Exposure Triangle 2/3: ISO.
You are in a fairly lit room and need to capture a special moment, you have the shutter speed already decided and the aperture widely open to get a shallow depth of field (refer to this article) but your image is not bright enough or maybe too dark. What went wrong? What could you have done to get a perfect the image.?
The aperture and shutter were already decided, so you are left with the flexibility of adjusting the ISO value on this scenario.
Just as I described in a previous article we can compare the exposure triangle to the human eyes
The shutter speed is the time duration our eyes is open to let light enter, the ISO is the sensitivity of rods and cones behind our eyes to the light and the aperture is our eye’s iris that opens and constricts to control the amount of light entering.
In earlier times before DLSRs we had various films with different sensitivity to light. The ASA (American Standard Association) accepted a set of definite numbers provided by a camera film manufacturer for the sensitivity of films to light, consequently this set of definite numbers now used by the ASA got accepted by the ISO (International Standards Organization) and has been referred to ISO ever since. ISO is basically a set of definite numbers given to the sensitivity of film to light. The emulsion of some film reacted slowly to light and others faster, the films with a slower reaction (i.e. sensitivity) had lower ISO values i.e. 25, 64, 100 and the faster films had higher ISO values i.e. 320, 400, 800.
What this meant was to get a similarly exposed image you will need more light and/or a slower shutter speed for a film with lower ISO value and less light and/or a faster shutter speed for a film with higher ISO value.
Fast tracking back to DSLRs though they do not make use of films ISO is as important as ever and the principle behind it is still the same.
Generally, as the ISO values increase by stops the brightness of the images are doubled accordingly i.e. an Image with ISO 1600 is brighter than an image with ISO 100, 200, 400 and 800. When I say “increases by stops” I mean when you dial your knob it moves from ISO 100 to ISO 200 this is an example of a stop. Knowing the function of moving in stops can be very useful, in situations where you have to need to maintain the same exposure (brightness of an image) but need to adjust to a faster/slower shutter speed. Increasing the shutter speed from 1/100 to 1/400 will take 2 stops and can be compensated by an increase in the ISO from ISO 100 to ISO 400 (another 2 stops). The Exposure Values EV remains the same but you can get the image you desire.
We have the common ISO Values as
ISO vs Image Quality
Most DLSRs have a base ISO value and is usually ISO 100, this value gives the maximum Image quality as far as the ISO value is concerned. What does this mean?
As the the ISO value increases from the base value (ISO 100) the image quality decreases, this is in form of digital noise appearing on the image giving it a grainy finish. On a small print increasing to ISO 800 may not look so bad but as the size of the print increases the grainy feel becomes more obvious and it destroys the beauty of the image.
Even with this disadvantage of using higher ISO it is still very commonly used in different types of photography and the reasons include lighting condition and subject speed.
Deciding the right ISO Value
I personally use between ISO 100 – ISO 200 most of the times because I shoot more portraits in a studio condition (controlled lights). I have enough lights to compensate for the sensitivity of low ISO values and still get properly exposed images. This simply means in a properly lit location maybe Outdoor or a Studio you can afford to use lower ISO values and retain the maximum quality of your images.
Also in scenarios where you have a Tripod stand and a stationary subject, you can afford to use a lower ISO value and a slower shutter speed. The image quality will be perfect and so will your exposure as long as you meet this conditions.
I mentioned earlier that Higher ISO values increase brightness of images but come with the side effect of noise and grainy appearances so why would you need to use it? Well in low light conditions for a start you have no other option but to increase your ISO value to get a brighter image. Also in sporting photography or wildlife photography were the subject is moving at an incredibly fast speed you need to use a fast shutter speed maybe 1/2000s. The only way to compensate for such shutter speed is to increase your ISO depending on what your aperture value is reading.
How to set ISO Value
Attempting to change the ISO is a sign that you are ready to gain creative control over your images. The first step is by switching the Camera mode from “Auto Mode” to either “Shutter Priority mode” or “Aperture Priority mode” for a start and tweaking the ISO values to see the corresponding results. Then using “Manual Mode” when you have become comfortable with controlling both your ISO and Shutter speed.
In our Exposure triangle series, we have so far discussed on shutter speed and ISO, detailed explanation on why we use them and the best time to use them. Recall that photography is all capturing images and the Exposure triangle is the key to having creative control over your images. Keep practicing until you can’t get it wrong