Tutorial Photography - Lesson One - Part Two, ISO

CT

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Photography - Lesson One - Part Two - Photography - Lesson One - Part Two

ISO Setting.

It's intended that you read the tutorial on understanding apertures and shutter speeds before reading this second part of the tutorial which deals with ISO settings. Hopefully the two tutorials should give you a good undertanding of the relationship between shutter speed, apertures and ISO which is an absolutely basic requiremnt to starting to progress with your photography- you must grasp these basic principles.The ealier Part One of this tutorial can be...
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ISO Setting.

It's intended that you read the tutorial on understanding apertures and shutter speeds before reading this second part of the tutorial which deals with ISO settings. Hopefully the two tutorials should give you a good undertanding of the relationship between shutter speed, apertures and ISO which is an absolutely basic requiremnt to starting to progress with your photography- you must grasp these basic principles.The ealier Part One of this tutorial can be found here...

http://www.talkphotography.co.uk/forums/showthread.php?t=185536

Let's have a look now at your cameras ISO settings and what they mean in practice.. It's actually very simple, but a brief look at the history of these settings will help to understand how we've arrived at the system we use today and will help even today if you pick up older film equipmennt.

Up till about the mid 1980's ( pre-digital) if we bought film it was marked with two separate sensitivity ratings, and all film camera with any form of metering and also hand-held exposure meters required that you set one of these speed ratings to match the sensitivity of the film,

ASA (American Standards Association)

A simple system where doubling or halving the ASA number either doubled or halved the senitivity of the film - as the ASA number increased, the film became 'faster' ( more light sensitive) and as it decreased it became 'slower' (less light sensitive). Eminently simple and easy to understand.

DIN ( Deutsches Institut für Normung)

This was a German system where a setting of 21 DIN was equal to a setting of 100 ASA, but an increase or decrease of 3 in the DIN scale doubled or halved the film speed, so that 24 DIN was equal to 200 ASA - 18 DIN was equal to 50 ASA and so on.... This was a pretty pants system - complicated and confusing, but with the politics surrounding the devotees of each system, it meant that all film was marked with each system, and all camera metering equipment was provided with both scales by which to set film speed. Almost everyone preferred the ASA system as being more logical and simple to understand and the DIN scale was largely ignored, despite still being provided on film and metering equipment.

Eventually in the mid 80s after much eye gouging and ear biting, common sense finally prevailed and both ASA and DIN ratings were swept away in favour of a new universal sensitivity rating which was assigned the new designation 'ISO'.

ISO (International Standards Organisation)

What they actually did in arriving at this new standard was to simply dump the DIN system in the bin where it belonged, and adopt the ASA system under a different name. The ISO scale is identical to the old ASA scale in all respects - only the name has changed, so.... a victory for common sense, even if the implimentation was a wee bit crafty!

So the old ASA rating doesn't matter one jot to us today if we're using modern digital equipment which is marked with the only scale used these days - ISO. If however we pick up an older film camera or perhaps use an older vintage exposure meter, it's likely to still be calibrated to the old ASA and DIN speeds. All we need to know is that we can ignore the DIN scale completely and that ASA is the same as ISO in all respects.- only the name has changed.

That's the historical bit out of the way then, let's have a look at what altering the ISO on our cameras actually does for us.

The Relationship Between Shutter Speed, Aperture and ISO

Let's assume that we've taken a meter reading of a scene/subject with our camera and that the indicated exposure is 1/125th at f/8, but we'd like a smaller aperture for a greater depth of field- our current ISO setting is 200. Remember - doubling the ISO number doubles the sensitivity of your sensor. If we set a new ISO speed of ISO 400 it will therefore allow us use to use a one stop smaller aperture, therefore we can use f/11 and still retain correct exposure for the subject while obtaining a greater depth of field with the smaller aperture. Increasing the ISO again to 800 will enable us to use a one stop smaller aperture again, so we can now set an aperture of f/16, still retaining correct exposure and gaining an even greater depth of field.

In the same situation and with the same initial reading of 1/125th at f/8, our shutter speed might be a more important consideration than the aperture if subject movement is our main concern, or photographing any sort of action is our intent. In this situation as we increase the ISO speed we can increase the shutter speed instead of the aperture, so that ISO 400 would now give us a shutter sped of 1/250th, while- ISO 800 would give a shutter speed of 1/500th, with both settings retaining the same exposure value as the original reading.

Halving or doubling the ISO speed therefore needs either the aperture or the shutter speed halving or doubling to retain the same exposure for your subject.Hopefully this explains how ISO, shutter speed and aperture work together to enable us to make the settings we prefer in a given situation while still being able to maintain correct exposure. It's actually a pretty basic principle once you've grasped it, but halving or doubling the ISO gives a one stop difference exactly the same as increasing or decreasing the aperture .or shutter speed makes a one stop diffence in either direction. IT'S THE WAY WE BALANCE ALL THREE ELEMENTS TO SUIT OUR PARTULAR SUBECT NEED WHICH IS THE IMPORTANT CONSIDERATION.

Grain/ Noise Considerations.

Unfortunately, as we increase the ISO beyond a certain level there is a price to paid, and in the case of film it's grainy looking images. Film is basically just a strip of gelation coated on one side with light sensitive crystals. and the more sensitive the film the larger the crystals - the larger crystals just capture more light than smaller ones, but beyond about 400 ISO film can start to get very grainy - the more you enlarge the negative the more visible those large grains which form the structiure of the image become. Film grain isn't always unattractive in a an image and many film users deliberately manipulate the development to increase the grainy appearance. Many of those black and white harsh gritty shots from the Vietnam War deliberately had the grain enhanced during the film development to suit the stark harrowing nature of the images.

We don't have grain with our digital images - in it's place we have image noise - an awful pitting effect in images which can show itself at higher ISO settings, and unlike grain it's never attractive, so it's mostly to be avoided at all costs if we can. I said earlier that increasing the ISO on our digital cameras alters the sensitivity of the sensor, and while it does no harm to think of it in those term, it's not quite true. - we can't actually change the sensitivity of the sensor at all. When we increase the ISO on our digital cameras all we do is increase the voltage going to the sensor to make it capable of capturing more light in any given prevailing light conditions. As we increase the voltage, so the sensor produces more noise, and the noise will increase proportionally to how many pixels are crammed onto the sensor and how close they are together. Smaller sensors with a high pixel count therefore produce more noise at a given ISO setting than large sensors with a lower pixel count, or where the sensors are less packed together. It's an over - simplification in some respects, but that's the bones of the problem.

The good news is that ISO performance of modern cameras is improving all the time and image noise is a lot less of a problem than it was just a short few years ago. The thing to avoid is under-exposure which will increase noise in any image, but the general principle is the same as it always was - for best image quality use the lowest ISO setting that the prevailing light and subject matter allows.
 
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