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???

Guys, 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.

Mojo - Great! It's not exactly what I intended, but the only real attraction for Y!A for me these days is hoping to find a question that makes me LEARN something in order to give an answer.

Answer 1

You're correct-it is a bit of a misnomer to describe digital cameras as having an ISO.

As per international convention, the ISO speed of a film is defined and measured under a very specific set .

According to these, to find ISO you need to plot the density of film vs. log of exposure. You then find where density exceeds base+fog by .1. Divide the exposure(or log of exposure) at that point by .8, and you get the linear, or ASA, film speed rating.

By the way, the ISO speed technically should specify both ASA(linear) film speed and the DIN(logaramithic) film speed. Thus, a film with an ASA of 100 should really be specified as having an ISO speed of 100/21.

In any case, though, since you can't take a densitometer to a digital sensor, there's no way to measure the optical density, so there's no real way to measure the ISO of a digital sensor according to convention.

The best we can do is provide an "ISO equivalent", stating that the sensor will behave like a film of a certain ISO. For most purposes, this works fairly well. We can, however, change the gain on a digital sensor and get and increase the ISO equivalent, while there is no way to change the ISO of a film(pushing does NOT change the ISO).

This was a great question, Dr. Sam, thanks for bringing it up.

Answer 2

Not much Sam.

The functional similarity between setting the ISO in digital and the actual physical sensitivity to photons of a film emulsion has benefits for explanation and understanding that far outweigh any technical correctness of your explanation.

Besides that, in terms of learning photography and making sense of the wealth of material related to media speed, aperture, shutter speed and exposure, the understanding reduces to adjusting the sensitivity of the sensor.

Explaining that setting the ISO changes the sensitivity of the sensor is intuitively understandable. Even if it is not immediately understood, you can bridge from one domain of understanding and knowledge (some form of common understanding or experience) using analogy to make the concept clear.

Not knowing a technically accurate explanation isn't a hinderance to good technical photography and knowing it isn't an asset.

Sorry, Sam.

Vance

Answer 3

While I agree with your definition of ISO pertaining to digital cameras, I think the reason it was coined sensitivity was so that it could be easily related to film speed, as the principle itself is the same.

An excerpt taken from the nikon microscopy, fundamentals of digital imaging:

A third camera acquisition factor, which can affect image quality because it modifies the CCD readout process, is the electronic gain of the camera system. The gain adjustment of a digital CCD camera system defines the number of accumulated photoelectrons that determine each gray level step distinguished by the readout electronics, and is typically applied at the analog-to-digital conversion step. An increase in electronic gain corresponds to a decrease in the number of photoelectrons that are assigned per gray level (electrons/ADU), and allows a given signal level to be divided into a larger number of gray level steps. Note that this differs from gain adjustments applied to photomultiplier tubes or vidicon tubes, in which the varying signal is amplified by a fixed multiplication factor. Although electronic gain adjustment does provide a method to expand a limited signal amplitude to a desired large number of gray levels, if it is used excessively, the small number of electrons that distinguish adjacent gray levels can lead to digitization errors. High gain settings can result in noise due to the inaccurate digitization, which appears as graininess in the final image. If a reduction in exposure time is desired, an increase in electronic gain will allow maintaining a fixed large number of gray scale steps, in spite of the reduced signal level, providing that the applied gain does not produce excessive image deterioration. As an example of the effect of different gain factors applied to a constant signal level, an initial gain setting that assigns 8 electrons per ADU (gray level) dictates that a pixel signal consisting of 8000 electrons will be displayed at 1000 gray levels. By increasing the gain through application of a 4x gain factor to the base setting, the number of electrons per gray level is reduced to 2 (2 electrons/ADU), and 4000 gray levels are distinguished by the digitizing electronics.

As an electronics technician, and a person well versed in semiconductor technology, I can understand the principles of electrical gain and signal to noise ratio, but to the average person those terms may as well be hyroglyphs. And for the purpose of the mass of laymens involved with modern digital photography, I think the consensus is that we [the masses] would feel more comfortable with relating SNR with ISO considering the two eventually lead to the same outcome.

Bravo, Dr. Sam on this question. I had to do some research myself because I can't say I've actually sat down and thought about it like that.

Answer 4

Dr. Sam, I believe using the definition:

"ISO is a measurement of the sensitivity to light of a light sensitive surface, either film or digital sensor" is sufficient and can be understood by most people.

Answer 5

how can it be explained fully to people that have never shoot film?? iso to me means noise on digis - lots or not much

film iso means grain to me

i would answer fully but im sure some digi user who has never shot film and doesnt understand why anne geddes and such shot film will get upset

so we know what it is, i think we explain it ok


OK Dr an EDIT:

iso on film is a useful creative tool and a tool to assist speed, iso on digital gives more speed like a race car\, the by product is a noise producer, with a trottle/volume control cleverly called iso


a

Answer 6

Well, I am freaked.

I was hoping my version was a simplistic answer, considering how much I dont know, for a segment of the group that knows even less than me.

I dont know that much about amplifiers and associated gadgets either, other than I miss that 11 setting, hi-fi's, 33's, 45's, AND 78's, dimes keeping the arm down and needles from skipping, and screaming old people (that were younger than I am now)....,hehe.