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Understanding Digital ISO

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Digital ISO and film ISO are two different things. I don’t have any film that was shot with the ISO I’m going to be comparing, but I will try to find a few examples.

In film ISO or also called ASA measured the sensitivity of the film that was being used. So depending on what film you chose to use you where limited to that ISO and couldn’t change it unless you changed the film physically. The lower the ISO value the lower the sensitivity of the film and thus the finer the grains of that film would have been. Higher ISO means the opposite to that.

In digital photography ISO measure the sensitivity of the camera sensor. As in film the lower the ISO the less sensitive the sensor is going to be to light and vice versa. Most of the user’s out there will try to use the lowest ISO as possible to decrease the amount of noise in the digital image.

So how do I apply ISO?

I rarely use higher ISO then 100 because I’m often shooting outdoors and there is always enough sun around so i can shoot at a shutter speed of 1/6 the least. But when I move indoors or I’m doing flash photography I will crank up the ISO so I can decrease the power of the flash (thus giving me flexibility). I use it indoors when there is poor lightning, and I know my camera will choose a shutter speed that I cannot hold a steady shot.

ISO is directly correlated to the shutter speed of your camera and so is aperture. As I stated in my previous article bigger apertures or smaller f-number will give you a faster shutter speed so will higher ISO.

So in theory:
Higher ISO = Faster Shutter Speed
Lower ISO = Slower Shutter Speed

But here are some combinations with apertures as well:

Lower ISO and Big f-number = Slower Shutter Speed
Lower ISO and Small f-number = Faster Shutter Speed then above

High ISO and Big f-number = Kind of evens out but leaning towards the Slower Shutter Speed direction< High ISO and Small f-number = Fast Shutter Speed What I mean above is that if you were to take the same image under the same light conditions those would be the results that you would have to use to maintain the same settings so to speak. Here is this in practice, yes the orange again Camera: Nikon D40x Lens: 50mm f/1.8 Shutter Speed: Varied ISO: 100 and 800 Mode: Aperture Priority Mode Image Quality: JPEG, fine size small

ISO: 100 Shutter Speed: 1/5sec Aperture: f/2
ISO: 100 Shutter Speed: 20sec Aperture: f/22

ISO: 800 Shutter Speed: 3sec Aperture: f/2

ISO: 800 Shutter Speed: 1/40 sec Aperture: f/22
I tried to maintain the light conditions constant but I cannot guarantee that its was 100% constant. But it gives you and idea of what I was talking about above. Note that aperture will affect the “look” of your image as you can see here and as I explain in the previous post.If you want to maximize the quality of your image you want to choose the lowest ISO possible or the optimal ISO for you camera.A member of the Camera Labs forum whom know alot about the technical things about camera and he shared this means, which is really what takes place on the DSLR sensor very in depth.

“Although, strictly speaking, the sensitivity of the sensor, as measured by its quantum efficiency, remains constant. What changes when you alter the ISO is the selected gain of the readout amplifiers or, in other words, the way the photoelectrically released electron count is mapped onto the 12 bit (or 14 bit) output from the analogue to digital converters.”

Reprinted from Bläcker Photography

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