Photography 101: ISO numbers – how smooth do you want your photos?

ISO speeds need a slightly longer explanation and a lot of comparison talk about analogue/film cameras plus some peanut butter chat but bear with me, it’s all useful.

This is the third in my ‘Photography 101’ series, aimed at helping people feel more confident with their camera.You don’t need to read the posts on shutter speed and aperture first, as long as you read all three of these before you go any further. For best results, follow the ‘Homework assignments’ and try the exercises. You’ll get to know your camera a lot quicker if you do!

ISO numbers are the most abstract part of understanding your camera’s internal gubbins. It needs a slightly longer explanation and a lot of comparison talk about analogue/film cameras and some peanut butter chat but bear with me, it’s all useful.

Some peanut butter chat

In the olden days, when we all thought Top of the Pops was the best thing on TV, cameras used film. Film has a layer of emulsion, and suspended in that emulsion are tiny bits of silver halide crystal – like bits of peanut suspended in crunchy peanut butter.

The silver halide crystals are the important bits. They react to light. They start off dark, and get lighter the longer they are exposed to light. The bigger these crystals are, the faster they react to light.

And now a pointillism analogy

You remember studying pointillism at school? Those paintings made up of thousands of tiny dots? That’s really what your film photo is. Just thousands of tiny dots, and each one of those dots is one silver halide crystal. 

A Sunday on La Grande Jatte (1884) by Georges Seurat. Original from The Art Institute of Chicago. Digitally enhanced by rawpixel. Creative commons. Do not adjust your set; it’s meant to look dotty.

We said a moment again that the bigger the crystal, the faster it reacts to light. Returning to our pointillism example, it would be like painting with a bigger, fatter paintbrush. You’d complete your picture much faster with larger dots, right? The downside is that your picture wouldn’t look so good – the dots would be much more noticeable.

‘Pointillist balloon’ by Eden, Janine and Jim on flickr ( Creative commons.

If you want your picture to look it’s smoothest, you have to paint with the tiniest dots possible, but that takes longer. It’s a trade-off between speed and quality. A bit like masturbation.

ISO numbers

The speed that film reacts to light is called its ISO number and the lower the number, the better. Even though it was created for film cameras, we use the same principle for digital cameras because apparently everyone in photography prefers abstract concepts. The sensor uses more and smaller dots to create the photo. The speed with which the digital sensor reacts to light, and whether it uses larger or smaller dots in your photo, is exactly the same basic principle in analogue and digital cameras. Let’s break this down:

The lower the ISO number >>>

The smaller the dots in your photo >>>

The longer it takes to react to light >>>

The smoother your photo looks. 

ISO 100 is usually considered the highest regular quality; depending on the brand of your camera, it might be ISO 200. You can get better, such as ISO 80 or ISO 50, but they’re rarer, especially in digital cameras. Take a read of the ‘It’s all relative’ section below which talks more about the quality at different ISO numbers.


Keep in mind that pointillism painting. To recap: I can buy film with larger silver halide crystals. It will react faster to light, but the dots will be more noticeable. The faster the film, the more noticeable the dots will be. Up to a point, those visible dots might even look nice – when photographers talk about ‘grain’, this is what they mean (grain is another virtual Viagra for camera neckbeards). Grain can add an atmospheric quality to a photo and make it seem quite nostalgic or raw, depending on your subject.

If you want to see what grain looks like when it’s used to creative effects and adds greatly to the emotional impact of a photo, I urge you to track down Masahisa Fukase’s photobook “Ravens“.

Public Service Announcement: Fukase seems to have been a deeply unhappy man who led a traumatic life until he fell down some stairs, spent 20 years in a coma and died. This is reflected in his photography.

If you want something slightly less unhappy, obsessive and self-destructive, then “1-2-3-4” by Anton Corbijn is a great way to go, because that only features rick stars like Dave Gahan, Nirvana and The Rolling Stones. Corbijn’s portraits are joyful and lively but still atmospheric (link, very extensive link).

In Photoshop and various editing apps, you can even add the grain effect artificially, which to me is a bit like buying jeans and then cutting holes in them to make them distressed. But what do I know, I’m just some git with a camera.

After that though, the dots – the grainy effect – starts to look like you tried a pointillism painting with a whiteboard marker, or possibly by gluing Lego bricks together. Anything between ISO 400 and ISO 800 is usually good. After that, you start to take your chances.

The same principle is true of your digital camera. Depending on how new it is, you might find your camera goes from ISO 100 to ISO 1600, ISO 12800, maybe even ISO 104000 (if you have one of those, you probably don’t need this lesson from me. But it’s nice to have you here). But the higher that ISO number goes, the lower the quality of the image because, just as with a pointillism painting, the bigger the digital dots will be. 

ISO 80. This is the first CD cover from the same example shots as the aperture lesson. This is the rear cover of 90s’ funsters The Happy Monday’s greatest hits package. One of them at least.

ISO800. The surface texture of the CD inlay is becoming more pronounced


ISO 8000. It’s still usable. I shouldn’t have done this exercise with a camera that was released less than a year ago, the high ISO performance is too good to make the point.

ISO25600. It’s starting to get worse now, especially at the edges of the photos. Look how indistinct the 7 is on the second line.

First time I’ve put my camera in ISO51200 and I’m pretty pleased! I mean, it’s really minging – look how indistinct “Lazyitis” is now. But in an emergency, if it was ISO51200 or nothing, I would use this. But remember this is (at the time of writing) one of the newest mirrorless SLRs on the market – the Fujifilm X-T30 – and if your camera is even a year older (or newer) , you’ll probably see a marked difference at higher ISO speeds. ISO is one of the areas that we see drastic improvement in on a regular basis, both in terms of the highest possible ISO and the quality of the ISO numbers underneath. Shout out to The Happy Mondays – I don’t think I’ve ever got so life-endangeringly drunk to any other band and to whoever carried me out of that toilet in 1990 to stop me drowning in my own vomit – cheers!

It’s all relative

There’s one more thing to say, as if ISO numbers weren’t abstract enough. ISO speeds are relative. If your camera only goes from ISO 100 to ISO 1600, then ISO 1600 is probably going to be pretty bad. If it’s dark and going to the very highest ISO is the only way to get your photo, then do it, but don’t expect a great work of art.

If your camera goes to ISO 104000, then you’re probably not going to notice a difference between ISO 100 and ISO 1600 unless you look close-up on a computer.

And this is an important note worth reiterating – ISO speeds are relative. The quality of ISO 1600 is not a universal constant across all cameras, just like with clothes – the most expensive sweater in Primark is not going to bear much resemblance to the most expensive sweater at Gucci, right?

If you put your camera on auto, it will pick the best ISO speed for you. It will calculate the highest quality (lowest ISO number, remember) whilst making sure that the photo is properly exposed (not too light, not too dark) and making sure that nothing in your photo is moving and blurred. 

With some cameras and especially with mobile phones, the software will do its best to add smoothing to the picture no matter what the settings. When your ISO starts to get a bit extreme, the picture may look very muddy and indistinct up close.

As De La Soul helpfully pointed out in a pleasingly singalong fashion, three is the magic number and now you know the three basic concepts of using your camera. Next, we’ll look at how you can start to combine them.

Homework assignment

Yes, we’re taking pics of the same scene and comparing them again.

  • Work out how to change ISO.
  • Work out the best quality (lowest number) and fastest ISO (highest number).
  • Set up your camera indoors, somewhere that’s slightly dark like a hallway or staircase, maybe with an area in shadow. Put the camera on manual, set the shutter speed to 1/100th of a second and the aperture to F5.6. Then take identical pictures at various ISO numbers. When you compare them, zoom in very close so you can see the dots/grain up close. Do the dots look different in areas of shadow as opposed to areas of light? Does the grain start to look like it has a colour not related to the colours in the photo, like purple or yellow?

Quick snaps

  • Photos, whether analogue or digital, are made up of dots.
  • The bigger the dot, the faster your camera can take the photo.
  • The smaller the dot, the smoother the photo.

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