Are we defining ISO correctly?

Question

I just want to throw this in to freak everyone else out.

ISO is NOT an adjustment to the sensitivity of the sensor. A sensor can either detect photons or not. It can not vary the sensitivity to those photons.

ISO in a digital camera refers to the gain applied to the signal sent from the sensor to the processor. Think of the ISO adjustment as an analogy to a volume knob on an amplifier. Adujsting the ISO upwards boosts the signal sent from the sensor to the processor. More signal ("louder" signal, if you will) leads to digital noise as some of the signal from one individual diode jumps over to an neighbor, creating noise. Think of it as being analogous to distortion as an amplifer runs out of headroom and eventually starts to clip the signal because it can't handle it. There's your "hot" or blown pixel in high ISO settings.

I want to start using this as part of my answer when someone asks for a definition of ISO in a digital camera. What do you think of that???

The difference in response to this question in this section vs. in Photography is rather telling. I understand the nuance of telling the effect vs. telling the real deal. I'm just trying to open a discussion.

Actually, thinking of ISO as a "power boost" helps ME understand why increasing the ISO increases the noise.

Mason, I like your idea of saying that ISO adjusts the sensitivity of the whole system rather than referring to only the sensor. It's an answer for both aesthetists and techno freaks - neither of which I feel I really am.

Teef, I've heard that joke, but the conclusion is that the helicopter has reached its destination, which is the Apple Computer headquarters. The logic is the same, though. :-)

Answer 1

Doctor, you're thinking too hard about this. Put it this way, to adjust the sensitivity of a microphone, we adjust the gain of the amplifier. But we still call it adjusting the sensitivity of the mic. Besides, who said we are adjusting the sensitivity of the sensor anyway? We are adjusting the sensitivity of the camera. And what exactly is the sensor? The chip? The photocell? The gold wires leading from the chip to the IC? Or the whole bloody lot?

Answer 2

you got it right
copied from dp review, sonds pretty much right on
Learn : Glossary : Digital Imaging : Sensitivity (ISO)

Sensitivity (ISO)
By Vincent Bockaert

Conventional film comes in different sensitivities (ASAs) for different purposes. The lower the sensitivity, the finer the grain, but more light is needed. This is excellent for outdoor photography, but for low-light conditions or action photography (where fast shutterspeeds are needed), more sensitive or "fast" film is used which is more "grainy".
Likewise, digital cameras have an ISO rating indicating their level of sensitivity to light. ISO 100 is the "normal" setting for most cameras, although some go as low as ISO 50. The sensitivities can be increased to 200, 400, 800, or even 3,200 on high-end digital SLRs. When increasing the sensitivity, the output of the sensor is amplified, so less light is needed. Unfortunately that also amplifies the undesired noise. Incidentally, this creates more grainy pictures, just like in conventional photography, but because of different reasons. It is similar to turning up the volume of a radio with poor reception. Doing so will not only amplify the (desired) music but also the (undesired) hiss and crackle or "noise". Improvements in sensor technology are steadily reducing the noise levels at higher ISOs, especially on higher-end cameras. And unlike conventional film cameras which require a change of film roll or the use of multiple bodies, digital cameras allow you to instantly and conveniently change the sensitivity depending on the circumstances.

Answer 3

I think that's a very good explanation. Better than I could have done.

A person who used film cameras for years would have no trouble with ISO, but the concept seems a little strange in a digital camera.

Back in the day, I used to 'push' film by overdeveloping it to effectively boost its speed. This gave amazing speed abilities (like shooting Tri-X, usually 400, at 4000!) But the pictures looked very grainy and muddy and 'noisy'. A few months ago I thought it might be interesting to try this with my digital camera. I took a picture deliberately underexposed, then put it into Adobe Photoshop and boosted levels, brighteness and contrast. There was a surprisingly good picture there, but it was noisy, in fact it was really almost analogous to 'pushing' film!

Answer 4

A man was in a hot air balloon. After a while he realized he was lost. To find out where he was he decided to lower the balloon's altitude to try and talk to a man he saw standing in a field underneath him. He managed this and so called out to the man standing in the field "Excuse me sir, can you tell me where I am?" The man on the ground replied "Sure, you are in a balloon hovering 30 feet above this field." The man in the balloon said "You must work in IT." The man from below replied "That's amazing, how did you know that?" From above came the reply "Because whilst the answer you gave was technically correct it was completely useless to me!"

All humor aside I wonder if the people asking such a question are not ready to hear a technical answer?

Answer 5

might be my only opportunity to answer one of your questions, so even if I sound like a total twit, here's my answer....

when I think of ISO, from what I've read in newspapers and from what the guy at best buy said, it means something about sensitivity to light, small ISO good, large iso bad because of noise. It's some kind of standard in the photography world. thanks for the question.

Answer 6

A rarity! Dr. Sam asking. Anyway, too technical for me. In Simple terms I still equate ISO setting with an ASA of 35mm film setting. Take care, Teacher.

Answer 7

iso on film provides speed and the creative option of grain

ios on dige offers little noise or big niose and speed

a

Answer 8

Hahha the sensor is on roids... :)

Answer 9

Please refer to my answer in the Photography section.