[Leica] Another M9 question

[henningw at archiphoto.com (Henning Wulff)]

You were both wrong and right, Mark.

At first you said that you had a two stop drop with each halving of 
light. That'ts not right; you have a one stop drop, but that halving 
of light comes with each 1.414x factor of distance increase. I know 
you meant doubling of distance.

Then you said that you had a one stop drop for doubling of distance, 
which is wrong again, as that would lead to a two stop drop.

Reading between the lines, I can tell you know how it works, but both 
times it came out wrong.

In any case, for a point source of light like a flash, you have a 
drop of two stops for every doubling of distance.

If you have an infinite line source of light, it drops as the linear 
factor; ie, twice the distance, half the light or one stop. If you 
have an infinite plane of light, there is no drop in light. That's 
why under some studio situations you don't have to change exposure 
when the model moves away from the light bank.




At 9:59 PM -0400 10/5/10, Mark Rabiner wrote:
>I was wrong about me being wrong. Is that a double negative?
>Don't know what could have come over me.
>It's two stops like I first said. Its a bad idea to listen to me.
>
>The inverse square law works like this:
>Say your guide number is 110 which means you've got f 11 and be there at 10
>feet.  110/10 = 11
>
>If you want to go twenty feet you have to open up two not one stops. F 5.6
>110 / 20  = 5.5
>Double your pleasure double your fun and open up two stops everyone.
>
>split that distance which is 15 feet that's when you have one stop you open.
>f8
>
>If you want to half your ten feet and shoot at 5 feet you stop down two
>giving you f 22 and be there.
>Spilt that distance which is 7.5 feet and that's your one stop stop down.
>Which is f16.
>
>You read it here first.
>
>
>
>
>
>--------------------
>Mark William Rabiner
>Photography
>http://gallery.leica-users.org/v/lugalrabs/
>mark at rabinergroup.com
>
>
>>  From: Mark William Rabiner <mark at rabinergroup.com>
>>  Reply-To: Leica Users Group <lug at leica-users.org>
>>  Date: Tue, 05 Oct 2010 20:49:18 -0400
>>  To: Leica Users Group <lug at leica-users.org>
>>  Subject: Re: [Leica] Another M9 question
>>
>>  Your right its one stop of light for half or twice distance.
>>
>>  --------------------
>>  Mark William Rabiner
>>  Photography
>>  http://gallery.leica-users.org/v/lugalrabs/
>>  mark at rabinergroup.com
>>
>>
>>>  From: Philip Forrest <photo.forrest at earthlink.net>
>>>  Reply-To: Leica Users Group <lug at leica-users.org>
>>>  Date: Tue, 5 Oct 2010 20:40:25 -0400
>>>  To: Leica Users Group <lug at leica-users.org>
>>>  Subject: Re: [Leica] Another M9 question
>>>
>>>  Its a 1 stop difference, not two.
>>>  Phil Forrest
>>>
>>>  -----Original message-----
>>>  From: Mark Rabiner <mark at rabinergroup.com>
>>>  To: Leica Users Group <lug at leica-users.org>
>>>  Sent: Tue, 05 Oct 2010, 20:10:42 EDT
>>>  Subject: Re: [Leica] Another M9 question
>>>
>>>  DOF (depth of field) is a mixed bag when you have things perfectly in 
>>> focus
>>>  but dropping off fast into the darkness with the inverse square law. Or
>>>  whitening out before you the same as things get closer.
>>>  With each doubling or halving of light you gain or loose two f stops.
>>>  That's the inverses square law. As I understand it. Having flunked math.
>>>  You can not miss nailing your focus with flash as it allows you 
>>>to stop down
>>>  a bit or stop in fact way way down in fact:
>>>  But there is only one point front to back in which its exposed right.
>>>  Not that you even it out like crazy with Photoshop and grad masks.
>>>
>>>  Bounce flash off the ceiling is a good way around this.
>>>
>>>
>>>
>>>  --------------------
>>>  Mark William Rabiner
>>>  Photography
>>>  http://gallery.leica-users.org/v/lugalrabs/
>>>  mark at rabinergroup.com
>>>
>>>
>>>>  From: Tom Schofield <tomschofield at comcast.net>
>>>>  Reply-To: Leica Users Group <lug at leica-users.org>
>>>>  Date: Tue, 5 Oct 2010 15:03:19 -0700
>>>>  To: Leica Users Group <lug at leica-users.org>
>>>>  Subject: Re: [Leica] Another M9 question
>>>>
>>>>  Very true!  I bought the sf20 when I bought the M7 and maybe used it
>>>  twice!
>>>>  Which is to a large part why I don't want to spend $500-800 on a new
>>>  flash!
>>>>
>>>>  Sometimes when I take shots of my wife's preschool class, I want more 
>>>> DOF
>>>  than
>>>>  available light allows, and I am thinking of trying macro with it 
>>>> either
>>>  with
>>>>  a viso or a macro adapter, if I could find one used.  (I can get pretty
>>>  close
>>>>  with the 75mm, too.)
>>>>
>>>>  Of course, a cheap canon dslr with an R adapter might be an easier 
>>>> choice
>>>  for
>>>>  macro.
>>>>
>>>>  Tom
>>>
>>>
>>>
>>>  _______________________________________________
>>>  Leica Users Group.
>>>  See http://leica-users.org/mailman/listinfo/lug for more information
>>>
>>>
>>>  _______________________________________________
>>>  Leica Users Group.
>>>  See http://leica-users.org/mailman/listinfo/lug for more information
>>
>>
>>
>>  _______________________________________________
>>  Leica Users Group.
>>  See http://leica-users.org/mailman/listinfo/lug for more information
>
>
>
>_______________________________________________
>Leica Users Group.
>See http://leica-users.org/mailman/listinfo/lug for more information

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    *            Henning J. Wulff
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