How to avoid noise in Aperture Priority mode arising due to auto-ISO?

Question

I was taking photos of birds in a cloudy day and in an area of forest where light was even low under a tree as compared to other areas.

I used 70-300 mm kit lens. When trying to take photo even at 1/10 or 1/20 with f/6.3, the images were coming too dark. I even increased ISO to 500 but still the photos were dark. I wanted to avoid using higher ISO value.

When this didn't work, I set the camera to Aperture priority mode. This time the photos were clear, at least on camera screen, but when I transferred on computer, I realized the camera automatically increased ISO to 25600.

This resulted in too much noise in the image. Even with Lightroom, I am not able to enhance the quality of the image. The photo came out blurred.

I checked the values which camera used with Aperture Priority mode:

1. 1/400 sec
2. f/16
3. 300 mm
4. ISO 25600

I am using Nikon D 5600 with 70-300 mm kit lens.

How to adjust exposure and maintain sharpness without increasing ISO in manual mode?

The attached image is one such sample of the poor quality image.

Answer 1

Regardless of what mode you use, it's helpful to know how aperture, shutter speed, and ISO relate to each other to keep exposure constant.

• Open the aperture (smaller F-number) ⇒ more light reaches the sensor ⇒ increased exposure.
• Increase shutter speed ⇒ shorter exposure time ⇒ less light ⇒ decreased exposure.
• Increase ISO ⇒ increased sensitivity ⇒ increased (effective) exposure.

To simplify the math, photographers work in "stops" that correspond to halving or doubling light.

• Aperture stops are 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22... Just memorize the sequence.
• Shutter speed and ISO. Multiply by 2 to increase a stop. Divide by 2 to decrease a stop.

Now the equation to balance is just:

• Exposure = E(aperture) + E(shutter speed) + E(ISO)

Where E(x) is the contribution of x to the total exposure, in terms of stops. It's not necessary to know how to calculate E(x), only that if one parameter is increased a stop, another parameter has to be decreased a stop to keep the equation balanced.

So starting with f/16, 1/400 sec, ISO 25600, you could:

• Decrease ISO by a stop. f/11, 1/400 sec, ISO 12800.
• Decrease ISO by 2 stops. f/8, 1/400 sec, ISO 6400.
• Decrease ISO by 3 stops. f/5.6, 1/400 sec, ISO 3200. You probably can't open the aperture any more at 300mm on your variable aperture zoom.
• Decrease ISO by 4 stops. f/5.6, 1/200 sec, ISO 1600. With long exposure times, a tripod would probably be helpful.
• Decrease ISO by 5 stops. f/5.6, 1/100 sec, ISO 800.
• Decrease ISO by 6 stops. f/5.6, 1/50 sec, ISO 400.

When using manual mode, you can set ISO to the max acceptable value. Then choose an aperture or shutter speed. Set the other according to the meter.

• Choose aperture based on desired depth of field (F16-22), background blur (F1.4-2.8), or optimal sharpness (F5.6-11).
• Choose shutter speed based on camera shake prevention. (1 / (crop-factor * focal length))
• Choose ISO based on whether it would be worse to have a blurry image with less noise or a sharp image with more noise.

See:

• What is the "exposure triangle"?
• What is the relationship between ISO, aperture, and shutter speed?

Answer 2

In the general case, if you find that the images as taken by the camera are too light or too dark, you can use exposure compensation.

If you shoot subjects that are very different from their background (crows are rather typical) you can use center-weighted or spot metering to make the camera set the exposure for the subject and not the whole image.

Answer 3

Why did you select an aperture of f/16?

That's fairly narrow and will reduce sharpness due to diffraction on your D5600, which has a diffraction limited aperture (DLA) somewhere around f/5.6. If you open up to even f/8, four times as much light will be allowed through the lens and the ISO can be two stops lower. At f/5.6, the ISO could be three stops lower with the same shutter speed.