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.
Three blurry choices, inside.
Dedicated speedlights and high-speed sync is one way to get rid of your aperture problems. But because of the inefficiency of focal-plane flash (lots of energy falling on mostly closed shutter curtains) your flash power is greatly reduced.
You can add extra flashes (cue McNally's Tree of Woe) or you can do everything at f/16 or so and then bleed some aperture with a neutral density filter.
Couple of things: First, you put the filter on the lens, and not the flash. And second, there are a few routes to take, with some being better than others.
The upside about ND filters is that you can use them with any dumb flash. Anything you flash can do balancing with the sun at f/16 it can also do at f/2.8, or even f/1.4 with ND filters. Big flashes, small flashes - doesn't matter. You are simply taking aperture limitations out of play.
You could wrap three speedlights around an athlete for a very cool mid-day portrait. All hard lights -- high front key, and two back/side rims. At a 250th of a sec, you'll need to light your subject to ~f/16 or so (depending on the ISO) to balance the sun. More if you want to overpower it.
That takes a lot of light. But close-in, bare speedlights can absolutely do it. And any decent monobloc or pack-and-head could do it without breaking a sweat.
The problem is your backdrop -- maybe you want to blow it out. Maybe it looks like crap at f/16. Maybe your sensor dust is starting to look like a scene from Jurassic Park.
The important thing to know is that the flash/ambient relationship is not going to change. You need the power to compete with the sun. But now we want to bleed some aperture out and lose the background focus.
To take your exposure from 1/250th at f/16 to 1/250th at f/2.8, you will need to place five stops worth of neutral density filter over your lens. This will maintain the flash-to-ambient balance, while knocking the light down.
(If you put it on your flash, you would be giving yourself big problems -- unless maybe your flash has a thermonuclear setting or something. And you still would not be able to get your aperture down.)
So, what kind of ND filters to consider?
As always, there are choices. And some of those choices depend on your wallet. Here are three.
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 -- 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?
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 NDs.
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 -- or 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" 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" gels and his photos of course look amazing.
He also uses it for wide-open movies with his Canon 7D. Above, he is shooting footage from inside a seaplane over Dubai earlier this year.
Pretty intimidating looking setup, if you ask me. Very Cecil B. DeMille.
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 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 ne plus ultra of ND filters. It gives you a "dial-in" setting of anywhere from two to eight stops of neutral density -- 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 filter is $340. 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.
I'll probably catch some crap in the comments for such a pricey filter, but I tried the cheap version and that was $50 for nothing. To me, that's more expensive.
And yeah, I have given the old Visa card a pretty good run over the last year. But I shot for 20 years with someone else's gear, and I wanted the next 20 to be with that of my choosing.
And frankly, it feels very good to finally be done with major purchases for the foreseeable future.
The photo up top, of my daughter Em, was when I first started using it. Just some learning time with no pressure.
It is a straightforward shot, done mid-afternoon with a single Profoto head 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.