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Welding glass filter - how to...

October 25, 2013

One of my frequently used photographic techniques is long exposure, this mystical dark art to those not in the know, is one of the most eye catching effects available to photographers. Winding car light trails disappearing into the darkness on a deserted carriageway, or funfair rides streaking bright colours into the night sky are a staple of any quick image search for long exposure... not to mention awe-inspiring and mind spinning star trails!

A while back I started to discover an even darker art, excuse the pun, that is daytime long exposure. How is that possible?... even with a camera stopped right down to f/22 in broad daylight, a photographer will be lucky to shoot at slower than 1/100s, not even enough to catch some decent motion blur! The effect had me hooked and I had to know more.

The answer... a neutral density filter, these handy (and often expensive) pieces of glass fit to the front of the lens, or in a filter holder much like any other UV, polariser or colour glass filters.

The problem... cost, producing a piece of glass that stops every colour of light in the visible spectrum equally takes a lot of time and effort. To shoot long exposure in daylight, you will generally need a 9 or 10 stop ND filter (one stop is equivalent to one value of exposure change - f/2.8 to f/4, f/8 to f/11... and so on, a good tutorial is available here). A rough finger in the air price for a good quality 77mm thread 9 stop filter is around £80... a bit steep if you're thinking of experimenting with the dark magic of long exposure and want to save yourself the dark void of an empty wallet!

So... the hack! Cheap heavy darkening glass is widely used and easily available if you know what to look for, a welding mask for example is a practical application of this glass. A quick search on eBay gave me a slice of 'shade 11' glass for £1.40, another search for '77mm UV filter' gave me a cheap UV filter to use to hold it, £1.99. Note; 'shade 11' refers to the darkness of the glass, it's hit and miss what shade you will need, but generally the range between shade 9 to 12 isn't too great for our application and you should be able to change some camera settings to make them all easy enough to use. (By that I mean getting an exposure you can use in around 30 - 240 seconds, any longer and you'll just get bored and cold standing around too long!)

Time for the catch... cheap welding mask glass has a very poor colour transmission quality, this glass is used to shade welder's eyes from bright sparks and as a result, perfect colour transmission isn't the top priority. You will almost certainly notice a heavy green cast in the image colour, this can be rescued to some extent in post processing, or at least can be minimized. In some scenes, this colour cast can even become a feature. I'll explain a little more about minimizing the colour cast towards the end of this post.

Total so far: £3.39 delivered. Now the biggest cost, if you don't have some way of cutting glass, you may have to invest in a circular glass cutter (less than £10 online), or a hand held glass cutter for about £1 with a very steady hand. Note... there are other ways of cutting the glass, for example a power hobby multi-tool, this is the method I have used in the past, with varying levels of success (chipped rough edge glass is a common outcome here). You could also get a plastic filter thread mount, carefully white tack the outside of the mount to the glass and stick it on this way, no need to cut any glass (Note the white tack acts as a great light seal). In fact, this was my first attempt at the hack and it served me well for a very long time. The only negative to this is that you will end up with an over-sized piece of glass which is awkward to handle. Be sure to tape up the edges of the glass sheet with black tape to keep stray refracted light out and to save yourself from a nasty cut!

The method...

Step 1, measure and cut the glass to fit your UV filter. Use your preferred method, glass cutter, hobby tool or handheld. Be careful of sharp edges and glass fragments. Watch some YouTube glass cutting videos first for best practice advice!

Step 2, remove the UV filter glass and place the new glass into the filter ring. Do this by carefully unscrewing the retaining ring of the filter with a thin flat blade screwdriver, or use a lens spanner wrench if you have one.

Step 3, attach the filter to the camera

Shooting Tips: Use bulb mode, a good tripod, and turn off image stabilisation. Be conscious that exposure time calculation will be a very manual process until you get used to the effect of the filter. Shooting in live mode usually activates the sensors high ISO sensitivity and the camera will battle the effect of the filter to display a preview image on the screen. Choose a pretty small aperture (f/8 to f/11), low ISO, try using live mode to focus, but if this fails, focus at infinity using the lens marking. Use an aperture around f/11 as mentioned to compensate for the lack of focal point accuracy. It goes without saying that auto focus will not work with the darkening effects of this filter. Compose, then focus manually in bulb mode and use a remote release cable - this is essential. Try to stack your images if you can, eg. 15s, 30s, 60s, 120s and 240s, then combine them in Photoshop or Photomatix HDR later. This method should help balance out some of the negative highlight and shadow effects that the colour cast can throw in.

Try to avoid: ...focusing and composing the shot without the filter attached, then gently screwing on the filter later. If you can create the shot with the filter attached by any other technique then I'd suggest you do it. There's too much opportunity to knock the camera or disturb the focus ring.

Step 4, results.... but before that it's time to talk about some colour correction. When using the welding glass, I shoot in raw and set the white balance in Lightroom later. This usually corrects the colour enough to make it useable, a hint of highlight and shadow colour usually does the trick after that. I've heard some people say that Canon's Digital Photo Professional Software bundled with most SLRs is better at correcting colour, also some people use Photoshop with varying success. But as this is a hack, it's best to find your own style of correcting the colour, or like I do, live with some of the colour cast and incorporate it into your final image, it is art after all.

Tip for HDR... if you are comfortable using the High Dynamic Range - HDR technique, then it can be used to great effect in these kind of shots. Remember not to overcook the style and end up with shadows that are too bright and highlights that are too dark, keep it subtle enough to balance the exposure over the entire image. Use HDR to enhance the look, tone and balance of the photograph... not to make the photograph a HDR shot.

A great example where I managed to correct most of the colour cast, but left a hint of it in for effect is my shot of Tenby Harbour, before and after, shown below...

The image is a HDR sequence combined in Photomatix, then processed in Lightroom.
















...and after, click to enlarge...


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