Do You Use AUTO-ISO? You Should Be!

What is Auto-ISO anyway? Let’s build a foundation to understand why it is so valuable.

Time Travel

First we will get into the Wayback Machine and venture into the early days of commercial film stock. At this point, there were a variety of films available and as part of the chemical formation of the emulsion, a standard (actually two) was established to represent the sensitivity of the emulsion to light, where the higher the number, indicated increased sensitivity.

Black and white films of the day were ASA 125, while colour print films were most commonly ASA 100, or in Europe referred to as DIN 21. ASA stood for American Standards Association and DIN stood for Deutsche Industrie Norm. While the numbers and scaling was different, both performed the same job. They allowed a photographer to choose a combination of shutter speed and aperture that fit the current lighting situation in the context of the film emulsion. The ASA/DIN also specified the development sequence and timing. Very useful stuff.

Eventually, photographers started demanding great light sensitivity, so we first started “pushing” film to a higher ASA/DIN in camera and making appropriate changes in the development process. Not all labs would accomodate this, but those with home darkrooms revelled in pushing Kodak Tri-X Pan from ASA 400 to ASA 800, or Ilford HP4 (at the time) to ASA 800 from its standard of ASA 400.

Eventually the two different sensitivity conventions came together under the new banner of ISO or International Standards Organization. For film, and later digital, the ISO defined the sensitivity to light, pretty much fixed in the context of film, but in the digital world the ISO was only there to give a photographer coming from film to digital a rough idea of potential use cases. Today in digital, the ISO number does indicate light sensitivity, but ISO ranges are so massively expanded, that we can no longer really align them to film once we pass ISO 1000.

Noise Rears Its Head

A challenge that carried over from film is that to get a film to be more sensitive to light, the size of the silver halide crystals had to get larger and as the particles got bigger, they became more apparent in prints. We referred to this as film grain. It made sense to us all. A film with a higher ISO than another, would also have larger particles or “more grain” and the images would look “grainier”. Once we started to see digital cameras on the market, while people still used the term grain, it was and remains a misnomer. What we saw then was digital noise. The early CCD sensors were pretty amazing but as you increased the ISO, you pushed more voltage to the sensor, reducing the signal to noise ratio and this resulted in images that appeared noisier at higher ISO settings. When CCD transitioned to CMOS sensors, the amount of noise at any given ISO dropped because the CMOS signal to noise ratio at any ISO was better than that of a CCD based sensor.

Computer displays and editing tools also improved, allowing users to sit inches away from the digital image and zoom in to hundreds of percent of the real size and despite (or because) screen resolution was so low compared to the sensor resolution (this has not changed, it’s actually gotten worse in the chase for megapixels) those who abused the magnification and viewing distance started to whine about noise. Thus created the market for noise reduction software, which to my enormous amusement actually softens images by reducing contrast, so all that money on those extra megapixels just went out the window. This is why we can accurately say that sharpening software which increases the contrast between pixels is the opposite of noise reduction software which decreases the contrast between pixels. Of course, mcmarketing wins over data most always.

Digital Noise Is Now Mostly Irrelevant

Back to today. Most of use making photos or videos are using cameras with CMOS sensors. The higher the megapixel count, the smaller the pixel surface area, so the lower the tolerance for really high ISOs. It’s that same voltage to the sensor problem redux. This is why most “pro” cameras offer what some would call a low megapixel count but provide superb images at incredibly high ISO settings. My now old Canon 1Dx Mark II records superb data with negligible noise at ISO 21,600. My Canon 5Ds has a much higher pixel count, but is also prone to noise at lower ISOs. It’s a balancing act. While sensors improve over time, we still have not reached the point where you can have a really high megapixel count and a really low noise floor at the same time, mcmarketing spin regardless. You cannot get something for nothing, no matter what advertising says.

Which brings us to AUTO-ISO

AUTO-ISO is a very simple thing. It takes as input the meter reading from the camera and then based on the settings for shutter speed and aperture computes the optimum ISO for the meter reading.

Consider this incredible value proposition. You choose a shutter speed because you want to stop action, or avert camera shake, but also are determined to achieve a certain depth of field and not have it vary. If you think that this implies shooting in manual, you are mostly right. In pure manual, you would then have to manipulate the ISO so you achieve a proper exposure. This could take some time or require test shots. Or you could set the camera to AUTO-ISO and let the incredible computer in the camera compute the best ISO for the current meter reading at the instant that you squeeze the shutter. If you change position and that changes overall exposure value, the ISO is altered to compensate. If a cloud rolls in, the ISO is altered to compensate and you do not give up on your preferred shutter speed or aperture.

We would not have liked this in the early days of CCD or even early CMOS sensors, but that technology has come so far, that the concern of digital noise is really a non-starter. Your sensor in any modern camera has a ton more resolution than any display that any normal human could buy, even if one existed. That sensor has a ton more resolution than any 300dpi print can possibly render. To be blunt, the ISO is irrelevant in most all cases on a modern sensor.

Let’s consider making video clips. The shutter speed is fixed because you want a filmic or video look and don’t want to see any jitter from too high a shutter speed. You have chosen a certain depth of field for the current clip or clip sequence. But, you don’t have a generator, a truck of lights and a team of gaffers to keep the light identical for each clip. You do have AUTO-ISO which will compensate for light changes automatically without messing up your shutter speed or aperture choices.

When an ISO Setting is Not a Real ISO

I remember when the first digital sensors arrived that could actually record at lower than ISO 100. Yes, I know that lots of sensors have an L mode for ISO as well as an H mode or multiple H modes. None of those are actual ISOs, they are programmatic settings to tr to fix the image or clip before it is written to the card or the disk. This is true whether you shoot in RAW or JPEG. It’s not a real ISO. However there are digital sensors that will actually use ISO 80 or ISO 64. Do you get higher resolution here as you would from film? Perhaps, but you will not see it. Use these super low ISOs when there is so much light for your selected shutter speed and aperture that the shot would be overexposed at ISO 100. Or use a neutral density filter to impart the same effect.

Believe me, as a former Kodachrome shooter, I have hundreds, maybe thousands of ASA 64 and ASA 25 slides as well as a ton of K64 shot at ASA 80 because the 1/3 stop of underexposure made the colours punch off the page. That was film. This is digital. If you absolutely want the look of film in your digital stills buy a copy of DxO Filmpack and you are set. If you are a videographer and need filmic looks, there are commercial LUTs built to give you the look of cinematic film. Dropping the ISO lower does not improve saturation or reduce grain, because we are talking digital now. I am sure that you can find some article on the Internet that says what I am sharing is not so. There is freedom of speech out there, so save us both time and don’t send along links, I will be quite happy sticking to the science of sensors.

When to Stick With Manual ISO - It’s the Meter’s Fault

So, if AUTO-ISO is so freaking awesome, why have the ability to manually choose an ISO at all. Well first of all, we spend a lot of money on cameras and if buyers felt something was being taken away, some of them might have a stroke. Some think that modern digital sensors behave like film, and the fact that this is not true, escapes them or they voluntarily ignore it. Perhaps you are doing commercial work where you cannot have any colour shifting happening before processing. Large ISO shifts can and do change colour rendition, but if you are shooting commercially and not using a colour checker and a colour calibrated display, you won’t be doing commercial work for long.

The caution here is not the ISO, it’s the camera meter. The reflected light meter in modern cameras is superb, better and more flexible than any handheld reflected light meter, but it is a reflected light meter and will be influenced by the reflectance and the colour of the subject being metered. If I am doing studio work where I control the lighting, I lock in the shutter speed I need, the aperture for the depth of field and the ISO, all manually in accordance with metering off a grey card with a reflected light meter, or more often, using a handheld light meter that measures incident light including flash. Then if the subject changes but the lighting stays the same, I have locked everything else in place for a correct exposure and variations measured by a reflected light meter do not change the exposure.

Summarizing the Idea

So I conclude by proposing that your go to ISO should be AUTO-ISO and you should only set the ISO manually when AUTO-ISO is failing you. By the way, if you think that the manufacturer is being “overly optimistic” about the low noise performance of a sensor (Canon I am speaking to you in reference to the 7D Mark II), you can set the upper and lower range for AUTO-ISO in your camera menu system.

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