[rollei_list] Re: Rolleiflash and Guide Number
- From: CarlosMFreaza <cmfreaza@xxxxxxxxx>
- To: rollei_list@xxxxxxxxxxxxx
- Date: Mon, 30 Nov 2009 01:09:28 -0300
Marc, my main interest for a flash GN is to know the flash power to
compare them, otherwise I use the flash in automatic and I only need
to work with the GN for manual mode sometimes.
Scales in 1955 and today are the same, only was changing the GN that
the manufacturer considers more convenient to show in the basic Flash
specs, but any good flash manual has the GN for different ISO, for
example the Rollei E 36 ER manual has Guide Numbers for ISO 12 to 1600
including every intermediate number in the ISO scale between these two
numbers, but please note that only 12, 25, 50, 100, 200, 400, 800, 1
600 are emphasized because doubling the film speed each time the
difference produce significant changes for the exposure with flash and
then you have, f.e., that for ISO 25 the guide number is 25(metres)
and for ISO 50 is 36(metres), ISO 100 GN is 51 (metres), ISO 200 GN is
72, etc., etc. (You can convert metres in feet using an online
conversor), and then I think the Flash manual is the ideal means to
know any Flash GN for any ISO speed for feet and metres.
But if you don't have the Flash manual and you only have a little
manual flash indicating one GN, for example GN 20(metres) for ISO 100
and you have an ISO 400 film, the quick formulae is: 20 x 1.4=28 for
ISO 200 and 28 x1.4 for ISO 400=39.2 or 39 for practical purposes, and
then you have that your GN 20 for ISO 100 became GN 39 for ISO 400.
The same way if your GN is 26 for ISO 25, the quick formulae is; 26 x
1.4= GN 36.4 for Iso 50 and 36.4 x 1.4=GN 50.96 or 51 for ISO 100.
In feet would be about 82 (ISO 25) and 118 (ISO 50) and 168 (ISO 100).
Carlos
<marcsmall@xxxxxxxxxxx>:
> At 07:50 PM 11/29/2009, you wrote:
>>Thank you very much Charlie, I used the wrong number to multiply the
>>first time, now: 36 (ISO 50) x 1.4 (to ISO 100)= 50.4, in other words
>>51 (167 for feet) like the flash manual indicates for ISO 100.
>>
>>BTW, Charlie does not participate in the list topics very much, but I
>>can assure he owns an impressive knowledge and practical experience
>>about photographic technical issues, analog and digital, I suspect he
>>could only be a great pro photographer.
>
> Contemplate the square root of two, Carlos. All will be made blindingly
> clear.
>
> We have these discussions all the time on the Rollei List. I had my hair
> singed when Ed set me right around 1995 about the proper relationship
> between the European and International f-stop scales.
>
> Carlos, please give us a quick formula for converting, say, a Guide Number
> calculated in feet at an ASA of 25 in 1955 to a current scale. Bear in mind
> that the ASA scale shifted in 1959. This is not a trick question: I am
> curious. I do almost no flash work but, when I do so, I tend to use Guide
> Numbers even when I have some sort of more sophisticated system to suggest
> an exposure -- and GN's are vital in doing contre-jour exposures, unless you
> have a one-degree spotmeter at hand.
>
> But, shucks, I still regard the move from the 154-game season to he 162
> games and the addition of that horrid "designated hitter rule" to have been
> the death of US Baseball. It will collapse. Just give it time.
>
> Marc
>
>
>
>
> msmall@xxxxxxxxxxxx
> Cha robh bàs fir gun ghràs fir!
>
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