Photography Basics 2: What is ISO or ASA – Camera/Film Sensitivity AKA Filmspeed

ShaolinTiger posted this at 9:50 pm on Sunday, January 21, 2007 —

Introduction – ISO Basics

ISO or ASA in the most basic terms is the speed with which your film or digital camera responds to light, so the higher the ISO/ASA rating the more sensitive the film or CCD/CMOS sensor is to light.

In terms of film those with with lower sensitivity (lower ISO speed rating like 50 or 100) requires a longer exposure and is thus called a slow film, while stock with higher sensitivity (higher ISO speed rating such as 400 or 800) can shoot the same scene with a shorter exposure and is called a fast film.

The same holds true for digital camera, but you are adjusting the sensitivity of the CCD or CMOS not actually using different film, this is one of the beauties of digital cameras, you can change ISO on every shot if you wish, you don’t need different physical films!

The basic rule would be a higher ISO gives a higher shutter speed with the same Aperture settings, so less blur. The trade-off is that higher ISO also gives more noise or grain to your images, which can be a bad thing if it’s not a look you appreciate.

Slow shutter speed will give you pics like this:

Blur

Because you just can’t hold it steady enough!

or

Blur

Because your shutter speed isn’t fast enough to capture someone in motion!

ISO or ASA

ISO is the term generally used on Digital Cameras, the standard was ASA and in the later years ISO.

To get a bit more technical it was known as the ISO linear scale, which corresponds to the older ASA scale, doubling the speed of a film (that is, halving the amount of light that is necessary to expose the film) implies doubling the numeric value that designates the film speed so 50, 100, 200, 400, 800 and 1600.

Nikon digital ISO ratings tend to be exactly the same as the real film counterparts where as some others like Sony Alpha A-100 are slightly off, I think the Canon 350D was out too.

As for other manufacturers I’m not really sure.

For example with the Alpha:

The first column is the indicated sensitivity the second is the actual.

ISO 100 ISO 125
ISO 200 ISO 250
ISO 400 ISO 500
ISO 800 ISO 1000
ISO 1600 ISO 2000

And for the Canon 350D:

ISO 200 ISO 250
ISO 400 ISO 500
ISO 800 ISO 1000
ISO 1600 ISO 2000

Noise or Grain

Noise is what happens when you crank the ISO up, it’s because you are making the sensor more sensitive to light so you are also making it more sensitive to it’s flaws. The contrast, ISO and grain/noise are all linked.

speed rating sensitivity contrast grain
50 ISO/ASA low low low
100 ISO/ASA medium medium medium
200 ISO/ASA medium medium medium
400 ISO/ASA high high high
800 ISO/ASA very high very high very high

Visual Noise is what occurs on digital cameras where as Grain is what occurs on film, Grain tends to be a lot more pleasing than noise, especially in Black & White photography.

In terms of film, the ASA or file speed is roughly related to granularity, this is the size of the grains of silver halide in the emulsion, since larger grains give film a greater sensitivity to light. Fine-grain stock, such as portrait film or those used for the intermediate stages of copying original camera negatives, is “slow”, meaning that the amount of light used to expose it must be high or the shutter must be open longer. Fast films, used for shooting in poor light or for shooting fast motion, produce a grainy image.

In digital pictures the image noise is a random, usually unwanted, fluctuation of pixel values in an image. Image noise can originate in film grain or in electronic noise in the input device (scanner or digital camera) sensor and circuitry.

Noise in digital images looks like this:

Noisy Image

Now this looks ok in a small size, but if you blow it up the noise is very visible, here is a crop from the image:

Noise

Noise is also directly related to sensor size, so camera phones generally give terrible noise even under quite reasonable lighting conditions. Digital SLRs have much better noise performance than compact cameras and even better than that is full frame sensor Digital SLRs like the Canon 5D.

Also note noise tends to be more obvious in shadows or underexposed pictures, so do be careful when shooting.

We will discuss noise more in detail later, what causes it, what are the types of noise and how to combat it. There a few options for software based noise reduction, which again we will discuss later.

Interestingly the most sensitive sensor common in commercial photography may be the Silicon Intensified Target Vidicon, at ASA 200,000, used in TV cameras.

Rule of Thumb for ISO

The most basic rule is keep the ISO as low as possible at all times, especially on cameras like the Panasonic Lumix range and other super compacts with small sensors, keep the ISO at 50 or 100 even if you are shooting at night because the noise these cameras generate can be quite terrible.

When shooting in low light or darkness with a camera with decent light sensitivity (the best compacts on the market now for high ISO shooting are the Fuji cameras after that would come Canon), adjust the ISO up one setting until you get a clear shot.

You can check the sharpness of your shot by zooming in on the LCD.

You can check your camera manual or have a look around online to find out how you can change the ISO, most compact cameras have it inside a menu somewhere and are often using auto-ISO.

For example here is the Canon S3 IS ISO button and it’s usage.

S3 IS ISO

Every digital SLR from medium range and above has a dedicated ISO button.

Be careful when you start shooting something that your ISO is set correctly as well, as the night before you may have been shooting at ISO800, if you keep shooting like that in daylight you will have a lot of overexposed pictures and unnecessary noise in all your shadows.

I’ve made this mistake before! I blame it on an oversight in the D70s user interface..

If it’s really dark and you are taking still scenery it’s always best to use a tripod and the lowest ISO possible.

Summary

Just think a little bit before you take your shot, don’t just keep shooting and hope for a lucky shot :) Understand your camera and it’s limitations and learn to get the most out of it.

ISO is an important tool in getting us clear and sharp shots by getting the optimum shutter speed out of our camera for any given situation, but it’s also something that can cause a lot of noise and terrible artifacts.

These can ruin your picture if you aren’t careful.

So do learn to use ISO, just be wary of it’s effects as like most things it’s a double edged sword.

You can read more here:

Film Speed

If you need and clarifications please leave a comment below, do check out the previous article in the series on What is Aperture or f-stops/f number & Depth of Field and next to come is Shutter Speed!


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16 Comments »

Comment by Intensecure

January 22, 2007 @ 8:21 am


Nice, comprehensive article, ST.
Of course, as a Pana FZ30 user I cannot go higher than ISO200 under any conditions. Image stabilisation helps, but there are still times – moving objects as you point out – which people really need to consider if that is a reason for buying a camera – so often parents are disappointed when trying to shoot their children’s indoor activities, or school plays without flash, when compacts just won’t do, however expensive – with the exception of some of the new Fujis.
BTW, slightly OT – I am currently using DxO software to correct my images, it is stunningly good, at removing noise as well as CA and perspective correction. Highly Recommended.

Comment by Albert Ng

January 23, 2007 @ 1:44 am


Very well-written article with good examples!

Up until recently, even Canon compacts were actually 1/3rd of a stop more sensitive. Such inaccuracy was rampant.

I look forward to your noise-reduction methods, as I generally dislike the output of NeatImage.

Comment by Silencers

January 24, 2007 @ 7:01 am


Albert: I have to agree on the point on NeatImage, but sometimes I just have to rely on it when processing noisy images in bulk. Makes for a good mass resampling tool, too.

Comment by ShaolinTiger

January 24, 2007 @ 3:27 pm


Intensecure: Aye the only real problem with Panasonic cams, it’s even present in their first SLR, although it’s not SO bad. Yah DxO is excellent, will be including it in the comparison I think. Not sure if I have a working version though.

Albert: Thanks man :)

Neat Image is not bad in the way it works, especially how it can read profiles from the EXIF and match the predefined parameters for each camera you have at each ISO. I don’t particularly like the way it does noise reduction though, some others are better. Will do some comparisons later.

Comment by Intensecure

January 24, 2007 @ 3:43 pm


@ ST – The trial versions from DxO, from 3.0 to 4.0 themselves were very buggy, would only process about half my pictures correctly. I think their trial software still sucks AFAIK.
I have a copy (and yes, I admit a “copy”) of Elite Pro 3.5, which is rock solid and processes everything perfectly. The changes that it makes to perspective distortion from the zoom is simply fantastic! It makes my FZ30 twice the camera that it was before.

Comment by ShaolinTiger

January 25, 2007 @ 10:36 am


Intensecure: Thanks bro I’ll look for that version, all in the same of trials of course :)

Comment by Alex

July 25, 2007 @ 11:52 pm


It is a very nice article but some paragraphs confused me.
For example the first line of the 4th paragraph when it says:

“The basic rule would be a higher ISO gives a lower shutter speed…”

Don’t you think that higher ISO gives faster shutter speed? Higher ISO setting makes the image sensor(talking about digital cameras) more sentive to light so an image can be captured with a quick shutter.
I am thinking again and again and cannot understand how a higher ISO gives lower shutter speed!..

Please correct me if I am missing something. I am not a professional photographer at all.

Thank you so much…

Comment by ShaolinTiger

July 26, 2007 @ 5:55 pm


Alex: You’re correct, that was a typing mistake on my part. Lower ISO gives slower shutter speed, higher ISO gives faster shutter speed. I have amended that sentence in the post – Thanks!

Comment by krutika chaturvedi

December 13, 2007 @ 1:13 am


heyy the data you gave was informative but the statements you used are contradictiong each other, for eg. you say that Rule of thumb for ISO is to use lower ISO all the time but then you also mention that if higher shutter speed is required which helps the image not becoming blurred needs higher ISO.

Comment by ShaolinTiger

December 13, 2007 @ 4:52 pm


krutika chaturvedi: It stands as correct, you don’t go shooting at ISO800 in bright daylight, or ISO100 in pitch black (without a tripod). If possible always shoot at the lowest ISO your camera allows, but if the shutter speed is too low you have to increase the ISO to reduce the chance of handshake.

Comment by Adam Parker

December 4, 2008 @ 2:33 am


Great writeup, this is a good place for beginning photographers to come and learn more about their camera. Balancing all the configuration of a camera takes practice and time to master. Between white ballance, ISO, shutter speed, aperture, RAW/jpg, etc., the more references the better.

You did an excellent job illustrating the differences between the ISO values regarding sensitivity and grain . Only when photographers learn how/when to adjust the various settings on their camera will they have accurate expectations and adequate results come from their shoots.

Here is one more article covering ISO, it’s benefits and a description of how/what the ISO values represent.
http://blogs.adamparkerphotography.com/blog/What-does-the-ISO-setting-on-my-camera-do/16/

Comment by jeff randall

May 31, 2011 @ 9:16 pm


I have an older sears automatic SLR 35mm camera with electric eye. If I use 200 asa film ,the camera will not take a good picture outside. Why???? Also should I use only 100 asa outside and if so where do I find ,……100 asa film…..By the way I think my camera is from aproximately – 1974…

Thanks for your time——-Jeff Randall

Comment by Dave Erickson

April 5, 2012 @ 1:11 am


I got a problem.

I have a yashica j7, old one if the film I bought has a ISO/400/27, what would the asa setting be on my camera, and is the some kind of chart to go by?

Thank you
Dave Erickson

Comment by Dave Erickson

April 5, 2012 @ 1:14 am


Help please
I have a yashica j7 that has asa settings and the film you buy no longer has it on the box,it carries aISO number. The on I bought is 40/27. what will I set my asa at and is there some kind of chart to aid us dummys.

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