Exposure Part III – Graduated Filters

In Part I, I covered how to fix a good mid point by spot metering off a suitable subject and adjusting for the colour. In Part II, I discussed how to manage the highlights and shadows of your image – whether in digital or film.

The problem that is often encountered is that the dynamic range of many scenes is greater than some films or digital sensors can cope with. Colour transparency film has about 4 stops of light from the darkest to the lightest point. Digital sensors have 5 (6 if you post process the shadows). Many landscape scenes that include the sky have 6, 7 or even 8 stops of light.

So in these cases, if you meter for a good mid point, the highlights will blow out. If you meter for highlights, the image will be too dark and the shadows will lose detail. One option is to switch to colour negative film which has 8 stops of dynamic range. But do you have another option?

Graduated grey filters were invented to help. They work by darkening part of the image to bring the highlights into the range of the film – this allows you to use a mid point to get a good exposure and retain highlight detail. An example is my picture 'Dawn Over Loch Cleat'. I used a two stop graduated filter across the horizon to darken the sky by two stops so that I was then able to meter off the sunlit grass to get the exposure right and still retain the detail in the sky.

A graduated filter is a piece of glass which is placed over the lens. One half is clear and the other half reduces the exposure by a fixed amount (e.g. 1, 2 or 3 stops). A ‘hard’ grad filter has a very quick transition between the clear and dark regions. These are good for placing over a horizon to darken the sky. Digital users will need to use these. ‘Soft’ grad filters have a more gradual transition. I carry a set of Lee Filters at -1, -2 and -3 stops in both soft and hard types. Lee Filters make an excellent holder which allows several filters to be attached at once. Each lens I use has a lee ring permanently attached to it which makes putting the filter holder on the lens at speed a piece of cake. Lee’s filters are quite large and easy to grip which means that handling, even in the cold, is straightforward.

To position the filter correctly, step down the lens to your desired aperture (using the aperture preview button found on most SLRs) and, looking through the viewfinder, lower the filter until it is in the required position. You stop down for two reasons – this guarantees the filter will be in the correct place when the shutter fires and also the darker image enables you to see the effect of the filter more clearly. For most shots I will position the filter over the horizon.

What strength filter needs to be used? There are 4 steps:

  1. Identify and meter for a midpoint.
  2. Spot on the highlight.
  3. Calculate the difference.
  4. Work out whether a one, two or three stop filter is needed to bring the highlights into the range of the film or sensor (+1.7 for transparency film, +2 for digital).

For example, using transparency film, if the mid point is 1 second at f11, the highlight is 1 second at F32 1/3, then the difference between the two is 3.3 stops. A two stop filter is needed and the highlight will fall at +1.3.

Try another example on digital. If the mid point is 1/60 at f11, the highlight is 1/400 at f11, the difference is 2.6 stops. A one stop filter will be needed and the highlights will fall at +1.6.

My handheld spot meter calculates the difference automatically which lets me identify that I need a grad and which one at a glance. Put the correct grad on the front and adjust as indicated above. Re-meter on the mid point and fire when ready.

Make sure your midpoint subject is in the clear part of the filter otherwise you will need to adjust for the effect of the filter by increasing exposure.

Of course there are some side effects to using a graduated filter which means that they don’t work for all images. If, for example, you have a shadowed object (like a tall tree) in the same region as the highlights (e.g. the sky), then the grad will also darken the tree. This can look odd or even ruin the image. Similarly, you have to make sure that you do not over grad (a common mistake) by darkening the sky so much that other parts of the image (e.g. a reflection of the sky) are brighter. This looks unnatural. In such situations a wider dynamic range film, digital blending or later post processing of levels might be the only approach.

I also often find that I place two filters over the scene (for example a 2 stop hard and a 1 stop soft) at different positions. This helps me, for example, to lighten poorly lit foregrounds, to manage highlights lower in the image (e.g. water) and to compensate for light falloff due to large format lens movements.

In Part IV , I shall look at other final adjustments to exposure such as increasing the exposure for filters and (on large format) bellows length.

© Jon Brock 2007