How does ISO work?

Can someone explain what ISO numbers mean? Which numbers are best for which type of shots?

In the context of photography, people use "ISO" to refer to a film's (or a digital sensor's) sensitivity to light.

For a given lighting situation, a smaller ISO will force you to do more things to capture light (opening up the aperture, reducing shutter speed, adding flash).

Conversely, a higher ISO may let you stop down the lens, increase shutter speeds, reduce/eliminate dependency on flash, or capture a scene that is too dark to shoot any other way. Note that quality usually decreases with ISO level.

Unofficial ISO scale
--------------------------

25 -- Slow, requires lots of light, can produce very detailed images. Found mainly on slide film (though Kodak once sold ISO 25 negative film). To take best advantage of 25-speed film, you probably need a tripod (so you don't lose to camera shake what you gain in resolution).

100 -- The old "standard" film ISO, before Kodak started really pushing films in the 200 to 800 speed range.

200...400 -- These ISOs produce decent results with current film and with current DSLRs, but may reveal noise problems with digital point-and-shoots.

800...3200 -- The exclusive territory of film and of DSLRs. (1600 and 3200 might be what you'd use if you're in a museum that allows taking pictures, but that does NOT allow the use of flash.)

Use the lowest ISO number you can for the type of shooting you are doing.

In broad daylight use the minimum your camera allows - this will give you the least amount of noisein, and the smoothest rendition of, your pictures.

As the light gets dimmer, use the lowest iso that lets you shoot with an acceptable shutter speed. If you see that you were wiggling in the photo, use a higher iso, and a faster shutter speed.

In the dark, use the highest iso, and brace the camera against something.

If you have a tripod, use it, and lower the iso to get the smoothness back.

Other than that, the numbers are pretty much arbitrary, but up to 200 will usually have no noise, 200-400, very little noise, 800-1000 some but acceptable noise, over 1000 will show noise unless you process the photo to get rid of it.

--
Read my blog -> http://radio.weblogs.com/0101365/

When I started 12 ISO was slow (like colour film) and medium was 50 or 80. Medium to fast was 100 and anything above 150 or even (gasp!) 200 was fast. Of course we didn't call it ISO in those days but used degrees. So a film would be rated as 21° or ASA or DIN or merely "fast".

Somehow we managed to take pictures and often maximum shutter speed was labeled 1/500 and was really 1/300th and a maximum aperture of f/2 was for rich amateurs or else proffessionals and the rest of us made do with f4·5 lenses.

And as for taking pictutres in the dark...

Regards, David

More ISO means more sensitivity. Increasing ISO a camera amplifies signal which also means higher noise. Higher smooth ISO in modern cameras becomes a crucial feature of a camera. Consider a smooth 3200ISO shot - you don't need image stabilisation or an expensive bright 1,8 lens anymore.

Anyway it is important to use a rule of thumb - 1/300 sec for a 300mm lens for example. For moving objects a 1/500 sec would be nice. Then use the lowest possible ISO to keep the best image quality. Problem solved when your camera has an Auto ISO mode (that really works thru the whole ISO range - in compacts Auto ISo usually means ISO 100).

CarbonCamera515 said:

Can someone explain what ISO numbers mean? Which numbers are best
for which type of shots?

Click to expand...

--
mc

David Hughes said:

Somehow we managed to take pictures and often maximum shutter speed
was labeled 1/500 and was really 1/300th and a maximum aperture of
f/2 was for rich amateurs or else proffessionals and the rest of us
made do with f4·5 lenses.

And as for taking pictutres in the dark...

Click to expand...

It's all relative, David, part of continuously rising expectations (and watch out -- your age is showing).

Must admit, I do have a bit of a giggle when posters state that noisy performance above 400 ASA/ISO is reason to reject a small sensor camera as USELESS!!

Perhaps you feel the same?
--
Regards,
Baz

What exactly does changing ISO do physically?

Is 50 ISO a faster shutter speed, and 400 a slower shutter speed (thus gathering more light), or is shutter speed a separate thing?

The camera I am looking to buy has 50 - 400 ISO. Is this pretty good for a relatively cheap ($300 or so) point and shoot?

Tom_N said:

800...3200 -- The exclusive territory of film and of DSLRs. (1600
and 3200 might be what you'd use if you're in a museum that allows
taking pictures, but that does NOT allow the use of flash.)

Click to expand...

... and the Fuji F10 and F11, which have very little noise at 800 ISO and moderate, but treatable noise at 1600 ISO. Fuji is mounting a pretty good attack on that territory with their latest P&S cameras. Looking forward to seeing their next generation.

I think you should google for a decent photography 101; you can easily find comprehensive guides on the basics, teaching you a lot more than some random forum posts on this topic.

I can still remember how hard it was for me to figure what the hell this aperture thing is - I wished the internet had already existed in these days....

Pretty standard for small sensor P&S. Expect to be able to see "noise" on the picture at ISO 400. You'll want to use 50 to 100 most of the time, unless available light doesn't let you for that type of camera.

Test it out and see what you can expect.

CarbonCamera515 said:

What exactly does changing ISO do physically?

Click to expand...

Nothing changes physically with a digital camera. It all happens in the electronics.... but hey, let's forget about that for the moment.......

CarbonCamera515 said:

Is 50 ISO a faster shutter speed, and 400 a slower shutter speed
(thus gathering more light), or is shutter speed a separate thing?

Click to expand...

Yeah, shutter speed IS a separate thing... so let's forget about that too, just for the moment. ;-)
-------------------------------------------------------------

It seems we must explain ISO rating from the ground up, and that means returning to how things were when we were shooting with film.

I'm starting with the basics, OK? And I'm going to number things as I go through so I don't forget anything. ;-)

1) In order for a film to take a photograph it needs a PRECISE amount of light to fall on it from the lens.....

and......

2) It has to be precisely the SAME amount of light EVERY time. Indeed, film has very little latitude to incorrect amounts of exposure. Too much light, and your picture is over-exposed (doesn't "come out"). And similarly with too little light, your picture is underexposed (also doesn't come out).

3) It is ESSENTIAL that only the precisely correct amount of light hits the film, even under a wide range of lighting conditions -- from very bright indeed (say, bright sun on snow or sand) to very dim (indoors without a flash).

It turns out that.....

4) This huge range of brightnesses is MORE than can be accommodated by adjusting shutter speeds (the time the lens is open) and apertures (the size of the hole in the lens) alone.

Because of this limited range of adjustment on the camera......

5) Film is made in a RANGE of sensitivities to the light, so that shutter speed and aperture combinations can be adjusted to be (relatively) convenient whatever the light level happens to be..

6) "Slow" film [50 ISO and lower] is made for bright light, and yields the sharpest, most detailed and 'grain free' images.

7) "Medium" speed film [100-200 ISO] is made for average outdoor conditions, and indoors with flash. It yields high quality images, even if they are not quite so good as slow film.

8) "Fast" film [400-800 ISO] is for low light levels, indoors without flash, and at dawn/dusk outdoors. It also can be used in bright street lighting sometimes, or in shopping malls, say. Grain is more noticeable, but is the price that must be paid for the extra sensitivity.

9) "Ultra" speed film [1600-3200 ISO] is very grainy indeed, but permits pictures in light that is so dim nothing else will do -- or maybe use it for action pictures where ordinary fast film would need too slow a shutter speed to freeze the subject motion.

Please note: To maximise the image quality with film we use the LOWEST ISO that can be conveniently used in the prevailing light conditions.

So how is ISO changed with digital, (which was your original question)?

Well, the sensor has very much the same lack of latitude as film does, and it is fixed in the camera so we can't swap it out.....But we can change the effective ISO sensitivity it provides, by changing the degree of electrical amplification applied to the output signal.

Sometimes this is done to the analogue current from the sensor sites themselves, and sometimes it is done in software by applying mathematical functions after the analogue to digital converter.

Either way, boosting the signal to higher ISOs produces very similar results to boosting ISO in film.There is a distinct rise in "graininess" at higher ISOs, although it's called "noise" in digital. Indeed, the whole process is very similar to turning up the gain on a hi-fi amplifier, where noise is heard as hiss.

Please note again: The same rule of using the lowest possible ISO consistent with convenient shutter speeds and apertures still applies to digital -- but we do have the huge advantage of being able to change ISO sensitivity BETWEEN individual shots, if we wish.

Now, I do hope it's all clear now, but if not, get back here and I'll give it another shot. [Is that an offer, or is that an offer!?]

God bless!
--
Regards,
Baz

Here is pictorial explanation. Click the thumbnails for more detail.
http://www.pbase.com/otfchallenge/the_basics
--
BruceMck
http://www.pbase.com/brucemck

CarbonCamera515 said:

The camera I am looking to buy has 50 - 400 ISO. Is this pretty
good for a relatively cheap ($300 or so) point and shoot?

Click to expand...

There are a great many factors wrt ISO on digicams. One of the most important issues is sensor size. If the pixels are really tiny, then they collect less light and must be amplified more in camera. This causes a sometimes dramatic rise in noise.

Most digicams are very noisy at 200 and above. But some are very good as you go higher. This page is from the first technical review of the Fujifilm F11 against several competitors, and shows the wide variance among digicams wrt noise. (Scroll down for the noise comparison.)

All the test results I have seen confirm that the F10 and F11 are the leading non-dSLR cameras for high ISO at this time. Fuji builds their own super-CCDs, and they have some good in camera processing to go with them. Other 5th gen CCD Fujis that do pretty well at 800 and 1600 are the S5200/S5600 and the S9000/S9500. Two compact cameras that do ok at 800 are again Fujis, the E900 and the E550.

Going outside of the Fuji brand makes for fewer good high ISO noise performers outside of the dSLR class. One of them (the Sony) is shown in the review referenced above.

Bottom line ... having the ability to shoot at 400 ISO means nothing. You need to look at lots of pictures to determine if you like what you are seeing out of that particular camera. Try the camera database at http://www.pbase.com (click search, then camera database.)

Good luck.

Barrie Davis said:

It's all relative, David, part of continuously rising expectations
(and watch out -- your age is showing).

Must admit, I do have a bit of a giggle when posters state that
noisy performance above 400 ASA/ISO is reason to reject a small
sensor camera as USELESS!!

Perhaps you feel the same?

Click to expand...

Hi Baz,

You only have to look at the white hair to guess my age, although people are then puzzled as to how I got such a young wife...

Couldn't agree more. Think I only ever used 400 ASA once (or HPS as it would have been) and stuck to FP3 for years and years. Used to buy it in cans and cut off a length the same length as the window sill in the darkroom, then stick it to a cassette spool and wind it up by hand...

Those were the days, my friend, we thought they'd never end.

What worries me is all those newbies asking about low light performance, when they could use flash or pick their moment and handhold for ½ second. Seems a lot of technique has been thrown out the window with digital. Or do they expect the things to do it all?

Regards, David

David Hughes said:

Hi Baz,
You only have to look at the white hair to guess my age, although
people are then puzzled as to how I got such a young wife...

Click to expand...

[Hmmm.. It's impertinent to comment, but presumably she knew a good thing when she saw it...? ;-)]
snip

David Hughes124436 said:

What worries me is all those newbies asking about low light
performance, when they could use flash or pick their moment and
handhold for ½ second. Seems a lot of technique has been thrown out
the window with digital. Or do they expect the things to do it all?

Click to expand...

Well, we mustn't be too curmudgeonly.

The high ISO, high quality performance of the latest kit has opened up new classes of pictures we have never seen before. I'm thinking of action pix of indoor sports, which were quite impossible before now. The fact that those pictures get published has increased the popularity of some sports, basketball for instance.

And then, when spiffingly fast AF is added to high ISO, wild life photography takes on a whole new dimension. I have recently seen so many really GOOD pictures of birds on the wing (sorry, can't find a link) that I had to admit these guys have a point!

Thanks for posting. Please have a good Christmas.
--
Regards,
Baz

When talking about film it has to do with light sensitivity to the physical chemicals in the film. This is from an online source info:

The term “speed” refers to the speed at which the film emulsion reacts to the presence of light. Fast film is very sensitive to light - it reacts rapidly and so can be used in low-light conditions or with fast shutter speeds. Slow film is not as sensitive and so reacts more slowly.

There’s a complex relationship between film grain (resolution, in a sense) and film speed. All things being equal, faster film is grainier than slower film. This is because the speed at which film emulsion reacts to light is related to the physical size of the silver halide grains in the film. Larger clumps react more quickly but have greater grain size.

When talking about digital ISO, this is done by amplifing the electrical signal which has been gathered from the photons on the photodiode. This also includes extranious electrical signal as well
Will

CarbonCamera515 said:

The camera I am looking to buy has 50 - 400 ISO. Is this pretty
good for a relatively cheap ($300 or so) point and shoot?

Click to expand...

It's actually pretty standard for a point & shoot digicam. A given ISO rating implies a certain sensitivity to light, and fits into the established exposure system. Ever hear of the "sunny f/16 rule" ? It says that you get good exposures photographing subjects in sunlight at f/16 with a shutter speed of 1/ISO. So if shooting ISO 100, you'd shoot 1/100th (or as close as you can get) second. ISO 200 is twice as sensitive to light, so you can shoot at 1/200th. ISO 400 allows you to shoot same scene at 1/400th.

If you can shoot at ISO 100, 1/100th and f/16 in bright daylight, but indoors in the evening, need ISO 1600, 1/15th at f/2.8, it becomes obvious that there's a huge range of brightness you'll want to photograph in. There's some minimum shutter speed you'll want to use in a given situation. When you get to that shutter speed (let's say you can get a reasonably sharp image at 1/30s) at ISO 100, then ISO 400 gives you the ability to shoot in a little bit less light before you need to turn on the dreaded flash. With a DSLR or the Fuji F10, and the option of ISO 1600 or even 3200, you have a bigger "window" ... you can shoot in lower light w/o needing flash.

When you increase the ISO, you're amplifying the signal that the sensor receives. You're also amplifying the noise. Images get noisy at higher ISOs. Larger sensors with larger pixels have a higher signal to noise ratio at any given ISO setting, so you can shoot ISO 1600 or even 3200 on a DSLR as cleanly as ISO 400 on most any small sensor digicam.

If you feel the need to go above ISO 400 on a digicam, look at Fuji, particularly the F10/F11. Note, though, that those cameras have slower lenses than some competition, so they're not all they're cracked up to be for low light. (You won't find faster lenses on ultracompacts, but you will find them on slightly bigger cameras).

There are also cameras with antishake. These let you shoot at slower shutter speeds and get a steady image, but the image may still contain blurry subjects if they were moving during the exposure.

Finally, within the realm of digicams with ISO 50 to 400, there are sensors with more or less noise. Reviews here & elsewhere demonstrate the differences. I find it's worth paying attention to, not to the point of nitpicking, but to be aware of what you can expect from an 8x10 at ISO 400.

• Dennis

--
Gallery at http://kingofthebeasts.smugmug.com

and you didn't even mention push processing or reciprocity law failure. I thought I had forgotten all that stuff about film. Not touched a roll for over five years. But it all came flooding back..

Well done.

Bob

Bob Hellen said:

and you didn't even mention push processing or reciprocity law
failure. I thought I had forgotten all that stuff about film. Not
touched a roll for over five years. But it all came flooding back..

Well done.

Click to expand...

Well thank you, Bob. What a charming bouquet!

However, something tells me your powers of recall are pretty good without much prompting from me. Besides, you mentioned reciprocity law failure , instead of just "reciprocity" -- that's surely the mark of someone who was also paying attention back whenever! ;-)

Have a good Christmas, please.
--
Regards,
Baz