Using ND Filters to Kill Depth of Field

It's simple math. If you are shooting outside in the sun and limited to 1/250th of a second sync (or worse) you are are going to be shooting through a tiny hole as your aperture. Even if you crank your ISO down as low as it will go, you'd better like that background. Because you are going to see it in pretty sharp focus.

Or maybe not. In addition to high-speed sync, there are a few ways to bleed some aperture from your exposure settings in full sun.

Keep reading for three, blurry-backed choices.


HSS: More Limiting Than you Think

Using the combo of dedicated speedlights and high-speed sync (HSS) is one way to get rid of your aperture problems. But high-speed sync robs power progressively as you go up the shutter speed scale, because more and more of that flash's rapid-pulse light is falling on partially closed shutter curtain. Further, you have to have a flash (and remote) that is dedicated to your HSS-capable camera.

The good thing about using plain-old, one-pop flashes with ND filters is that you can use them with any camera and any flash. (Although to be clear, big flashes obviously will give you more working distance than small flashes.)


How to Use Manual Flash and ND

First, you shoot at your camera's maximum shutter speed for a full flash sync. (For most cameras, this is somewhere between a 1/250th and 1/180th.)

At this shutter speed, you then find an aperture to either properly expose, or underexpose, the background. In full daylight, even at a low ISO, this is going to be at like f/16 or f/22.

Second, you'll set your flash to a high power level, and move it in until it can light your subject to your chosen aperture. Your maximum working distance will be limited by your flash's power. Big flash = more working distance.

Speedlights, especially working through a diffuser, will struggle at all but the closest working distances when trying to compete with full daylight. But if you are willing to use your flashes bare, you can easily work in full sun at reasonable portrait lighting distances. See more here.

Third, once this shutter speed/aperture/flash relationship has been established at a tiny, sharp-background aperture like f/16 or f/22, you use a neutral density filter on your lens to knock down the light from both the flash and the ambient until you get to your desire aperture.

The important thing to keep in mind is that the flash/ambient relationship is not going to change. You still need the power to compete with the sun. The neutral density filter just allows you to have that balance of light at a wider aperture.

If you are going to go the ND and manual flash route, here are three choices to consider.


The Budget Option

You can get a typical-brand ND filter for about $50 or less, which is very tempting. This is the route you will probably try first, as I did. Unfortunately, it was a learning experience. Here's why.

I bought a Tiffen 77mm ND filter that cut three stops of light. Cheap, fit my lenses and solved the problem, right?

Wrong.

The sharpness was not what I had hoped it would be. But there was also a color shift — it was a kind of weird warm that sucked the color out of the sky, which was exactly what I would typically be using as a backdrop with the ND/flash portraits.

Granted, it is very difficult to make an optically pure ND filter of that strength, and maybe $50 was a pipe dream. They got the "density" part down. The "neutral," not so much.

But on top of that, it was like my first microwave oven. It had two settings: off, and nuclear. What I found is that I needed variability to solve different problems. Sometimes 3 stops was okay. Often I needed more (and sometime, less.)

In the end, it went into a drawer. $50 lesson learned.


The Pay-As-You-Go Plan


Being younger and wiser, my friend JoeyL skipped the dime store version and went for a set of Lee 4-inch polyester ND filters. The good news -- they are both sharp and cheap -- on an absolute basis.

The bad news, they are basically a consumable. They will scratch, and you will have to replace them.

This is the way the Hollywood folks roll when making movies. You'll need a 4" gel holder (probably "pro shade" combo) and a filter for every ND value you use. If you always work in full sun and want to go to f/2.8, this might be a good option for you. But you will use up the filters and have to replace them.

If you need variability in your ND filters, it could get to be expensive pretty quickly and do so in an ongoing way. That said, Joey seems very happy with the 4-inch gels and his photos of course look amazing.

He also uses it for wide-open apertures when shooting video. Above, he is shooting footage from inside a seaplane over Dubai earlier this year.

If you want to use ND sparingly (and you are very careful by nature) the 4" polyester filters can be a very reasonable option that will give you very good results. You'll probably want to buy one that will get you from your full-sun aperture to your wide-open aperture. Then maybe a second which will do the same on a cloudy day.

This will also give you the option on a sunny day (with the second, less powerful ND filter) to go to only f/5.6 if you want moderate depth of field.


The Buy-It-Once Plan

If you want optical quality, durability and continuously variable densities, there is one option. And it is expensive.

The Singh-Ray Vari-ND is the gold-standard for ND filters. It gives you a "dial-in" setting of anywhere from two to eight stops of neutral density, with color that is actually neutral. And it is sharp, too.

Singh-Ray filters have an outstanding reputation, for which one pays dearly. For example, the 77mm Vari-ND thin-mount filter is $390. As far as I can tell, it is two high-quality polarizers that used together form somewhat of a "dimmer switch" for light.

But it is a thing of beauty, both in operation and performance. After paying my $50 Newb Tax above, I at least was able to experiment enough to know that I wanted to have the ability to mix flash with any level of sunlight, work at any aperture and with any piece of flash gear. That's what the Vari-ND let's me do.

For God's sake, only buy one size of this filter: get it to fit the biggest diameter lens you'll use. Then use step-down rings for your other lenses.

If you think of it as a $390 filter, it is obscene. But if you think of it as a magic disc that allows you to use any flash and any lens in daylight at any aperture, it is more tolerable.
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Walk-Thru

The photo up top, of my daughter Em, was when I first started using the Singh-Ray. Just some learning time with no pressure.

It is a straightforward shot, done mid-afternoon with a single big light in a 60" Photek Softlighter II. But the neutral density adds a third variable to your normal f-stop and shutter speed duo. Here's how I keep from getting too confused by that.

First, I choose my shutter speed. If I am pushing the limits of my lights (i.e., maybe when using speedlights) I would choose 1/250th. In this case, I had plenty of power so I started at 1/125th. This was simply to give me the ability to alter the ambient background levels with my shutter speed while shooting without hitting my sync limit. And in the end, I shot this frame with a darker background at a 1/250th. Nice to have the option.

Next, I close down my aperture until I get a background that is the exposure tone that I want. It will be very much more in focus than the final shot will be.

Now adjust the power on the flash to light the subject. In this case, Em was in the shade of a building (background in full sun) so I was adding light to a nice, dark starting point.

The flash and ambient relationship now are set. Placing the Vari-ND on your lens will allow you to remove as much light as you want from the photo, and you compensate by opening up the aperture. Rather than go wide open to f/1.4, I stopped at f/1.6 because I know my 85 is sharper there.

I went pretty wide open here, but I could have shot at any aperture. And I absolutely love that I can do that now.

The idea of crisp, multi-hard light wraps mixed with squishy backgrounds at high noon gets me a little tingly. Which is much needed, after the numbing effect of buying a filter that cost as much as a car payment.