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What is ISO.....

Discussion in 'AP Magazine Feedback & Suggestions' started by PC60, Jan 13, 2018.

  1. PC60

    PC60 Active Member

    Nice reminder of the tradition diagram. But I don't accept the professor Newman's agument that it is showing that 'ISO is defined in terms of exposure', rather that Exposure is influence by the three variables shutter speed, appature and ISO.

    So be it ISO or the older film term ASA, it is the light sensitivity of the recording medium, be that film or ccd sensor etc. that we are interested in.

    With film the chemical emulsions used altered the sensitivity and gave variations in texture, colour or shade (panatomic/panachromatic) rendition of reflected light in an image.

    ISO for me in digital photography is the adjustment of the camera's sensor circuits. The circuits have been tuned to mimic or match traditional film light sensitivity values, I think because that is what users were used to. What this does not do is match the differing film emulsion characteristics.

    I feel a comparison of the charateristics of film and digital to that of valves and transistors, one is organic and almost warming the other clinical brash and potentially quite harsh. Guitarists often all these years on since the invention of the transistor still often choose valve amplification.

    There is room for more film and clearly as the star letter of the week in this issue showed, people are coming back to it.
  2. PeteRob

    PeteRob Well-Known Member

    Whether by ISO (international Standards Organisation) or ASA (American Standards Ass0ciation)a standard is a standard and should be written to allow no room for interpretation. That's the point of a standard.

    What he didn't do was explain is the corresponding ISO standard applied to film. I expect that requires a certain density of film to ensue from exposure and a given development

    Basically it says that the definition is Response x Exposure = Constant, where Response is output from the sensor controlled by the ISO setting for digital and, as above, I'd expect density (or density above base) for film.

    I would expect the standard(s) to be defined for a prescribed spectrum of light but I'm not going to buy it/them to find out.

    Colour response is something else. From a digital system it is largely a matter of processing although the spectral properties of the filter(s) will have some effect on the raw output and arguably the mask design has an effect, plus there is the Fovean approach. For films there are clear differences in colour response between manufacturers for the same ISO, and different ISO films also had different colour responses. You chose the film you wanted to suit the subject matter.

    It is a personal choice whether you prefer a print made the analogue way or by the digital route but that has nothing to do with the ISO definition of sensitivity (fixed response to a given exposure under reference conditions)
  3. daft_biker

    daft_biker Action Man!

    In digital photography you can change the ISO after the shot has taken as on modern cameras it is just a parameter to tell software how bright to make the image. The sensor stays at it's native ISO.

    It is more like pushing individual frames of film than changing film.
    Terrywoodenpic likes this.
  4. Andrew Flannigan

    Andrew Flannigan Well-Known Member

    It's generally better to aquaint yourself with the subject before blethering. ISO in a digital camera is short for "ISO 12232:2006 standard" and is simply a statement of how a manufacturer may define the sensitivity of the camera's electronics to a quantity of light. This page may be of assistance https://en.wikipedia.org/wiki/Film_speed#ISO_12232
    Roger Hicks likes this.
  5. PeteRob

    PeteRob Well-Known Member

    Yes this is how it works. But in the sense of the standard. If you set the dial to a particular ISO rating and expose to a particular amount of white light you should get a particular grey-scale reading which is the same for all cameras. I'd have to read the article again but I think it was a mid-tone (118 out of 0-255). So going from ISO 100 to ISO 400 you would reduce the amount of light by 4 times to get the same 118 reading.

    In post processing you can change this of course by analogy with pushing/pulling.
  6. PC60

    PC60 Active Member

    As may well be but the diagram used 'centered' on exposure and what affects achieving exposure so I stand by the statement made.
  7. Andrew Flannigan

    Andrew Flannigan Well-Known Member

    You're free to stand by anything you want but in technical matters the standard definition is what matters. In this case you're wrong.
    Roger Hicks and EightBitTony like this.
  8. PC60

    PC60 Active Member

    So I am and will, have a lovely day.
  9. Andrew Flannigan

    Andrew Flannigan Well-Known Member

    Thank you. As the weather's clearing up here I hope I will.
  10. Chester AP

    Chester AP Well-Known Member

    A while ago I tried to help somebody with exposure when using a decent (DSLR) camera on its manual settings. I think the diagram in AP would have helped me explain - somewhere in the triangle is the correct combination of settings for the image you want. If using manual exposure because you want full control, all it takes is years of experience to pick the right place in the triangle...

    She was really confused (one of the many young people who refuse to read any kind of reference book, camera manual or even Wikipedia), but all I could say was:

    For any particular picture there are 2 variables that control the amount of light reaching the sensor - the lens aperture controls the size of the 'hole' through which light passes, so a larger hole passes more light, and shutter speed which controls how long the light is allowed to reach the sensor (so a longer shutter speed allows more light to pass). You are controlling many light 'particles' reach the sensor, and different combinations of aperture and shutter speed with will allow the same number of light 'particles' to reach the sensor. Playing with these variables allow control of image brightness/depth of field/risk of camera shake, etc.

    I said that a sensor's ISO/ASA is a guide to how many light 'particles' it needs to record a correctly exposed image. So in the days of film this was not a variable (it was fixed when the film was put in the camera), but with a digital sensor it is now a variable like aperture and shutter speed. Like these two, adjusting it allows more control because a higher setting means that fewer light 'particles' are required, at the price of decreasing image quality because the data generated by the sensor is processed more as the ISO increases.

    Since she was only using an 18-55 kit lens, I suggested leaving the ISO at 125 or 250 for a few weeks and learning all about aperture and shutter speed adjustments first, and only then experiment with the ISO setting.

    For a beginner, 3 variables to juggle is more difficult than 2. And, of course, the camera body's various automated exposure settings are there for users who can't cope with 2 variables (or even 1 if they use 'program auto').
    Andrew Flannigan likes this.
  11. Andy Westlake

    Andy Westlake AP Staff

    Clearly there's still a lot of misunderstanding here.

    The technical definition of 'exposure' is purely about how much light reaches the sensor, which means that it's a function of shutter speed and aperture alone. Open the aperture wider, or extend the shutter speed, and the exposure is increased. Keep the shutter open for double the time and halve the aperture area, and the exposure stays the same, so 1/30sec at f/5.6 gives the same exposure as 1/60sec at f/4. This is true regardless of the ISO you choose to set on your camera, be it ISO 100 or ISO 100,000. It simply doesn't matter if you prefer to think differently - this is the technically correct definition.

    The ISO setting then defines how bright the image will be for any given exposure. The higher the ISO, the brighter the image, so 1/60sec at f/4 and ISO 200 generates a brighter image compared to 1/60sec at f/4 and ISO 100, despite both having the same exposure. However, in-camera ISO settings can only be described with regard to the camera's JPEG processing. Raw files don't have any inherent ISO rating of their own, because they're not visually-meaningful images and require demosaicing, white balancing and gamma correction before they become actual photographs. Raw files can be quite happily developed to a wide range of different brightnesses and therefore ISOs, especially given the huge dynamic range of modern digital sensors.

    Another myth is that ISO is intrinsically linked to in-camera electronic amplification of the signal from the sensor to increase its 'sensitivity'. In reality, the camera manufacturer's image processing can use any combination of hardware gain and mathematical manipulation to achieve the desired JPEG image brightness. Many in-camera dynamic-range expansion systems work by changing this balance, as excessive gain can irretrievably clip highlights.

    Because ISO is so difficult to pin down, the current standard (ISO12232:2006) contains no fewer than five different definitions of ISO, which can potentially all give different answers. The most-used is probably 'Standard Output Sensitivity', which in effect states that an 18% grey card should be rendered as a mid-grey in the camera's JPEG output - no more, no less. But once you turn on adaptive dynamic-range balancing systems like Canon's Auto Lighting Optimiser, Nikon's Adaptive D-Lighting or Sony's Dynamic Range Optimiser, even this definition stops working and you're left with only one choice, 'Recommended Exposure Index', which broadly translates as 'Use this ISO and your images will look right'.

    Increased noise at high ISO settings doesn't come as a result of extra processing of the signal by the camera. Instead, it simply reflects the fact that to get an image of a standard brightness, you use a lower exposure at high ISO. So if you're shooting at ISO 1600, you'll have used 16 times less light to make your image compared to shooting at ISO 100. But the less light you use, the higher the intrinsic 'shot noise' within the light itself turns out to be: this simply reflects the quantum or particulate nature of light. (Some 'read noise' is also introduced by the electronics along the way, but on modern cameras it's very low indeed.)

    Incidentally, one logical consequence of all this is that the 'Exposure' slider in Adobe Camera Raw and Lightroom is incorrectly labelled - it should really be 'ISO', as on some other raw converters.
  12. Chester AP

    Chester AP Well-Known Member

    I was trying to offer helpful advice to somebody wanting to use manual exposure, and it was later confirmed that my advice had been both helpful and comprehensible, which is more than can be said for some of the articles about 'ISO' in AP.

    A clear and helpful explanation in AP, written in comprehensible English, is long overdue (unclear and technically very complex articles are possibly the cause of the misunderstandings).

    Perhaps, because 'the current standard (ISO12232:2006) contains no fewer than five different definitions of ISO', this is impossible. We are in the realms of pure and applied mathematics - the difference between 'accurate' and 'close enough for all practical purposes', where 'sensitivity' is close enough for all practical purposes and certainly easier to understand. When we used film, did we discuss the chemical formulation of the film's emulsion and its physical structure, or did we just talk about sensitivity?[/QUOTE]
  13. Andrew Flannigan

    Andrew Flannigan Well-Known Member

    Photographers can always take a tip from the days of film. When in doubt about the speed of a batch of film it was common practice to make a series of exposures at different ratings of a familiar scene. We would then pick the image that looked best as the speed index. It's even easier with digital and probably worth trying with all your cameras.
    RogerMac likes this.
  14. Learning

    Learning Ethelred the Ill-Named

    Andy W has just given a sensible rule of thumb.
    Never mind how the camera manufacturer fiddles about with analogue gain or digital multiplication in order to achieve a particular result; only the result matters. Provided that the exposure, as defined by AW above, does not saturate any photo sites, but comes close then you should get the maximum information into your image at base iso. If you are short of light and have to reduce exposure then no photo sites will be full. You can get a brighter image by raising the iso but you are not getting any more data; the dynamic range will reduce.
    You can see why the "expose to the right" applies for images intended for raw processing. This may (nearly always?) cause jpegs to be too bright.
    I would suggest that if you are shooting jpegs then enable auto iso, set D-lighting (or other manufacturers equivalent) and matrix metering plus any other 'scene modes' that might be applicable.
    If you want to achieve special types of image such as high key, low key, silouettes or dramatic sunsets, then shoot raw, expose to the right at the lowest iso (but not 'extended low') that gives you a sensible combination of shutter speed and aperture. To get the benefit of raw one must be able to understand the histogram from test shots.
    Last edited: Jan 17, 2018
  15. Chester AP

    Chester AP Well-Known Member

    This has been scanned from pages 108-109 of my 1969 edition of 'The Complete Colour Photographer' by Andreas Feininger.

    Even without making allowance for its age, I cannot fault its advice (apart from the advice about images for printing, where digital sensors have forced a change, which I refer to below).

    It should also make younger readers realise how fortunate they are to have equipment that will record decent colour images at 1600 or 3200 ASA/ISO. The American Standards Institute (formerly the American Standards Association or ASA) was one of many national bodies that eventually evolved into the International Standards Organisation (ISO).

    I know from personal experience with Kodachrome that the advice about exposure of tranparencies for printing is true - my rule was always 'if in doubt, underexpose', and I have some large Cibachrome prints that show it worked. Part of the learning curve for digital is that the reverse is now true, because this gives the better shadow detail and we can usually recover detail on 'blown' highlights in the RAW images.

    Any comments about the text below?

  16. Learning

    Learning Ethelred the Ill-Named

    That would seem to be very sensible.
  17. swanseadave

    swanseadave Well-Known Member

    "I feel a comparison of the charateristics of film and digital to that of valves and transistors, one is organic and almost warming the other clinical brash and potentiallyquite harsh".

    There is a program from DXO called Film Pack.Mine is version 3 but I believe it`s up to 6 now.
    You can apply to your photo a simulation of just about any film you can think of to mimic that film`s characteristics.
    I find it does a reasonable job.You can probably find a free trial to see If it suits.
  18. PC60

    PC60 Active Member

    Finally after a year plus we get a better followup.

    We need to follow the combined set of articles. Nice to see acknowlegement to electronic noise associated with sensors. Also to see some reference to the limitations of molecular chemistry in film emulsion and ccd sensor sizes and counts.

    Don't worry Mr Flannigan its a comment not 'blethering'. I won't be chatting with you.
    PentaxManiac likes this.
  19. Petrochemist

    Petrochemist Well-Known Member

    It's worth mentioning that the standards for ISO are themselves somewhat variable. There are at least 2 ISO standards for speed rating of film - ISO 2240 for Black & White) & ISO 5800 for colour negatives (I've not found one specifying colour slide film but all the references I've seen for 5800 specify negatives.)

    AFAIK the is only one ISO standard from digital rating but it's 2006 edition had 5 different procedures, that did not give the same results. It also didn't define any official values above ISO 10,000. The Latest 2019 edition may change either of these but I'm not going to splash out and buy it!.
    PC60 likes this.
  20. Petrochemist

    Petrochemist Well-Known Member

    No there is a third, the amount of light present. Perhaps not something that can be suitably adjusted with all shots but for studio shots lighting it is usually totally within the control of the photographer, (as well as adding light, shading the subject, or adding a ND filter etc. also effectively change this parameter.). The subject brightness will definitely adjust the amount of light reaching the sensor, unless you adjust your two parameters to compensate.

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