Nikon SB 800 vs Sekonic L558

[MattKing]

In the Vivitar 283 service manual the automode is calibrated with target that is 36% refectance.

That is the target - not the environment that the target was present in.
Most use of Guide numbers came from the days when people used flash to take photos in social situations - indoors, with floors, ceilings and frequently walls providing the benefit of extra light reflecting on to the subject before it reflects back to the camera.
If you used a flashbulb or amateur electronic flash in your dining room to take photos on slide film of your 5 year old's birthday party, and used a Guide Number that did not incorporate the effects of light bounced from those other surfaces, the result ended up being over-exposed. So most amateur Guide Numbers did incorporate that.
We used to give advice to amateurs that if they were photographing in a high ceilinged room, or outside, or anywhere else that didn't offer any reflective surfaces, the Guide Number recommendation needed to be modified, because without the reflected light, the exposure would be a bit insufficient.

I believe that’s what Matt was referring to as a “guide number” for “commercial/professional “ strobes when he wrote, “Guide Number was more a measure of the light output itself”.

I assume he meant studio strobes. In any event, there aren’t multiple types of guide numbers.

Perhaps he was implying that different strobes may have different assumptions regarding reflectivity…

No, but there also isn't a standardized environment for where Guide Numbers are measured, and the characteristics of the scene surrounding the subject have an important affect on the amount of light that ends up hitting the subject.
Guide Numbers are designed to help photographers use flashes when taking photographs. A lux-second measurement with a pulse photometer is designed to measure the light output of a flash bulb or electronic flash.

 

[Chan Tran]

That is the target - not the environment that the target was present in.
Most use of Guide numbers came from the days when people used flash to take photos in social situations - indoors, with floors, ceilings and frequently walls providing the benefit of extra light reflecting on to the subject before it reflects back to the camera.
If you used a flashbulb or amateur electronic flash in your dining room to take photos on slide film of your 5 year old's birthday party, and used a Guide Number that did not incorporate the effects of light bounced from those other surfaces, the result ended up being over-exposed. So most amateur Guide Numbers did incorporate that.
We used to give advice to amateurs that if they were photographing in a high ceilinged room, or outside, or anywhere else that didn't offer any reflective surfaces, the Guide Number recommendation needed to be modified, because without the reflected light, the exposure would be a bit insufficient.


No, but there also isn't a standardized environment for where Guide Numbers are measured, and the characteristics of the scene surrounding the subject have an important affect on the amount of light that ends up hitting the subject.
Guide Numbers are designed to help photographers use flashes when taking photographs. A lux-second measurement with a pulse photometer is designed to measure the light output of a flash bulb or electronic flash.

The procedure calls for non reflective environment except the target which should have 36% refectance.

 

[MattKing]

The procedure calls for non reflective environment except the target which should have 36% refectance.

If a non-reflective environment is used by manufacturers to determine a recommended Guide Number, then when users go to use that that Guide Number in a more typical environment, they will end up with over-exposed results.

 

[Chan Tran]

If a non-reflective environment is used by manufacturers to determine a recommended Guide Number, then when users go to use that that Guide Number in a more typical environment, they will end up with over-exposed results.

But it also calls for 1/3 stop less light.

 

[BrianShaw]

ISO 2827 governs measurement of strobe ouput. Guide Number is defined there, and in other standards. GN is only applicable when inverse-square is applicable. And the criteria is “an acceptable exposure “; rather subjective. Along the same lines as you write but more scientifically described versus application based (amateur vs pro) description. The standard has a method for measuring pure flash output. Interestingly, the environmental reflectivity aspect isn’t described in the standard, yet environmental reflectivity assumptions are real and important, especially for auto mode flash exposure. For all flash modes, GN is always understood to be approximate.