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In Layman's Terms ...

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Postby Grover1 » Tue Oct 03, 2006 4:56 pm


Please, can someone explain to me ISO ...

Everyime I have tried to play with it, mostly around dusk, I get the 99% white snowy frame with very little image ... I understand you use it in low light situations ... is there a formula?... or is it a feel? ... is it just talent gained from practice and knowledge?

And usually, when the geek stats get posted, the ISO , "bumped up", is usually combined with a time exposure ...

Is there an exercise I can experiment with? Seems there is more to it than trial and error ...

HELP!

(Thanks in advance!)
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Postby Hersh » Tue Oct 03, 2006 5:35 pm


Not an art at all, ISO is pretty much purely science.

ISO is the measurement of film speed, or sensitivity. The higher the ISO number, the more sensitive the film. I use ISO 50 velvia and ISO 100 provia slide films most of the time, which are generally considered pretty slow. At work I use ISO 400 print film, which is about the standard for print films. In digital, they use ISO as a sensitivity measurement because it's familliar to photographers, and it works the same way as with film.

The ISO is a set measurement, so a bump of two 'stops' of ISO would require a compensatory two stops of adjustment to your exposure.

Let's say I'm shooting a scene outside under stable light conditions. I set the ISO to 200 and the meter tells me the proper exposure is 1 sec at f3.5. If I turn the ISO up to 1600, under the exact same lighting, the exposure would then be 1/250 at f3.5. Same lighting, same final photograph, but a much faster ISO speed to get me there.

Is that as clear as mud? :D


Here's an article by Ken Rockwell that explains exposure, including ISO. Ken's articles have been referenced on here bfore and he knows what he's talking about.

http://www.kenrockwell.com/tech/shutteraperture.htm
Last edited by Hersh on Tue Oct 03, 2006 5:42 pm, edited 1 time in total.
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Postby AL » Tue Oct 03, 2006 5:41 pm


With my digital camera I'll bump the ISO up to 1600 or 3200 for shots where flash is not permitted, like the grandkids events. The high ISO will permit enough light to capture a image indoors, the quality falls (noise) off sharply with the higher ISO.
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Postby Larry » Tue Oct 03, 2006 6:27 pm


Hersh wrote: same final photograph


except for the noise

But, yeah, everything Hersh said.
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Postby Hersh » Tue Oct 03, 2006 6:56 pm


Actually Larry, with digital noise doesn't increase with high speeds as much as with film. We just discussed this a bit in another thread, I'll have to find it... With digital noise is caused much more by underexposure of certain areas within the frame than faster ISOs.
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Postby Gary Martin » Tue Oct 03, 2006 6:59 pm


Ken Rockwell's comment ...

"Use the lowest ISO that gives you the apertures and shutter speeds you need. Pump up the ISO up to get smaller apertures and faster shutter speeds. Unlike film, digital interchangeable-lens SLRs usually look great even at ISO 1,600."


I have to disagree with Rockwell... shooting at an ISO of 1600 will let you get an image but it will hardly look "great even at ISO 1,600" especially if you print it :!: Looking at the image on a computer screen will let your eye cover a multitude of sins, but they tend to stick out like a sore thumb when printed.
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Postby Gary Martin » Tue Oct 03, 2006 7:13 pm


BTW, Hersh, check your math on your 200 to 1600 ISO exposure argument. That's a three stop increase in film speed, which would only drop your exposure from 1 sec to 1/8 sec at a constant aperture.
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Postby AL » Tue Oct 03, 2006 8:20 pm


Hersh, noise is more pronounced with a digital camera then with a DSLR. I know that Barry did not specify which type camera but in light of the camera he spoke of possibly purchasing (Sony) I assumed it meant a non DSLR.

I agree with Gary. I have taken many photos using high ISO's and printing them produces grainy (noise) photos.
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Postby Hersh » Wed Oct 04, 2006 3:26 am


You're right on the computational error Gary, I did that one quick & dirty in my head. As for grain, I shoot a lot at 1600 ISO, especially snapshots of the boys, and I've noticed that if I can control my exposures, I can control my grain to a high degree. It's still there, but I can manage it. One big factor is that if there's a lot going on in the photo grain tends to be hidden, while in a large blank space you will see it much easier.

And Al, I didn't even think about the noise difference between a P&S digicam and a full blown dslr, thanks for pointing that out.
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Postby Jen » Fri Oct 06, 2006 7:47 pm


Mike, you hit the nail right on the head in your last statement. You won't see as much noise at the higher ISO with the proper exposure. And Neat Image or Noise Ninja can help with some of that. The Canon 20D is SO much better at the higher ISO's than the 10D due to the improvement in the sensor.

The reason the noise is so much higher in the p&s digicams over a D-SLR is due to the smaller sensor in the digicams.
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Postby Grover1 » Fri Oct 06, 2006 8:10 pm


(My edit ... Just too convoluted a question ... I didnt even understand it after I wrote and posted it ... I'll try it again later)
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