Paul C. Buff, Inc. Technical Forum

Okay what am i missing? AB-400, is 6 feet from subject, shooting through a Paul B. umbrella softbox, umbrella is centered on subject, just above eye-level. Settings are f5.6, t250, ISO 200, metering with a Gossen Digipro F on the chin of the subject. Measured output is between f4.0 +9 and f5.6 +3 consistently, mesured light varries with every shot/strobe trigger. I have 4 AB-400's all of them do the same thing. Is this normal?

So it could be one of two things, or a combination of them.

1. Flash units are varying the output

2. Flash meter or metering technique is causing inaccurate measurements.

I think at the lowest power levels you may see more of an output variation. What output level are you using on the strobes?

Also are you sure that it couldn't be how the measuring is taking place? That is a 0.4 f-stop range of difference, which isn't huge. I could see a portion of that happening from the light meter being held slightly differently in measuring it.

You might want to compare actual images and see if you can see the light output difference.

Thanks for the reply, and ideas, I have noticed a slight difference in some of the shots. Also I did some testing last night, and your comment about how I am holding the meter might be part of it, I will have to pay attention to that and see. As far as the power setting, I am down just less than 1/8th power. With the main Brolly umbrella about 4 feet away from the subject. I was trying to shoot at f5.6, but read some articles today and they suggested the one might want to be around f11 for main, and f8 for the fill. So I’m going to do another test tonight and see what happens if I raise the power a bit. I am going to do a shoot this Saturday at my wife’s company dinner, so testing to see what will work best for both couples and larger groups, possibly as many as 4 people, so a bit more light might help as well.

Place the flashmeter in a fixed position and take multiple readings. At 1/8 power it should be consistent within +/- 1/10f on each shot. Below 1/16 power you may see some of this inconsistency - typically 1 or 2 shots out of 10 might show this degree of variation.

Thank-you Sir Luap, easy to do, I will try it.

Sir, i just finished a test, and i believe the resultrs are what you would expect, fixed the meter and pointed it directly at the center of the modifier from the point where the subject would be standing. (AB400-1 white bounce umbrella w silver/black backing, AB400-2 28" Brolly)
AB400-1 main @ 1/8 was f5.6 did 20 shots range was between -.1 to +.1 with 11 of the shots at 5.6.0
AB400-2 fill @ 1/16 was f2.8 also did 20 shots, 15 of the shots were at +.3 and the remaining 5 were between +.5 to +.2
When I centered the meter towards the camera and did test shots with both strobes firing, the the measured output was 5.6+3, and it was fairly consistent, enough so that I would just add the +.3 and go.

So I think I have figured out what I was doing i.e. how I was pointing the (pickey) meter, and have a workable solution.

Thank-you for your assistance, and the forum to ask for help!