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FORVM`s Classical Numismatics Discussion Board  |  Numismatic and History Discussions  |  Coin Photography, Conservation and Storage  |  Topic: Choice of ISO equivalent setting 0 Members and 1 Guest are viewing this topic. « previous next »
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moonmoth
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« on: August 22, 2006, 11:40:20 am »

Another comparison set.

ISO setting on a digital camera mimics the behaviour of different films which required more or less light to create an image.  The downside was that the faster a film was, the more grainy was the resulting image.  Digital cameras have similar characteristics for similar reasons, except of course that you can change ISO setting from one frame to the next.  The higher the ISO rating, the shorter the exposure. Less light is used to create the image, and you are more likely to see a grainy result. 

With my little Ixus 400, a setting of ISO 400 gives a noticeably grainy result.  However, look at these comparison strips from my EOS D60.  These are at the natural magnification of the sensor, and I can see very little difference from one end of the scale to the other. 

There is perhaps a little more coarseness in the colour tones of the coin in the ISO 1000 strip.

Normally I would unhesitatingly recommend using the best quality option, ISO 100.  But having made this test, I think that with a decent camera you could use ISO 200 or even ISO 400 with no problems at all.  That might be useful if your setup makes it hard to avoid camera shake on longer exposures - for example, if you live next to a busy road.  But please do this test with your own camera first.
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slokind
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« Reply #1 on: August 22, 2006, 12:15:29 pm »

It's a bit like those ads on my regualar TV set for HD digital transmission.  Or the old ads on radio for how RCA shellac 78rpm recordings sounding just like your live broadcast (10 secs. purportedly of each follow).  Of course they did!  Consider the microphones of the 1940s and the AM transmission with its cut-offs of high and low frequencies, not to mention the speakers in home radios.
At 72 dpi on a monitor, or 82 on some, when with virtual ISO we're talking pixels, what in the world CAN we see??
Pat L.
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moonmoth
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« Reply #2 on: August 22, 2006, 01:20:59 pm »

Well, that's a good theoretical point, but I have a practical answer to it.  Here are two shots I have just taken with my IXUS 400, one at ISO 50, one at ISO 400.  I cut them down to about the same size and desaturated the yellow to compensate for the light source, more or less.  No other adjustments.

These aren't model shots - they are hand-held, for one thing.  But they show clearly enough what can happen with the wrong ISO choice.
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bruce61813
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« Reply #3 on: August 22, 2006, 03:06:19 pm »

   There are two factors working here. The first, as Pat mentioned, is that monitors are limited as to what they can show. The DPI is set, and generally 73 for the most part, but that only limits the finest detail that can be shown, and if you compare the pictures, tha same detail is there. The other part is the light amplification. Going from "ISO" 50 to 400 is a 8 x step in amplification, however amplifiers can't be selective as to what they are amplifing, and there is always background noise, this is increased also, and becomeds more noticeable with the higher ISO settings. It is the electronic equivalent of "grain" in traditional photography.

  If you want to improve the overall quality of the picture, us good strong lighting, the weake the light sources the less information is transmitted to the camera sensors and and detail is lost. In engineering terms, it is:  signal quality = log [ signal + noise] /noise, when the quality number is 10 or highter, it is all signal, and the noise does not produce a significant factor, but below that, noise is an important factor, and color, saturation, and detail all suffer from poor light.  Changing the ISO can have some effect, but changing the lights or increasing the brightness has a bigger impact. There are trade of issues, too much light or too harsh, will burn out details, too little light, the details are not registered, as it is all shadow. Take some time and read Ansel Adams book on the Zone System, it is harder to work with in the digital form than in traditional photography.

  The intensity of the light is an a inverse square to the distance from the object, halving the distance, doubles the intensity and is equal to 1 "F" stop or one shutter speed change, but it can make a difference. Also being able to place secondary reflectors to change or modify shadows can help.

Bruce
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moonmoth
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« Reply #4 on: August 23, 2006, 03:26:25 am »

Bruce -

In your comments on light intensity, in this and the aperture thread, are you suggesting that the same amount of light (number of photons) will produce different results depending on whether it arrives quickly or over a longer period?  (We're talking in the region of a second here, not what I'd usually consider to be long exposures.) I'll certainly try some experiments and get back later today.

The amplification of background noise is, as you say the reason why using high ISO equivalent settings can affect the end result.  As I said, less actual light is used, and graininess results.  What we see above in the ISO 400 shot from the IXUS 400 is inaccurate colour rendition in a noisy pattern, presumably because insufficient information is reaching the sensor at high ISO equivalents and the errors are being magnified.  This noise is very visible on my screens, and I'm seeing it from two different PCs, so I don't think the screen resolution is important in this context.  That background seems to be made up of blue and red patches, whereas actually it is neutral grey, and I can see the same patchiness on the coin itself.

Equally important to note is that this effect differs with different cameras.  I used the same light source and distance with both.  (I also use a secondary reflector.)
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« Reply #5 on: August 23, 2006, 08:46:04 am »

Here's a better comparison set using the IXUS 400.  I took these with a tripod and the timer, using natural light. 

The first is a straight comparison between ISO 50 and ISO 400 in colour.  The second is the same image, but this time showing only the blue channel.

ISO 50: 1/320 sec at f2.8.
ISO 400: 1/400 sec at f7.1

Graininess occurs in each of the colour channels at ISO 400, but the grains do not coincide, so the end result is poor colour rendition.  The "grains" and the colour effects are clearly visible on my monitor.

With a better camera, and a better sensor, I expect you wouldn't get anything like this graininess except perhaps at the extreme end of its ISO equivalent range.  But this demonstrates the nature of the effect quite well.
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« Reply #6 on: August 23, 2006, 10:24:42 am »

Here's a blowup that shows what is happening at the pixel level.  This is the letter T from "AVGVSTA."
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bruce61813
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« Reply #7 on: August 30, 2006, 05:04:07 pm »

Sorry about the apparent confusion, by the way your work is super. It does not matter how "fast" light arrives, but the correct amount.  To much light, causes an area to "burn out" or be over exposed, this is normal for the extreme highlights, the opposite is no exposure and total black.  With any electronically amplified circuit there is a basic level of noise, we hear it as hiss over a stereo, you cannot get rid of it, but you can make it such a tiny percentage of the final signal that you can't hear it. Keep that in mind. If this background noise level was 1 volt, and you added a 1 volt pure tone, you would "hear" mostly the hiss of the noise. As the amplitude of the tone is increased, the presence of the tone is more noteable.

   Here comes the bit with digial photography. The sensor cell requies a certain number [amount] of photons to produce the optimum output. Let use 5, so if we give the unit 5 photons , we get a gray like the back ground of this page. 10 photons would be pure white, 1 photon -pure black.
ISO comes into this because it is amplification, but it amplifies all signals, noise and tone, eaqually. Assune ISO 100 is no amplification, and 5 photons produces a middle gray. ISO would make 4 photons appear the same gray, but the noise would be equally amplified, and  you would start seeing things that aren't really there, they are called artifacts. You last set of picturs should this in the appearence of the red and other colors. ISO 400 is amplifing a little light 8 times. The problem is that the camera change the shutter speed to adjust the total number of photons to be equal. IT is not speed, but amount of light that the system is adjusting.  A lot of your light gets through, but the noise is there and the two are mixed and amplified. A slower shutter speed allows the main light source to become 90 % or more of the total signal that make up the final photograph.

  Sorry, this is so clumsy, there is a lot of theorey that is being skipped. The simplist thing is to see a tradional color print make from and under exposed color negative, you see all the odd color spotiness, because the grain or color molecules are the background noise and have not been over come by the needed amount of light.

Bruce
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