What are ISO Numbers in Photography and Why it Matters

What are ISO Numbers in Photography and Why it Matters

2400 1350 James Tocchio

Even though ISO is a major part of the three main factors that make or break a photograph, it’s something of a neglected middle child. ISO sullenly stews between Shutter Speed and Aperture, its more dynamic and interesting siblings.

But understanding and controlling ISO is just as crucial to making good images as Aperture and Shutter Speed, especially in the age of digital cameras in which ISO is so immediately switchable (with film, it was a single ISO per roll of film – more on that later).

There’s no reason to be intimidated by ISO numbers. The concept, once illustrated, is simple, and understanding it will improve any photographer’s images.

What is ISO

In the early days of photography a standard was sought to denote the speed of photographic film; that is, how sensitive a film is to light. The American Standards Association established a widely used system, as did the German Deutsches Institut für Normang. These scales were abbreviated ASA and DIN respectively, and they were used all over the world for many years (an obvious truth for anyone who’s looked at an old film camera on which the conversion scales can often be found).

In the 1980s, these scales were widely superseded by the standards set forth by the International Organization for Standardization. Today we follow this scale for measuring film speed, hence the abbreviation “ISO” and the numbers that follow. The value of the ISO used by film is different compared to the value used to show digital sensor sensitivity, but the practical principals of the two scales are the same regardless of their numerical individuality.

How ISO number impacts photos

In simplest terms, the light sensitivity of an image medium is denoted by an ISO number. With both digital sensors and film, the higher the ISO number the more sensitive that film or sensor is to light. What this means is that all things being equal (exposure time, aperture value, and environmental light of a scene) a film of ISO 800 will absorb more light than a film of ISO 100 and therefore show a brighter scene. The principal is the same with digital cameras. Setting a higher ISO number will increase the sensitivity to light of the camera’s sensor, while lower numbers will decrease that sensitivity.

While high ISO values allow for easier captures in low-light situations, they come with a cost. In photographic film, particles of light sensitive material suspended in an emulsion are what react to light and make the image appear after developing, so more sensitive films require larger grain. This use of larger grain makes each individual grain more visible, resulting in grainy photos at high ISOs.

Increasing the ISO setting of a digital camera comes with compromises to image quality. These digital anomalies manifest in the form of image noise. This noise presents as discolored pixels most visible in the darker areas of a low-light, high-ISO image. Similar to highly-grainy film prints of high ISO films, digital noise is undesirable to most photographers.

In the sample shots just below, my dog Cooper was laying in the same amount of low light. The settings of aperture and shutter speed remained the same through each photo – aperture set to F/1.8, shutter speed set to 1/60th of a second. The only change between the two shots was the ISO setting on my digital camera. 

In the gallery just above, we see two film shots. The one on the left was made with Kodak’s super fine, low ISO film Ektar 100, the one on the right was made with Fuji’s high ISO Superia 1600. Note the difference in grain.

ISO normally starts at a base number, usually around 100 or 200, and increases through the scale by a power of two. A typical ISO sequence would run : 100, 200, 400, 800, 1600, 3200, 6400 and on (today’s digital machines can reach into the hundreds-of-thousands). With each incremental increase of the scale, the effective sensitivity doubles; so an ISO of 800 is twice as sensitive to light as an ISO of 400. This information may seem of little interest in practical terms until one realizes that this directly influences the amount of time required to capture a set amount of light.

Best ISO Practices

So how does a photographer know when to use low or high ISO? As a general rule it’s best to always use the lowest ISO possible in order to attain the highest image quality and cleanest detail. To get silky smooth shots, low ISOs are a necessity. In bright light, during studio shooting, or when using a tripod, keep the ISO set as low as possible. This will yield beautifully fluid images with rich blacks, and great contrast.

Handheld shots start to get wiggly at a shutter speed around 1/60th of a second, so when the light starts fading it’s time to turn up the ISO. Increase the ISO incrementally until the sensitivity is high enough to allow crisp captures with a relatively quick shutter speed. Modern digital cameras are so good at eliminating high-ISO noise that quality images can be produced even in the highest ranges, but the photographer needs to be aware that with every increase in ISO comes an increase in noise or grain.

One notable advancement in the digital age has been the advent of customizable Auto-ISO. This relatively new system is implemented in many modern DSLR and mirror-less cameras and it allows the photographer to dictate the behavior of the camera’s Auto-ISO. The shooter can now set the default sensitivity, limit how high the ISO is allowed to go, and the minimum allowed shutter speed. While setting these parameters intelligently can effectively solve the ISO problem, it’s still crucial to understand the principals of ISO. Not understanding the principals will make the chosen parameters nothing more than guesswork.

As with everything in photography, finding balance is key. Good photographers know how to manipulate the ISO setting of their camera to achieve the lowest grain or noise possible, while still allowing quick enough shutter speeds to capture sharp images. A complete understanding of ISO and how it impacts images is crucial to creative photography. This knowledge may be the difference between capturing a usable shot, or getting nothing at all.

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This post was originally published in 2014. It has been reposted after countless questions regarding ISO from new shooters. Hope it helps!

James Tocchio

James Tocchio is a writer and photographer, and the founder of Casual Photophile. He’s spent years researching, collecting, and shooting classic and collectible cameras. In addition to his work here, he’s also the founder of the online camera shop Fstopcameras.com.

All stories by:James Tocchio
  • One of the other factors on RAW digital images is whether it is better to increase the ISO and thus the amplification of the sensor output in the camera or to put up with a dark original image and apply the amplification in post production. This could be on the basis that the software and computing power used for post production is greater than at the disposal of the in-camera processing. Probably a bit of both is the best answer.

    The real way to teach yourself about ISO is to use two film cameras at the same time, both without built in metering. The first camera using say 25 ISO film and the second a much faster say 400 ISO film. The easiest way IMHO, is to think in EV levels of illumination (with EV 16/Weston 12 as the lighting for sunny 16) of the subject and then mentally transfer this into shutter speeds and apertures. My problem is that if I am thinking about all this, my framing and other imaging criteria tend to suffer.

    My father was one of those irritating people who could do this automatically without even thinking about it, with Kodachrome 1 in his Leica IIIa (10 ASA) and Tri-X in his Super Ikonta B (400 ASA). I asked him a few times to explain to me how he did it in all sorts of lighting situations with very few errors but he couldn’t do it. He said he just knew what the aperture and shutter speed should be for any ASA/ISO and lighting and working backwards from sunny 16. The one thing he did say is that you must learn your aperture stops/EV steps off by heart and not even have to think about what a stop up or down should be. (f1.0/1.4/2/2.8/4/5.6/8/11/16/22).

    • These are nice tips, cheers. I’ve thought of bringing around 2 cameras with different speed films in before but never done so due to laziness essentially. I just want to bring one camera on a trip so it’s usually Foma 100 or 200 and I’m stuffed if the light gets too low.

    • On this same note – Can someone explain exposure compensation to me? I don’t quite understand the use…If I rate a 400 speed film as 200 and set my ISO to 200 in my camera isn’t that just like a +1 exposure comp? I noticed, at least on my analog cameras adjusting exposure comp really just changes my shutter speed? Again, what’s the point?

      • From what I gather, you are right wrt to the relation of ev to iso. It’s useful in combination with a light meter. On film, say you have a 100iso loaded but you end up shooting indoors. Or 400 in a sunny daylight. Instead of running off the scale with your meter needle (I use a pentax K1000 with a little needle indicator for correct exposure) you change your ISO and over or underexpose the whole film, then correct for it in the darkroom. If you go to the detailed specifications for the Fomapan films, they mention how much over/under exposure the films can take. So you can use a smooth low iso film in the dark. You get a different result then loading a more sensitive film.
        I reckon it’s similar on digital. Obviously there you can choose your iso but high iso comes at a cost: grain. So you stick to a low iso and EV correct it so that your exposure times fall within a reasonable range. Then correct everything in photoshop.
        It’s how I use it, I’m not a pro though.

      • I think you are talking about two different things. Exposure compensation is when you override what the exposure meter is telling you. For example if you meter a bright scene, snow or white sand in bright daylight, the meter will put this scene toward middle grey resulting in under exposure. The snow or sand will tend toward grey. You can compensate exposure by overriding the meter reading (by a +1 or +2 etc. exposure comp) giving more exposure to the scene than the meter suggests thus rendering the sand and snow white.

        Rating film speed, from what I understand, is how much exposure a film needs to render shadow detail. Lets say you have an average scene with equal parts shadow, mid tones and highlights (not the beach or snow scene) and you are shooting Tri-x at box speed, ISO 400. If the photos turn out ok but the shadows are just black, no detail, you may want to rate the film at a lower speed (ISO 200) to give more exposure and lift the shadows out of the black area. Of course you risk blowing out the highlights but this can be controlled to a certain extent during film development (another topic). Film speed ratings of the same film can differ for different photographers depending on their individual processes (the exposure meter used, film developer used etc.). You may get good shadow detail at box speed but another photographer may not because of these differences. These different ISO ratings for the same film is many times referred to as individual exposure index or EI for that particular film.

        I hope this helps

  • Merlin Marquardt June 22, 2018 at 9:20 am

    Nicely and clearly said. Thanks.

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James Tocchio

James Tocchio is a writer and photographer, and the founder of Casual Photophile. He’s spent years researching, collecting, and shooting classic and collectible cameras. In addition to his work here, he’s also the founder of the online camera shop Fstopcameras.com.

All stories by:James Tocchio