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08-05-2020, 07:55 AM   #1
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ASA vs. ISO

when asked what do I know, my standard response is:

not as much as I might claim, but more than some people give me credit for

now I claim no expertise in photography I am just some one who posts a lot


here is my question:

back in the film days I was told it was best to use a low ASA film if I wanted to have enlarged prints

due to " grain " issues

is the same true with low ISO numbers or has the digital era changed all that


Last edited by aslyfox; 08-05-2020 at 09:38 AM.
08-05-2020, 08:07 AM   #2
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Lower iso generally has more dynamic range and yields let noise, overall the best results are available at low iso - but depending on the subject, higher iso may still provide excellent results up to a point.
08-05-2020, 08:38 AM   #3
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UncleVaya's reply said it best. The sensor in a camera captures a given range of levels when given all the light it needs. When you increase ISO, it means that you're starving the sensor for light to some degree (the higher the ISO, the more you starve it). The sensor output would otherwise look darker and darker (with fewer levels), but to compensate, more electronic gain is applied which stretches the sensor output (along with any noise the sensor produces) so the image approximates what would be seen with a full amount of light being applied (but now with a smaller number of levels). That leads to what UncleVanya described. Higher ISO allows smaller apertures and/or faster shutter speeds, but lower ISO provides for a better "electronic version" of the image and better IQ in that respect.
08-05-2020, 08:40 AM   #4
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https://www.techwalla.com/articles/what-is-the-difference-between-asa-iso

08-05-2020, 08:45 AM - 3 Likes   #5
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In general yes a lower ISO in digital is better than a higher one. Wtih film it was becasue faster (higher ASA/ISO) films had larger grains to make the more sensitive to limited light. In the digital world it is the amount of analogue signal amplification applied. A higher ISO shot has more signal amplification applied before it goes through the A-to-D converter so some additional noise or better term error is added in there. However what most people consider noise form high ISO is actually what would be better termed shot noise since photons are arriving randomly. To compound matters by boosting the digital ISO the dynamic range is reduced. A good explanation I have heard on how to think about it is to picture a bunch of buckets on nice level ground with an opening of 12 inches these are your pixels on your sensor. When choosing the low ISO setting these buckets are as deep as a 5 gallon bucket, when you choose an intermediate ISO they are a 1.5 gallon ice cream pails, and when you choose a high ISO they are dinner plates. Now to properly expose the image you need to fill the containers on average 1/2 full by throwing handfuls of marbles up in the air.

In the case of the dinner plates after one or 2 handfuls you will likely have "filled" the containers which were dinner plates. However a bunch of the pates have no marbles but bunch are so full they have marbles spilling off of them. This is your shot noise and loss of dynamic range. The marbles spilling off the over full plates is the highlights being clipped.

Now with the ice cream pails you may have had to throw 100 handfuls of marbles however now every pail has some marbles in it and only a couple of pails are completely full. However most of the pails are pretty close to 1/2 full.

Going over to the 5 gallon buckets you find that you had to throw 400 handfuls of marbles but none of the buckets are overflowing and that most of the buckets are really close to half full.

As we went with deeper containers the randomness got averaged out but required more time spend throwing marbles. The same thing happens with photons so the lower the ISO the more accurate the sample is.

I hope this helps clear some things up for you.
08-05-2020, 08:48 AM   #6
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I use to shoot Kodachrome 25 and 64 asa slide before Fuji produced the super speedy 100 asa stuff. If you sent your Kodachromes off in Germany they came back in cardboard mounts, in the UK it was plastic. I thought the cardboard was cooler. I don't remember buying above a 400 asa film but I do remember pushing them in processing to 1600 or so on occasion. Reciprocity failure was a thing, as was underexposing and overprocessing for sunny conditions, giving slide film an extra third of a stop, Cibachromes, the list goes on including devving fp4 in print developer for grain. Digital was a completely alien world when it came along, I know a few of my peers never made the leap. Digital changed a lot but the principles of asa/iso remain largely the same.
08-05-2020, 09:14 AM   #7
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Yes, as already noted. High ISO is not 'free'. You pay with lower dynamic range resulting in less tonal scale in the image. If you want larger prints with less grain with film, using a larger format film is the way to go.
08-05-2020, 09:25 AM - 1 Like   #8
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QuoteOriginally posted by MossyRocks Quote
(Snipped for quote)

As we went with deeper containers the randomness got averaged out but required more time spend throwing marbles. The same thing happens with photons so the lower the ISO the more accurate the sample is.

I hope this helps clear some things up for you.
Mossyrocks, you rock!
I somewhat understood the concept, but this sealed the bargain. It also gave me a point of reference for using filters, both UV and color.
Thanks for this.

Kirk B.

08-05-2020, 09:30 AM   #9
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For me digital changed things. On BW film, mainly Tri-x, the grain, even when pushed, was not objectionable and many like the aesthetics. Now that I probably can't even get film on to a roll anymore and rarely convert my digital images to BW, I don't think noise is ever wanted so the lower the ISO the better.
08-05-2020, 10:27 AM   #10
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Some cameras offer low ISO setting that are below the native performance of the sensor. (Digital Leica M models for example) This allows shooting fast lenses wide open in brighter light. In this case the image suffers at the pull setting by reduced dynamic range. Best IQ is at the true native ISO of the sensor.
However, I’m amazed at how well some of today’s cameras can do at higher ISO settings.I shoot mainly indoors, and now shoot my M10 often at 3200 with very nice results - I think better than my M9 at 640.
08-05-2020, 10:27 AM   #11
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QuoteOriginally posted by crazy4oldcars Quote
I somewhat understood the concept, but this sealed the bargain. It also gave me a point of reference for using filters, both UV and color.
Thanks for this.
Glad it helped. I wish I knew where I read it originally to give credit as it wasn't my original idea but I did switch up the example using things people would likely be familiar with for added clarity on the subject. I've found this to be a good explination when I teach the photography merit badge to help the scouts understand how things work as I find it to be a good analogy.
08-05-2020, 10:43 AM - 1 Like   #12
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QuoteOriginally posted by aslyfox Quote
back in the film days I was told it was best to use a low ASA film if I wanted to have enlarged prints
due to " grain " issues
is the same true with low ISO numbers or has the digital era changed all that
In 1974 the American Standards Association (ASA) and the German (DIN) numbers were both changed to the International Standards Organization's ISO number. Thus ASA#=ISO#.

Lower ASA gave you less apparent grain (more grain, but smaller, thus less visible), and higher resolution. Higher ASA did the opposite.

With different ASA film emulsion, they were engineered for optimal exposures at different amounts of light. If you wanted to shoot that a certain film at higher or lower than the recommended ASA, you had to under or over develop the film (pull or push).

With digital ISO, the sensors have a native ISO (generally ISO 80 or ISO 100). If you wish to "push" the sensor to a higher ISO, the camera will boost the sensor electronically for greater sensitivity, but similar to film, the trade-off is increased noise and more of a grungy like grain appearance.

So to answer your question? Yes, the same is true with low ISO numbers, but sort of for different reasons.
08-05-2020, 11:36 AM   #13
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QuoteOriginally posted by TomB_tx Quote
I’m amazed at how well some of today’s cameras can do at higher ISO settings.
My K10D does best at ISO 400 or below. My K-5IIs does best at ISO 1600 or below. I'm still experimenting with my KP, but ISO 6400 images are quite good, and I've seen others shoot at ISO 10,000 with good results. We are all waiting to see what the ISO specs are for the K-new. No doubt another step forward for good IQ at high ISO.
08-05-2020, 01:26 PM   #14
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with film a lower ASA/ISO = smaller grain (sort of lower noise)** and lower DR (higher contrast)

with digital a lower ASA/ISO = same size "grain" [pixel] and higher DR and lower noise

_____
** except higher acutance and thus higher apparent sharpness with larger grains [clumps] at higher ASA/ISO.
08-05-2020, 01:40 PM   #15
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it's all about the noise-to-signal ratio, really...
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