Here's a rundown of how the Nikon D3400's Auto ISO feature works.
ISO is one of the three settings that is part of the crucial exposure trio in photography. The other two–shutter speed and aperture–control how much light hits the senor. Shutter speed does it by controlling how long the shutter iis open and exposed to the light. Aperture controls how large the opening is–a larger opening lets more light in. ISO refers to something a bit different: how sensitive the sensor is to light.
Back in film days, you’d buy a roll a film that had a specific ISO rating (or ASA rating, if you really want to go back). The most common was ISO 100, which was good for general-purpose photography and people photos. If you were shooting sports or street photos you might have used ISO 400 or 800 or faster. If you were shooting the iconic Kodachrome film, there was a good chance you were using ISO 64 film. Basically, the lower the number the less sensitive it is, and vice versa. So an ISO 800 film was far more sensitive to light than ISO 64 film.
Those ISO ratings have been transferred across to digital cameras. A default with many cameras is ISO 100 or 200. But advances in the technology of digital sensors has also opened up opportunities for very high ISOs that are extremely sensitive to light. The D3400, for instance, can go all the way up to ISO 25600, which is much higher than any standard consumer film was.
The catch with those very high ISOs is that the photo can become grainy (film) or noisy (digital), detracting from the sharpness of edges and smoothness of tones in the photos. The dynamic range also decreases, so the depth of the colors and tones decreases. So it’s always a trade-off of having enough sensitivity to get a decently exposed photo while minimizing noise and maximizing dynamic range.
Like many other DSLRs and mirrorless cameras these days, the Nikon D3400 has an Auto ISO setting. That takes advantage of a feature digital has over film. With film, you were pretty much stuck with using the same ISO for a whole role of film. With digital, there’s no such limit; you can change ISO between every shot.
As it suggests, the Auto ISO setting takes control of it so you don’t have to worry about it. But in doing so, it has some clever features.
For one, it defaults to the lowest ISO rating it can. So it starts with the lowest setting and only moves up higher if it has to. This helps to preserve the maximum image quality and minimum noise.
For another, you can set the lower and upper limits. If you don’t want to use the full range of ISO 100 through ISO 25600, you can narrow the range. A common reason you might want to do this is because you’ve decided that the image noise at high ISOs is too much. So you might set a range of ISO 100 through ISO 1600, for example.
And there’s one more neat feature: you can set the minimum shutter speed to its own Auto setting. This applies to the A and P shooting modes; it’s not relevant to the S and M shooting modes because you specify the shutter speed in those.
Why is this under the ISO options rather than somewhere else? It’s because the shutter speed is one of the three crucial factors in calculating the exposure. If there’s no limit to how long the shutter stays open, then the ISO rating becomes irrelevant. You could keep it set to ISO 100 and keep the shutter open for minutes to let me light in, if you wanted.
But long shutter speeds cause problems, especially if you’re shooting without a tripod. Even the tiniest shaking can blur the photo. And that effect is magnified with longer focal lengths of zoom and telephoto lenses. A useful general rule of thumb is that when doing hand-held shooting you probably want the shutter speed to be at least around the focal length number. What I mean by that is that if you’re shooting with a 50mm lens, you probably don’t want shutter speed to be around 1/50 of a second or faster. With a 100mm lens you probably want it to be at least 1/100th of a second or faster. That by no means a hard-and-fast rule; with modern lenses with vibration reduction you can often get away with slower than normal shutter speeds, or you might need even faster if you’re shooting with cold hands or windy or bumpy conditions. But it’s a reasonable place to start.
So the D3400 has a neat trick that helps with this: it will detect the focal length being used on many modern lenses and automatically use that in adjusting the minimum shutter speed. If you’re using the 18-55mm kit lens, as an example, when you zoom out as wide as it will go, the minimum shutter speed will be 1/30 sec. If you zoom in as close as you can go the minimum shutter speed will automatically change to 1/100 sec. And there’s a sliding scale in between.
That’s a very handy feature, because it greatly reduces the chances of blurred photos from camera shake. On higher-end models in Nikon’s range you can fine-tune it even more by adjusting the sensitivity of the auto minimum shutter speed setting itself to be faster or slower than the default, and even though I’m pretty comfortable hand-holding the camera at slowish shutter speeds, I often set it to be one step faster than the default as a safety precaution to reduce the chances of motion blur.
It works in all the main shooting modes: M, A, S, and P (manual, aperture priority, shutter priority, and program).
The Auto minimum shutter speed setting works only in the A and P modes.
The Auto ISO setting can be really useful, and it’s something I leave on much of the time for general shooting. But there are still times when you might not want to use it.
When mimimum noise is critical. The D3400 has excellent low-light performance for its class and is practically noise free up to reasonably high ISOs. But in situations where you want as little noise as possible, you’ll want to keep the ISO setting at the bottom of the range (that is, around ISO 100 or so).
There are two ways to accomplish this. One is to turn off the Auto ISO setting and set the ISO manually to something like 100. The other is to keep using Auto ISO but set the maximum to something like ISO 200 or 400.
When using a tripod. There’s nothing inherent about using a tripod that means you can’t or shouldn’t user Auto ISO–if you want to, knock yourself out. But if you’re using a tripod you have vastly more leeway to use longer shutter speeds and can therefore adjust that while keeping the ISO at the lowest possible setting.
As always with photography, there are of course exceptions. When shooting this photo of stars on top of Mt. Kilimanjaro, for instance, I was using a tripod but still want to keep the shutter speed relatively brief to keep each star as a small point, so I still used a relatively high ISO setting of ISO 1600 (30 seconds at f/1.8). A lower ISO and smaller aperture would have meant a longer shutter speed, which would have created slight motion trails in the stars and make them less distinct. A similar principle might apply if you’re shooting traffic or city lights.
When shooting panoramas. When you’re shooting panoramas, you generally want each of the tiles to use exactly the same exposure settings if possible. Variations between frames can lead to some ugly joins between the tiles.
Again, there are a couple of ways to do this. One is to turn off Auto ISO and set the ISO manually. The other is to lock the exposure using the AE-L/AF-L button on the back of the camera. I have a detailed guide to shooting panoramas on the D3400 that goes into more detail.
Here are some popular accessories for the Nikon D3400.
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