What does ISO (in DIGITAL photography) means?

Question

Hi,

I've lately being interested in the 'advanced' photography.
I know what is the shutter speed (the amount time that the camera lets the light get inside the photo).
On my camera (SONY DSC S70) I've noticed that the ISO is always 100, but the shutter speed changes.

I'm about to buy a new digital camera, and I'd like to know what ISO stands for, and why in higher ISO there's lots of noise, what is the ultimate ISO for photographing in particular situations (close-up nature, buildings, moving objects (you're taking a train and photographing out of the window)).

I payed my attention to Panasonic Lumix DMC-FZ50 (new model should come out soon), if you got another suggestion (price should be upto 1,000 including the lens ((optical zoom should be x12 or more)).

Thank you freaking very much!
Arcady

Answer 1

ISO, in the photographic sense, is a standard measure of light sensitivity for your imaging medium. There are actually TWO standards to speak of (and before ISO became *the* standard, there were *other* standards, like ASA...but let's not get sidetracked). Folks will always refer to "film speed," which is a reference to the ISO standard for light sensitivity in *film* imaging, which technically not the right definition to refer to anymore. Actually these days, most people refer to *digital imaging* ISO, which is similar, but different than film ISO. The term itself--ISO--It's really a shortened reference to the International Organization for Standardization's, which documents hundreds of standards, not just film speed and digital still camera exposure index standards.

ISO earned the name "film speed" because a film emulsion that is more sensitive to light would require less exposure time to capture an image (or alternatively, a smaller aperture). A film that took less time to expose a shot was considered a "faster" film, hence, a higher ISO rating. Faster films were useful to shorten the exposure time necessary to catch low-light shots. But a downside to those film stock was that the grain of the emulsion was rather noticable in development--literally, bigger fatter crystals vs. slower film emulsions. You would get grainy pictures. If you ever shot an old b&W 3200ISO film, you would remember how grainy the images were compared to the same image shot with an ISO100 film.

The digital photography equivalent ISO standard is actually an entirely separate set of ISO standards. But it serves the same purpose--a method for assigning and identifying ISO standard speed ratings, ISO standard output sensitivity values, and recommended exposure index values, for digital still cameras. The ISO rating says that a given photosensor array, must be able to produce a particular image quality for a manufacturer to be able to say that it takes images at that ISO rating (technically called a "digital still camera exposure index").

Unlike film, which is capable of working at only one ISO rating, photosensors are capable of operating at various ISO sensitivity ratings because their output signal can be electronically amplified. Is your sensor actually "becoming more sensitive to light?" No. You can't change a sensor's native sensitivity. You're simply turning up it's output signal, like you would turn up your radio if you could barely hear the station broadcast. So, when you set a digital camera to take pictures at an ISO1600 setting instead of ISO100, you're telling it to amp up the weaker signal so that it will still produce an image of a particular light intensity given a specific amount of incoming light.

But here's the key for you...just like film speed ISO, photosensor ISO loses image quality when you force the photosensors to work at at higher sensitivities and with less light.

The root cause isn't emulsion grain (as it is in film ISO's problems), but signal noise. Since you have less light to work with, the camera tries to "turn up the volume" to reach higher ISO sensativities, as if you were trying to hear a very quiet song over your radio. The "image signal" becomes more and more difficult to distinguish from the "static," or noise. You'll see mottled patchwork of variations in the color of a blue sky, for example, when in real life, the sky is a smooth blue color. That's image noise, and it's significantly more noticable in higher ISO settings.

There's the crux of your answer.

When should you put it in high ISO?

If photodocumenting reality is your objective, the answer is simple: as seldom as you possibly can. Keep the ISO as low as possible to avoid unnecessary noise. This, of course, is only an issue when you're trying for low-light shots without flash. You'll find that in brightly lit scenes, or when using a flash, you can always stick to ISO100 or ISO80 (some cameras go as low as ISO 50). But when the lights dim, and you have an artistic preference for not using flash, that's when you'll have to start pushing the ISO up. What happens is that the shutter speeds start to get so long that you can't keep the subject from blurring in the picture. That's when you know that you're going to have to push the ISO up. If your subject is inanimate (doesn't move), then you can avoid pushing ISO up by putting your camera on a tripod, and use those longer shutterspeeds to get more light in the camera. But if your subject is moving, then you'll have no alternative but 1) push ISO up, or 2) use a flash. If you use a flash, you'll change the artistic impression of the shot. If you push the ISO up, you'll concede image quality. Both are a mixed bag. One thing you could do if you had the option, is switch to a camera/lens that is capable of wider aperture settings, letting in more light. Lenses that can stop down to f2.0 or less are highly sought after for it's ability to catch images in low light without going up to a noisy ISO.


There is one camera company (currently) that is changing the relationship between noise and higher ISO. To get to the higher ISO indexes, instead of just turning up the volume on a regular sensor, Fuji has put the work into developing a more sensitive sensor. Somebody smart is working at that company. They've been blowing away competition with their Super CCD sensor, by getting very low noise in high ISO shots, and as far as sensor technology goes, theirs is the best sensor out there to date. Personally, I hope to see other manufacturers follow its lead.


One last note regarding noise, and photography with high-ISO. It can be argued that *some* noise is not only an acceptable part of imaging, but can lend character that can add to an image's artistic qualities rather than detract. Would old world-war II era photographs posess quite as much nostalgia if they didn't have that grainy texture? That sepia tone? That point is of course debatable ad nauseum, when imaging steps away from photodocumentation, and into photography as an art form.

Answer 2

Mr. photography student above will fail his course if he thinks ISO is the shutter speed! An aperture is a hole through which light passes through. In camera terms this is the optical system that lets light through the lens. A narrow aperture (small hole) will let less light in but allow a photo to be focus throughout the scene whereas a wide aperture (large hole) will let more light in but only a certain focus point of the scene will be in focus. Apertures can typically vary from: f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 and so on with the smallest number being the widest (lets more light in) aperture ISO on digital cameras replicates what the ISO/ASA rating was on film camera. Essentially with film the ISO rating determined how sensitive the film was to light. ASA800 therfore could be used in cloudy overcast conditions and still produce an acceptable photo whereas on a sunny day you might choose to use ASA100 film. A films granularity also related to it's speed so as you moved up to 'faster' film i.e. ASA800 there would be a noticible increase in grain. All of this is replicated on a digital sensor. The higher the ISO you select the more sensitive the sensor is to light but the more 'noise' will be introduced into the picture.

Answer 3

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Answer 4

ISO is the International Standards Organization (or something like that), but the pertinent definition for photography is the light sensitivity.

ISO 100 (and below) is for photography where you have a lot of ambient light- like outdoors during the day. Use the lowest ISO you can get away with, as when you increase ISO, you add noise (i.e. grain) to the image. The Panasonic FZ50 that you suggest is a great camera, but not good at high ISO shots, so if you want to capture fast action in anything but great light, or if you want to shoot in lower light situations (like indoors), it's not the best choice. (look at the Fuji's instead).

DSLRs can shoot ISO400 very cleanly, and even very nice ISO800 shots. That gives you 8 times the light sensitivity of an ISO 100 setting. Or, 1/8th the shutter speed. Most compact cameras start to get noisy over ISO200 (except the Fuji F-series cameras).

Choose a camera which you can manually select the ISO setting.

Answer 5

The ISO is the old 'Film Speed'.

It's is the sensitivity of the sensor. In low light for example, you would generally set the ISO higher to make the sensor more sensitive. This in turn would allow you a quicker shutter speed.

the downside is that you may get more 'noise' on the photo which would need photo shopping

Answer 6

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Answer 7

If you are getting into more serious photography, I suggest you get a dSLR. The high ISO performance is much better than any digicam.

Canon and Nikon both have entry level dSLRs that are within your price range with the kit lens. Olympus also has one with 2 lenses, including a longer one if you're looking for a long zoom.

Look on www.dpreview or www.stevesdigicams to research different camera features and specs, and read reviews.

Good luck!

Answer 8

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