Are you stopping? ND filter strengths in photography.

In a post a few days ago, I mentioned the idea of Stops in the context of the Aperture triangle. When you use neutral density (ND) filters, the idea of Stops becomes a central issue. You use filters to reduce light entering the sensor in situations when using the shutter speed setting alone doesn’t fit the bill. ND filters are available in a range of strengths – these can be called by different names, but the easiest one to remember is the number of Stops of light which the filter offers. You have to choose, by looking at the situation, how strong a filter you want to use. If you are photographing a waterfall in bright light, perhaps 3 stops will do. If you are photographing waves then probably 10 stops would be your minimum – and you may want to stack a 10 stop plus a 5 stop. You are the judge. What you then have to do is to take the shutter speed that your camera recommends without a filter, and look it up in a table – such as that below, or by using a phone app – to find out what shutter speed you will need when you have a filter installed.

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So, if you’ve taken a test shot without a filter and the preview tells you that your exposure is OK, have a look at the shutter speed that you used, say 1/125 seconds. Now, let’s say you want to use a 6 stop filter, the table tells you that, with the filter installed, your shutter speed will need to be 1/2 seconds. At this point, you need to lock your focus, remember your settings and switch to Manual mode. Enter those settings – ISO, Aperture – and your new shutter speed (the 1/2 seconds one in this case), pop in your filter and you’re almost ready to go (I’ll say more about that tomorrow.

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I said above that filters are known by different names by different manufacturers – for example as a 6 stop filter, a 1.8 optical depth filter or as an ND 64 filter. The table above provides the equivalent types of naming for each strength. The table also shows the equivalent fraction of light that is admitted. So, a 6 stop filter admits only 1/64 of the light that would have been admitted without a filter. Think back to the first table. Without a filter you would have opened the shutter for 1/125 of a second. With the 6 stop, ND64 filter you will open the shutter for 1/2 second – ie 60/125 or roughly 64 times longer.

The table examples above are just to illustrate the principle. There are excellent phone apps, such as ND expert, where you just enter the base shutter time and the filter type and which also provide a timer.

Today’s featured image is of the lighthouse at Penmon Point, Anglesey, North Wales with Puffin Island to the right of the image.

This photograph was taken only slightly later with a 10 stop filter plus a 2 stop polariser. The light had also changed. The base shutter speed was 1/200 seconds, the filtered time was 10 seconds ie 2000/200 equalling a factor of 1000 (roughly 1024)

Tomorrow, I’ll provide a detailed checklist of everything you need to do to prepare for your long exposure outing.

Long exposure photography

Most cameras have a default maximum shutter speed of 30 seconds, so I guess the range of shutter speeds from about 1/8000 secs to 30 secs must be classed as the normal exposure range. By that reckoning any shutter speed longer than 30 seconds would count as a long exposure, but, for the purpose of this blog, I’m going to count anything longer than 1 second.

Why, though, would anyone ever need to open the shutter for longer than 1 second? One answer would be to take photographs in low light conditions where. Alternatively, say that you want to use a wide aperture in bright sunlight eg to blur the background behind a shrub that you’re photographing in your garden. In order to avoid the photo being over exposed you could use a neutral density filter (ND filter) to reduce the light entering the camera. Another would be to take photographs to smooth out movement – for example waterfalls, waves or clouds.

Compare this shot at Cleveleys beach, Lancashire with today’s featured photo in which wave motion has been smoothed to focus attention on the rocks. I took the photo on the left at 1/200 of a second but the featured photo at 8 seconds using a 9 stop filter and an aperture half a stop smaller.

I took this shot, mid morning on a bright day with a 10 stop ND filter. My shutter speed was 30 seconds at an aperture of f/8 and ISO 100.

This image was taken within minutes of the one above but the shutter speed was 1/60 secs at the same aperture and ISO. Notice how the movement of the water in the stream was blurred by using a filter.

I won’t be able to explain even the basics in a single blog post, so I’ll spread it out over a few days. Today, I’ll just show a couple of examples of long exposure. Tomorrow, I’ll say something about the various types of filter that photographers use. Then I’ll post a blog about calculating shutter speed for using different strengths of filter. Finally, probably, I’ll go into how to use filters.

Having said all that about filters, in low light conditions you won’t need them normally, your main equipment other than your camera will be a tripod to enable you to take a stable, steady shot.

For this photo of Liverpool by night I used a tripod but I didn’t need a filter. My settings were shutter speed 6 seconds, aperture f/8, ISO 100 and focal length 23 mm. Because I used a tripod I set the image stabilisation to off. Other than that, the effort was simply waiting until all the drivers were using their lights and that there were enough of them moving quickly between two sets of traffic lights.

I hope that these examples may inspire you to look at this blog tomorrow to learn more about the various types of filter.

Enjoy your day.

Photography – Aperture Words, words, words I’m so sick of words

Any newcomer to photography, picking up a modern digital camera is immediately confronted with a mystery. On the lens he or she might see something like SMC Pentax DA 1:1.8 50 mm. Turning to the camera – on a dial on the top they might see some of the following – B, M, AV, TV, SV P, AUTO. On the sides and rear of the camera there are acronyms such as AF/MF, RAW Fx1, AF/AE-L, ISO, WB, LV. If they were to ask what these initials mean, they’ll hear words like Aperture, Focal Length, Shutter speed, ISO. Pressed further, their mentor might mutter terms such as Depth of Field or Exposure triangle. As Eliza Dolittle sang to Freddy in the musical ‘My Fair Lady’, ‘Oh, words, words, words, I’m so sick of words…is all that you blighters can do?’

In the next few blog posts, I’ll try to explain just a few of these terms to the best of my ability. But you can find more detail by ‘googling’ any of the terms. Anyway, I’ll start with aperture. Essentially the word ‘aperture’ in photography refers to the hole in the lens through which light reaches the sensor – the bit that actually records your image. The hole in the lens is at the centre of the shutter which opens when you press the button to take a photo and closes when the shot has been taken. The shutter is made of ‘leaves’ that move both to do the opening and closing, but can also be set to change the size of the hole to create different effects.

This is a photo that I took in my garden this morning of a heather plant. I set the opening to create a wide hole – as wide as this lens would allow. Notice how only part of the plant is in focus and everything behind the plant is blurred.

The aperture – or size of hole – that I set for the the photo was f/1.8. Let me simplify that. 1.8 is almost 2 so when you see that f/ it refers to a ratio of 1/2 or half. So if the aperture size were f/16 the ratio would be 1/16. You know from simple arithmetic that 1/16 is much smaller than 1/2 – a sixteenth is smaller than a half. Why did I choose such a big aperture? I wanted only the plant to be in focus – I wanted the background to be blurred. That’s an effect known as a shallow depth of field where sharpness is confined to a very narrow band of depth.

I shot this photo at f/8 – an eighth, so I only allowed in a quarter as much light as in the first image in one way, but the camera compensated by opening the aperture for a shorter time, 1/5 seconds, to provide the same level of exposure.

The image looks different. In a way I’ve let in about the same amount of light, but I’ve changed the recipe for doing so. I used a smaller aperture but the shutter was open for 1/5 seconds whereas in the first photo it was open for 1/100 seconds. Look at the difference between them. In the second photo, the whole of the plant is in focus and the back ground is less blurred. In other words there is a greater depth of field. The field of sharpness extends further forwards and further backwards as well.

In my final photo, the settings are an aperture of f/22 and a shutter speed of 8/5 or one and three fifths seconds. Now look at the depth of field – how sharply the back of the garden has come into focus.

I don’t want to stray much further into depth of field, diffraction, ISO, angle of view and so on. Today’s blog is just to provide a simple introduction to the idea of aperture and the kind of role it plays in photography. I’ll just provide two examples below of deliberately choosing one size of aperture over another to change the depth of field.

I don’t do portraits of people, but that’s a definite area for wide apertures. This is just a handheld photo of my daughter’s dog, Ted. It was taken indoors in low light conditions, so I had to boost the brightness of the photo in camera using the ISO setting – I don’t have flash either. I took the shot at f/3.5 – quite wide with a shutter speed of 1/320. If you were to enlarge this image you’d be able to see that only the dog’s left eye (the one on the right of the photo) is in focus. You can almost see my reflection in the eyeball.

This final example is of a lone tree at Lake Wanaka in the South island of New Zealand. I shot this at f/14 (quite narrow) and 1/80 of a second. See how the image, though handheld, is fairly sharp from the beach to the mountains.

Tomorrow I intend to post about exposure, or more specifically about what sort of shutter speeds you might use for different types of photograph.