To this day, one of the most common questions that I get asked by beginner videographers & DPs that are new to the DSLR world is – ‘What is ISO’? I get that question so much that I wanted to answer it in this article once and for all, helping to explain the basic fundamentals of ISO to those that are just starting out. And for the regulars on this site that are at more intermediate and advanced levels, this article may be for you too. The idea here isn’t just to explain what ISO is, but also get to touch on how to best work with the various ISO settings on your specific camera to get the best possible images out of your gear. So while some of this may be old news to you, there are relevant points for shooters of all levels.

What Is ISO?

On the most basic level, you can think of ISO as the amount of sensitivity that your cameras sensor has to light, represented by a numerical ISO value. The average DSLR today will have an ISO range of 100 to 25,600 or so, but there are some that top out as high as 409,600. A low ISO setting (100 for instance) indicates a very low sensitivity to light, so if you were to shoot in a dark, candle lit room at ISO 100, it would likely appear completely black. Conversely a high ISO setting (let’s say 6400) makes the sensor significantly more sensitive to the light hitting it, so in that same candle lit room will appear much brighter and more visible. If you come from any sort of background in video production and have ever used a camcorder with a ‘Gain’ setting on it, you’re already most of the way to understanding ISO. Gain on a camcorder is usually just three levels (low, medium, and high) that allow varying sensitivity of the sensor, which is essentially exactly what ISO is.

Whether you shoot video, or still photography – ISO is one of the 3 pillars of exposure that you need to nail down in order to get your images exposed properly. The other two being shutter speed & aperture. Exposing and creating an image that looks the way you see it in your head is very much a balancing act between these three variables. Let’s say you want to achieve a deep depth of field in an interior shot, so that everything is in focus. You might need to set your aperture to F8 or somewhere in that range, which will mean that the inside of the location you’re filming at will be very dark. Assuming you’re shooting video, your shutter speed will need to be double your frame rate (so 1/48 for 24p video), which leaves only one option for making your image brighter – turning up the ISO. This is a situation when you should consider turning up your ISO, but be very careful about taking it too far. The higher the ISO value, the more grain your image will have. Most cameras perform really well at up to 800 ISO (1600 is usually acceptable as well), but past that point you will likely start to have lots of noise and digital artifacts in your image. That is, unless you are shooting on a full frame camera, or a DSLR that has really great low light ability.

Below are a couple sample images – click on them to enlarge and see the difference in grain/noise.

The first shot at F1.4/ISO 800 on my Lumix GH4:

And the second shot at F8/ISO 25,600 on the same camera with identical lighting:

Base ISO

Since ISO goes hand in hand with noise and grain, you ideally want to shoot at an optimal ISO level so that you get the cleanest possible image. Every camera is different, so it’s really critical that you find out what the base ISO of your camera is, and try to shoot with your camera at that level as much as possible. My Blackmagic Cinema Camera has a base ISO of 800, so 99% of the time when I am shooting with that camera, it’s set to 800. And since my shutter speed is always set to 180 degrees (1/48), that means the only three ways I can adjust the exposure of my image are by opening up the aperture on the lens to let in more light, using an ND filter to cut out light, or using a lighting kit (or practical lights) to brighten up the scene. That is of course, if I don’t want to bump up the ISO to 1600 or higher, which in some cases is unavoidable.

Most cameras perform much better in regards to dynamic range at their base ISO than at other ISO settings as well, especially when compared to very high ISOs. So even if you have a camera like the Canon C300 which can shoot extremely clean video at really high ISOs (25,000 from my experience can be very noise-free), you still don’t want to make a habit of shooting at that setting, since the camera isn’t performing it’s best in regards to dynamic range at that point. Cameras will just produce better images at their base ISO across the board – take a second look at the photos above and note that it’s not only the grain but also the way that the colors are rendered and the overall IQ that is better in the image shot at ISO 800.

It’s also important to know your base ISO, because some cameras have issues using ISO settings that aren’t exact multiples of the base ISO. For example, my old Lumix GH2 was fine at any ISO that was a multiple of 200 (400, 800, etc.), but had issues with multiples of 160 (640 in particular was really noisy). Not all cameras have these quirks, but as a general rule if you can’t shoot at your base ISO, try to shoot at an ISO that is at least a duplicate of that number as it may yield better results.

ISO & Film Speed

ISO (or as it was once more commonly refereed to, ASA) comes from the film days, when you used to buy film stocks partially based on their ‘speed’. If you went out and bought a film stock with an ISO of 800, that was essentially the same as having your camera set to ISO 800 today. The biggest difference being that in the film days, you couldn’t simply flip a switch and change your ISO setting. You picked a film stock and stuck with it, making sure to light and expose your image around the sensitivity of that particular stock.

If you’re going after a more cinematic or filmic look for your video footage, try to make an effort to treat ISO much like you would have if you were shooting film. For starters don’t change it all the time, from shot to shot – leave it set on your base ISO setting as much as possible. And when you do need to change it, don’t go too far past ISO 1600 (that’s the fastest speed many film stocks went up to), as it will force you to light your scene cinematically, rather than just turning up a dial on your camera and settling for a duller and noisier image.

Summary

ISO is one of the most crucial parts of exposing your image properly, but ironically to get the best end result you don’t always need to do a lot. In an ideal world, if you were able to set your ISO to the cameras base setting and never change it, only adjusting aperture, filters and lighting to expose properly, you would get the best possible images… Assuming you can light well of course! Play around with your cameras ISO settings to see what works best for you and how far you can push it in extreme circumstances, and you’ll be much more fluid and confident when shooting on set.

I wrote an article a couple months back on the Canon C100’s New 80,000 ISO feature which is worth a read!

Also, for those of you looking to take the next step and develop your craft even further, be sure to check out my Guide For Capturing Cinematic Images With Your DSLR.