Ok, I think I might get it! about ISO sensitivity

[snapsnap1973]

I had a revelation this morning! I know really nothing about photograhy and am trying to "start at the beginning", but I guess you can approach it from any angle and "still get there". I should probably be starting at the VERY basics and reading about how a camera works .... I mean a basic camera, not even a digital one and go from there.

Anyways, today I was looking at "A Great Graphic for understanding ISO, APerture and Exposure speeds" or whatever online and got to thinking about ISO to start the day. I was thinking "what's the point of changing ISO, why can't it just be set at ISO 100 and then forget about it?

So I sort of had an instant revelation and remembered that I read somewhere that ISO had to do with the sensors "sensitivity" adjustment. Am I correct? So obviously it would make sense that the HIGHER you put the ISO you are in turn "cranking up" the sensitivity of the sensor and allowing more NOISE in? Am I on track in my understanding?

So if you're out on a nice sunny day you ideally would want your ISO set as low as possible for the very reason that you would want the less "noise" in your photo that you could get? So the IDEAL situation is to always have ISO as low as possible for the ONLY reason that you want as low of noise as possible in your picture?

Of course in very low light you have no choice, but to bring up the ISO to the higher side and you have to compromise the "ideal" setting?

Am I getting this? LOL ....... I think I'm gonna head off now and read "Photography life's" "Understanding ISO" article .... LOL

 

[snowbear116 47]

That is correct. If you change one setting, you need to adjust at least one other in order to maintain the overall exposure setting. Aperture is the size of the "hole" in the lens, shutter speed is how fast the door to the sensor (or film) is open and ISO is basically the sensitivity (in digital it's more the amount a signal is amplified but sensitivity is usually easier to understand).

 

[sashbar]

Your idea is more or less correct, but the term "sensitivity of the sensor" is quite simplified and can be misleading. The ISO is a product of a camera, not just a sensor.

The photodiodes in a sensor have an inherent sensitivity to light. To get higher ISO a camera digital engine applies a gain, i.e. amplifies the signal. The amplification process also brings digital noise. Different cameras use different digital amplification/conversion/noise reduction algorithms. That is why cameras from different manufacturers have different high ISO image quality, even if they use the same sensor. So the ISO is a product of a sensor sensitivity, gain algorithm and A/D conversion programme as well as in-camera noise reduction.

But to simplify it all you can think about the ISO as sensor's sensitivity.

 

[JustJazzie]

It sounds like you get it. photography is often about trade offs, and the photographer has to choose what trade offs will help him/her achieve their artist vision the best. Luckily on modern cameras, iso1600+ is extremely useable so for me it's not much to consider until you've reached the limit of your camera.

 

[snapsnap1973]

Thanks guys!!

So I was gonna ask about NOISE and you guys sort of already answered my question, but what exactly IS noise other than what we end up SEEING in an image?

Is noise simply parts of an image (pixels) that don't receive information due to something going on somewhere in the hardware?

Why is noise inherent in digital cameras? Wasn't it also inherent in film, since film had ISO ratings too?

Isn't ISO a standard?

Thanks

 

[sashbar]

Thanks guys!!

So I was gonna ask about NOISE and you guys sort of already answered my question, but what exactly IS noise other than what we end up SEEING in an image?

Is noise simply parts of an image (pixels) that don't receive information due to something going on somewhere in the hardware?

Why is noise inherent in digital cameras? Wasn't it also inherent in film, since film had ISO ratings too?

Isn't ISO a standard?

Thanks

Digital noise is the product of the amplification of a digital signal that camera gets after the analogue to digital conversion of a light signal. It is not a part of an initial image, it is a digital artefact. That is what we pay with by amplifying the signal digitally.
If we could change the sensitivity of the sensor, that would be wonderful - it would have meant no digital noise at high ISO. But this is impossible, so the gain is applied in the digital domain. Pixels can only take a certain amount of photons or light, the excess is being spilled over and it creates a different sort of problem.
With film you would not get any digital noise obviously, but you will get grain.

 

[limr31]

Yes, with higher ISO film, you'd get more grain (film=grain, digital=noise), but how much more would depend on the film and the development. For example, Kodak Tmax films are finer-grained than Tri-X (for black and white) so if grain bothers someone, they can try films that have finer grain. And there are different developers to use with film, some of which are better at controlling grain, even if the film emulsion itself isn't particularly fine-grained.

Of course with film, you can't change the ISO in the middle of the roll, but you could "push" it to a higher ISO by developing the film longer once it's been shot. I've shot at night with Tri-X which is ISO 400, but knowing that I was going to push the development (extend the time), I shot the roll as if it were at ISO 1600. And yes, I expected more grain from that roll than what I would have gotten had I shot at 400 ISO. But I also would have had a lot of black pictures if I hadn't "increased" the ISO.

Film is generally dealing with lower ISOs in general. There's an Ilford Delta 3200, but that's the highest ISO film I know of, and have heard that it's really a lower ISO film that pushes well. Ilford also makes Pan F, a 50 ISO film. Most are between 100-800 ISO.

Digital cameras can go to much higher ISO levels than film.

As for ISO being "standard" - I think of the standard as being a general guideline, but that still doesn't predict exactly what a sensor or film emulsion will do. Some films, for example, have a higher tolerance so you can shoot it a little higher or lower and still get a decent exposure. Two days ago, I forgot that I had ISO 160 film in the camera and shot 2 shots at ISO 400 before I realized my mistake. The film I had, though, has good latitude so I'll probably still get something usable. Other films, like Ektar, are pickier and you have to be more exact. If I'd messed up with that film, it's likely the pictures will be too overexposed to be usable.

With digital, I imagine you have to discover the quirks of the camera - does it shoot a little over or a little under what you expect for a certain ISO.

 

[Overread17 5]

You are partly correct.

Firstly lets talk about noise. Now it is true that if you raise the ISO you increase the amount of noise that will be present in the photo. An ISO 100 and an ISO 800 shot will show the 800 shot with more noise - assuming in both cases that you had a good exposure.

Now that last point is important. So long as when you come to edit you're not adding in any brightness to the shot you'll follow that higher ISO = more noise pattern. However if you have to brighten the shot in editing then the pattern reverses. Adding light to the shot in editing will reveal more noise in the shadowed areas. Indeed you will actually get more noise if you brighten a shot up by around one stop than if you were to have used a higher ISO when you took the shot.

This is the backbone of the "expose to the right" theory which you might read about and is tied to how to take a good digital photo from a technical point. It links into learning how to read the histogram when reviewing your photos (search on the expose to the right theory for more info on it and detail).



Also never ever think that ISO 100 is the best. In fact one of the worst bits of advice to pick up is to "keep the ISO low". Because what tends to happen is that you end up underexposing (remember we just spoke that that will give you more noise) or using too slow a shutter speed (blur can't be fixed in editing) etc.... So remember you've got the ISO - make use of it. It's not just for dark situations its for any time where you have an ideal aperture and shutter speed that you want to use; but the light is not sufficient to give a good exposure. You then have that ISO to raise up as high as you desire - although in real world situations you might find that its a juggling act of balancing your desire/need for a certain aperture and shutter speed combo against the higher noise that higher ISOs will bring. Sometimes you will go "eh ok I'll use a wider aperture or risk a slightly slower shutter speed - keep my good exposure - and let the ISO be a little lower to save on noise"

 

[KmH]

There are a variety of causes of image noise.

A single image sensor pixel is a very small array of electronic devices.
The light gathering element is a photodiode and transistors in the array perform a variety of tasks like turning the pixel on/off and amplification.
A photodiode develops a voltage proportional to how much light hits it. More light produces a higher voltage.

Unfortunately a photodiode produces a small amount of voltage even if no light hits it. That is the cause of dark noise.
Electronic/electrical devices resist the application of electrical power and that resistance creates heat, the source of thermal noise.
When the ISO is increased the voltage the photodiode develops is amplified more and the increased amplification reduces the signal to noise ratio (SNR)