In a previous blog, we examined how adjusting the lens’s aperture helps determine the final exposure of an image. But if you recall the exposure triangle from earlier on in this series, there was a third element that was vital for completing the exposure puzzle. Today’s blog will focus on this very factor. It is,  of course, the ISO settings of the camera.

Quite simply, ISO refers to how sensitive the camera’s sensor is to light. Back in the days of 35mm film cameras this would have been the equivalent of the film’s ASA rating - remember that? One would have to buy a roll of film with a high ASA rating for low-light indoor use, or a lower rating for brighter outdoor conditions. The same is now true for your digital camera’s ISO rating. Instead of having to shoot off a whole roll of film at that rating all of the time, you can change it from one shot to another as circumstances dictate.

In a nutshell, as one increases the ISO and makes the camera more sensitive to light, the camera is able to achieve faster shutter speeds. As we will learn in the following blog, faster shutter speeds decrease the chance of blurring in the images (caused by subject movement or camera shake), and as a result will assist with getting sharper photos. Easy huh? Well, not quite. If it was as simple as increasing your ISO to get less blurry, better images, we would probably not be needing a whole blog to describe this aspect of digital photography!

In very low light, this leopard decided to hoist it’s kill up a tree. To ensure that there was a fast enough shutter speed to freeze some of the action, the ISO was set to 5000,  allowing me to get 1/400 second shutter speed. The problem with increasing your ISO, is that as you increase it,  you also increase the noise in the image.

Now this 'noise'  that we are talking about is not the sort that one can hear blaring out of a teenager's AirPods, but rather it is the digital equivalent of what we used to experience in the days of film cameras. If you ever look back at the photos shot in low light with a fast ASA film (something like ASA 400), you will see how the photos have a certain grainy finish to them. Compare that to a print from a slower ASA 100 film, and you will notice that there is very little evidence of grain on the photograph. The same phenomenon happens in today’s digital age, except rather than calling it grain, we call it noise.

The sacrifice I made to get a fast shutter speed can be seen at this 100 percent crop of the above image, where the noise caused by the high ISO is very evident.

This is a 100 percent crop of an image shot at a low 200 ISO, and as can be seen from this, there is no noise at all in the image, and it is full of rich colours.

In addition to having noisier images when shooting with a high ISO, there are a couple of other small niggles that this causes. Firstly, high ISO settings do not capture colours as well as the lower ISO settings do and this manifests itself in images having less vibrance, and sometimes unwanted (albeit subtle) colour tone shifts.

A second negative is that the dynamic range tends to be lower at higher ISOs, and this is particularly true when it comes to post production and efforts to get more detail out of the shadows (or overall image exposure increases). When attempting this, these shadowed areas become excessively noisy.

Of course, if the correct exposure is attained in the field, this becomes much less of an issue, and if anything, when one is shooting at a high ISO it is better to slightly over-expose the image (making it lighter) and then to drop the exposure during post-processing. In doing this one is able to better control the noise levels in the darker parts of the image.

With no light left, and presented with a rare sighting of a male lion walking across a river, I shot this at a high ISO, but left the image a little under-exposed; although there is some noise, it is not too terrible.

However, in attempting to get more light and detail in the dark areas of the high ISO image, when lightening it in post-processing, the high ISO led to a great deal of noise coming through in the processed image.

Photographically, this image isn’t much, but it is a great example of how overexposing the original high ISO image (shot at ISO 12,800) and then dropping the exposure in post-processing does wonders for controlling noise that is barely evident  in this heavily-cropped image.

Most cameras have an ISO range from 100 to 6400. Higher-end cameras take this up way higher and can be pushed all the way up to 102,400 ISO, or more! For now anyway, such high ISO ratings are merely gimmicks and it is not yet possible to get usable images from such high ISO settings.

So, now that you know a little bit more about ISO, the question becomes what is the best ISO setting to use? The general rule of thumb is to use the lowest ISO possible when shooting whilst still ensuring a fast-enough shutter speed. This will assist you in achieving the lowest noise and best colours in the image for that situation. But, situations change...

If the light is good, and the subjects are very static, then low ISOs are no problem at all and should be used. However, as light starts disappearing or the animals start moving, slower shutter speeds just aren’t going to cut it so it is time to increase one’s ISO. Another big deciding factor is the camera being used. Newer, more modern cameras have far better high ISO performance than those made only a couple of years earlier, so this too will have a bearing on what ISO you use. For example, with my first digital camera body, I was reluctant to shoot at ISO 800, and would almost never use ISO 1600.

Today, I will happily shoot at ISO 3200 and seldom go lower than ISO 400 when shooting in the morning and afternoon, because even at 400, there is no perceptible noise in the image.

This is a 100 percent crop of an ISO 800 on an old Canon 1DmkII – where the noise is very noticeable.

Compared to the older cameras, the newer models do so much better when it comes to handling noise. Here is an ISO 800 image shot on a newer Canon 5DmkIV where the noise is barely visible.

Even at ISO 6400, the noise in this image taken with a Canon 5DmkIV is about on par with what older models got from ISO 800.

Taken at a ridiculous ISO 25,600 this 100 percent crop shows how the high levels of noise clearly make such images unusable.

It is therefore  easy to see that choosing an ISO is not as straight forward as it appears! My only advice would be – and I learnt this the hard way – is to remember that it is possible to improve on a noisy image (with post-processing software), but there is no way of improving on a blurred image caused by being too conservative with your ISO settings. In other words, rather err on the side of a higher ISO!

Bear in mind too that the ultimate end-use of the image will determine what level of noise is tolerable. If you just want to share images on social media, then at such a small image size, noise will not be an issue, but if you intend on making massive enlargements, then the noise factors become increasingly more important and will determine what ISO you should be using.

Noise removal software is very good at removing both chrominance (colour) noise, as well as luminance (grain) noise. This is a 100 percent crop of an earlier image where the noise has been removed in post processing.

As mentioned, the usability of a high ISO image depends on the size of the image. This is an ISO 12,800 image that is very noisy, but if only being used for social media, it shows no noise and is completely useable.

There is one more advent in the world of ISO that is worth discussing. Personally, I am not a user of this function, but that should not stop anyone from giving it a go. What I am talking about is AUTO ISO.

As one of the main benefits of using higher ISO is to increase your shutter speed, the camera has a pre-programmed mode that allows you to shoot away without having to worry about your shutter speed slowing down. If such a point is reached, the camera automatically increases your ISO to compensate for the loss of light, and ensure that your shutter speed stays relatively high.

Depending on your camera, this AUTO ISO setting is more or less programmable to tell the camera what ISO-range it can automatically change within, as well as what the slowest desirable shutter speed is that you would be happy with.

If you are not selecting the latter, the camera typically decides on the shutter speed which is generally accepted to be the slowest shutter speed for hand holding a camera, namely 1/focal length of the lens. As a result, if you have a 100-400mm zoom lens, with AUTO ISO set to on, when you zoom out to 100mm, the camera will ensure that your shutter speed stays above 1/100 second, and will increase the ISO (making the camera more sensitive to light) should the shutter speed drop below this point. If you zoom all the way in to 400mm, then AUTO ISO will ensure that your minimum shutter speed is now 1/400 second.

This can be very useful if you don’t typically pay much attention to your shutter speeds, but sometimes, even 1/400 sec is just not enough to freeze the action you are wanting to capture, and some manual adjustments of your ISO are still necessary. The most effective time to shoot in AUTO ISO is when you are after particularly fast shutter speeds, and your camera is set to shutter priority (Tv-mode in Canon and S-mode in Nikon).

In such a case,  you dial in the shutter speed you desire (perhaps you are shooting birds in flight and want a speed of say 1/2000 second) and the camera will determine the desired aperture to give you the correct exposure. If there is not enough light even after the camera opens the aperture all the way up, AUTO ISO will kick in and increase your ISO settings to ensure that your fast shutter speed is still attained.

Better still, for fuller control, shoot in Manual Exposure mode, with AUTO ISO on. Here you tell the camera exactly what shutter speed and depth of field you want, and it will then choose the ISO to give you the correct exposure – this seems to be one of the more widely used applications of this ISO feature.

There you have it; hopefully this has explained ISO well enough for you without too much confusion for a simple concept, and that you are now aware that it is one of the easiest ways to help prevent blurry images whilst out there taking photos... just do be aware of the downsides of relying too much on high ISOs!

By courtesy of the Tanda Tula Safari Camp http://www.tandatula.com/blog/

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