ISO Explained

ISO (Also known as Film Speed or ASA if you’re really old!)

Setting the correct ISO for the prevalent lighting conditions is the first step in shooting manually.

ISO refers to the film or camera sensor’s sensitivity to light. Generally, when the light is good – i.e. outdoors, on a bright sunny day – you will set the lowest ISO available on your camera e.g. 50, 100 or 200. If the light is still good, but not overly bright, you will use an ISO of 400. Normally, you will also use an ISO of 400 and higher to capture fast action. One will start using an ISO of 800 if the light is deteriorating and 1600 or even 3200 in really low light.

Whenever you shoot manual or in one of your camera's creative modes, like Aperture or Shutter Priority – or your camera does not have an “auto” ISO setting – you will have to set your ISO manually. In some cases, you may find that Auto ISO may not be accurate as well and you best revert to set the ISO manually. This is where many photographers who made the brave leap to shoot manual falters. It normally happens when you have changing light conditions; i.e. when shooting indoors, in dimmed light, you may set your ISO to 400 or 800. Then later when you go back outdoors, under bright conditions, you may forget to change the ISO back to 100 or 200. Suddenly your photos are totally overexposed as the camera’s sensor will be too sensitive for the bright conditions outside!

The opposite is also quite true. If you shoot outside under sunny skies with an ISO of 100, and suddenly the sun’s rays are blocked by a giant, stormy cloud you will have to set your ISO higher i.e. ISO 400 to cope with the deteriorating light. If you don’t, your images may come out blurry or underexposed.

The relationship between ISO & Noise

This brings me to the next point; the relationship between ISO and noise. Think of a TV set that’s not tuned in well, all that grain and flickering makes it nearly impossible to see what’s happening on the screen. Camera noise is the same; it spoils your image. Noise is that coloured specks you often see on photos that were taken in low light or with high ISO speeds. Noise is more visible in the dark or shadow areas of a photo and may totally ruin it. Noise is the digital equivalent of grain that was found in film cameras. However, modern digital cameras are far less susceptible to noise than film cameras. Most digital cameras are equipped with a noise reduction feature to curb noise in shadow areas during long exposures. Some of the newer high-end cameras perform exceptionally well in low light, with barely any noise visible at even ISO 1600 and 3200! With film cameras, this was not possible as ISO speeds of 800 and higher yielded very grainy images.

Fortunately, you may use digital editing software to sufficiently reduce the noise in an image. On the other hand, noise reduction may also negatively affect the quality and sharpness of the image and especially the edges may appear soft. For this reason, you will have to use sharpening software such as Photoshop’s “Unsharp Mask”, after you applied noise reduction, but within bounds.

I would recommend that you enable your camera’s noise reduction feature from the menu command if you shoot anything at 800 ISO and higher. Also note that the “in-camera noise reduction” function will certainly need more processing time, and your camera will take longer before it’s ready for the next shot, especially with very long exposures, but it’s best leaving it on. Then again, be mindful that digital noise is really more evident and distracting when you print or view images on a large scale; and if you are going to need only smaller prints, you may just get away by shooting with a high ISO sensitivity.

On closer inspection of the above image (if zoomed-in or enlarged) it is clear that the image to the right (taken at ISO 3200) clearly suffers in terms of noise; especially in the shadow areas. The noise will not be apparent at this size, but will become annoying as soon as you print or view it in larger sizes.

To give you a better picture of how noise degrades the quality of an image, I have cropped and enlarged samples of the shadow areas from the above images.

At ISO 100, noise is barely visible in normal print. On the other hand, at ISO 3200, the shadow area definitely suffers from noise; in particular the red and green channels.

I also included series of snips covering ISO 100 right through to ISO 3200. Click on the image to get a closer view:

 

How to combat noise?

·        Some cameras or lenses offer image stabilizers, which will allow you to shoot handheld with really slow shutter speeds i.e. 1/30^th sec without any image blur. Modern image stabilizers may save you between 3-4 stops and if you don’t need to capture any movement in low light you may keep your ISO speed at a reasonable level, i.e. ISO 400 / 800. Take note that no camera will perform the same as another camera, and you may have to test your camera to obtain the best possible results.

·        Set you camera’s noise reduction to “On”.

·        Use noise reduction and sharpening software afterwards, and edit the image to obtain a good balance between noise and sharpness.

·        If you are forced to shoot with a high ISO and really need the best possible quality, shoot in RAW. RAW shots have a higher dynamic range and you will have more options in fixing the noise with your RAW software afterwards.

·        Another option to deal with high noise is to apply a software filter that simulates grain effects to obtain an old fashioned look. 

·        Digital SLR cameras have less noise than Digital Compacts at the same ISO setting because of their larger sensors which are more light sensitive (a large window will allow more light in than a small one). For the same reason, full frame SLR’s will nearly always outperform smaller APS-C sized SLR’s in terms of noise. 

Summary of various ISO sensitivities 

ISO 50-80	For shooting in bright light; excellent for close-ups, landscape, and portraits. Produces fine detail and image quality.	Not many cameras offer an ISO of 50 or 80.
ISO 100	Shooting in bright light; excellent for close-ups, landscape, and portraits. Produces fine detail and image quality. If you’re into landscape, or night photography ISO 100 is the only way to go, but you have to use a sturdy tripod and preferably use your camera’s self-timer or a remote control to ensure zero camera shake. You should also enable noise reduction to reduce very noisy, shadowy areas.	ISO 100 or 200 is normally the lowest ISO setting you get on modern cameras. As long as your exposures are fine, use the lowest ISO settings to produce the best possible quality. Note, ISO 100 is too slow for fast action.
ISO 200	Cloudy and slightly overcast days (yet still bright enough). Acceptable image quality, with some visible noise. With certain modern day cameras it yields the same quality as ISO 100 with barely any noise.	ISO 200 is the next best quality setting on modern cameras and you may use it to capture slow movement, i.e. a person walking etc.
ISO 400	Suitable for dimly lit scenes, heavy overcast days, or shooting indoors with flash. Fast enough for action and sports photography in bright light. ISO 400 is pretty much the most common ISO speed used for general purpose photography with its ability to capture action and a good quality versus speed ratio.	If you have the extra benefit of an external flash gun, use ISO 400 when you shoot indoors or in a dark environment. This will normally yield the best possible results. If you use powerful flash (studio lighting) you better minimize the ISO to 100 to obtain the best possible quality. Ensure that you stand close enough to your subject so that sufficient light will reach them. However, watch out for ugly shadows against the backgrounds. Take note that with most Digital Compacts, ISO 400 is still too noisy for any quality work.
ISO 800	Suitable for dark scenes and moving subjects in deteriorating light.  In high-end modern cameras, noise only really becomes evident at this sensitivity.	With certain new digital SLR’s, ISO 800 pose excellent quality and mostly perform better than ISO 200 or even 100 with digital compacts.
ISO 1600	Suitable for scenes where an acceptable shutter speed may not be obtained with ISO 800 and for very dark scenes.	Be careful to use ISO 1600. It is normally impossible to take high quality images with Digital Compacts on this setting. However, new SLR’s may provide sufficient quality for general shooting purposes, especially if you intend to print smaller sizes.
ISO 3200 - 6400	These two ISO sensitivities should only be used in really low light conditions; say for night-time photography or to freeze the action at a concert if you are too far away to use flash or want to be unobtrusive. However, if you own a very high-end camera such as the Nikon D3 with superb low light capabilities, you may not have a problem. Lucky you!	Only use ISO 3200 to ISO 6400 if your camera can handle noise exceptionally well, and when it is more important to bag the shot than to achieve outstanding quality i.e. when capturing a newsworthy event.

Important reminders to consider when selecting an ISO speed

·        When setting your ISO, make sure that you obtain the correct Shutter Speed. General rule for wide-angle to short telephoto lenses are between 1/60^th to 1/25^th of a second. Longer lenses may need much faster shutter speeds, say at around 1/200^th of a second and higher.

·        Test your camera and lens combination for optimum ISO versus Quality ratio.

·        If your camera or lens has an image stabilizer, use it, especially when shooting hand held.

·        Choose a low ISO setting (50-200) when shooting in bright light to ensure best quality.

·        Choose a medium ISO setting (400) to capture action in bright conditions.

·        Choose a high ISO 800-1600 when shooting dark scenes or when you need to capture action in deteriorating light.

·        ISO 3200 – 6400 is normally a no-go area, except if you have a really high-end camera like a Nikon D3, D700, or Canon EOS 1Ds Mark III, 5D mark II, Sony Alpha 900...