Improve Your Shots Using Optical Filters
Linux Format|November 2021
Mike Bedford looks at the benefits of photographic filters and how to emulate them – but some effects can only be achieved optically.
Mike Bedford

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One of the most extreme optical filters removes all visible light, allowing only infrared to pass through to the camera’s sensor. This provides the option of infrared photography which has been described as otherworldly or even spooky. If you want to know more, see our tutorial in LXF248.

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Some optical filters produced quite extreme vivid that might seem to be the sole domain of digital manipulation. Included here were multiple image and kaleidoscopic spiral filters; we even came across a rainbow filter. Many of these are still available, although duplicating the effect digitally would be preferable.

Here we’re going to be looking at photographic filters – but first, a word of explanation. The term ‘filter’ has taken on a different meaning in recent years. If our quick Google search is representative, a filter provides a means of digitally altering a photo. Commonly requiring little more than a single click, such filters can be used for artistic effect or for pure entertainment.

Before the advent of digital photography, though, a filter was a device that screwed onto the front of the camera’s lens, and applied some effect optically. These effects were usually not nearly as extreme as those offered by some digital filters, but enabled film-based photographers to improve their work substantially. What’s more, the benefits of optical filters or their digital equivalents can be just as important today.

In this tutorial we’re considering only those effects that can be achieved optically, and so we’ve excluded the more bizarre effects that really don’t need much in the way of instruction. Paradoxically, using filters to achieve more subtle effects often requires more understanding, but those more refined effects often make the difference between a photo that’s just okay and one that’s really good. What’s more, in many cases, a really good photo is often one that doesn’t appear to have been edited or filtered in any way.

Optical or digital?

First of all, we need to address the question of which is best: an optical or digital filter. As a general rule, we advise against applying any effect at the time you take the photo, and this applies not only to screw-on optical filters but also to in-camera or in-phone effects such as black and white or sepia conversion. The reasoning is simple. If your original photo has some effect applied, it will usually be difficult or impossible to remove that effect later by photo editing if you decide you don’t like the result. If you avoid applying any effects while you’re shooting, though, you have the option of applying whatever effects you like afterwards, while always keeping the original intact.

Despite this general recommendation, we need to point out that some effects that can be achieved with an optical filter can’t be duplicated digitally. In this case, you’ll need to use a physical screw-on filter and we’ll cover these types of effect also. After all, despite our commitment to all things digital, it would be a questionable approach in our coverage of filters to avoid discussing several useful techniques just because they’re optically-based.

Contrast enhancement

Some of the first and most widely used filters formed an important part of the black and white landscape photographer’s arsenal. They were coloured yellow, orange and red and had the effect of darkening blue skies to increasing effects, thereby providing more contrast with white clouds. Similarly, green filters were used in portrait photography for darkening skin tones. These effects can be duplicated digitally, as we discovered in our tutorial on black and white photography in LXF274, so these optical filters are now really only for the traditionalist.

Filters have also been used in colour photography to enhance skies, but with a difference. The sky is usually much brighter than the ground, and this can cause problems. In particular, if the rest of the shot is correctly exposed, the sky will often look somewhat washed-out and, in the most extreme of cases, it will be overexposed so clouds could almost disappear.

One way of addressing this digitally is to take two photos, one with the ground correctly exposed and one with the sky correctly exposed. These can then be combined to provide the best of both worlds – see LXF257 for information on HDR techniques. However, because this requires two shots of the same scene to be taken, it’s necessary to use a tripod, and moving objects can cause problems, so this isn’t for everyone.

This is where the graduated neutral density filter comes in. This is a filter that is clear at the bottom and grey at the top, the two regions merging one into the other, and which can be moved up and down and/or rotated so that the division between the clear and grey regions line up with the horizon. It might seem that this can be duplicated digitally, and in some cases it can. However, this doesn’t apply where the difference in brightness between the ground and the sky would cause the sky to be totally overexposed, because there’s no way that can be reversed. In the most extreme cases, therefore, this is really something that should be accomplished with an optical filter. If you didn’t use a filter when you took the photo, though, by all means see what you can do by photo editing – here we see how to do that in GIMP.

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