ISO & ASA

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#1
Can anyone help me to understand the difference between ISO and ASA. The "Auto" ISO setting on my Olympus 8080WZ is ISO 50, so what does that equate to in ASA.

I remeber when using my Minalta x700 many years ago that I always purchased either 200 ASA or 400 ASA films and never really bothered about the ISO number, now I wish I had.

Am I right in thinking the higher the ISO, the less light you need and the better quality of picture you will get.
 

Steep

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#2
They are actually the same, so ISO 50 = ASA 50

ISO = International Standards Organisation ASA = American Standards Association
 
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ronb
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#3
Steep, thanks for your reply but does that mean then that I should set my ISO to say 400 in brither light and 100 say in poor light??
 

CT

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#4
As Steepsays ASA (Now defunct) and ISO are the same - the numerical values denote the same film speed, or with digital - sensor sensitivity.

There wasn't actually an ISO number in your Minolta days, you're probably thinking of the DIN rating (Deusche Industry Norm) which was a German system running alongside ASA for years. When you bought a film it would have both ASA and DIN ratings on the carton. 400ASA = 27DIN - 200ASA = 24 DIN. An increase of 3 under the DIN rating doubled the film speed. DIN was a crap system and after a big dust up between the Yanks and the Germans, both systems were swept away in favour of the single ISO system we now use which is really still ASA by another name. :wink:

You are right in saying that the higher the ISO the less light you need, but you're wrong in thinking you get a better picture, in fact the reverse is true.. This applies equally to film or digital.

A film is basically just a gelatine strip covered in silver halide crystals which are light sensitive. The only difference between a slow film and a fast one, is that a slow film has very fine silver halide grains, so you need good light to use it, but you get very good enlargements due to the fine grain. With fast film the bigger grains of silver halide catch more light - it's really that simple. The downside is that as you begin to enlarge your iimage that large grain structure becomes visible much more quickly.

With digital we don't have grain, we have pixels, but you'd have to enlarge an image much bigger than 1:1 to see them. The problem we have with digital is image noise. Image noise is that pitting you see in your images, particularly in the shadow areas and in shots taken in low light. The practical problems are just the same as with film and grain. The higher the ISO number the lower the light you'll be able to work in but the penalty is noise in your images. As with film, we get the highest quality images with digital by using the lowest ISO number we can in any given circumstances. Circumstances often force us to use a higher ISO than we'd like. One of the many advantages of shooting in RAW format is that RAW processing software is usually pretty good at filtering image noise out to get the best noise free images you can if you're forced to use high ISO.

It's important to understand that each time you double the ISO number, it's only the equivalent of being able go one stop quicker on your camera. In other words if you were shooting at 100 ISO with your lens wde open at f4 and a shutter speed of 1/30 sec, changing to 200 ISO would only enable you to set a shutter speed of 1/60sec, but that would be a far safer bet for hand holding even with a standard (50mm ) lens. The differences can be that fine sometimes. :D
 
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#5
ronb said:
Steep, thanks for your reply but does that mean then that I should set my ISO to say 400 in brither light and 100 say in poor light??
no m8, its the other way round,
the higher the asa/iso the more sensetive the film/sensor reacts to light,
therefore shortening exposure times in lower light conditions,
asa 100 is for bright sunny days,
and say asa400 could be used in lowlight conditions in the evening ,

however, the higher the asa/iso the grainier the image becomes, this can be seen in increased noise on a digital image

hope this helps, but these are the very basics of the theory

MP


lol, just saw CT's post, much better than mine, :LOL:
 

CT

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#6
Or just wordier than yours! :LOL:

The noise problem has really increased with digital due to the battle between manufacturers to cram more and more pixels onto the sensor. As the pixel count has increased image quality has improved, but in low light, image noise has increased. Manufacturers are now getting to grips with the noise issue, and it's not the problem it used to be in the latest cameras. Canon now use a noise filter sited down on the sensor itself. The first camera which really impressed me for low image noise was the 300D, and things have only got better since.
 
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ronb
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#7
Phew! Thanks guys for all that information, wonderful, now I understand it all much more. My pal who has the same camera as me has had the same query so i will print this out and show it to him.

Once agan, a big thank you to all who replied.
 
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#8
ronb said:
Phew! Thanks guys for all that information, wonderful, now I understand it all much more. My pal who has the same camera as me has had the same query so i will print this out and show it to him.

Once agan, a big thank you to all who replied.
better still, send him a link and tell him to join us :D

glad u found the info usefull ,

and i think i can speak for all those who replied by saying it was our pleasure

MP 8)
 
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ronb
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#9
OK, have sent him the link and asked him to join us.

Cheers for now.
 
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