Nikon Auto ISO

Started Sep 19, 2012 | Discussions thread
Graystar Veteran Member • Posts: 8,373
Re: Nikon Auto ISO

stevesayskanpai wrote:

Hi all,

Maybe I should have put this q in the beginner's thread. If so, I apologise.

Ever since I bought my D40 4 years ago I've left it in Nikon's AUTO ISO mode, with a min shutter speed of 1/15, and max ISO of 1600. I've come to rely on this mode too much perhaps.

But now I'm getting ready to buy the D5100, I was wondering how many more experienced photographers ACTUALLY use this mode? Do you leave the camera in this mode when, say, it's handheld, or do you always select the ISO yourself, using the exposure guide in the viewfinder to help get the correct exposure?

E.g. if I was shooting inside and wanted the lowest shutter speed I could handhold, e.g. 1/50, would you then select a reasonable aperture, e.g. f/9, and set your own ISO to get a good exposure in this situation, OR would you use the AUTO ISO to let the camera make this decision itself?

Nikon has the best implementation of AutoISO, and you should use it. However, I think you need a little better understanding of it.

The camera doesn't make any decisions. As with A, S, and P modes, the camera is merely acting on your behalf. That is to say, the camera is simply following your instructions. With AutoISO the instruction is, "keep ISO at the setting I've set until lighting conditions require a shutter speed that is slower than the Minimum Shutter Speed, which I've also set. When that happens, maintain the MSS, and increase ISO to make up the difference in exposure."

You see...the camera isn't making any decisions from rules that you have no control over. You set the conditions, and the camera obeys. The benefit is that you get the very same exposure settings you would have set yourself, were you in manual mode, but you get them automatically. This frees you from ISO management so that you can spend more of your brain power on your subject and composition.

AutoISO works best with A mode...therefore, it is most effective when your shooting conditions favor A mode. It also works in other modes. It works in S mode, but while you're able to say, "no slower than..." in A mode, there's no equivalent, "no wider than..." in S mode. AutoISO works with P mode but the operation is a bit confusing so I don't recommend it.

You can use AutoISO with M mode. This gives you direct control of both shutter and aperture, and ISO will simply adjust itself to give you standard exposure. There's one problem with this. The problem is that if ISO is at its lowest setting and the lighting conditions require an even slower ISO for standard exposure, your image will be overexposed. Therefore, you still have to monitor ISO, just in case you have to adjust shutter speed to avoid an overexposure. But lets take this one more step, and say that after adjusting the shutter, the light level is reduced once again. Now, you're going to get an ISO that is higher than necessary, simply because you previously increased shutter to account for ISO.

This is why AutoISO with A mode is better. You tell the camera the slowest shutter speed that you're willing to use for the current subject matter, and you let the camera control the shutter speed to provide standard exposure. If ISO needs to increase, then it will. If ISO can then drop down again after increasing, it will. In short, A mode with AutoISO ensures that your shutter speed is never slower than the slowest speed you're willing to use, and your ISO is never higher than it absolutely needs to be. This gives the best IQ.

By far and away, the two biggest problems with AutoISO is getting to the settings, and deciding on what values to use. To solve the problem of getting to AutoISO quickly, many shooters use a function that Nikon provides. First, you set the ISO settings as the first item in MyMenu. Then, you set the Func. button to open the first item in MyMenu. With this done, you can now open the AutoISO settings by pushing the Func. button.

Now that the menu is open, you have to decide on what to set. The purpose of the Minimum Shutter Speed is to protect against blur from subject motion. Therefore, you should set the MSS to the slowest speed that is appropriate for the subject matter. So if you're shooting people sitting around at a picnic, you can set 1/125s. If you're shooting sports, set to 1/250s or 1/500s, depending on the sport. If you're shooting birds in flight then set to 1/800s.

The other setting is Maximum Sensitivity. Here, you tell the camera the max ISO to use. I don't really understand why this is needed. Lets say that a person doesn't want to go beyond ISO 1600 because of noise. So he sets a Max. Sen. of 1600. Now, when AutoISO reaches ISO 1600, the camera will go back to reducing shutter speed to get standard exposure. So you don't get the noise, but now you get blur from subject motion. That doesn't make sense to me. A sharp image with more noise is better than a blurry image with less noise. For this reason, I always keep the Max. Sen. to the highest ISO in the list...which is "Hi 1" for my D90.


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