How Automatic ISO Sensitivity Control Works on Nikon DSLRs

This article will be useful for Nikon Digital SLR owners. Automatic ISO sensitivity control is a very interesting thing, I’ll even say that this is one of the key things in modern Nikon digital SLR cameras. If you understand how automatic ISO sensitivity control works, you can simplify your work very much with a camera and pay more attention to other key points when photographing. It is very strange, but I have never seen a detailed explanation of how automatic ISO sensitivity control works.

How Automatic ISO Sensitivity Control Works on Nikon Cameras

In order not to write the phrase 'Automatic ISO sensitivity control' for a long time, I use a shorter and more understandable expression Auto ISO (in Russian it reads as 'auto iso'). This function can be found in the camera menu under the 'ISO Sensitivity Options' setting. Each Nikon camera hides Auto ISO differently.

The essence of Auto ISO is very simple. The camera itself selects the desired ISO sensitivity for the correct exposure.

In the 'Auto ISO' setting menu, set maximum ISO и maximum excerpt... The 'Auto ISO' function can be either enabled or disabled. Please note that on the CZK, automatic ISO control on cameras occurs only in modes P, A, S, M. In the future, I will write about the work of Auto ISO in these modes.

The maximum ISO value makes it clear to the camera that you cannot set the ISO above this value. Often the maximum ISO value is called the threshold, which can not step over the camera automation. For example, if you set the maximum value to ISO 3200 (the ISO threshold is 3200 units), then when working with the Auto ISO function, the camera will not exceed this threshold. Under no circumstances will automation be able to use an ISO above 3200. Maximum ISO Is a very important parameter for setting the Auto ISO function, as it allows you to control the noise level. Photographs suffer from digital noise at high ISOs. For each camera, I recommend calculating a certain ISO value at which you can get a good image result, after which this value should be set for the Auto ISO function.

If, with the Auto ISO function turned on, it is forced to set the ISO value higher than the maximum set for the Auto ISO function, the camera will only work at the forced ISO value. For example, when I shoot in Auto ISO mode, and I really miss the maximum ISO value, I force the value to be greater than the desired value and the camera uses only this one forced ISO value. Important: this principle does not work on all cameras.

You can set and Minimum ISO. This is done not entirely obvious and not for all cameras. When you manually set some ISO value on the camera, it immediately becomes the minimum automatic value. For example, if the camera had manual ISO control, and the ISO value was equal to 800 units, then when Auto ISO was turned on with a maximum ISO value of 3200, the camera will only work at values ​​from 800 to 3200 units. It is very convenient to use in low-light conditions, with telephoto lenses, etc.

Another key point in setting automatic ISO is setting maximum excerpts. Maximum excerpt important in A (aperture priority) and P (program) modes. The Maximum Shutter option allows you to tell the camera the shutter speed to which the ISO value should adjust. It's quite difficult to figure it out without an example.

Auto ISO in aperture priority mode (Mode A)

In 'A' mode, the aperture value (F-number) is set, to which the excerpt and ISO. In Auto ISO mode, the camera tries not to use a shutter speed slower than the Auto ISO function specified in the settings, and the ISO itself will be set only in the selected range (from minimum to maximum).

Example. Let's set Auto ISO like this: the minimum ISO will be 800, the maximum ISO will be 6400, and the maximum excerpt 1/100 second, set the aperture to F / 5.6. The camera algorithm will be something like this:

1. F / 5.6 calculated excerpt at a minimum value of ISO 800.
2. If calculated excerpt at ISO 800 longer than 1/100 of a second, the camera raises ISO by a minimum step.
3. At the new increased ISO again the shutter speed is calculated, if it is 1 / 100s, then the camera was able to choose the optimal ISO. For example, the camera picked up ISO 2500. Then the shooting will be carried out at a value of ISO 2500, F / 5.6, 1 / 100c.
4. If the camera went over all ISO values ​​from 800 to 6400 and the calculated shutter speed was constantly longer than 1 / 100s, then shooting will be performed at ISO 6400, F / 5.6 and the calculated shutter speed with these parameters. In this case, the ban on the minimum shutter speed will be canceled and the camera will set the shutter speed longer than 1 / 100s, for example 1 / 40s. In this case, the picture will be taken at ISO 6400, F / 5.6, 1 / 40c. A 99% camera can select the desired shutter speed, as the shutter speed range is very large. This allows you to almost always get a correctly exposed frame.
5. If the calculated shutter speed at ISO 800 is shorter than 1/100 second, for example 1/250 second, then the camera does not raise ISO, and shooting will be performed with the settings F / 5.6, ISO 800, 1/250 second.

In fact, the camera does not go through the ISO value, but calculates everything in its own way, but the effect is exactly the same as I described. In words, this turned out to be a very long algorithm of work, but in practice it is easier to understand. It is very convenient to use the aperture priority mode with the Auto ISO function, it is enough to try several times.

Auto ISO running in program mode (P mode)

In P mode, the camera itself determines the optimal aperture, shutter speed, and ISO. The algorithm of Auto ISO in P mode is quite difficult to write. The meaning of it in a few words is this: the camera tries to lower the ISO so as to 'fit' into the minimum shutter speed and aperture the lens. If the maximum aperture of the lens is not enough, the camera raises the ISO. If the maximum aperture and maximum ISO are not enough, the camera lowers the shutter speed below the set. If on the contrary, there is a lot of light, then the camera can set any shutter speed and aperture value with a minimum ISO value. Auto ISO in P mode is intuitive after a few test shots.

Auto ISO in shutter-priority mode (S mode)

In S (shutter priority) mode, the photographer sets the shutter speed, and the camera determines the optimum aperture and ISO. Let us have the same initial data: the minimum ISO value will be 800, the maximum ISO will be 6400, and the maximum aperture The lens will be f / 2.8. We set the shutter speed to 1/320 second and shoot in low light conditions. The camera algorithm will be something like this:

1. When the shutter speed is set to 1/320 second, ISO 800 is set and the desired aperture value is calculated.
2. If the desired aperture value is less than F / 2.8, for example F / 7.1, then the picture will be taken at ISO 800, 1/320 s, F / 7.1
3. If the desired aperture value is required more than F / 2.8, then the camera raises the ISO. Let the camera choose the value of ISO 4000, while the picture will be taken at ISO 4000, 1/320 s, F2.8. As soon as the camera starts raising ISO, the aperture value will always be maximum.
4. If the camera was unable to find the right ISO, then the image will be taken at ISO 6400, 1/320 s, F2.8 and the image will not be correctly exposed.

Auto ISO in manual mode (M mode)

In M mode (manual camera control mode), the photographer himself determines the shutter speed and aperture value, and the camera only calculates the optimal ISO value. The algorithm here is very simple. For example, we set F / 16.0, 1/1000 s, the camera will simply try to pick up the ISO value from the given range at which we can get the correct exposure. The mode is very, very convenient in low light. When there is a lot of light, Auto ISO mode is useless when operating in M ​​mode.

Important: modern Nikon cameras have the ability to automatically determine the maximum shutter speed for each individual lens. To do this, specify the value 'Auto' in the minimum exposure settings field. Only some Nikon CLCs have this feature: D3200, D800, D5200, D7100 etc.

How does the camera select the maximum shutter speed for setting Auto ISO? Very simple, the camera adjusts the maximum shutter speed to the focal length of the lens. For example, take a camera Nikon D3200 with lens Nikon 50 mm f / 1.4G AF-S Nikkor. For a focal length of 50mm, the camera will select an automatic maximum shutter speed equal to 1/80 second.

Shutter speed in '1 / x second' is roughly calculated as: x = mm * 1.5, where mm is a numerical value for the focal length of the lens. In our case, x = mm * 1.5 = 50 * 1.5 = 75, which is approximately 1/80 of a second. The camera cannot set the shutter speed to 1/75 second, as this is not the standard shutter speed.

If your camera does not have automatic maximum shutter speed when setting the Auto ISO function, then ask her yourself easy peasy. You need to take an approximate shutter speed of 1 / x, where x is numerically equal to the focal length times a certain factor. For example, I do this if I have a lens Nikon 28-200 mm f / 3.5-5.6D AF Nikkor and a camera like FX, then I take a larger number, which is responsible for the focal length of the lens - 200, and I look for a shutter speed equal to 1/200 second in the setting. If I have a DX camera, then I take 200 * 1,5 (mm * coefficient) and get 1/300 second, you won’t find 1/300 second in the camera menu, so I set 1/320 second.

Important: when working with lenses with a stabilizer, the coefficient value must be selected based on the situation. The same goes for tripods.

Maximum shutter speed is very important when a person photographs while holding the camera in his hands. I have a separate article about this, you can find it here.

Auto ISO has some interesting features.

• Auto ISO can accept any intermediate ISO sensitivity values ​​with a very small step from a given interval. This will help to squeeze the maximum quality out of the existing lighting level in photos. For example, when Auto ISO is running, the camera can independently set the values ​​of ISO 110, ISO 360, ISO 1400, etc. These ISO values ​​are not available with manual ISO sensitivity settings. Some amateur cameras have an ISO sensitivity step of 1 step. This means that you can manually set only ISO 100, 200, 400, 800, 1600, 3200 and the like, that is, the ISO sensitivity values ​​in such cameras differ by half, and you can not set something in between. These cameras include Nikon D40. On more advanced Nikon cameras such as Nikon D90, D7000 etc. You can set the ISO sensitivity step equal to 1 \ 2 and 1 \ 3 steps, this allows you to use the manual ISO setting with intermediate values. For instance, Nikon D80 can afford to set the following range of ISO values: 100, 125, 160, 200, 250, 320, 400, 500, 640, 800, 1000, 1250, 1600, 2000 (hi0.3), 2500 (hi0.7), 3200 (hi1). For example, when in poor lighting conditions ISO 1250 is too low for the shutter speed and aperture required by the photographer, and ISO 1600 is already unacceptable for obtaining low-noise photography - then the automatic system can help the photographer and set a compromise average value of ISO 1400 for the exposure. photo, I do not resort to an ISO value higher than necessary.
• Automatic ISO is just very convenient in that the camera can select the very lowest ISO on which you can get a photo.

Every single Nikon camera has a lot of its subtleties when working with Auto ISO. Here are the ones I've come across:

• Камеры Nikon D1x, Nikon D1h, Nikon D1 does not support the auto ISO function.
• Nikon D100 supports function only in S, M. modes
• Nikon D2H, D2H support the function only in modes S, M. For modes A and P, you need to update the firmware.
• On most cameras, in automatic camera modes (Auto, portrait, sport, macro, cute cat with a flower) Auto ISO always works and its work cannot be controlled. That is, you can not set a threshold or shutter speed. Usually the camera sets the lowest possible ISO value and uses the built-in flash.
• Some cameras, such as Nikon D100 they do not allow you to set the maximum ISO value for the auto-ISO function and at the same time use the entire range of basic ISO values. In case of Nikon D100For example, this is ISO 200-1600.
• Auto ISO often cannot use lower ISO values, for example Nikon D90, D700 in Auto mode, ISO cannot use the lower limit of ISO 100 (lo1) Also, some cameras, for example Nikon D80cannot use upper ISO values ​​such as Hi1 for Auto ISO to work.
• Most cameras allow you to set only the limits of the ISO value, which differ by one step (twice). For example, you cannot select a ceiling for ISO 5000, but only ISO 3200 or ISO 6400.
• It is very strange, but for different Nikon central control centers, the Auto ISO setting is in different menu items. This is not always convenient, for example, I have several cameras and sometimes I get confused in the menu of different cameras.
• Many cameras have a very meager range of minimum shutter speeds for setting Auto ISO. For example, for Nikon D80 You can select the maximum shutter speed for Auto ISO from 1 second to 1/125 second. For Nikon D200 - from 1 second to 1/250 seconds. Believe me, 1/125 seconds - in many cases this is a lot, for example, such exposure is sorely lacking for convenient adjustment when working with telephoto lenses. At the same time, many Nikon cameras can use almost the entire range of maximum shutter speeds, for example, Nikon D700 can take a maximum shutter speed from 1 second to 1/4000 second - it's damn convenient for working with telephoto lenses.
• Auto ISO often behaves unpredictably when working with flash. Usually, when the external or built-in flash is turned on, the camera selects the minimum set or available ISO value, but sometimes when shooting with the flash, the camera automatically picks up the ISO value. Usually, ISO is raised at the maximum focal length of the zoom lens and with a long focusing distance.
• Many cameras have one or more programmable buttons. It is very convenient to configure the programmable button to turn on and / or control the Auto ISO function. This is sometimes difficult or not possible at all.
• Some cameras show 'ISO AUTO' in the optical viewfinder, and some cameras also show the value at which the picture will be taken. This can be used very well, for example, immediately change the shutter speed / aperture and achieve the lowest ISO values.
• Auto ISO does not work on younger Nikon cameras when using non-chip lenses, such as AI-S lenses type
• When working with external flashflash shows distance of his work (GN) only for the maximum ISO that is selected for the Auto ISO function. This is not always convenient.
• Some Nikon CZKs themselves choose a threshold for the minimum ISO value, for example Nikon D7000, D5100, D3200 when Auto ISO is on, it always uses the ISO 100 - ISO maximum range.
• In camera control mode M, the Auto ISO function may ignore set minimum ISO and use ISO from 100 (or 200) to the specified maximum value.
• When viewing data for pictures that were shot using the auto ISO, ISO value is highlighted in red (usually highlighted in red if the ISO is greater than the minimum value for this function, but there are nuances for different cameras).

If you know some of the nuances of Auto ISO for your Nikon camera, leave them in the comments.

Personal experience

I understand that there is a lot of boring text, but just understand how Auto ISO works on Nikon cameras and everything will become very clear. Instead of rereading the algorithms I have described, I advise you to simply experiment on your camera. Impressions and questions ask in the comments.

Conclusions

Auto ISO - very useful feature, access to Auto ISO on Nikon cameras is carried out only in P, A, S, M modes. The function works quite specifically, but intuitively.

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Material prepared Arkady Shapoval. Training/Consultations | Youtube | Facebook | Instagram | Twitter | Telegram