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I so need to know digital ISO

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:lol: I understand the basic principle of digital ISO. The higher the ISO setting the more energy applied to the sensor essentially increasing its sensitivity to light...as well as noise. What I've read about it has told me to avoid high ISO's (above 200) at all cost unless you don't really have another choice. However, every time a new digicam comes on the market which possesses some higher ISO capability it is raved about by a few of the veterans here. This leads me to the obvious question: When do you use an ISO higher than 200 and why? :blush: Thanks!

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Hi Gary

 

Well to put a few answers to your questions.

 

to obtain the finest image quality was used to use ISO 50 (velvia) or like whilst shooting macro, whilst shooting wide we used to use ISo 100 (provia) or like.

 

if you striving for image quality then use the lowest ISO speed you can get away with.

 

By get away with, you must understand how exposure are made and what effect ISO plays on the exposure.

 

I'm sure you have heard of Stops of light.

 

well, it would be simple to say the ISO 200 is one stop faster than ISO 100, ISO 400 is 2 stops faster than ISO 100. by faster i mean that it actually requires less l(its more sensitive to light) light to make what used to be the chemical change in order to make an exposure on our film, dont get bogged with digital at this piont, you need to learn the basics of photography here firstly

 

for example.

 

if i loaded up the camera with ISO 100 film and took a meter reading of a reefscene and i had returned an exposure value of F8 1/60. if i Used ISO 200 instead of ISO 100 i gain 1 stop of light back to my image.

 

so in theory i could now choose to do the following to make the same exposure

 

1. Use F11 instead of F8, because this stops the light down by 1 stop, if ISO200 requires 1 stop of light less to make the expsoure then you must see that if i reduce the aperture opening by 1 Stop i will achieve the same exposure as ISO100 F8 1/60.

 

2. Use 1/125 shutter speed, Again , you can see that i'm not allowing the light to touch the film for as long as i would if i used 1/60 ( 1 stop) so my new exposure could be F8 1/125.

 

when we shoot images, we have a couple of choice's that need to be made before we actually press the button, and these choices are made based our available light at the time. the numbers you see around are not just plucked out of the air, the photographer usually selects his or her exposure setting based on the following.

 

1. Depth of field, The distance infront and behind the subject that falls into focus from our selected focus point. This actually changes with Aperture setting, the smaller the aperture opening the larger the DOF.

 

2. Subject Suitablity , I.e its no use trying to capture fast moving animals with 1/8 second Shutter Speed. So taking a look at your subject and its movement more often settles the shutter speed your going to use.

 

3. if exposing with Manual flash alone and not taking into account any ambient light exposure we must adjust our strobe power output in order to make a suitable exposure and again APERTURE here is the determind factor, the simple equasion of

 

F-stop = GN of strobe / Strobe to subject distance.

 

I could go on and on Gary, but i'm not going to teach you a photography course within these small pages.

 

but i hope you can see that one answer to your question would be that, if i dont get the exposure settings i wish to use on my image because the available light is not sufficient then as a trade off to image quality i can Up my ISO settings to give me back a few stops of light in order to try and achieve the settings that i require to capture my subject.

 

Photography actually looks easy, and with todays digital cameras it is quite easy, however like most things in this life, if we have a total understanding of what we are doing and know how our tools work then we can become at one with the tool, it is only then will you become a master of that tool.

 

a wise man once said. :blush:

 

He who does not know and does not know he does not know is a fool, so shun him

He who does not know, but knows he does not know requires teaching, so teach him.

He who knows and does not know that he knows is asleep, so wake him.

He Who Knows and knows he knows is wise, so follow him.

 

Best regards

 

Craig

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The higher the ISO setting the more energy applied to the sensor essentially increasing its sensitivity to light...as well as noise. What I've read about it has told me to avoid high ISO's (above 200) at all cost unless you don't really have another choice. However, every time a new digicam comes on the market which possesses some higher ISO capability it is raved about by a few of the veterans here.

 

 

Great post from Craig, but I'd like to clarify a few things.

 

Setting a higher ISO setting does not:

- change the energy (light) applied to the sensor: you need to open aperture or slow down shutter speed for that

- increase its sensitivity: that was the case with film, but not with sensors (you don't physically change sensors when you change the ISO setting)

 

What setting a higher ISO does do is boost the information that was received by the sensor. The key issue with this is that as you boost the information, you also boost any parasite sugnals that may exist (e.g. due to imperfections in welds or electronic components, etc.).

 

When the signal is strong (e.g. bright conditions), and you use a low ISO setting (50 or 100), the signal-to-noise ratio is very high, and the quality of the picture is at its highest.

 

When the signal is very low (e.g. at dusk or at night), and you use a high ISO to compensate for this, the signal-to-noise ratio is small, and the quality of the picture is at its poor.

 

The parasite signals, once boosted by using a high ISO setting, appear in the form of bright coloured dots in the picture. This is called "digital noise".

 

When new cameras on the market are said to perfom better at high ISO, it means that camera manufacturer have done a good job at improving the signal-to-noise ratio (e.g. less welds, better components, gold wires, etc.).

 

As explained by Craig, the good thing about this is that having the option fo using a high ISO setting with minimal loss of quality (little noise) means you have more lattitude when taking a shot, especially in extreme conditions.

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"boosting" means that there is a digital amplifyer which amplifyes the signal from the sensor..my understanding

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Last titbit to add to these excellent explanations; when new cameras are announced and they 'raved' about the higher ISO settings, it is because newer cameras are getting better at doing higher ISO's with less noise, some cameras are better than others for any given ISO etc. However for any given camera, the same rule applies, as you raise ISO, noise increases.

 

Think about noise like the old fashioned 'grain' in film. Increasing ISO with film generally meant increasing grain, increasing ISO with digital generally means increasing noise.

 

My understanding is that the digital amplifier increases the sensitivity or gain of the sensor.

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"My understanding is that the digital amplifier increases the sensitivity or gain of the sensor." This is a better way to say what I was actually thinking.

 

Matt - "change the energy (light) applied to the sensor"

I didin't articulate that correctly. I was actualy referring to the circuit energy, not light and you are correct, you can't change the energy applied to the sensor without adjusting the aperture. However, the output of the sensor is variable and the signal can be boosted/amplified at the cost of image quality.

 

Thanks everyone for the responses. I think I have a better understanding of when to crank up the ISO but it might take some experimentation for it to fully sink in. I've always cranked up the strobe power and tried to keep my ISO at 200 or less.

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But:

Does anyone ever shoot UW with higher iso than say 400? :blush: I have never done it myself...

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I shot a whole dive at 1600 by accident once - does that count? :-) Yes, I'm man enough to admit my mistakes...lol

 

I feel perfectly comfortable shooting ISO400 with the Canon 5D underwater. It is very very hard to see a difference between that and ISO100. Above water, I've sold many photos shot at ISO1600. Probably most of the sports and event photos being sold today were shot at ISO1600.

 

Cheers

James

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But:

Does anyone ever shoot UW with higher iso than say 400? :blush: I have never done it myself...

 

ISO 1600. I wanted the ambient light background and it was dark. This was shot during a one day beach dive comp. Shot in JPEG with no post processing. See EXIF in photo.

 

IMG_0548.JPG

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This is a nice photo, but it doesn't quite illustrate the issue. Downsampling an image for the web has the effect of averaging the noise away. This makes high ISO images look quite clean. If your goal is shooting for the web this is fine. If your goal is to make large format art prints, it isn't. It just depends on your application. Remember, noise isn't always a bad thing. Sometimes it makes things look more "realistic" since people are used to looking at film images, which suffers from the same issue in terms of crystal grain size.

 

Down sampling by a factor of 2 in both directions means that each downsampled pixel is an average of 4 pixels. The noise goes down as the square root of the number of pixels averaged, so the the overall noise is reduced proportional the reduction in the linear dimension, in this case by a factor of 2.

 

One reason I love my 5D is that the pixels are physically larger since the sensor is full frame. Therefore it collects more light per pixel and the resulting noise is correspondingly lower to start with. I too have shot at ISO 800 and higher underwater.

 

Another issue to consider is that noise has different visual inpacts depending on whether it is over a smooth blue background or over a complicated pattern. In the former case it is very visible, in the latter it is usually hidden by the pattern. Photoshop's smart blur actually does a good job of smoothing out the background while keeping the foreground sharp.

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