Fun with filters 5…

Neutral Density Grads

 

As I mentioned in the previous posting, neutral density filters are simply dark filters that are used to control the amount of light that enters the lens, without introducing any colour cast to the final exposure - hence "neutral". In this post we're going to explore ND graduate filters in a bit more detail and how to use them to your advantage.

A graduated filter is basically a piece of glass or acrylic resin that is dark at one end graduating to clear at around the mid point. As mentioned before, this is the ideal tool for controlling light in areas such as the sky, to balance the overall exposure and prevent it from blowing out. This can be controlled by bracketing and combining exposures to make an image with the best exposure for each area, but by using an ND grad filter, you can control your exposure in-camera and achieve the results you want without a lot of post-processing in photoshop.

ND grad filters are available as screw in or slot in filters, but by the nature of these filters work, the screw in versions are really not ideal. The graduation typically ends across the diameter of the circle and this limits your compositions so you have to keep the horizon line straight through the centre if the frame. Ideally, you'd want this to fall on a line following the rule of thirds, and the filter doesn't allow for this. A slot in filter system allows you to drop and lift the filter to ideally suit your composition. Plus, you can combine more than one graduate for a more complex effect. The filters also add more definition to cloud forms in your skies and boost up the colour, although not in the same way a circular polarizer would.

Grad filters are available as hard and soft grad versions - in use this basically means that, although the tinted areas stop at the same point, in the soft grad version, the gradation of the tint to clear is over a larger area and the hard grads taper to clear very quickly in a small area. In use, this means that a hard grad is ideal for use where you have a large flat horizon that you can sit the grad on - such as a sea scape. Soft grad filters are a bit more forgiving and you can use them on more uneven horizon lines without too much trouble. I've read a lot of magazine articles on ND filters and most people suggest that the hard grads are easiest to use for the novice - I only have soft grads as I think they're more flexible in use and I've not had any problems composing with them. In fact, the softer graduation curve means that they're more forgiving.

 Here's an example of one of my images where I've deployed a 2 stop ND grad (ND4) to hold back the sky areas against the foreground. As this was shot into a hazy sun, the filter was essential to stop the shy from just blowing straight out to white in the centre.

This was taken using a set of really cheap ND filters I bought on E-bay and I didn't realize how brown the filter tint was until I purchased a better quality set of replacements recently. That said, I like the colour cast it's put in the sky - it was early morning and the light was quite golden anyway, so it's resulted in this desaturated "tobacco" colour that's not unpleasant!

This is one thing to bear in mind when buying any ND filters - there are loads of cheap versions available, and these are great to practice with to fine tune technique, but they may not be manufactured to the same standards and calibration as a more expensive branded filter. Also, the resin sheets may not be the best optical quality. I first noticed about a month ago, that my cheap ND grads were introducing some weird patterns into the sky that look like banding. I put this down to a combination of the tint and maybe inferior acetate and replaced all my filters for Kood branded versions, that are far more "neutral" in colour. I used the ND4 grad on the image at the top to capture the dawn light 2 days ago. The light was so low at that time of the morning that even a relatively light grad delayed the exposure long enough to show the motion blur in the clouds, and added a nice atmosphere. You can see that the pinkish colour cast has gone and the blue pre-dawn light is more evident - I wanted the shot to look cold (it was bloody freezing at 5.45 am setting up).

Here are a couple of quick pointers for using ND grads in the field:

• Keep your filters to hand, but safely stored. They pick up dust really easily due to static and are fingerprint magnets. I tend to take one filter out at a time and keep the rest packaged up in a wallet in a pocket or my bag. Try and hold the filters by the edges to keep your fingers off them.

• Always blow the filters and your lens front over to make sure you get as much dust off as possible. Remember to do both sides of the filter so there's no dust trapped on the lens side.

• Always slot in your filters starting with the slot nearest to the lens front - then if you want to add a second, slide that in front. Quality may be affected if you add 3 or more filters at one time.

• Compose your shot and focus with the filter holder attached but nothing slid into the slots. This makes it easier to get a correct focus for your image. Once you have this how you like - switch to manual focus to lock it in place. If you leave autofocus on, the camera will keep trying to focus on the filter and defocussing itself (leading to much swearing until you get with the program). For slot in filter systems - manual focus is king!

• Try not to drop a filter, especially on the beach. If you do drop a grad, don't put it back into the holder as it will make that dirty and probably scratch it up as you slide it in. I dropped one into the snow once which was not quite as tricky, but still meant that I had to stop using it for that shoot as you can get drying marks.

• If there's any moisture in the air (fog, rain or sea spray) this can sit on the filter front and result in strange light or dark dots appearing in your image. These will have to be cloned out, so it's good practice to check the filter front regularly to make sure it's clean.

• Be sure that the filter is slotted all the way across the lens as even the clear end of the filter sheet will leave an unpleasant line across your image if it's not slid down enough.

There we have it! If you're thinking about getting a set of ND grad filters, hopefully this has given an overview of how they can aid your exposure and how to best use them. I really like working with them and think they add more to an image than simply bracketing and merging exposures can offer. If image quality is a concern - go for Lees products (if it's good enough for the world's best landscape photographers it'll be fine for the rest of us). If you want to try them out without breaking the bank, try some cheap Cokin P sized versions for a few quid and if you like using them - buy better quality replacements that will fit the same holder (Cokin, Kood, Hitech - they're all good). Give them a go - you won't turn back!

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