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.