Re: f number adjustment to increase light level

[Kim Mosley <mrkimmosley@xxxxxxxxx>]

How's this (from a Mensa-like friend who read my post):

The way I see this, the amount of light is proportional to the square of the f# and the smaller the f# the greater the light, so in your case, if you want four times light then you have to increase the f-stop by a factor of two (=sqrt(4)) but f-stops are in the denominator so f8 becomes f4 as you said. 

f-new =  f-old / sqrt(light increase factor)

On Fri, Jan 21, 2011 at 4:21 AM, Christopher Strevens <chris@xxxxxxxxxxxxxxxxx> wrote:

In fact this was one of the first computer algorithms I wrote back in the ‘60’s when I started to study computing.

 

 

Chris

 

From: owner-photoforum@xxxxxxxxxxxxxxxxxx [mailto:owner-photoforum@xxxxxxxxxxxxxxxxxx] On Behalf Of Kim Mosley
Sent: 21 January 2011 03:52
To: List for Photo/Imaging Educators - Professionals - Students

Subject: Re: f number adjustment to increase light level

 

Here's o:p>

 

16 x 16 / 10 = 25.6    

Square root of 25.6=f/5 (rounded off)

 

Or suppose you are at f/8 and you want 4 times more light.

 

8x8 (or 8 squared) / 4 = 16

Square root of 16 = f/4

 

We are just talking about the area of the aperture, right?

 

No logs!

On Thu, Jan 20, 2011 at 8:31 PM, ADavidhazy <andpph@xxxxxxx> wrote:

I was intrigued by a statement I made during a review of a sample exam
session and this afternoon decided to find out what the new f# should
be given that one wants to increase the light level at the image plane
over that available at some starting f#. After 4 hours of pondering
came up with this (whole increments would be quite easy ...):

http://www.davidhazy.org/andpph/text-fnumber-aperture-adjustment.html

I am sure there are easier ways to do this and if so a hint would be
most welcome! Of course fractional or "decimal" f#s are not easy to set!

Andy

--
Kim Mosley
mrkimmosley@xxxxxxxxx
Website: http://kimmosley.com
Blog: http://kimmosley.com/blog

--
Kim Mosley
mrkimmosley@xxxxxxxxx
Website: http://kimmosley.com
Blog: http://kimmosley.com/blog