Now that I understand Aperture and Shutter Speed – how they work independently and together to expose an image – let’s move to the last of the 3 major components of exposure, ISO. Let’s start out with the definition. (www.photographytips.com & www.digital-slr-guide.com & wiki)
ISO - Film speed is designated by a single, almost universally-accepted common system developed by the International Organization for Standardization which uses the initials “ISO” before the film-speed number – e.g. ISO 100. (The letters ISO come from the Greek work for “equal” (isos). These three letters have been adopted by the International Organization for Standardization, but ISO is not an acronym for the organization)
FAST FILM - High speed film, i.e. film that is more sensitive to light, meaning less light is needed to obtain a properly-exposed image.
FILM SPEED - is the measure of a photographic film’s sensitivity to light. Film with lower sensitivity (lower ISO/ASA speed) requires a longer exposure and is thus called a slow film, while stock with higher sensitivity (higher ISO/ASA speed) can shoot the same scene with a shorter exposure and is called a fast film.
Okay that makes sense, let’s see if I can come up with a real world explanation that has nothing to do with photography. Ok got it… You’re car has an MPG rating of 20 miles per gallon in the city, so to go 40 miles in town, would take you about 2 gallons. Now imagine you were driving a Mini Cooper, which gets close to 40 MPG in the city. That same trip would only take you 1 gallon. So how does this relate? Imagine that your car of choice is the ISO (film speed or sensor’s sensitivity to light), and the distance is the image you are trying to expose. The variable that would be directly affected by the CAR and DISTANCE is the gas needed. The gas needed equates to what we learned in lesson 1 and 2. Aperture and Shutter Speed together effect the amount of time the shutter needs to be open to get the proper amount of light to expose an image. So if you pick a high ISO setting, it would take less shutter time to produce an image. Let’s jump back to our UPS truck story from Lesson 2, we said that it takes 100 boxes to fill the truck. Imagine that each box has a size measured as 100 (ISO 100). If we increased the box size to 200 (ISO 200), it would take half the time (amount of light) to fill the truck.
So what’s the problem with shooting in high ISO all the time??? Noise.
The reason noise shows up is that because the camera on a higher setting has less time to capture an image, it isn’t able to capture all the information as it would if it was on a lower ISO setting, like 100. This lack of information shows up as noise in the exposed image.
Here’s a real world example. You’re blind folded and led into a bare room with only a table. They turn off the lights, take off your blind fold, and it’s completely dark. The table has 10 items laid out on it. Now, they quickly turn the lights on for a brief second. Most of us would not be able to remember exactly everything about every detail on the table, let alone 2 or 3 of them. Let’s say the room is dark again and the items are switched out with another item. This time the lights are turned on for 5 minutes. Most of us now, could easily remember and describe every item in detail after 5 minutes.
Another lesson that can be learned from this is why better cameras cost more or have bigger sensors. My D70s is an average C+ student, while my wife’s D300 is a straight A gifted student. The D300 is able to more correctly “remember” more information (items laid out on the table) than the D70s at the same ISO (time the light are on), and produces less noise (false information out the stuff on the table). Now the D3 would definitely be Rainman and Einstein combined, being able to remember a lot of detail at very fast light on and off sequences (ISO). If you probably notice, the more expensive/pro cameras tend to have bigger sensors. Imagine the sensor of a camera to be the size of the brain of the person looking at the items on the table.
There’s not really an exercise for this lesson as much as there is an experimentation.
Find an object (preferably with a large area of a solid color) and shoot it with at least 7 different ISO settings. Note the different setting, and notice how the noise increases in the solid area as the ISO increases. Here are my results below. I shot in Auto mode, but locked and changed the ISO for each shot.
ISO 1600 / 1250 / 1000 / 800 / 600 / 500 / 400 / 320 / 250 / 200 (In order as they show up in the gallery)
Here are some standard ISO settings and what they are most commonly used for.
Best Light / Studio (dealing with very still or inanimate object) – ISO 50-100
Outside – ISO 100-400
Inside – ISO 400-800
Speciality – ISO 800 – whatever your show off camera can do.
Here some basic tips if you’re looking to buy a camera, and you want to know how ISO plays in your purchase. (www.digital-slr-guide.com)
Buying Tip
Look for a camera with low noise at high ISO if:
- You enjoy taking photos in dim available light
- You enjoy natural light images and don’t want to use your flash a lot
- You don’t want to lug around a tripod all the time
Don’t make ISO noise into a big issue if:
- You intend to take photos during the daytime in plenty of light
- You don’t mind carrying a tripod to keep the camera stable
- Faced with a low-light scene, you’d rather pop the flash than fiddle with ISO
Again thanks for taking the time to learn with me,
Chris.
