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I have a new pair of Apkina TTL 568 flash with soft boxes. I am operating them with trigger. When I take the picture, Flash lights do not give me same light for all the pictures, like for one picture they give normal light and for other picture (on the same setting) they reduce or increase the light. Is there any way to have same result every time? Help will be appreciated.

Camera Canon 700 D

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Is there any way to have same result every time?

Turn off the TTL and use manual flash power control. Your flash from shot-to-shot will be much more consistent. There will still be some relatively minor variation, but it will be a lot less when the flash is attempting to output the exact same power each time than when the camera is computing a new power level for each exposure based on the measurement of the pre-flash and any changes to the scene being metered. Since the pre-flash will also have minor inconsistencies, those inconsistencies can be compounded with the main pulse.

For example, say the preflash is 7% weaker than intended. The camera measures the light from the pre-flash reflected from the scene and calculates the needed power based on an assumption that the preflash was stronger than it really was. But say the main flash is then 5% stronger than the camera set it. The compounding of the two errors means the flash will be about 13% too strong. (1/0.93=1.0753, 1.0753X1.05=1.129). Sometimes such errors are self canceling. SOmetimes such errors compound each other. Higher end flashes generally have less variation for the same power settings from shot-to-shot. Cheaper lights can have more variation in both color and power output.

Beyond that, if the contents of your scene change, the targeted flash power will change when using TTL. It could be as simple as reframing the same scene so that more of the frame is taken by darker (or lighter) objects or it could be that the subject has changed and reflects the same amount of light differently. In TTL mode, the camera will try to make a black cat and a white cat both look gray. The black cat will get significantly more power than the white cat. Setting the flash power in the middle (ideally using an 18% gray card) will give the same power for both shots and the black cat will properly appear very dark and the white cat will properly appear very light.

  • Thank you so much for your answer. I will test it on Sunday and will let you know. – muhammad usman May 30 '17 at 1:04
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If you're using the flashes/camera in TTL mode, then stop doing that and use Manual mode.

TTL (through-the-lens) is a way for the camera to automate the power setting on the flash based on metering. The camera tells the flash to send out a low-power "preflash" pulse of light so it can be metered. And then, based on the meter reading, the flash power output is then adjusted. Vary your distance to the subject; vary the way the subject is framed, vary the background behind the subject, and you could vary the meter reading and the resultant power output setting.

Just as reframing an image in Av or Tv mode can get you different exposure settings, simply shifting the frame with TTL can get you different flash power output. That's why there's a variance. If you want to lock down the flash's power setting you must put the flash into M mode and dial in an explicit power setting that's a ratio of the flash's full-power pulse (1, 1/2, 1/4, 1/8, etc.)

Another possibility, if you are getting a strong pulse on one shot and a weak pulse on the next is that you're simply not waiting for the flash to fully recycle between shots. At full-power, most speedlights can take anywhere from 2 to 7 seconds to fully recharge the capacitor from the batteries. A speedlight is only powered by AA batteries, after all. Most speedlights will indicate if they are recharging and when they're reading to fire again with a light or a beep. Check your owner's manual to see if you're waiting long enough. Using something like burst shooting will typically only work with flash if your flashes are set to very low power (say, 1/8 or below). You may have to compensate with a higher ISO or wider aperture setting to bring the power down to where the recycle time works for you.

  • Thank you so much for your answer. I have read it, i will test everything on sundayand will let you know. thanks – muhammad usman May 30 '17 at 1:05
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Flash units work by trickling an electric charge into a storage device called a capacitor. The capacitor is best described by comparing it to a fire bucket. Water is trickled into a bucket which takes time, maybe 40 – 60 seconds to fill. The bucket sits with its load, waiting until needed. When called to action, the bucket of water is tossed onto the fire. The electronic flash circuit fills a capacitor with an eclectic charge. It takes a few seconds to fill this capacitor. When the shutter button is depressed, the charge is dumped into the flash, tube causing it to fire and output a brilliant blitz. These units have a monitor circuit that turns on an indicator light when the capacitor is charged. The fact that this light is on is your indicator that the unit is fully charged. My experience tells me that the indicator circuit likely glows a little premature. I advise, for consistency, to give the unit a little more time between exposures. Also, I know that units that operate, plugged into the household electric grid always display some typical variations due to line voltage changes. I can tell you with confidence that to get uniform results, waiting a little while longer helps, as does placing a constant voltage transformer between the household main and the unit.

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