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    The polariceing on the cake
    From: Peter Fogg
    Date: 2006 Jan 28, 11:10 +1100

    Earlier excerpt of mine:

    “So two (polarizing filters) used together would already have quite a significant minimal darkening effect, which could be halved by using only one.”


    To which George has taken great exception, leading to this sweet inquiry:

    “He doesn't appear to have withdrawn it since, although it has been questioned, and was contradicted by Bill's numerical observation. Perhaps he will tell us whether from that, and from the recent web reading he has told us about, he has since changed his mind.”


    to which the answer can only be:

    No. The platform from which George is gloating prematurely seems to have somewhat shaky legs, to put it kindly. It appears to be solely based on a single experiment, that Bill himself qualified on presentation as:

    “a quick series of tests”


    For George to want to run with this as a fait accompli seems rather naïve, or is it a case of embracing evidence that supports his preconceived notion, while ignoring evidence that questions it, as did my own? Hardly very scientific or rational. Tut tut.


    Now that this – in itself fairly unimportant – idea has been thoroughly worried and chewed at, the only change I would make is to insert a qualifier; an ‘about’ or ‘roughly’. Here it is:

    “which could be roughly halved by using only one”


    Happy now, George?

    The ‘about’ is necessary because there are so many variables in practice, some detailed earlier.


    The ‘about’ qualifier can be taken (thus improving the work of both authors) from a recent passage of George’s otherwise excellent exposition on f/ stops which seems to have rather too many ‘abouts’:

    “About the most efficient lens you will come across in an ordinary camera has a diameter equal to ABOUT 0.5 f when wide open (so aperture = f / 2). It can be "stopped down" to a smaller diameter of say 0.35 f  (so aperture is f /2.8 ), which will let through ABOUT half as much light.” 


    My understanding is that there is nothing imprecise about f/ stops; they need to be and are quite precise in order to function, ie; to permit an accurate exposure. Just try telling Schneider or Rodenstock or any other lens maker that its finely calculated then engraved f/stops are only ‘about’ right and I suspect you would be met with withering scorn.


    If the bear is no longer required he may retreat to the woods for a nice peaceful sleep …


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