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ISO

In this post, we'll take a closer look at the ISO setting, what it does for our exposures, and how it can affect our fire scene images. The term ISO stands for International Standards Organization. In the 1970s, I.S.O. (the organization) standardized the light sensitivity rating of camera film. Commonly used films started at ISO 50, then increased to ISO 100, ISO 200, ISO 400, doubling with each step, up to ISO 800. The light sensitivity of the film doubled with each step, giving the photographer the equivalent of one extra stop of light. While using more sensitive film gave the photographer the ability to shoot in lower light conditions, it came with a tradeoff. The higher the ISO, the more grainy the developed image would be.

The digital ISO setting is analogous to film ISO. Each full ISO stop (100 to 200, 200 to 400, etc.) gives the photographer the equivalent of one extra stop of light. Just as it does with film, raising the ISO allows for shooting in lower light conditions, but does so at the expense of image quality. There is a difference, however, in how a digital camera compensates for low light conditions. Despite what some sources will tell you, digital ISO does not change the light sensitivity of the sensor. ISO doesn’t affect the sensor at all. Instead, raising digital ISO boosts the signal coming from the sensor to the camera's processor. As discussed in the article on the Exposure Triangle, the surface of a camera’s sensor is covered with millions of light sensitive photosites. The photosites, also called pixels, collect the light and convert it into an electrical charge. The electric charge carries color and brightness information from each photosite to the camera's central processing unit (CPU). The CPU uses this information to construct the digital image. Borrowing terminology from the realm of audio engineering, we refer to the light information coming from the sensor as the signal. We call the random variations in the electrical signal noise. We can describe the quality of the light information going to the CPU in terms of the signal to noise ratio. In shooting conditions where there is lots of light, the signal is strong and noise is low. In this case, we would say that there is a high signal to noise ratio. A strong signal to noise ratio results in a high fidelity image. But as the light decreases, and the sensor struggles to gather information from the dark scene, digital noise increases, and the signal to noise ratio is diminished. In this case the noise will start to manifest in the image as blotchiness, graininess, overly bright, or mis-colored pixels.

ISO can be a great help at fire scenes where poor lighting is the norm. Especially when we are limited as to the apertures and shutter speeds that are available to us. We know that to maintain adequate depth of field, we need to use small diameter apertures. While this ensures that we get the most detail throughout our images, it also limits the amount of light reaching the sensor. We also know that we are limited in how long we can keep the shutter open. If our shutter speed is too slow, we’ll get soft images due to camera shake. We can overcome these limitations by amplifying the signal coming from the sensor, to the CPU. Another audio analogy will help explain how this works. Imagine we listen to two audio recordings of a live music performance. The first recording comes from the musician’s on-stage microphone. Because the mic is close to the source, the audio signal is strong and background noise is minimized. As a result of the high signal to noise ratio we hear a good quality recording. The second recording is bootleg audio of the performance recorded by an audience member. Because the recorder was far from the stage and because the crowd noise is more prominent, the signal to noise ratio is low. In order to hear the musician better we can turn up the volume, and thus, amplify the signal. By raising the volume, we hear the musician more loudly but the crowd noise is louder too. By amplifying the signal, we also amplify the noise. The result is a noisy, low quality recording.

Example of film grain. This photo was taken with ISO 400 film.

The reptile exhibit. Olympus Tough TG-6. Digital ISO-12,800. The extremely high ISO and small image sensor resulted in very noisy image as you can see in the crop above.

Lightroom Denoise

Raw Image

The original image was shot in very dark conditions at ISO 25,600. The photo is extremely noisy.

After Adobe Lightroom Denoise

This is the same photo after using the AI Denoise tool in Lightroom. While the results can sometimes look overly-smooth, it's a major improvement over the first image.

Raising the ISO is like turning up the volume of our image. As with the audio example, when we boost the signal, we boost the noise. If we try to amplify a poor signal too much, we end up with a noisy, low fidelity image. How high an ISO setting you use can depend on the purpose of the image, the equipment you’re using, and also on personal preference. There are different requirements for different types of images. For instance, If I need to take photos of a dark basement, just to demonstrate that there was no fire damage in that part of the house, I am not too concerned about image noise. I’ll crank the ISO. If the fire started in that dark basement and all of my evidentiary photos will be taken in those conditions, then I will try to get more supplemental lighting so that I can keep ISO levels reasonably low. Finding out what ISO levels will give you acceptable results may take some experimentation.

The camera you shoot with affects how much noise you see in your images. Generally speaking, larger sensors will produce less noise. This is because the larger surface area gathers more light, resulting in a stronger signal. The age of a sensor’s design will also have an impact on noise performance. With every sensor generation produced, high-ISO noise performance improves. The camera model I use for fire scenes was originally released about 10 years ago. When it came out, the high ISO noise performance was among the best in its class for a cropped sensor camera. While it’s still no slouch in the ISO department, the latest generation of mirrorless cameras are significantly better. If you have a newer camera, you may be able to shoot at higher ISO with better results than can be achieved with an older camera at the same ISO setting. The camera’s processor can also affect the amount of noise visible in a photo. Most cameras have built-in noise reduction features that the user can enable through the settings menu. Phone cameras have automatic denoise algorithms that significantly reduce visible image noise. In-camera noise reduction generally only applies to images captured as JPG and does not affect RAW images. Denoise can also be applied in post-processing using photo editing software like Lightroom, Photoshop or Capture One. These programs are very effective at reducing noise but typically can only be used with images captured in RAW format. Some advanced noise reduction programs, such AI Denoise from Topaz Labs will work with JPG images but are still more effective with RAW files. Keep in mind that any post-processing technique adds an extra step to your workflow. It’s always faster to get the image you want in-camera. While denoising technology can be a great asset to fire investigators who often find themselves in dark scenes, there is a tradeoff. Denoising techniques can remove texture and fine image details along with image noise. If applied too aggressively, denoising can lead to overly-smooth, unnatural looking photos that are lacking in detail. One of the downsides to in-camera denoise is that the photographer has little control over how much noise reduction is applied.

ISO Examples

Crop at ISO 200

The crop here is pretty extreme. Even so, with an ISO of 200, the details are very sharp.

Crop at ISO 12,800

At ISO 12,800 the noise is wiping out image details. It's no longer possible to read the print on the sidewall of the tire.

Recommended ISO Settings

As I've said previously, what ISO setting you choose will depend on a number of factors. Their are no set rules here. I can't tell you what ISO setting will be best for your work. But I can tell you what I do and give you some options if you have not used the ISO adjustment on your camera before.

When I take daytime exterior shots, I set the ISO around 200 to 400. In good light, that will will give me a fast enough shutter speed to get sharp images with very little noise. As I move inside and there is less is less light available, I'll change the ISO setting to around 800. With a powerful external flash and some ambient light coming in from windows, ISO 800 will usually give good results. If windows are boarded or sooted up, or if the space is very large, images might turn out underexposed at ISO 800. In that case, I'll bump up the ISO to 1,600 or 3,200. With my camera, by the time I hit ISO 6,400, the noise is noticeable and I find it distracting. If a scene requires this high of an ISO setting, I'll try to get more supplemental light on the scene. If that isn't possible, then I may have to deal with the noise. The priority should be to ensure that the image has proper depth of field and a fast enough shutter speed to avoid camera shake. We can deal with a noisy image. A blurry one is useless.

If adjusting the ISO manually seems like too much work to you, there is another solution. Most pro and semi-pro level cameras have user specified Auto ISO settings. You can access the Auto ISO set-up through the cameras menu system. See examples below. When enabled, the camera will automatically adjust the ISO for each shot based on the light conditions. The camera will typically allow you to set a maximum ISO for your camera. For instance, if you don't want the camera's ISO to go beyond 3,200, you can set that as the upper ISO limit. This is a very convenient setting and a lot of photographers appreciate having one less thing to worry about.

One thing to keep in mind though, is that the camera may prioritize exposure settings differently than you might wish. For example, let's say that we want to shoot a dark interior scene with a shutter speed of 1/60 and a aperture of f/11. The camera has two options for increasing the exposure — increase the ISO or increase the flash output. Most cameras will increase ISO first and leave the flash output as low as possible. This decreases the flash recharge time and keeps the flash operating temperature cooler. The downside is that it can result in an unnecessarily high ISO. I prefer to set the ISO where I want it and let the flash's TTL system (TTL will be topic for another post) adjust for proper exposure.

What's Next?

At this point, we’ve covered the elements of the exposure triangle, how we can use the three parts of the triangle to achieve proper exposure, and how these elements affect our images. But, how do we put this information to use? What are the best settings to use at the fire scene? These will be the topics of our next post where we will discuss camera shooting modes.

Auto ISO Settings - Nikon

On most Nikon cameras you'll find the Auto ISO setting under ISO Sensitivity Settings in the Photo Shooting Menu (green camera icon).

The shooter can set the upper ISO limit by changing the Maximum Sensitivity setting. Here the limit is set at ISO 6400.

Auto ISO Settings - Canon

Canon users will find the Auto ISO settings under the Shooting Menu (Shooting Menu III on this 7D).

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