6 of 6 people found the following review helpful
Does the job,
This review is from: External Flashlight Battery Pack EP-N4 (Replace Canon CP-E4) for Canon Speedlite 600EX, 550EX, 580EX, 580EX II, MR-14EX, MT-24EX (Electronics)
This product is about as third party as they come; the one I received is branded as "Shoot" rather than GTMax. Very little planning and design has gone into it and it is really cheaply made. However, it does the job.
In brief, the unit is basically a battery pack which has a sliding end that allows access to a tray which can accommodate 8x AA batteries, giving you 12 if you include the ones in your flash (I use this with a 580EX II). The unit connects to the flash by a 22" (56cm) curly cable which can stretch to over 60" (152cm). It has a sticker on it warning not to fire more than 20 consecutive flashes.
It comes with an odd little case which has a press-stud closure at one end, a velcro closure at the other, an adjustable velcro loop on one side, and a fixed belt loop on the other. Also packaged in the plain white box was a set of gibberish instructions (wouldn't be the same without them), and a long thumb screw for connecting the battery pack to the base of your camera via the tripod mount.
So the all-important part: tests.
- I used a 580EX II mounted on a Canon EOS 7D in gloomy but not dark conditions, high speed shooting, 2.0 aperture and 1/6400 shutter speed at 35mm, ISO 100.
- The fast shutter speed was chosen so that any frame not supplied enough light by the flash would be noticeably under-exposed or completely black.
- I allowed the flash to use full ETTL-II because this is how I am most likely to use it in any circumstances where I want to use high speed shooting with flash (kind of implies I won't be metering and setting manual power levels between shots).
Using just the batteries in the 580EX II, the flash was able to provide light for three frames with the third noticeably underexposed. There were then three black frames, one underexposed, another three black frames, and one normally exposed frame. It was obviously struggling to provide power.
Adding in the battery pack, the flash was able to provide light for five exposed frames, then one black frame, then two exposed frames, one black, another two exposed frames, another black, another two exposed frames... I think it would have continued like that.
(E=Exposed, U=Underexposed, B=Black)
580EX II Only: E E U B B B U B B B E
With the pack: E E E E E B E E B E E B E E
Using the battery pack, the exposed frames were more or less exposed correctly; slight variations were visible on the 7D's LCD screen and using the histogram but these were not anywhere near as obvious as the variations seen when using just the four 580EX batteries. Clearly, having 12 batteries instead of 4 allows the flash unit to supply more power per flash and to shorten the period between flashes ('recycling' time).
If you want to capture rapid-fire sequences lit by flash in dynamic conditions, this is a cheap way to increase the number of keepers you'll get. Although I'd obviously prefer the more expensive original Canon version, I've found this cheaper brand adequate for non-professional purposes.