fill flash

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!

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.

Enough with the megapixels and ISO!

What I want from the camera manufacturers

I was talking with Jared Platt and Jim Schmelzer after the classes I helped them with at Glazer's Camera's PhotoFest 2017 in Seattle today. After some technical discussions about high speed sync, hypersync, and old ASCOR strobes the conversation turned to camera features. Here are two, no make that three, changes we would like to see in ISO and megapixels. I have no idea if these are practical or feasible, but hey! Let's at least put it out there.


The discussion of high speed sync led to our agreement that we don't need any more super high ISO settings. For portraits, especially outside with fill flash, we want LOW ISO settings. ISO 32, ISO 25, and even ISO 10 would be so welcome. Then we could more easily balance daylight exposures with fill flash and have some headroom to adjust shutter speed to control the ambient light levels without having the resort to high speed sync. High speed sync (HSS) is great. But it comes with a price--lower output power. Our lights have to be very close to our subjects with HSS. If we could go to a lower ISO we could keep the lights in regular sync mode and have the power available to back them up out of the frame or to use a larger light modifier with them. Right now the only way to do this with studio power level strobes that don't offer HSS is to use neutral density filters, and that has other complications, like difficulty focusing and dealing with color shifts from ND filters that aren't quite neutral. Are lower ISO settings too much to ask for?

If you aren't familiar with what is sometimes called syncro-sun flash, it is basically using a flash unit to supply fill light on a sunny day to lessen the shadows on your subject. You take an ambient light reading without the flash (let's say that is 1/60 at f/22 at ISO 100) and then you set your flash power to give an appropriate amount of light to open up the shadows. I picked the 1/60 shutter speed so that I have some headroom in case I want to vary the shutter speed to darken the ambient light exposure vs the flash. So, I could go as far as 1/200 to darken the ambient by 1 and 2/3 stops. But I am at f/22 and would much rather be somewhere around f/4 to lessen the depth of field and make the background less distracting. F/22 to f/4 is 5 stops. I could use a 5-stop neutral density filter on the lens to bring down the ambient light level to allow the f/4 aperture. 5 stops is a lot of light being cut out. It is going to give you a very dark viewfinder for composing and focusing. And it might be too dark for autofocus to work.

If, however, ISO 12 was available, the ambient exposure would be 1/60 at f/8 (3 stops different) and then only a 2 stop ND filter would be needed to get the exposure to f/4. Much easier to look through the viewfinder to compose and focus.

Enough with the megapixels!!

How many of us need more than about 20 - 24 megapixels? What if a camera manufacturer took a high megapixel sensor and used some of the pixels to extend dynamic range?

There are lens arrays that let you adjust focus after making the photo (see plenopticsa). And think of the Bayer filter array currently used to create color images. You have a red, a blue, and 2 green pixels that are used to create our color images. What if someone made an array of pixels that, in addition to color, produced an output of a dark, normal, and bright pixels that could be combined to create a kind of high dynamic range image without having to resort to combining multiple images in post processing? All the information would be in one file, so no worry about any movement during the bracketing sequence.

ISO Bracketing

Normally, we would bracket by taking a series of images in quick succession changing the f/stop (which changes the depth of field, which might affect alignment of the multiple images) or by changing the shutter speed (which changes the ability to stop motion, affecting alignment, and which won't work with flash exposures) on each of the images in the bracket. A third bracketing option is to bracket the ISO, but that usually requires additional equipment, like the CamRanger until camera manufacturers get wise to the need for ISO bracketing and add that to their cameras. If the bracketing could be done within one image you wouldn't have to worry about the change in depth of field or about subject movement between images. OK, so that's three things I want from the camera manufacturers, not two.

I haven't upgraded my Canon 5D MkIII or 6D bodies to newer higher megapixel models like the 5D MkIV or 5DSR. I don't need more megapixels. But if a new camera was introduced with better dynamic range and lower ISO settings, I'd be looking hard at purchasing the new model.

How about you? Do you see the benefits of lower ISO (they don't have to remove the higher ISO settings, those can stay, too), better dynamic range, or ISO bracketing? Please comment below.