ND filters are used in two common scenarios …
- to slow down the shutter speeds to get water to show movement or
- to increase an aperture to decrease DOF
What an ND filter does.
In both cases, the filter is darkening the scene. But another scenario is when you are shooting into the sun, and don’t want to wash-out your background. To achieve this more dramatic look, you’ll need a flash to fill in your subject. But how do you achieve a slower shutter sync speed and maintain a large aperture?
Slap on that ND filter and suddenly everything goes dark. It’s like, well not like, it IS putting a dark piece of glass in front of your camera and shooting thru it. It’s neutral (the N part) so it doesn’t add or take away the color/tone etc from what your viewing, and it’s got a measure of density (the D part) which determines how dark it is.
An example
This portrait was shot at f8, a shutter of 1/200 sec to be in sync with the flash and iso 100. I grabbed two flash units to fill in both my subjects equally.
It’s fine, but my DOF is quite “deep” and the model behind my main subject is in focus. I want him to be soft. If I shoot at f2.8 my entire scene will go 3 stops brighter. I want to maintain the look I have here but just with a shallow DOF. Let’s add the 3 stop ND filter and shoot at f2.8
They are remarkably similar but you can see the difference in DOF. The model in the background has gone soft (out of focus). Without an ND filter this would be impossible. I could have changed my shutter speed BUT this would throw my flash and camera out of sync. Not an option. I’m at my lowest iso, so there is no getting where I need to be by changing iso either. All I got is my f stop. ND filter to the rescue.
Here is what I did.
I knew I wanted to shoot at f2.8 for this portrait. So I decreased my f stop to f8. That’s 3 stops, which is what my ND filter is. Filters come in various densities but I only use a 3 stop version. I set my camera to f8 @1/200 sec (for sync) and iso100. f8 is 3 stops “darker” than f2.8. I looked at the sky and decided that f8 was good for me. I set my flashes at the same distance away from each subject so that my settings were all the same. I set both flash power outputs to 1/8 power and in my case, metered the output on both flashes to verify f8. I also like 1/8 power on my flash so I can get a decent recycle time. Once I had everything where it looked good, I just needed to increase my f stop to f2.8 and add the filter. With everything set, I grabbed a couple of victims and put them in front of my camera.
Boom.
I recommend you do all your figuring before you put the filter on. Just play with it …. your not shooting film so you can cheat by looking at the playback (back of your camera) and adjust along the way. Just remember, your ONLY changing the f stop not the iso or the shutter.
I hope this helps you begin to understand ND filter use with flash. If you got a question, don’t hesitate to contact me and I’ll do my best to answer. Who knows, maybe it will lead to another post.