What was the power of the Flash?

April 16, 2018  •  1 Comment

I refer to a post about a year ago called Lighting Product Photography. In that post I was explaining how products sold online could benefit with addition of a nice photo of the product. I was recently asked a question relating to that post. What was the power of the Flash?

There are many ways to light a product and with the equipment at hand, I was very limited. The setup was very simple consisting of a Speedlight/Flash shooting through a 5 in 1 reflector using the diffusion portion as seen below. I'm showing this to give a sense of where we are going with todays topic. What was the power of the Flash? 

I used to ask this same thing when reading/watching a tutorial and thinking that if I had exactly the same settings I would get exactly those results. The problem was, I wasn't getting those results. But why? Well there are a whole host of reasons for this, one being the camera settings, but in reality the first exposure should have no ambient light in the shot. That way you only light the product with the flash.


There are so many other variables like type, power and distance. If using a modifier, it would also depend on the type and how many stops of light-loss the diffusion creates. Flash/Speedlights range in different power/intensities. For instance, the Nikon SB5000 AF Speedlight  @ full power is approximately +0.59 EV exposure, 1.51x power or intensity, 1.23x distance or fstop number of the Nikon SB700 AF Speedlight using the guide numbers @ 35mm zoom. Now the SB5000 would need to be set at a lower setting to achieve the same exposure as the SB700 @ full power. 

To help understand the power value of each speedlight. The SB5000 Guide # is 34.5 m/113 ft (at 35 mm zoom head position)  (FX format, standard illumination pattern) (at ISO 100). The SB700 Guide # is 28 m/ 92 ft. (at 35mm zoom head position) (FX format, standard illumination pattern) (at ISO 100). I would refer to Understanding Guide Numbers. It gives an excellent detailed explanation and has a Flash Comparison Calculator and a Guide Number Calculator.

For or a bit of fun, here is an example using the Cameron W600  Manual Flash vs the Orlit RT 610 TTL  Monolight.

The difference is the monolight is approximately +2.53 EV exposure, 5.79x power or intensity, 2.41x distance or fstop number to the flash, the monolight being either with a reflector or bare bulb and the flash being at 24mm zoom for widest coverage of each, with the distance to the subject the same. I should be really using a light meter to meter the Fstop value of light output to show you. I'm using a styro-head as my subject. 

This first shot is with the flash @ 24mm zoom, distance 6' aperture F11. This gives me a guide number of 66 and on the GN chart  @ 24mm zoom in the flash manual the closest is GN65 and the matching output is 1/2 power. Note the histogram for reference. 

The below exposure is the monolight bare bulb at half power same distance. It was surprisingly close to the flash, about 1/2 of an fstop over exposed. Not the two plus stops difference I was expecting but hey, remember its just for fun and not that scientific of an experimentation. There are too many unknowns like the monolight guide number, is it for bare bulb or the reflector that was supplied?  

The next exposure was taken with the reflector attached. I believe the manufacturer had the reflector attached when calculating the guide number for this model. Check out the histogram below. Notice how it is over exposed?

I dropped down two stops on the monolight to create the next exposure. Now we are back in the ballpark. I could come down even more, about 1/3 and be really close to the flash/speedlight as seen in the histogram below below. 


Back to the topic, What was the power of the flash? Judging by the photo, the flash is approximately 2' to 3' away from the glasses using an aperture of F11(at ISO 100). So using the guide number calculation GN = Fstop x Distance m/ft, at F11 and 2ft away the GN would be 22 (@24mm coverage). Now find the GN chart in your manual and the closest GN that matches, tells you what power setting to start at. You may have to adjust your EV some. Keep in mind most flashes have zoom coverage. You would absolutely need to know all the guide numbers if your flash has a zoom. The Cameron W600 zoom ranges from 24-105 and the chart in the manual has the full range of guide numbers for 1/1 down to 1/128 flash outputs. So @24mm, (widest coverage) the closest to GN 22 is GN 23, at 1/16th power. Now 1/16th is my starting point for that particular flash. Since its is going to be over exposed slightly you would adjust the power down. Most flashes adjust by 1/3rds while monolights adjust by 1/10ths giving a bit more control. Remember though, I was using diffusion so that would affect the light output by about one stop. So you could either zoom in the flash to compensate as zooming will increase the intensity (but the spread will be smaller), or adjust the flash power. 

What was the power of the flash?

I guess it depends.




Thanks a lot Geoff for explaining the concept so nicely. Thanks once again.
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