# [argyllcms] Re: Spotread -f and flash Guide Number calculation problem,

• From: Matthieu DUBAIL <matthieu.dubail@xxxxxxxxx>
• To: argyllcms@xxxxxxxxxxxxx
• Date: Wed, 6 Apr 2011 16:00:40 +0200

```Hi everybody,
For those who were interested by the subject.

I searched for other sources of informations:
- On this forum, there's a copy of an old ASA version of the GN calculation:
http://photo.net/photography-lighting-equipment-techniques-forum/00VsPr
It is there stated that M used in B4 is the reflector factor of the flash. M
(equal a value from 3 to 7) is directly related to the flash inner-reflector
geometry and reflectance: how light from the flash tube is concentrated.

- "It" (intensity per time) is often called BCPS Beam per Candle Power
Second in other GN formulas.
Constant value vary a lot from different official formulas and literacy
sources. Focal press encyclopedia is far too generous on this one.

- The 4pi used in equation B3 and B1 is used to convert lumen to candle
power

- Calibration constant is a very discussed matter and vary from light meter
brand to brand. ISO value is not very representativ of the results obtained
with light meters and cameras.

Considering all that and testing different light meters and digital cameras,
I ended with this formula:
GN = 0,052 * sqrt(Lumen * ISO)

I got decents results, but all photo flash specs are way too generous (from
1 to 1,5 photo aperture too generous).

thanks,

Matthieu DUBAIL

2011/2/11 Matthieu DUBAIL <matthieu.dubail@xxxxxxxxx>

>
> Interesting.  Was the flash mounted on a camera to set the zoom to 50mm? If
>> it is a digital camera, does the flash automatically adjusts the zoom to the
>> crop? 45*1.5 = 67.5?
>>
>
> Nope, the flash was on a tripod, setted manually (zoom factor included).
> Actually, only the screened focal number is changed if the DSLR  APS-C focal
> format option is chosen in the flash menus. Angles of light and GN doesn't
> really change.
>
```