first curtain of the shutter opens, the flash fires, then the second (rear) curtain closes. When you go above your camera sync speed (which could be anywhere from 1/60 to 1/250 second) the first curtain opens, the second curtain starts to close, then the flash fires.

With the shutter speed set at or below the sync speed

Shutter speed is set higher than the sync speed

flash sync black band

If you are just a step or two above your sync speed you get a dark band across one edge of your photo. At higher speeds the flash doesn’t register at all, as the second curtain starts closing as soon and fast as the first curtain opens. 

High Speed Sync fires a series of lower powered flashes in perfect synchronization with the moving curtains so that the image looks like it was made with one flash.

How flash and shutter speed sync with each other. Click on the image to view it larger.

Downsides to High Speed Sync

The series of flashes, while being great for letting you photograph at wide open apertures, does have some downsides. The most noticeable is the reduction in power output. Normally the strobe fires and needs a second or two for the capacitors to recycle and be ready to fire again. In HSS the flash doesn’t have any time for recycling, so it puts out a series of very low powered flashes. The light usually has to be close to your subject. You will lose a stop of power every time you double the shutter speed above the sync speed. The second downside is that it doesn’t help much with freezing action. Third is that it goes through batteries quicker. And fourth is that over time the rapid flashing can lead to a lower life expectancy of the flash tube. A note on that, though, is that I have some studio flash units here that are close to or even more than 40 years old and the flash tubes are still working on them (but they’ve never been used for HSS, which didn’t exist 40 years ago). 

*IGBT = Insulated Gate Bipolar Transistor
Different flash technologies go about controlling power output and flash duration differently. Large studio pack and head systems usually control power by switching banks of capacitors on or off (capacitor switching). Lowering the power on the pack will shorten the flash duration.
Until recently, most monoblock strobes would vary the voltage of the capacitors (voltage lowering) to control power. In these lowering the power leads to longer flash duration with some change in the color temperature of the light.
IGBT units have constant voltage, but have the ability to shut off the flash tube (tail trimming) when the desired amount of light has been emitted. Here flash duration gets faster as the power is decreased. Color temperature should be a little more consistent at different power levels and by cutting off the tail of the flash the ability to freeze motion is increased. When using IGBT flashes, going to half power or lower gives the faster flash durations. 

DISCLOSURE: I am part of the Interfit CreativePros Team