When people think of the use of Neutral Density (ND) filters it is usually in relation with landscape photography and particularly in terms of producing milky effects in water. This is because the ND filter allows you to set a slower shutter speed that would be possible due to the amount of light causing an overexposure.
But most people would not think of using them to control the aperture that you want to shoot at so that you can produce shallow depth of field (DOF). But they can be used exactly for that purpose. And what’s more you can use the cameras light metre to work it out.
It is really simple but you do need to work in manual for the final result.
Let’s assume that you have a lens that can shoot at f2.8 and that is what you want to use to achieve a shallow DOF to blur the background. If you have a ND8 filter then this will restrict light by the equivalent of 3 f-stops. On the f-stop range the full stops are:
1 1.4 2 2.8 4 5.6 8 11 16 22
So going 3 stops from 2.8 is 8.
Therefore to shoot this, firstly set you camera in aperture priority and set it at f8. Take a photo without the filter in place and in the image information look at the shutter speed that the camera chose.
Now go into Manual mode and enter that shutter speed, and then change the aperture to f2.8 and then attach the ND filter.
Now some of you may be wondering why when I was in aperture priority I didn’t just enter f2.8 and let the camera come up with the matching shutter speed. Very good questions indeed.
If I was shooting outside I could do just that, however I may wish to still have some movement in the shot. If a setting of f8 gave me a shutter speed of 1/200, then f2.8 will give me 1/1600 which will effectively freeze everything.
If I am in the studio under flash such a shutter speed would be impossible. This is because cameras have a maximum shutter speed that can be used with the strobes. This is either 1/200 or 1/250 depending upon the make and model of the camera. If you exceed that speed what happens is you get the shutter obscuring part of the image. This value is referred to as the maximum sync speed.
And in confined spaces it is often difficult to get the power level of the lights low enough to keep below the maximum sync speed when the aperture is wide open.
When using strobes you do the calculation the other way around. Set your camera in shutter priority and take photos your subject. From the information the camera produces alter either the shutter speed or the power and/or position of the lights until you achieve f8. Then go into manual select that shutter speed, set the aperture to f2.8 and put the filter on.
Obviously the process of setting a f-stop with studio lights is a lot easier if you use a separate light meter rather than relying on your camera.