# Neewer Macro LED Ring Light Flash RF550 – Guide Number

I purchased a Neewer Macro LED Ring Light Flash online for a modest price (~\$50).

It was described with specifications, but specifications for Chinese product are not to be trusted… so it deserves measurement (though shortfall would become evident in use in short time).

First observation is that the flash mode appears to be the “light” mode though with a timed burst. Since the manual suggests using shutter speed 1/100, the duration of the flash burst is of interest.

Above is capture of a test of flash duration with a phototransistor, it measures 142ms, and the intensity appears independent of the power setting on the flash unit.

So, in fact maximum light will be captured at shutter speed 1/8.

## Inverse square law?

The light intensity from a point source decays as the inverse square of distance ($$l \propto d^{-2}$$), but the ring flash is not quite a point source at the distances used.

Above is a plot of the luminous intensity vs distance from 0.15 to 1.0m, and a curve fit which suggests that the exponent is -1.9 rather than -2. Not a big difference, but a difference.

## Exposure calibration

The DSLR is used with ASA=1600.

Above, distance from flash to subject is 0.3m, the camera is set for f32 @ 1/8, ASA 1600.

The flash is set for +0.0 but it seems the variable power does not affect the FLASH mode output.

The light meter is set for ASA 1600 and indicates f32.7, which means f32 +0.7stops, so it is equivalent to f52. So, the calculated guide number (GN) at ASA 1600 is 52*0.3=15.6, or 15.6/16=1 at ASA 100.

The GN specified in the manual is 15 with no mention of ASA speed which most people would read as ASA 100 implied. To that extent, it is a fraud, the GN @ ASA 100 is close to 1.

Now the manual suggests using shutter speed 1/100, so that would reduce the GN by a further factor of (1/100)/(1/8)=8/100 so GN=0.08 at 1/100s shutter speed for ASA 100.

Above is a plot of the light intensity of the test pic above, it looks to be close to optimally lit / exposed.

In my use, the DSLR camera is set for ASA 1600, so at 1/8 shutter speed, GN=16.

The use of GN assumes inverse square law behavior, and it has been shown that is not quite correct for the ring flash at close distances.

## In practice

It is convenient to use the flash unit in FLASH mode, set the aperture for F16 for depth of field, and adjust the shutter time to 1/8s or shorter to obtain correct exposure at the subject distance.

The shutter time is adjustable in approximately third stops, so a table was constructed of the optimal subject distance at shutter times from 1/8 to 1/500 (ASA 1600).

This is a theoretical prediction, assuming that light captured is proportional to shutter time… but in fact it loses accuracy at shutter times shorter than 1/200s, probably due to rise time / synchronisation of the LED burst, mainly the latter considering tracking with the flash meter.

So, in use, the subject is composed, the distance from front of flash ring to mid subject measured, and shutter time found from the table for the nearest distance and camera set.

It is an interesting enough device, quite inexpensive. I may yet purchase a Yongnuo YN-14EX II for the TTL flash feature and external power connector.