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Thread: Greyscale and ISO

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    Greyscale and ISO

    Hi all, I'm just having a few problems with getting accurate greyscale readings at different ISO levels. I was wondering if, as I raise the ISO level, the Zone readings should become darker or lighter? ie
    at Zone V
    100ISO R112 G112 B112
    400ISO R109 G109 B109
    or
    100ISO R112 G112 B112
    400ISO R116 G116 B116
    I've had both but so must have to much light variation?
    Many thanks

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    Re: Greyscale and ISO

    Hi Victoria,

    Welcome to CiC

    You shouldn't see too much difference at low to modest ISO settings with a modern DSLR - but dynamic range typically drops off at least 2 to 3 stops at high ISO setting.

    I don't think that there's a right or wrong answer to your question; it's really a personal choice as to whether you want to protect highlights at the expense of shadow noise -v- having cleaner shadows and midtones at the expense of some highlight clipping.

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    Re: Greyscale and ISO

    Hi Colin, thanks for your reply. To be honest I'm only just starting with this whole greyscale thing and am finding it a bit hard to get my head around. So sorry if my questions are really basic but if I could just ask, on a standard greyscale, should the RGB numbers from 100 to 800 ISO increase or decrease?
    Thanks

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    Re: Greyscale and ISO

    Quote Originally Posted by Victoria View Post
    Hi all, I'm just having a few problems with getting accurate greyscale readings at different ISO levels. I was wondering if, as I raise the ISO level, the Zone readings should become darker or lighter? ie
    at Zone V
    100ISO R112 G112 B112
    400ISO R109 G109 B109
    or
    100ISO R112 G112 B112
    400ISO R116 G116 B116
    I've had both but so must have to much light variation?
    Many thanks
    Hi Victoria,

    Welcome to the CiC forums from me, great to have you join us, complete with first question

    OK I assume that these two 2nd exposures were shot at 2 stops shorter shutter speed (to counteract the 100 - 400 iso change)
    OR two stops wider aperture, or a mixture; intended to achieve the same exposure of grey card patch.

    Either way, the actual measured values like this (3 or 4 levels in 255) represent just 1.1 or 1.5% difference on zero change you might expect.

    I would say that mechanical (aperture and shutter timing) and electronic (aperture set, shutter speed, iso 'gain' and even sensor linearilty) are within acceptable margins, but also probably explains why you've had it both ways - it may depend which is the predominant error causing parameter. And/or, the light changed!

    Can you tell I was an engineer once?

    Anyway, now a non-engineer bit of advice; if you can't see it in an image (and I bet you can't),
    why worry

    That assumes you're not attempting some scientific measurement where it does matter.
    What was the light source?
    What is the camera?
    What mode (Auto, P, A, S, M) are you shooting in?
    If Canon camera: A = Av and S = Tv.

    Cheers,
    Last edited by Dave Humphries; 28th April 2010 at 02:11 PM. Reason: add questions

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    Re: Greyscale and ISO

    Quote Originally Posted by Victoria View Post
    Hi Colin, thanks for your reply. To be honest I'm only just starting with this whole greyscale thing and am finding it a bit hard to get my head around. So sorry if my questions are really basic but if I could just ask, on a standard greyscale, should the RGB numbers from 100 to 800 ISO increase or decrease?
    Thanks
    Hi Victoria,

    If in manual exposure mode and you don't alter shutter speed or aperture, if you increase the iso from 100 to say 800, the same grey step should increase (by quite a lot I would think).

    Cheers,

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    Re: Greyscale and ISO

    Quote Originally Posted by Victoria View Post
    Hi Colin, thanks for your reply. To be honest I'm only just starting with this whole greyscale thing and am finding it a bit hard to get my head around. So sorry if my questions are really basic but if I could just ask, on a standard greyscale, should the RGB numbers from 100 to 800 ISO increase or decrease?
    Thanks
    Hi Victoria,

    I suspect that "what you want to know" isn't quite the same as "what you're asking" here ... so are you able to give us a little more "thought between the lines" as to what you're trying to accomplish? or a little more background?

    Basically the difference between ISO 100 and ISO 800 is that ISO 800 won't be able to handle the same RANGE of brightnesses, but in the real world it becomes somewhat of a moot point in that most cameras capture a dynamic range in the region of 12 stops at their native ISO, dropping to around 9 stops (or less) at their highest ISO setting - but - most monitors only display around 6 stops and most paper can only print around 4 stops. With digital captures - if you have a relatively high dynamic range scene (say something with backlighting) then it might be more than the camera can handle - so the usual thing to do would be to decrease the exposure so that the brightest portion is still recorded correctly and the blacks are where they are (the blacks are usually just fine by the way - camera captures them, but we just have trouble displaying them without some kind of manipulation). At higher ISOs we often still capture more than we need and it's STILL a display / print issue ... but all this is probably getting too technical for you, which makes me think that I'm misunderstanding the question

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    Re: Greyscale and ISO

    Thank you both sooo much for your advise, some I understand some not so much. I'm currently doing a online course that comes with very little support. The current assignment asks for 3 greyscales to be made from a 18% greycard, 1 at 100ISO, 400ISO and 800ISO. With the aperture at f/8 for all, a correct exposure is to be taken followed by 6 over and 6 under exposed, 1 stop at a time. I have tried to attach my scales. I have done this test about 5 times now and get different results every time, and am now more confused than I was before I started Although I can see the greys get more muted, I cant really see the range in stops diminish?
    Thanks again for your time
    By the way I have a Canon 40D

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    Re: Greyscale and ISO

    Quote Originally Posted by Victoria View Post
    Thank you both sooo much for your advise, some I understand some not so much. I'm currently doing a online course that comes with very little support. The current assignment asks for 3 greyscales to be made from a 18% greycard, 1 at 100ISO, 400ISO and 800ISO. With the aperture at f/8 for all, a correct exposure is to be taken followed by 6 over and 6 under exposed, 1 stop at a time. I have tried to attach my scales. I have done this test about 5 times now and get different results every time, and am now more confused than I was before I started Although I can see the greys get more muted, I cant really see the range in stops diminish?
    Thanks again for your time
    By the way I have a Canon 40D
    Hi Victoria,

    Might be better to ditch the course and just stick with us (only kidding ... sort of!)

    It sounds like the experiment is designed to demonstrate the changing dynamic range of a camera at different ISOs. If that's the case you'll need to manually set the aperture + shutterspeed + ISO; if you have the camera set to aperture priority mode then you probably will get different results. So for starters I'd set those things manually, and then compensate as required (ie if you double your ISO then 1/2 your shutterspeed etc, in addition to varying your shutterspeed to vary the exposure anyway).

    If you do it right then the normal exposure should look the same, but at higher ISOs you'll have the lighter patches go to "all white" sooner as you start to over-expose the target, and darker patches will go to black sooner as you under-expose the target.

    It's actually very similar to what I do when building custom light meter profiles for my camera, so I can sort of see what they're getting at there (although I can't understand why they'd want to teach that as it's really not a topic for the faint hearted).

    Does that help?

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    Re: Greyscale and ISO

    Hi Colin, yes that helps alot!!
    But just so I've got this right - at high ISOs the lighter zones should be higher in RGB numbers and the darker zones should be lower in RGB numbers than the zones at 100ISO? As I said I've had both but need to know what they 'should' be so I can get on with this assignment. And yes I always work with manual settings.
    And I agree with you - it isnt for the faint hearted and its only assignment No3!!!! ......oh dear!!!

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    Re: Greyscale and ISO

    Hello Victoria,

    Welcome to the group. Wondering if you could tell us what course you are taking? If you are not getting the support you need, the information could assist future attendees.

    Quote Originally Posted by Victoria View Post
    Thank you both sooo much for your advise, some I understand some not so much. I'm currently doing a online course that comes with very little support. The current assignment asks for 3 greyscales to be made from a 18% greycard, 1 at 100ISO, 400ISO and 800ISO. With the aperture at f/8 for all, a correct exposure is to be taken followed by 6 over and 6 under exposed, 1 stop at a time. I have tried to attach my scales. I have done this test about 5 times now and get different results every time, and am now more confused than I was before I started Although I can see the greys get more muted, I cant really see the range in stops diminish?
    Thanks again for your time
    By the way I have a Canon 40D

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    Re: Greyscale and ISO

    I think some first principles might help, so let's look at the simple version to start with, the "correct" exposure of a grey/gray card.
    You set the camera up pointed at a flat card and use the meter on the camera or a (seperate one) to give you the settings for the camera.
    Lets say that the meter says that for your chosen (assuming here) 1/125s exposure that you need f/4 for 100ISO. You take the picture and you're done. Next, do the same at 200ISO. The meter will tell you to use one stop less on the lens, i.e. f/5.6 to get the same result. 400ISO should read f/8 on the meter etc.

    Once you have the pictures taken you examine them using software, and the deal is that they ought to be identical. If you shoot another test set at one stop over the meter reading, againthe results ought to be identical.
    I think that the purpose of the exercise is to demonstrate the different look of the image at high ISO settings where you will begin to notice a degradation of the image. This is called "noise" and is caused in the same way that a faint signal on an old AM radio sounds crackly when you turn up the volume to allow for the weak signal.
    The internal elecronic magic inside the camera "turns the volume up" when you increase the ISO setting to get an image from a "weaker" image.
    The weaker image is caused by the camera allowing less total light to reach the sensor at high ISO settings, though in some situations its better to get a noisy image than none at all.
    HTH

    Jonathan

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    Re: Greyscale and ISO

    Quote Originally Posted by JonathanC View Post
    I think that the purpose of the exercise is to demonstrate the different look of the image at high ISO settings where you will begin to notice a degradation of the image. This is called "noise" and is caused in the same way that a faint signal on an old AM radio sounds crackly when you turn up the volume to allow for the weak signal.
    Hmmm - I think we really nead to find out for sure what the test is all about ... I've been assuming that Victoria's shooting a gray SCALE, not a gray card. Victoria - can you confirm which it is? And does the assignment give you more of a clue as to what the objective is?

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    Re: Greyscale and ISO

    Wow thank you all so much for your help!! To answer you question Colin, I am shooting a 18% greycard at the different ISO levels then constructing a greyscale with these shots. I'm guessing the objective is to get the dynamic range at different ISO levels? However my camera seems to have the same amount of stops in dynamic range at 100 and 800ISO which as been very confusing. I had a look at your website and I luv your photos!! Especially 'One Day My Prince Will Come', such a beautiful photo, my favourite type of photo, one that tells an emotional story. Was it hard to achieve? Lighting etc?
    Many Thanks
    Victoria
    Last edited by Victoria; 30th April 2010 at 04:27 AM.

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    Re: Greyscale and ISO

    Quote Originally Posted by Victoria View Post
    Wow thank you all so much for your help!! To answer you question Colin, I am shooting a 18% greycard at the different ISO levels then constructing a greyscale with these shots. I'm guessing the objective is to get the dynamic range at different ISO levels? However my camera seems to have the same amount of stops in dynamic range at 100 and 800ISO which as been very confusing. I had a look at your website and I luv your photos!! Especially 'One Day My Prince Will Come', such a beautiful photo, my favourite type of photo, one that tells an emotional story. Was it hard to achieve? Lighting etc?
    Many Thanks
    Victoria
    Hi Victoria,

    I've just realised that your a fellow kiwi (I'm a bit slow some days). I love Christchurch (apart from the parking wars!) - used to live there for a year and a bit when I was based at Wigram in the Air Force.

    Most modern cameras will have a fairly similar DR at 100 & 800 ISO - but if you'd like to tell me the exact make and model then I'll look up the exact DR figures for you.

    Thanks for the kind words re: my gallery. The lighting for the one day my prince will come shot was by accident - I was playing around with a new piece of lighting equipment - mucked something up - and as a result the fill flash didn't fire - so the only lighting was some directional window light My daughter hates the shot ... so I figured it must actually be OK

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