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Written by Edward Eastman   

ISO and Your Sensor

All digital cameras, except cell phones, let you adjust the ISO setting or value.

How many of you believe that increasing the ISO value increases the sensitivity of your sensor? Did you say, "You mean it doesn't?" That's ok. It's a common digital photography myth.

Increasing the ISO value on your camera DOES NOT increase the sensor's sensitivity to light. Increasing the ISO value also DOES NOT allow your sensor to capture more light photons. Your camera's sensor has a fixed light sensitivity.

I could get all technical on you, but I won't. (I subscribe to the KISS principle. Keep It Simple - Silly!)

So exactly what does increasing the ISO value do?

When light strikes a pixel site on the camera's sensor, a device inside the camera converts the light energy (photons) to an electrical signal. The greater the number of photons hitting a pixel site; the greater the electrical signal.

Every electric signal creates a certain amount of static or noise. All forms of electricity create noise. The electric wiring in your house contains noise. Turn on an old analog radio, adjust the channel selector between stations and you'll hear static (noise). Camera and sensor manufacturers are always working on ways to minimize the noise in the sensor's signal.

Sensor pixel sites are designed to collect a certain amount of light (photons). Once a pixel site reaches it's maximum, it stops recording any additional photons that may hit that pixel site. (Think of the pixel site as a bucket. Once the bucket is full; it is full!) The conversion device also creates a maximum electrical signal. Part of the electrical signal is a record of the brightness of the light (the number of photons) and the other part of the signal is base noise. All digital cameras produce some base noise in every image.

So what happens when you increase the the ISO value?

The only difference between the following two images is one image was at shot ISO 200 and the second image was shot at ISO value 6400.

ISO 200 Image

ISO 6400 Image

Look at the ISO 200 Image and you'll see the detail in the colours in the birdhouse and the wood grain in the fence.

Now look at the ISO 6400 Image and you'll see the colours appear sort of fuzzy and the wood grain in the fence is not sharp and the detail is all but gone. The image noise is often referred to as "grain", but that term is a holder over from the analogue film days where there was actually tiny grains of silver on the B&W negative.

The digital images above have not had any image processing done to them. They were enlarged so the "noise" is more visible and converted to a JPEG file format so I could upload them to the web site.

By looking at the Histogram, you can see the lighting is rather flat. (The graph is centered and the range is evenly spread over the left and right of the grid.) The only camera adjustments were to ISO value and Shutter Speed - ISO 200 1/60th second; ISO 6400 1/2000th second.

Measuring a Scene's Dynamic Range

You can measure a scene's dynamic very easily. Simply set the camera's Mode Dial to M(anual), set the ISO value to your camera's lowest setting, e.g., 100 or 200, set the lens Aperture to say f8 and set the Metering Mode to Spot. Now position the meter over the brightest part of the scene and adjust the Shutter Speed until the meter register a proper exposure. Make note of the Shutter Speed value, e.g. 1/2,000th of a second. Now place the meter over a very dark shadow in the scene and adjust the Shutter Speed until the meter register a proper exposure. Make note of the Shutter Speed Value, e.g., 1/2 seconds. Now count the number of Stops between the two Shutter Speed values and you have the dynamic range of the scene. In our example, it is 10 Stops.

More on this later.

Back to ISO

In the example above, there are lots of photons to go around because the scene is very bright. So what about a dark scene with a smaller dynamic range, e.g., night photos? What's important to remember here is the concept I describe.

So now we have a digital signal that consists of two parts - light data captured by the pixel site and some base noise.

The more light data in the signal, the less visible the base noise is in the final image. (Remember, the amount of base noise is constant.) Light data and base noise are referred to as the Signal to Noise Ratio. The more light data; the less visible the base noise is.

In the ISO 200 image, you can see lots of image detail, the tones appear smooth and even and the image appears sharp.

In the ISO 64000 image, you can see the noise, the image details are blurred together and there are few if any sharp edges visible.

Remember the dynamic range thing? Well increasing the ISO may reduce an image's dynamic range (the difference between brights and darks). Go back and look at the two sample images. The one shot with the higher ISO value appears flat and dull. The darks are no longer pure black and the whites appear more grey than white. The image's dynamic range is less.

Increasing the ISO simply amplifies the signal created by the digital conversion device, thereby fooling the pixel site into thinking it is full when it is not. So the pixel site actually captures less of the available photons. And the base noise is also increased therefore the Signal to Noise Ratio is lower. Another unwanted feature is the image's dynamic range is reduced also.

Image noise appears in the shadow areas first. If you think about it for a second, it makes perfect sense. Image shadow areas are created by less light data (fewer photons hitting a pixel site) than highlight or bright areas. Therefore, the Signal to Noise Ratio is going to be lower in the shadows. Creating images in low, or very low, light conditions without a tripod means one of two things - either you use a wide open Aperture and a slow Shutter Speed and get little depth-of-field and/or a blurry image; or you increase the ISO so your Shutter Speed allows a hand held shot and you get more noise in your image. It is called give and take. But even if you use a tripod in a very low light conditions, the resulting Shutter Speed still may too slow to freeze any moving objects. More give and take.

Consequences

As I've noted before, photography is all about give and take.

Increase the ISO and image quality may suffer. Put your camera on Auto Mode and you are allowing the camera's software to determine the camera's Aperture, Shutter Speed and ISO settings for you, which may not be what you want.

One could say that photography is all about compromise - something like life! (That's a whole other discussion!!)

Modern image processing software and camera internal image processing software are getting better and better at reducing visible noise to a more acceptable level.

To better understand the consequences of increasing the ISO with your camera, I suggest you do the following simple test:

ISO Test

  1. Select a static subject and use a tripod if you have one.
  2. Set the ISO on your camera to its lowest value.
  3. Zero out your meter and take a picture.
  4. Double the ISO value, e.g. if your camera's lowest ISO value is 200; increase the value to 400.
  5. Repeat steps 3 and 4.
  6. Repeat steps 1 - 4 until your camera is at it's highest ISO value.
  7. Download the images to your computer and view them.

Repeat this test in low light conditions.

I'd also suggest shooting in RAW image file format and looking at the images in Photoshop, Lightroom or one of the other RAW image processing applications. You see JPEG image files are processed and compressed by the in-camera image processing software and so you're not getting a true picture of what you camera's sensor is capable of.

Study the images. Look at the shadow areas first. Note the ISO value for each image. Magnify the images 50% and 100% and look at the image shadow areas for image noise.

What is the ISO value of the image that first starts to show image noise in the shadow area?

What ISO value image shows noise in the highlights?

Check out the Shutter Speed and Aperture settings. Will the Shutter Speed allow you to shoot handle held? Does the Aperture give you the depth-of-field you want?

You now know what the highest ISO value you can set your camera to and get an acceptable image. (Acceptable is a matter of personal taste.)

For those who want a great in-depth read on ISO and noise, click here.

I hope this helps you understand ISO values and your sensor.

 

 

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