ISO or ASA in Photography – Sensitivities in Camera Sensors
ISO (pronounced “eye-sew”) is an independent, non-governmental international organization (International Standards Organisation or International Organization of Standardization as per their website), based in Europe that provides the standards for a wide variety of subjects.
For photographers the key standard is Film Speed ratings. In the past this was known as ASA or the American Standards Association (Now discontinued and replaced by the American National Standards Institute or ANSI), and you could buy your films in ASA 50, 100, 200, 400, 800 and 1600. There were specialist films that would go higher or use infra-red although these were generally known as the standard speeds.
Most decent cameras now have interchangeable ISO settings which is especially useful for digital photography because, as discussed in the previous chapter, you can change the ISO setting for every shot you take without the need to change film.
So what ARE the settings and how do they affect your photos?
The standard ISO that most people use everyday, giving accurate colour rendition and “clean” noise-free images is 100 ISO.
If your camera is able to set a lower ISO of 50 or less, you will notice that the images become a little more saturated in their colours. You won’t see too much difference in quality like you do with 50 ISO. Slide film, but a slow film speed or ISO. with digital photography has its benefits nonetheless.
50 ISO or less
There have been occasions when I have come across a particularly bright scene such as a sunny, white sandy beach or a sun kissed bleach-white property on a sunny day where I simply have too much light all around me.
I am already on the fastest shutter speed that the camera will allow and the smallest aperture that I wish to use (N.B. I never try to go smaller than F16 or F22 otherwise the image quality can start to deteriorate, or I may wish to create some depth of field with an aperture of say F5.6).
By reducing the ISO to the lowest, I can maybe save a few shots whilst enhancing the colours.
Also when shooting images that I want to be rich in colour such as a beach scene with blue skies and deep blue water for a holiday magazine, I use the lowest ISO possible (normally 50) coupled with a polarizer filter.
You may also want to purposely slow down the shutter speed for some creative shots. If you read enough photography magazines you will have undoubtedly seen images of waterfalls with milky smooth water and pin sharp rocks and foliage?
The milky water is created by a very slow shutter speed (normally a matter of seconds), which can sometimes only be achieved with the help of the smallest aperture and lowest ISO setting (note, using a Neutral Density filter will also allow super long exposures during the day to get the milky water effect as in the shot of the sea on this page).
The small aperture and low ISO block most of the light which means you need to compensate with a long shutter speed which in effect causes the flowing water to “blur” itself onto the sensor. Obviously when doing this kind of shot a tripod is essential.
Standard ISO – 100
As I mentioned earlier, this is the industry standard for most situations and subjects. Whether you shoot weddings, portraits, studio or commercial, 100 I.S.O. will do just fine and create nice, clean colourful images across the board.
High ISO – 400 and above
Being quite a perfectionist myself, as I think most photographers are, I like to produce the cleanest and noise free images as possible. I also like bold, colourful images.
Unfortunately, there are some times when you just have to use higher ISO.’s in order to get the shots you need:
- Weddings – For many interior shots of the church and ceremony, I don’t like to use flash as it causes unnecessary distractions from the service. Even with the aperture wide open you will struggle to get a fast enough shutter speed to freeze any movements especially when hand-held at ISO 100 or 200. Even using a tripod doesn’t help if you have a slow shutter speed and the subject is moving, even the slightest amount. I have used ISO’s as high as 800 and 1600 for the darkest of churches and have ended up with some great shots which, after using Neat Image, are quite acceptable and clean.
- With the more modern and technological digital SLR’s (especially in the Canon and Nikon camp, Canon EOS 1Dx Mark II, Canon EOS 5D Mark IV and the Nikon D5 or Nikon D810), the sensor arrays are becoming that much better with larger pixels, the quality at high ISO.’s is quite superb.
- Indoor sports – For events such as show-jumping or similar, where the subject can be too far away for effective flash use and the light is too dim for hand held photography at low ISO.’s, you need to obtain as fast a shutter speed as possible to freeze the action. When you are at the widest aperture, your only other option is to whack up the ISO to 800 or 1600. Get to a point where you have a shutter speed of 250th/sec or faster and “pan” with the action as much as possible.
The biggest problem when using high ISO’s is the quality deterioration. If you have ever used high ISO. film such as 800, the emulsion used on the film contained larger, more sensitive “grains” which were perfectly visible in your images thereby reducing quality.
Now, with digital cameras, the higher ISO’s produce digital “noise” which is similar in appearance to grain and caused by increasing the pixel’s sensitivity. As I mentioned before, there are more and more programs becoming available which help to reduce this noise to an acceptable level such as Neat Image.
Noise or grain can also be your friend if you intend to get creative at some point. Many black and white images can have their mood greatly enhanced by adding grain or noise to them and most editing software even has the facility to add noise. So learn to control noise and either remove it or increase it depending on what effect you need.
The main thing to remember is that your camera has 3 main controls for adjusting the amount of light in your images:
- Shutter Speed
Once you learn these in their entirety, what they do and the effects they achieve and when to use each one effectively, then you need to concentrate on your composition and subject matter!
You might also like the video on the link below. Auto ISO was a god-send to me when it came out and can get you out of a pickle when the light changes drastically…