Help me understand ISO - it's confusing the hell out of me?

classixuk · Jan 3, 2012

Hi everyone,

As I await the arrival of my shiny new Canon 600D, I've been using the time to read up a hell of a lot of the basics.

ISO is confusing me, and here's why...

Back in the 90's I used to use an Olympus Zoom camera that took film. I worked on cruiseships during this period so really took advantage of my (at the time) excellent consumer camera. One of the photographers onboard explained ISO to me in this manner to help me choose which film I would need for my camera every few days: ISO = Time of day. ISO 100 = 1.00pm when it's bright and sunny. ISO 200 = 2.00pm when the clouds have come out and the weather's cooled off. ISO 400 = 4.00pm when it starts getting dark in Winter and ISO 800 = 8.00pm when it's dark as the night sky in the mid-ocean blackness.

Now that might sound crazy to some of you, but for me, as the consumer it always got me through. I always planned ahead where most of the pictures would be taken on a roll and purchased with that scenario in mind.

It seems as though things have changed now. Looking through some of the galleries here and reading the settings for some pictures, the ISO is in the thousands! I cannot get my head around this and why the ISO would be so high (or needs to be)? Surely, unlike film, the same amount of sensors are always there on a digital camera? Why aren't they "always on, all of the time" and simply allow the aperture and shutter speeds to dictate how much light hits against them?

I could understand it with film, as the higher ISO meant more sensitivity and a different chemical composition, but it's not as if my Canon will suddenly get 400 more sensors because I set it at ISO 800 instead of 400.

So what am I failing to understand? And can someone give me a new metaphor/tale to remember my ISO's by, as it seems they go way past 8.00pm on a digital camera.

:er:

Many thanks.

Robin Usagani · Jan 3, 2012

Camera Exposure: Aperture, ISO & Shutter Speed

paigew · Jan 3, 2012

I put my iso that high when I am inside my house and I do not want my flash to come on. It can make them look grainy but I dont mind the grainy look.

MLeeK · Jan 3, 2012

It's a great way to remember how ISO works.
In short ISO is the film, or in this case now digital sensor's sensitivity to light. If it's 100 it's low in the numbers=low sensitivity. Bright sunlight would be too much if you were overly sensitive-you want that low sensitivity.
If it's a high number like 6400 or 12800 it's MUCH more sensitive to light than at 100, so it needs less light to work. Doesn't mean you can shoot in pitch black, but you can shoot in MUCH lower light.

There are downfalls with raising ISO though too-you want to shoot at the lowest you can AND GET PROPER exposure. NEVER underexpose to shoot at a lower ISO. As you go up in numbers you also raise the grain or noise in the image. If you underexpose and think "Oh, I'll fix it in post so I don't have to go higher" you will make more noise in your image than you would have had by raising the ISO to get proper exposure.

SCraig · Jan 3, 2012

It isn't the number of sensors, it's the sensitivity of the sensor. The higher the ISO the more sensitive the sensor is. And as it becomes more sensitive it also becomes prone to noise (the rough electronic equivalent of film grain).

I can't provide a metaphor for the numbers. Just learn to use them is all I can offer. They make sense after the first day or so.

rambler · Jan 3, 2012

With film you used the film ISO for the entire roll. (used to be ASA) With digital you can change the ISO at any time, or frame by frame. The higher the ISO the less light. ISO 200 is twice as sensitive to light as ISO 100, 400 twice more than 200...etc. But, because ISO can be changed so quickly, it can also be used as an exposure control or adjustment. As you increase ISO you also change the shutter speed. If a shutter speed is too slow to catch the action, increase the ISO and you can increase the shutter speed and still get a correct exposure. Watch out for "noise", however. Increase the shutter speed too much you will see a blurring or noise around the edges.

In general the lower the light, try a higher the ISO.

As you increase the ISO you will need to close your aperture or increase your shutter speed.

Think of it this way, when your eye becomes more sensitive to light (higher ISO), you will need to squint (close the aperture) or blink faster (increase shutter speed).

Big Mike · Jan 3, 2012

You need to remember that ISO is only one of three things that make up exposure. ISO, Shutter speed and aperture.

Photography almost always involves some sort of compromise involving the exposure settings. For example, the shutter speed controls how motion is captured...either motion of the subjects or motion of the camera. The faster (shorter) the shutter speed, the less blur you will have from that motion...so you could say that the faster the shutter speed, the sharper the photo can be.

So with that in mind, why can't we just use a fast shutter speed all the time?....because a faster shutter speed lets in less light (that we need to get a good exposure). So the first thing we do, is to open up the lens aperture, which lets in more light, allowing for a faster shutter speed. But in less then bright light, we soon reach the limit of the lens aperture.
So if our lens is at it's maximum aperture, and we need more light (more exposure) we have two options left to us. One is to use a slower shutter speed, which would result in more motion blur....or we could raise the ISO.

So the higher we can raise the ISO, the faster the shutter speed we can use (or the the less light we need)(or the smaller the aperture we can use).
So having the option to raise the ISO, gives us more creative options for what exposure settings we use.

The downside to this, is that a higher ISO means more noise. In the film days, higher ISO film has more 'grain'...so if you wanted clean images, you stuck to low ISO film (less than 400). Digital is similar, but rather than grain, it's digital noise. It's typically something that we want to avoid, so using a lower ISO is usually good advice, but if that higher ISO means a sharper photo (because of a faster shutter speed) then it's probably a good trade off.

As technology advances, we are able to get cleaner and cleaner images at higher and higher ISO settings....giving us more options with our settings.

5 years ago, 800 was a pretty high ISO on a digital SLR. Now, something like 3200 or 6400 is a high ISO, but still useable on many newer cameras.

KmH · Jan 3, 2012

Actually, the image sensor only has 1 level of sensitivity.

The ISO setting determines how much the signal (amount of light) the sensor recorded gets amplified.
At ISO 100 the signal is amplified less than the signal gets amplified at higher ISO settings.

Most of the image noise we see in a photo that was made at higher ISO settings is actually amplifier noise that is overwhelming the small signal (not much light) the image sensor was able to record.

As shutter speed gets longer, power is applied to the image sensor longer and because of it's electrical resistance it heats up, generating thermal noise that gets added to any amplifer noise caused from boosting the ISO setting.

jowensphoto · Jan 3, 2012

I kind of like the analogy OP is talking about. The easiest way for me to remember (and understand) ISO is that a higher number means greater sensor sensitivity.

Like Big Mike said (and Schwetty posted a link), you have to take all three sides of the exposure triangle into consideration.

Rephargotohp · Jan 3, 2012

And don't assume because you see a lot of people using high ISOs they know what they are doing. People use way too high ISO or they are setting thieir camera to Auto ISO because they themsleves are confused about ISO and want to choose a safe number.

You can usually tell when you see f1.8 1/4000 ISO 800...On a tripod...still life

You may have a better understanding then they do

classixuk · Jan 3, 2012

Thank you all for the great advice.

Some of you mentioned the very reason I was always very wary of ever using a higher ISO than necessary - grain (or noise). ISO 400 was only ever used in my camera for visiting aquariums or caves. ISO 200 was my 'safe bet' if I just needed a roll to last a few days and ISO 100 for everything else (we visited a lot of beaches - being on a boat

). ISO 800 was the special stuff I did not dare use incase my photos blurred (used to happen on ISO 400 and that was bad enough). So you can imagine my shock horror when I was reading about photos taken (in what looks like clear daylight) using 3200 and 6400 ISO

But just to be sure I have this right ... are you basically saying (to avoid unwanted noise as much as possible) that I should only increase the ISO once I have lowered my aperture to the lowest possible F-Stop for that particular lens? Because it does not make sense to me why I would choose to increase the ISO if I can still drop a stop on my aperture first and retain IQ? Am I over thinking this?

Thanks again for your patience.

naptime · Jan 3, 2012

GREAT info in this thread.

i've always known that a higher number was for lower light. and lower number for more light.

but, still wasn't really sure when to use what.

your 1pm, 4pm analogy is awesome !!!! thanks :thumbup: (particularly for me, because i am new, and we are shooting film)

c.cloudwalker · Jan 3, 2012

classixuk said:
One of the photographers onboard explained ISO to me in this manner to help me choose which film I would need for my camera every few days: ISO = Time of day. ISO 100 = 1.00pm when it's bright and sunny. ISO 200 = 2.00pm when the clouds have come out and the weather's cooled off. ISO 400 = 4.00pm when it starts getting dark in Winter and ISO 800 = 8.00pm when it's dark as the night sky in the mid-ocean blackness.

I'm glad I didn't learn photography from your onboard friends

I wonder what time of day they would have told me to use my 25 ASA film (2:30 am?) or my 50 ASA film (5am?)...

ISO = ASA by the way.

And all it is is the sensitivity of the film in the days of film and of the sensor today. The difference between film and sensor being that you can change it on the fly when it comes to sensors.

Now if you are trying to get good shots that is really all you need to know. If you're trying to build a digital camera, then we have to talk about more. Although you won't talk about it with me. I don't understand how it works either, lol.

And there is no grain in digital. Grain is with film only. Digital photography has noise.

Rephargotohp · Jan 3, 2012

classixuk said:
Thank you all for the great advice.

But just to be sure I have this right ... are you basically saying (to avoid unwanted noise as much as possible) that I should only increase the ISO once I have lowered my aperture to the lowest possible F-Stop for that particular lens? Because it does not make sense to me why I would choose to increase the ISO if I can still drop a stop on my aperture first and retain IQ? Am I over thinking this?

Thanks again for your patience.

The best way to use it, is almost a last resort.

Say you want to Shoot a landscape ISO 100 and you want a Deep DOF, so you want to shoot at f/16 However, your shutter speed is 1/25 and you are hand holding the camera (no tripod around) well you want to maintain that f/16 but you need get your shutter speed to /100. Raise your ISO to 400. Done

Opening the aperture may not be the solution because it would change your DOF

Big Mike · Jan 3, 2012

But just to be sure I have this right ... are you basically saying (to avoid unwanted noise as much as possible) that I should only increase the ISO once I have lowered my aperture to the lowest possible F-Stop for that particular lens? Because it does not make sense to me why I would choose to increase the ISO if I can still drop a stop on my aperture first and retain IQ? Am I over thinking this?

No, you can (should) raise the ISO to get the exposure that you want/need, if it helps to achieve your creative goal, which may involve certain shutter speed or aperture settings.

For example, if you are shooting a group photo with several rows of people. You want your Depth of Field (DOF) to be deep enough to get all the people in focus. DOF is mainly controlled by the lens aperture, so let's guess that you need to use F8 to get them all in focus.
Now, you are shooting hand held (and shooting people) so you don't want your shutter speed to drop below say, 1/200.
So depending on the light, you may need a higher ISO to achieve proper exposure at those two other settings.

So, it really has little to do with what time of day it is etc. It has everything to do with your creative goal and how the three exposure settings relate to that goal.