Today I want to talk about Auto ISO. A lot of photographers, especially those who advocate shooting in manual mode, also tend to advocate using a static ISO. It's almost treated as a badge of honor. Manual exposure, fixed ISO, full control, right? But from my experience, very often that's just not necessary, and I believe Auto ISO is the best way to go for the vast majority of photographic situations. Even as a professional shooter, I'm shooting in Auto ISO.
So I want to talk about why and when I use Auto ISO and why I recommend it to so many photographers. The first thing everyone needs to acknowledge is this. Camera manufacturers want you to come away with a great image. No manufacturer wants your photos to be noisy or grainy. If every image coming out of your camera looked terrible, they wouldn't be able to sell cameras. Because of that, the algorithms behind Auto ISO are designed with one primary goal.
Keep the ISO as low as possible while still producing a good exposure. Lower ISO means higher image quality, better resolution, and less noise. That's true regardless of brand. And that's the first big reason why I use Auto ISO. The camera is always trying to protect image quality. The second thing to understand is that on virtually all modern cameras, you can set both minimum and maximum ISO.
Having said that, I pretty much always set my floor as low as the camera will let me and my ceiling as high as it will go. And I don't use the extended ISO settings on either the low side or the high side. Depending on the camera, the lowest native ISO might be 64, 100, or even 200. And as I just said, I set my ceiling as high as the camera will go, again sticking to the native ISO.
On my current camera, that's somewhere around 51,000. Some Sony cameras are even going higher. Anytime I ever suggest this, I get a lot of pushback. People immediately worry about noise and grain. They're afraid the camera is going to jump straight to those high ISOs and ruin their images. But here's the reality. Your camera will not push the ISO that high unless it absolutely has to. No manufacturer wants you to have a bad shot if it can avoid it.
If you give your camera enough light, it will keep the ISO low. It only raises the ISO when it has no other choice. Think about it this way. If you step into a cave, the camera doesn't suddenly become irresponsible. It simply doesn't have enough light to work with. The same thing happens if you're shooting with a very fast shutter speed, a small aperture like f22, or a dim environment like your living room. So the camera has to compensate by raising ISO.
Short of these, ISO stays very conservative. Now let's talk about how ISO behaves in different shooting modes and the decisions your camera is making. In program mode, the camera chooses everything for you, shutter speed, aperture, and ISO. It's going to look for the sharpest aperture available for the lens, what's often called the sweet spot. That's assuming you're using an OEM lens. That's a lens made by the same manufacturer as the camera body.
The camera also assumes your hand holding and photographing static subjects. So it chooses a shutter speed that minimizes camera shake, which is based on the lens you're using. Then based on those two factors and the available light, it selects the lowest ISO possible to complete the exposure. That is, if you're using auto ISO. The algorithm balances shutter speed, aperture, ISO, and available light factors to get you to what it believes is the sharpest image possible.
In Aperture Priority, you choose the aperture and the camera chooses the shutter speed and ISO. Like Program Mode, the camera assumes your hand holding and photographing non-moving subjects, and it balances shutter speed and ISO accordingly, always trying to keep the ISO as low as possible while keeping the shutter speed high enough to prevent blur from handshake. If you're using a fixed ISO, you're limiting how the camera can respond.
In low light situations, a low static ISO will likely force the camera to use a shutter speed that's too slow to handhold, which can result in motion blur in your images. If you're using auto ISO, you won't have this problem. However, if you're using a high f-stop like f22, you are again limiting what the camera can do and will likely force it to an ISO that is very high, causing a lot of noise and grain.
As you can see, in both program and aperture priority, the camera is constantly balancing three things, shutter speed, aperture, and ISO, all based on how much light is available and continues to assume that you are hand-holding and shooting non-moving objects. In shutter priority, you choose the shutter speed and the camera selects the aperture and ISO.
If you set a very fast shutter speed and a very low ISO without providing enough light, the camera will open the lens to its widest aperture. If that still isn't enough light to achieve proper exposure, your images will be underexposed. On the other hand, if you're using auto ISO in that same situation, fast shutter speed and low light, the camera will again open the aperture as wide as possible in an attempt to keep the ISO low. However, if there's not enough light, the camera will raise the ISO to compensate. That may introduce noise, but it's far better than ending up with no image or an image that is unusable or missing the shot entirely.
Very often, people who are shooting very fast shutter speeds are shooting things like birds, and birds tend to be where? In trees, in the shade, where there's not a lot of light. By shooting in Auto ISO, you can compensate for the lack of light problem because of the fast shutter speed and the shade.
Now let's talk about manual mode, because this is where Auto ISO really gets misunderstood. In manual mode, you choose the shutter speed, aperture, and ISO. If you set a very fast shutter speed and very small aperture, like f22, while shooting in low light, and you're also using a low fixed ISO, you're almost guaranteed to end up with an underexposed image.
If instead you use an auto ISO, the camera compensates for those choices by raising the ISO to maintain proper exposure. That's actually a good thing because it means you'll come out with an actual, you know, image. But this is also where many photographers get confused. They see a high ISO and a noisy result and assume that something went wrong and they blame it on auto ISO. In reality, nothing went wrong.
Shooting in manual mode doesn't mean you can set your exposure to your heart's desire and expect a sharp, well-exposed image. You always, always have to compensate for the available light. If you don't do it through shutter speed and f-stop, the camera has no option but to do it with the ISO. That is, if you allow it to do it with the ISO. Again, if you set a static ISO and you choose the shutter speed and f-stop poorly, you're going to come out with either an overexposed shot or a grossly underexposed shot. Neither is good.
Put simply, the camera is responding to the constraints you gave it. If you're in low light or you cut off light with the shutter or aperture, ISO becomes the only remaining variable that can compensate for the other problems. That's not a camera failure. That's the camera doing exactly what it's designed to do.
When I shoot in full manual mode, I'm usually either in the studio or on a tripod. In the studio, I control the light, so if I need more, I just add it. That means I don't need to worry about high ISO or slow shutter speeds. If I'm on a tripod, camera shake isn't an issue. So using a low fixed ISO with a slower shutter speed works just fine. In those situations, the camera doesn't know I'm on a tripod or that I have enough light from a studio strobe. So if I'm in auto ISO, the camera will try to protect the shutter speed as if I'm hand-holding with ambient light.
Are there times when I'm hand-holding and don't use auto ISO? Yes, but they're very specific. A good example is photographing the moon. The exposure is fairly consistent. ISO 100, f16, and around 125th of a second. So auto ISO isn't necessary.
Outside of cases like that, I'm almost always using auto ISO. So the reason I advocate using auto ISO so often is because most people are shooting handheld. When you're hand holding, you need a faster shutter speed. You can only go so slow before camera shake becomes visible. And most people aren't shooting in environments with a lot of light.
By using Auto ISO, you're allowing the camera to protect your shutter speed while still maintaining proper exposure. I want to repeat for those in the back of the room still shaking your head. The camera raises ISO only as much as is necessary. So if you have a lot of light and you set your values so that the camera gets a lot of light, guess what? Auto ISO is going to stay low. That's what it wants to do.
That's why if I'm shooting handheld in program mode, aperture priority, or shutter priority, Auto ISO makes a lot of sense. It's not lazy. It's practical. It's one less thing I need to think about and one more thing the camera can handle better than I can in the moment..
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