To this day, one of the most common questions asked by junior photographers and DPs who are new to the world of DSLR is – “What is ISO”? I get asked this question a lot, so I wanted to answer it once and for all in this article to help explain the fundamentals of ISO to those who are just starting out. For intermediate and advanced regular readers of this site, this article may also be suitable for you. The idea here is not just to explain what ISO is, but also how to best use the various ISO settings on a specific camera to get the best images from your device. So while some of this may be old news to you, there are relevant points for shooters of every level.
What is ISO?
At the most basic level, you can think of ISO as the sensitivity of your camera’s sensor to light, expressed as an ISO number. The average ISO range for today’s DSLRs is around 100 to 25,600, but some go as high as 409,600. A low ISO setting (such as 100) indicates very low sensitivity to light, so if you were to shoot at ISO 100 in a dark, candlelit room, you might end up with total blackness. Conversely, a high ISO setting (let’s say 6400) makes the sensor more sensitive to the light hitting it, so it will appear brighter and more noticeable in the same candle-lit room. If you have a background in film production and have ever used a camera with a Gain setting, you already have a general understanding of ISO. Cameras typically have only three levels of gain (low, medium and high), allowing the sensor to change sensitivity, which is essentially ISO.
Whether you’re shooting film or still photography, ISO is one of the three pillars of exposure that you need to determine in order to properly expose your image. The other two are shutter speed and aperture. Exposure and creating an image that looks like the image you see in your mind’s eye is largely a balancing act between these three variables. Let’s say you want to achieve a deep depth of field in an indoor shot so that everything is clearly visible. You’ll probably want to set your aperture to F8 or somewhere in that range, which means the inside of the location you’re shooting will be very dark. Assuming you’re shooting video, your shutter speed needs to be twice the frame rate (so 1/48 for 24p video), which leaves only one option to make the image brighter – turn up the ISO. In this case, you should consider raising the ISO, but be careful not to raise it too high. The higher the ISO value, the more grainy the image will be. Most cameras perform very well up to ISO 800 (1600 is usually acceptable), but beyond that point you may start to get a lot of noise and digital artifacts in your images. That is, unless you’re shooting with a full-frame camera or a digital SLR with really good low-light capabilities.
Here are some example images – click on them to enlarge and see the grain/noise differences.
First photo taken at F1.4/ISO 800 on my Lumix GH4:
The second photo was taken on the same camera at F8/ISO 25,600, with the same lighting:
Basic ISO
Since ISO is closely related to noise and grain, you’d better want to shoot at the optimal ISO level in order to get the cleanest image possible. Every camera is different, so it’s important to know what your camera’s base ISO is and try to shoot as close to that level as possible. My Blackmagic cinema camera has a base ISO of 800, so when I shoot with that camera I set it to 800 99% of the time. Now I can adjust the exposure of my image in three ways: opening up the lens aperture to let in more light, using an ND filter to block the light, or using a lighting kit (or utility light) to illuminate the scene. Of course this is unavoidable in some cases if I don’t want to raise the ISO to 1600 or higher.
Most cameras perform much better dynamic range at base ISO than at other ISO settings, especially compared to very high ISOs. So even if you have a camera like the Canon C300 that can shoot extremely clean video at very high ISOs (in my experience, 25,000 can be very noise-free), you still don’t want to make a habit of shooting at that setting. Get used to it, since the dynamic range of the camera is not yet at its best at this point. The camera only produces better images across the board at its base ISO – look at the photo above again and notice that it’s not just the grain, but the way the colors are rendered and the overall better IQ in the image taken at ISO 800.
It’s also important to know your base ISO, as some cameras have problems using ISO settings that are not an exact multiple of the base ISO. For example, my old Lumix GH2 performed well at any ISO at 200x (400, 800, etc.) but had problems at 160x (640 in particular was really noisy). Not all cameras have these quirks, but as a general rule, if you can’t shoot at base ISO, try shooting with an ISO that at least duplicates that number, as it’s likely to produce better results.
ISO and film speed
ISO (or ASA as it was more commonly referred to before) comes from the days of film, when you purchased film stock based in part on its “speed.” If you go out and buy film at ISO 800, it’s basically the same as setting your camera to ISO 800 today. The big difference is that in the film era, you couldn’t simply flip a switch and change the ISO setting. You choose a film and stick with it, making sure to light and expose your image within the sensitivity range of that particular film.
If you want to give your video footage a more cinematic or cinematic feel, try treating ISO like you would when shooting a movie. First of all, don’t change it every time you shoot – keep it to the base ISO setting if possible. When you do need to change it, don’t go above ISO 1600 (which is the fastest speed many film stocks go to) as it forces you to light the scene cinematically rather than just turning the knob on your camera and selecting a darker setting , noisier images.
generalize
ISO is one of the most critical parts of properly exposing your image, but ironically, there’s not always a lot you need to do to get the best end result. In an ideal world, you would get the best images if you could set your ISO to the base setting of your camera and never change it, only adjusting aperture, filters, and lighting to get the exposure correct…assuming of course that you Makes for great lighting! Experiment with your camera ISO settings to see what works best for you and how far you can push it in extreme situations, so you’ll feel more fluid and confident when shooting on set.
A few months ago I wrote an article about the Canon C100’s new 80,000 ISO feature, it’s worth a read!
Also, for those of you looking to take the next step and further develop your craft, be sure to check out my A guide to shooting cinematic images with a DSLR.
