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Filters-ND Graduated Filters

Neutral density graduated filters or 'grads' allow the exposure to be varied over parts of the image. They are similar to ND filters, but rather than being square are oblong and around the middle fade from having no effect to cutting the light by a defined value. By sliding them up and down you can determine the part of the images to be effected. you can also rotate them so that one side or any angle can be selected.

The most common application is in holding back the exposure of the sky in landscape shots. Often the skies will be too light and be lost in the highlights (over exposed) or if you reduce the exposure the ground will be too dark (underexposed). With a 'grad', with the darkest part at the top, you have in effect put sunglasses on the sky, while the ground is still clear.

An example of when I use 'grads' is for railway photography. A steam train is running along amongst trees, and pushing up white smoke and steam into a bright sky. The contrast between the dark train in the shadows and bright steam/smoke and clouds is too great. We can expose for the train and loose the steam, smoke and clouds, or get that right and the train is very dark. In software we can lighten shadows but in this case its too extreme and we find a lot of noise (random patterns) in the darker areas when editing. By using a 'grad' I can darken the sky, and smoke etc, so that it comes within the exposure range that allows me to get good photos.

Grads cannot be used in all situations, for example if you were taking a photography of a windmill that was occupying most of the vertical height of your shot, and the highlight areas were in the sky behind, then using a grad would not only darken the sky but also the windmill and sails. Generally you can use them in scenic situations where you have woodland and the like as these are naturally dark and in most situations where you don't have very large dominant items in the skyline that you do not wish to affect. Even in these situations there may be occasions when you put up with the effect knowing that you can recover or reverse the effect on the foreground item in editing.

You will find three types of graduated filters, 'hard', 'soft' and 'hard reversed'

• Soft - the most common and most useful is the soft filter. This is the basic one explained above, it filters from a no effect over an area slowly into a full effect, there is therefore no definable line when used.
• Hard - This has a far more distinctive line, or step going from no filter effect to a ND effect. This distinct edge is lined up with the horizon. In practice it still graduates but over a very short distance. How hard it appears in the image in part depends on the aperture and focal length of the lens.
• Reverse hard - quite unusual and not widely used. This is similar to the hard but fades backwards to a lighter filter effect towards the top. Perhaps 3 stops hard fading back to 1 stop.

ND grads come in different densities or strengths, from light to dark, they are marked usually with either the ND value and marked graduated or with an ND factor.  The conversion between these and the stops of light lost at the darkest part is:-

You can use more than one grad, either in the same direction or at different ones. You could, for example on a beach scene, have a grad up from the bottom darkening the sand near to you, and a second holding back the sky coming down from the top.

Many only use 'grads' straight, but you can run them at any angle that works in your photo. Particularly if you are using a hard grad, or notice a line ending to form across the subject you will find that turning it at an angle makes it look both more natural and the eye tends not to pick up the graduation effect then. If you have more complex filter holders you can twist one grad against another, so perhaps having weaker graduations running in more than one direction to get the effect you want. In one of the sets I have built, I am able to run filters at three angles and have filters running up and down in two of these.

In many movies, and in TV dramas you will see an effect used that in affect puts a light steak through the main subjects eyes and often appears to come from a window or light, this can be achieved with two grads 1 up 1 down and the holder rotated.

The ideal effect is one that no one can see, but has controlled the exposure in a way that allows all the detail to show. Today of course we can do a lot in editing, but the controlling of extreme contrast is better managed at the point the picture is taken.

Photography is also an art, and as such you may want to use the graduated filters to produce some artistic effect. By this I don't mean its a good excuse for things that go wrong, but that you are able to balance up the light in a scene to get the effect that you require. Other parts of this management of the light will include the use of reflectors and fill in flash.

What should you get

If you don't have any and you are about to splash out then a soft ND4 (ND factor 0.6) 2 stop grad, will be the most useful. You don't necessarily need to have an holder you can just hold it in front of the lens.

You will find that as well as buying ND grads separately, you can get sets, one set has three grads of different strengths while another has an ND grad and two coloured grads. Some sets have a basic holder. In addition to this you will need an adaptor. To use it on more than one lens you may need more adaptors or to use stepping rings for the different lens filter sizes. Sets are often a lot cheaper than the separate filters. You can in some cases mix filters from one maker and holders from another, which may give you the ideal set at a lower cost. In the article on filter holders we look at the choice of sizes and makes.

Start with a few, or even one and get experience before expanding.

Deciding on the value of graduated filter to use

Lets look at the ideal solution then at some other ways to achieve this.

Ideally you would be able to use your cameras spot meter and deduce the filtering effect you require on various parts of the image to decide which 'grad' to use, or what combination would work best in this case. The Photography Skills Exposure Masterclass covers this and more, were you have a full day just looking 1 to 1 at getting the exposure right, including  metering and grads.  So with this skill, how would you then achieve the ideal results. To know what settings to select you ideally need to know the dynamic range of the cameras set up, including active D-lighting and other options set. You also need to be able to look at a scene and see the dynamic range or levels displayed before you, and imagine how you would like to see them represented in your image. For example, you may see a snow caped mountain range with deep valleys and dark woods, and by looking at this you can see the relative brightness of the snow caps to the darker parts and by using your cameras spot meter select the exposure to record this. The conversion from exposures on the camera to working EV values is done with the EV Table, making all calculating extremely easy. Where its not possible to set directly, you know the range that outside of what you want to achieve, and can drop in a grad or several to adjust the dynamic range of the image. The camera exposure is set manually to now get exactly what you want.

So if you don't have these skills how do you use grads? Well there are several methods:-

• A method devised from the days of film - its reproducible, constant and it looks as if you know what you are doing.  Set the camera on centre weighted metering with a large area or full screen average (set this in the camera menus for centre weighted metering) and then with the mode set at 'A' aperture priority, look at the difference in the average exposure between the general shot, less sky (camera titled down) and the sky, camera tilted slightly up, and take off two stops. The difference is the value of grad needed. We make a deduction as we don't expect the sky to be the same level of brightness on average as the ground.  Lets go through it again, look at the proportion of the speeds the camera comes up with for a set aperture. This is easiest done by changing the aperture on the first to be an easy number to manage.  Taking an example, suppose you tilt the camera down and you adjust the aperture so you get a reading of 1/125 sec, and then up and get a reading of 1/2000 sec then you have 4 stops difference  125, 250(1) ,500(2), 1000(3), 2000(4), so 4 stops - 2, and we use a 2 stop grad. Once you have the grad in you should be able to go back to matrix metering to get the exposure, although you may find you need to adjust the exposure variation by a bit depending on how much of the image is in the bright and dark image areas.  To make this method work for you with your setup, do the above but also try subtracting 0 through to 4 and compare the results you get. If its different to the 2 we have used above then use that instead.
• The simplest way, other than trial and error to get the value of the grade to us, is to take a test shot, look at the highlights screen to see what's flashing, adjust the exposure, say -1, then take another test shot. If its still flashing increase the variation and try again. When you can see what exposure variation was required for each part of the image you can see the ND filter effect requirements, the only complicated bit is that as its graduated, you need to think about the strength of the grad at different parts of the image in relation to the positioning. If however you take it as requiring a grad of around twice the variation required it wont be far out. So if a one stop negative variation overcomes the flashing highlights, use a two stop neutral grad.
• The trial and error approach, is simpler and just as quick but less accurate, stick in a two stop grad and try it, if you have highlights that are lost then reduce the exposure,  and use the curve to see if you have then lost detail in the darker areas. If not happy increase the strength of the grad or add a second.

Neither of the simple, or the trial and error approach is working out and dealing with dynamic range but the results will satisfy most people. Even if not fully scientific and you result totally to trial and error you will find that 'grads' can be used to overcome exposure problems.

See the article on filter holders to get a better understanding of the sizes and how they go together. The values in practice are variable according to the comparison of your lens size and filters and the aperture you use.

Concluding

ND grads effect the dynamic range, allowing you to capture higher contrast situations. While you can, in editing get most types of filter effect, we look at this in filters- other solutions, the ND grad is the effect that is best and can often only be achieved at the point of capture. Just about every landscape photographer will use ND grads, and some specialists like those involved in photographing steam railways could not get good results without them.

ND grads can be used with coloured grads,  polarizer's and many other skills to get exactly what you want.

Reading through this page it may look very complex, and have too many variables, but even if you use complete trial and error and allow your camera metering to work out the exposure with the grad in place, you will find you can improve results and its well worth a few experimental sessions to play with this. To start why not just get a single 2 stop neutral grad, no holder and just hold it in front of your lens to see what you can easily achieve.

See also: Filter Section for more articles.