just trying to understand..

Discussion in 'Beyond the Basics' started by SimplyMo, Aug 5, 2008.

  1. SimplyMo

    SimplyMo TPF Noob!

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    cirlces of confusion--when they are over sized, these are what cause an image to appear not sharp/out of focus (because the light rays coming through the lens have spread too widely before reaching the film/sensor causing the cirlces to be larger and overlap), is that right?

    grain/noise in an image is cause by the over sized silver crystals in the film/sensor (that were purposely made larger to allow for the light-sensitive material to be more sensitive to light), is that right?

    but just because a film speed may be fast, and appear grainy and not as sharp as a slower film, this is different than if the cirlces of confusion were to cause an image to be out of focus, right?

    does any of this make sense??
    i know this is really probably not that important --it's just a sudden random thought i stumbled across...;)
     
  2. reg

    reg TPF Noob!

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    A sensor has no silver crystals, adjusting ISO is similar to turning up or down the gain on a speaker, and as the gain goes up it gets kinda distorted (noise in a photo/noise on a speaker).

    Someone else will have to come along re: Circle of confusion.
     
  3. SimplyMo

    SimplyMo TPF Noob!

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    ohh you're right--of course sensors have no silver.. i didn't even notice that i said that..i tried to include both--because i know they're both widely used, but i really only had my mind on film-- haha...:blushing: thanks--
     
  4. Alex_B

    Alex_B No longer a newbie, moving up!

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    A strange way of describing it, but I would tick it as basically correct ;)

    simplified, but correct for film. As stated already, as sensor works a bit diffferent. The pixel signals from the sensor are amplified. Higher ISO means stronger amplification (gain) of a weaker signal (which has a bad signal to noise ratio). Also the noise is amplified more, and hence more apparent.

    grainy and not sharp are to different things. an image can be very sharp, but still grainy. of course the grain swallows some detail. but as you correctly stated, this is a thing totally different from the circle of confusion caused by bad focussing or shallow depth of field.



    So it looks like now you got the correct feeling for things and what they mean :D
     
  5. Tiberius47

    Tiberius47 TPF Noob!

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    Think of circle of confusion like this...

    Imagine a point of light. The light comes from a single point , so we can imagine the light being like a cone; the point of the cone is at the point of light, and the flat disk at the bottom of the cone is the aperture of the lens.

    The lens reshapes the cone, and kinda turns it around so there is another cone of light inside the camera. As before, the disk at the bottom of the cone is at the aperture, but this time the point of the cone is on the sensor.

    if the image is in focus, then the point of the cone inside the camera will be on the sensor surface, but if the image is out of focus, then the point will either be behind the sensor or in front of it.

    If the point of the light cone is behind the sensor, then the sensor is effectively slicing the top off the cone. And when you slice the top off the cone, you get a circle. So instead of the point of light being a nice crisp point in the image, the point of light is displayed as the circle.

    If the point of the light cone is in front of the sensor, it still forms a circle, because once the point of the light cone is reached, the light starts spreading out again, like two cones meeting point to point.

    Incidently, this is why a larger aperture creates larger circles of confusion. A wider aperture means the base of the cone is wider. So a narrow aperture will create a narrow cone, like an ice cream cone. A wider aperture will create a fatter cone, like a pile of sand.

    Now, imagine if you have two cones. Both are the same height, but one is really thin and the other is wide. if you slice the tops of both of these cones off at the same height, the narrow cone will give a much smaller circle than the wider cone.

    Likewise, the circle of confusion when you have a wider aperture is larger than the circle of confusion with a narrower aperture (if all other things are equal), so if something is out of focus, it will create a larger circle of confusion with a wide aperture than with a narrow aperture.
     
  6. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    Grain and noise can appear to be visually similar in some photos, particularly in out of focus areas or areas with no sharp detail. Grain is the structure of a film photograph, and it influences the look of a film photo in several different ways (such as accutance) than noise affects a digital photo. A completely grainless film photo is blank as the grains are what make the image, tones, and colors. Remove all the noise from a digital photo and you have a noiseless digital photo, but the image is still intact. Noise tends to be described as similar to grain, but grain also has visual characteristics that make it like the pixels that make up a digital image.
     
  7. Garbz

    Garbz No longer a newbie, moving up!

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    Dodgy analogy what you are describing would convert directly to sensor saturation (clipping colours) since that's what happens when you crank the volume and the amp hit's it's clipping point.

    I would liken it to a game of Chinese whisperers where each person talks a bit louder than the last. You can get a quiet version from the first guy, or you can get a louder version from the 5th guy but with probably some of the message lost along the way. (I couldn't think of a simple hifi analogy)
     
  8. Moglex

    Moglex TPF Noob!

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    The hi-fi analogy worked much better in the past!

    With tape or vinyl disks there was always noticable background noise and you would indeed notice it more if you needed to turn the level up to compensate for a very low recorded signal level. As you point out the analogy becomes confused once clipping occurs.
     
  9. snowalker

    snowalker TPF Noob!

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    Yeah, that's right!
    But, how can you explain the big noise, on ISO200 films let's say, from Advantix (APS)? Even ISO100 APS films has some inexplicable noise. How come?
     
  10. Moglex

    Moglex TPF Noob!

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    Films don't really have noise, they have grain.

    Just because a film is slow does not mean it doesn't have grain. It's still there but is (generally) less visible for a given magnification than it is for a fast film.

    If a film is 'push' processed it will have larger grain than one that is processed normally.
     
  11. Helen B

    Helen B TPF Noob!

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    The graininess of a particular film usually depends on the density - ie the exposure. For colour negative films, which have an image made up of transparent clouds of dye, the graininess is usually much greater at low exposure than it is at high exposure. The toe is grainier than the mid-curve and shoulder - the shadows will generally look grainier than the highlights.

    Even slow colour films show this property, so an ISO 200 film that is exposed well will look a lot less grainy than one that is underexposed.

    The reverse is usually true for B&W silver-image films - the more exposure they get, the higher the graininess tends to be.

    The appearance of graininess in the final image depends a lot on the amount of detail present (clear skies often look grainy because the grain is the only detail present) and the local contrast (contrast enhances grain).

    There is also a property that is similar to noise, particularly noticeable in the shadows - the uneven development of grains with low exposure. There is a degree of randomness when the film is close to the lower limit of its sensitivity. This affects the presence of the latent image in the first place, the stability of the latent image and the development of the image.

    In technical terms it is better to refer to granularity and graininess rather than grain - because you can't usually see the grain itself, but you can see something that looks like grain.

    Best,
    Helen
     

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