Long exposure photography – filters.

I promised yesterday to say more about the various types of filter used in photography. As the featured image shows they come in different shapes and sizes: circular, square and rectangular. The square ones in the photo are 100 mm. For an ultrawide lens I would need 150 mm filters because of the diameter of the lens. They come in different strengths to block different amounts of light. They have different purposes, and while the circular ones screw into the internal thread at the front of the lens, the square and rectangular ones drop into a special holder that is mounted on the lens.

In the photo, from top left you see a filter holder with slots for up to three 100 mm filters to be stacked. Next along is an 82 mm NISI circular polariser lens next to a NISI 82 mm adapter for the polariser to screw into the filter holder. Below those three items are three 100 mm square neutral density filters – 1 @ 3stops, 1 @ 6 stops and 1@ 10 stops. Top right is a 3 stop rectangular graduated ND filter – darkest at the top and fading towards the centre. Below that is a 3 stop reverse graduated ND filter having a sharply defined area of darkness at the centre, fading towards the top.

Firstly, the circular filters. They come in different diameter sizes, so if you have several lenses with different diameters and you need various strengths of filter it would be expensive to have all the combinations. The workaround is to decide what will be the largest diameter that you are likely to need – say 82 mm, buy all your filters to fit that diameter and then buy one set of step-up adapters to convert your various lenses to hold your 82 mm filters.

Most photographers will own and use circular polarising filters. These usually block up to 2 stops of light. I say ‘up to’ because they have to be rotated to be effective. These lenses reduce reflections, reduce the effect of haze, increase colour saturation and make blue skies bluer but they only work when the Sun is at 90 degrees to you. A useful tip, though, is to look through your viewfinder while rotating the filter and you should notice when it takes maximum effect.

The photo shows my camera with a filter holder mounted and an ND filter and graduated ND filter in their slots.

The graduated filters are used to reduce the brightness of the sky area of the image and bring out cloud detail. These can be purchased as hard edge or soft edge versions. The reverse graduated ND is usually used at sunset where there is a straight horizon such as the sea. It’s purpose is to reduce the intensity of the light from the setting Sun.

Neutral density filters simply block different amounts of light depending of their strength, They are called neutral density filters because the coating should not cause a colour cast in your images. The may be square or circular. Different measures are used by different manufacturers – but they map across consistently. I only want to know how many stops because that’s what I’ll use when converting my base exposure to my intended filtered exposure. Let me explain that.

I’m in position to take my photo, my camera is on my tripod securely. The lens is focused, I’ve set my ISO as 100, chosen my aperture. I switch to Manual Mode. I check that the preview looks properly exposed and, if not, I adjust the shutter speed until I get a good exposure. I make a note of the exposure. I then look up the strength of filter I’ll be using against that base exposure and that tells me how many seconds I need to keep the shutter open for. I can now insert my filter. Because it’s stopping light, I won’t be able to see through the lens with my viewfinder, that’s why I did everything else first. If the shutter speed I need is 30 seconds or less, I can set that in the camera, press the shutter button and wait. Otherwise, I need to use Bulb Mode and time it myself.

There are aids available for much of what I’ve said. There are free apps for your phone to calculate shutter speeds if you input or look up the number of stops against your base shutter speed. There are also free interval timer apps also available for phones. Remember though, this is not an exact science. You’ve taken a test shot at a specific moment in time. At various times after that, before and while you’re shutter is open, the sun can go behind a cloud or appear from the cloud, messing up your assumptions. You need to judge the situation as best you can. Check your preview after the exposure has been taken. If it’s really bad, have another try adjusting the shutter speed up or down a bit. If its almost right, you can do some tweaking in software afterwards.

Tomorrow I’d like to say some more about stop sizes and filters.

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