Daft Question - Objective measures?

Discussion in 'Open Gear Talk' started by stillshunter, Apr 11, 2011.

  1. stillshunter

    stillshunter Super Moderator Emeritus

    Nov 5, 2010
    Down Under
    Real Name:
    From a recent thread, I professed to being the asker of daft questions. Always aspiring to be a man of my word, here's another couple of gumbies for you.....

    We often make comparisons between cameras to determine which might better suit our 'needs'. Two areas that we often focus on are lens speed and low light performance. Hence, I wondered on whether objective measures underlie our thinking here?

    1. Lens Speed: so when you want to better light gathering or bokeh (OOF) capabilities we all seek a larger aperture. So f2 is faster than f4. I know this has to do with how 'open' the aperture is, but I wondered is this measure dependant on the relationship to sensor size or is it a constant measured in, for example, 35mm terms? Therefore, is "f2" different when you are talking in terms of a 1/1.7", APS-C or FF sensors? Accordingly, if you use an old 35mm f2 lens on a sensor half the size - e.g., m4/3 - does it remain f2 or is it twice as fast - maybe f1.4?

    2. Low Light Performance: Again we all must confess to comparisons on DPR and the like looking at how cameras perform at ISO 400, 800, 1600 and up to 51200 (jeez, I'm hard pressed finding a film of this rating). So, again, is this an objective measure? For example, is ISO 1600 the same between manufacturers - i.e, Olympus vs Sigma vs etc.? If so, how is each ISO rating baselined?

    I know these are Digital Photography 101 type questions, but not one's I know the answers to, nor one's that my Google searches are helping with, I'm afraid. :blush: So really appreciate your insights.
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  2. bilzmale

    bilzmale Super Moderator Emeritus

    Jul 17, 2010
    Perth, Western Australia
    Real Name:
    Bill Shinnick
    The focal length of a lens is a constant for that lens. A simple example, a magnifying glass will focus the sun's rays a fixed distance from the lens - this is the focal length. Speed or "f" number is a ratio of the focal length to the diameter. Camera lenses can vary their aperture diameter to control the amount of light and it is expressed as a fraction of the focal length
    eg a 200mm lens with an aperture of 50mm will be f/4 written that way as Focal length/Aperture =200/50 = 4 or re-arranging Aperture = Focal length/4.

    Because the aperture area is related to the sqare of the diameter (assuming it is circular) a set of 'f stops' is written as 1; 1.4; 2; 2.8; 4; 5.6; 8; 11; 16; 22; 32. As the f number increases by one stop the area decreases by half and lets in half as much light. Being based on physical dimensions of the lens, apertures are constant for the lens regardless of the camera.

    So what does change between a lens on a 35mm film camera and that same lens on a digital body?

    The variable item is the area of the sensor compared to the 36mmX24mm of a piece of 35mm film. A 4/3rds sensor is half the area of film so the lens only sees or has a point of view (POV) equivalent to a lens 2 times as long. The focal length is still 200mm but it captures the smaller amount of light equivalent to a 400mm lens. For an APS-C sensor used by Canon, Nikon, Sony ilcs and dslrs this 'crop factor' is 1.6x as the sensor is bigger than 4/3. For a point and shoot the crop factor is more like 6x making very compact super zooms possible.

    Hope this clarifies more than it confuses.

    As to low light performance or ISO this is meant to be a fixed scale but sensors will vary. Regardless of this small variation ISO is usually quoted as 100; 200; 400; 800;1600; 3200; 6400 and even 12800 and 25600. Notice the doubling of the sensitivity similar to the sequence of f numbers.
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  3. stillshunter

    stillshunter Super Moderator Emeritus

    Nov 5, 2010
    Down Under
    Real Name:
    Wow thanks Bill. I have to be honest and admit that much of this has flown over my head. So I'll need to ingest it a few more times before it really sinks in. Please don't think this has anything to do with your explanation, as it seems very lucid and comprehensive. Funnily enough, I just came back from a work-sponsored Executive Leadership course where we spent a lot of time talking over our Personality Profiles. Suffice it to say that my left-brain, and esp the "technical" quadrant, is rather lacking. Although it's some solace that I make up for it with an over-abundance in the right-brain, and the 'visionary' sector in particular. So I'll need a little more time to come to grips with what you have said....and hope I don't get too distracted by the next big shiny thing :blush:

    Suppose this accounts for why I'd not make much of a Mod. :blush:
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  4. BBW

    BBW Administrator Emeritus

    Jul 7, 2010
    betwixt and between
    Real Name:
    I sometimes wonder if I have a left brain.

    You'd be a fine Mod, if you had the time, Mark. Just you wait.:jedi:
  5. Luckypenguin

    Luckypenguin SC Hall of Famer

    Dec 24, 2010
    Brisbane, Australia
    Real Name:
    In terms of light intensity, sensor size is irrelevant to the f-number. A lens set to f/2.8 will give the same exposure on a full-frame camera as will a lens set to f/2.8 on a 1/2.3" compact, assuming they are capturing the same scene. The physically larger lens will let in more light because iris opening is much larger, but the intensity of the light on the sensor will be equal (more light spread over a larger area, less light spread over a smaller area -> equal intensities). Bill describes it correctly as a ratio that can be related to focal length, but it is best to ignore focal lengths and crop factors when considering the effect of aperture values.