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Old Jul 26, 2011, 8:51 PM   #1
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Default ISO is variable - so use it!

ISO is a measure of the sensitivity to light of a particular film or sensor. Low numbers show low sensitivity and high numbers indicate higher sensitivity.

The easiest thing to remember about ISO is that doubling the number buys you one stop. For example this can be 1/30 second to 1/60 second shutter speed if you keep the aperture constant or f/5.6 to f/4 if you keep the shutter speed constant. The intermediate numbers are 1/3 of a stop changes which barely shows so go for broke and double or half the ISO when playing with it! You can also pump up the ISO to get more range out of your flash unit.

To better understand ISO as regards your camera's image quality you should run a test series to see what the usable limits are. I used a simple nighttime desk scene lit by a ceiling fixture. Make sure there are shadow areas as well as well lit areas with light walls. I used incandescent white balance to match the lighting. The camera was on a tripod and I used P (program) mode. The actual shutter speed and aperture aren't important as long as we have some slow shutter speeds around half a second for the low end ISO. Shoot RAW or JPEG as you normally would.

The ISO series from 100 to 6400 was shot then the images uploaded and examined at 100% (or Actual Pixels). It's important to check at 100% otherwise you won't see the noise. I checked the lit walls and the shadow area for the tell-tale colour noise patterns.

My camera was good up to 1600, 3200 showed visible noise and by 6400 it was blatant! I now knew that 1600 would yield great image quality, 3200 in a pinch (my grandson at gym class) and 6400 was only for a "gotta get it shot" (Elvis in the doughnut shop parking lot at midnight). Even so I still try and keep the ISO at 200 most of the time.

Please refer to http://en.wikipedia.org/wiki/ISO_(ph...exposure_index for a discussion on how it's determined.
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Old Feb 29, 2012, 1:41 PM   #2
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Bob,

Could you change ISO to create long exposures?
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Old Feb 29, 2012, 2:30 PM   #3
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When you change the ISO number in a digital camera, you are not really changing the sensitivity to light of the camera's sensor. What actually happens, is that the output is amplified to appear brighter (or dimmer, for a lower setting) than the exposure settings would give at the base sensitivity. The same thing was done with film, by 'pushing' it to a higher ISO. This consisted of underexposing by a given number of stops, then overdeveloping to make it brighter. Very common using B&W film, among those obsessed with low light shooting.
When you underexpose and amplify the result, you amplify the noise along with the signal. The more it is underexposed, the worse the signal to noise ratio, so the pictures tend to have more visible noise. With some of the newer cameras, the noise floor is so low now, that the ISO setting can be increased to 6400 and still yield reasonably good photos. My old Pentax *ist D begins to show noise at ISO 800, and ISO 1600 is terrible. Of course, I have never been that interested in taking pictures of black cats in a coal mine at midnight, so it doesn't bother me much.

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Old Feb 29, 2012, 3:03 PM   #4
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Brian,

LOL @ black cats in coal mines at midnight.

Thanks for the explanation that it doesn't really change the sensitivity of the sensor. I was thinking that I could turn down the ISO to a point where it would give me a longer exposure time - sort of like putting a ND filter on. My goal would be photos of water where the movement of the water becomes that cool blur effect. I guess I would be better off closing down the aperture or using a ND filter.

Thanks
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Old Feb 29, 2012, 3:17 PM   #5
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G'day ewen

You are quite correct in asking/saying " could I turn down the ISO to a point where it would give me a longer exposure time ..."

As Bob says in his intro passage " doubling the number buys you one stop ie- 1/30 second to 1/60 second shutter speed " which in-reverse says " halving the number loses you one stop " ... therefore doubling your shutter speed

So while some may tell you to use ISO400 for after dark [ie- so that you can hand-hold etc] by dropping from ISO400 > 200 > 100 gains you 2 stops / 2 shutter speeds, which could well be from 1sec > 2 > 4sec or maybe 8sec > 16s > 32sec for a great long exposure

Hope this helps too
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Old Feb 29, 2012, 8:53 PM   #6
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You can lower your ISO setting to give you longer exposure times, but only to a point. Most cameras use ISO 100 as their lowest setting, while some will go to 80, and a few to 64. I don't know offhand of any lower than that. Sensors have a base sensitivity, which seems to be usually around ISO 200. Lowering that is the reverse of increasing it, which means you would be overexposing a bit, and under under amplifying the result, which would leave you with very washed out highlights if you went very far in that direction.

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Old Mar 2, 2012, 10:10 AM   #7
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My camera's base ISO is 200 and it will go to 100 which is only one stop. To really get slow shutter speeds I have to use neutral density filters, pick the time of day or both.
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Old Mar 9, 2012, 6:04 AM   #8
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Speaking of ISO, next week I'm going at the Geneva Motor Show.

Lighting is usually very good from all the spotlights, but do you think I should use an ISO value bigger than 400 (I have an A35 + 17-50/2.8)?
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Old Mar 9, 2012, 10:28 AM   #9
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Run the test I suggested above and judge for yourself.
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Old Apr 19, 2012, 5:35 AM   #10
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Before experimenting with ISO and Aperture .
I read lots of theories about it.But never got clarity on it.
Now after experiment of couple of months its clear that what setting should be needed.
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