Wireless flash question

[883robert]

Hi all, newbie here.
I'm using my A700 and HVL 42AM flash for portraits and can't balance the lighting as I'd like to.
I have the flash operating wirelessly, illuminating the subject from one side through an umbrella. My problem is that the on-camera flash which triggers the HVL is providing fuil illumination to the subject. I want the HVL to be the main light with the on-camera flash only providing fill.
I've tried flash exposure compensation but that reduces the light from the HVL. The only way I can get the results I want is to tape some fabric over the on-camera flash to attenuate its light. Is there another way to raise the output of the wireless flash relative to the on-camera flash? If I can find a solution to this then my next step is to incorporate a hair light and a back light using old flash units I have lying around.
thanks in advance
Robert

[Javelin]

The onboard flash shouldn't be influencing the scene at all .. it is possible to make it add some light using extreme settings but I notice i can eliminate that effect by choosing HSS and a higher shutter speed. what your saying is the subject is lit completely with the onboard while still firring the wireless and thats not been my experience with my 36. David know the system very well and might be able to shed some light (haha) on the subject...

[883robert]

Yes, you surmise correctly. The on-camera flash alone provides what looks like sufficient illumination while still firing the 42. However the camera is not telling the on-camera flash to provide stand-alone illumination as otherwise putting fabric in front of it would not diminish its output. Rather, the camera would increase its output to compensate. And at a subject distance of 1.5m, the on-camera flash would not be at full power.
So, at the risk of overdoing the obvious puns I'm still in the dark here. As you say, David could well be able to set me straight.
Robert

[stevecim]

Hi Robert

I don't know if this helps, but I remember reading , some where on this forum, (I think ) where someone used a piece of black developed film in front of the on-camera flash, which would block the visable light but not the IR light which is used to trigger the remote flashes. But I can't find the post, I could be wrong

[David Kilpatrick]

Your problem is that the umbrella is reducing the offboard flash light by about 75%, which makes the fill-in from the triggering onboard seem too strong. The best way to deal with this is to invest in an HVL-F58AM, which has to be used (regrettably) as an onboard commander - wasting most of its potential - in order to make proper adjustments in the ratio of lighting between onboard and remote units. However, it is not a total waste. The 58 flash, when mounted on the camera, can be aimed to the opposite side the brolly flash is situated, and bounced off a large reflector (a 1m square or larger panel is needed). The power can then be controlled to give you exactly the right level of fill in.

Also, with the 58, you can set options which may make it easier to sync up hair lights. At present, I think you would need to use the X-sync cable socket for this, and trail a cable to any fill or hair light because slave cells will trigger dumb flashes too early.

I am just testing the Elinchrom BXRi flash heads. These are amazing. You can teach the studio flash head to recognise the pre-flash control pattern of the Alpha wireless system and ignore it, only firing with the final actual exposure. I have more tests to do on these. They also come with a Skyport radio triggering system (needs an adaptor for Alpha hot shoe).

David

[pakodominguez]

other than what David said, you probably are working with hi ASA in order to get "enought light" and that's why the otherwise week on-board flash get into your photograph...

[youpii]

The other way is to go entirely manual with ebay wireless triggers.
You'll see a lot of info and tutorial on Strobist.
http://strobist.blogspot.com/" onclick="window.open(this.href);return false;

[883robert]

Thanks David for your informative reply. Can you explain one more thing? I don't understand how the umbrella can alter the ratio of built-in flash to off-camera flash. Regardless of the attenutation by the umbrella, is the HVL 42 not instructed by the camera to increase its output till the camera sees enough light reflected?
Camera to subject distance is 1.5 m and HVL to subject is 1.5 m also.
I have the HVL42's tube pointing into a reflector umbrella and the flash's sensor pointing back towards the camera. What's the sequence of events when I make an exposure? I assume the built-in flash instructs the 42 to commence firing and when the A700 (via TTL?) registers sufficient light reflected from the scene, it sends a shutdown signal to the 42.
The 42's sensor is pointing all the time towards the camera and not towards the scene so it doesn't register any reflected light from the scene.
Hope you can clear up my confusion.
Thanks in advance
Robert

[David Kilpatrick]

The remote flash receiver is completely immune to direction, except in terms of being too far or too close. All measurement is done through the camera via the pre-flash, all the sensor in the flash receives is a simple on-off pulse sequence to tell it what to do. No measurement, or transmission of values, is involved. By bouncing the remote flash, you cut its power just as if you had taped a 4X neutral density filter over the flash and aimed it direct. That makes the 1/16th power of the onboard flash (used for final exposure) relatively stronger. You can just detect the presence of the onboard in a normal shot, it will be maybe four times as bright in your set-up.

The HVL-42 can not be instructed to increase its output, only to cut its output. It may well be working at maximum power. And, what do you mean by 1.5m? Do you mean the flash is at 1.5m, but the brolly is sited further away (say 2m on its stalk) in which case the flash is actually 2.5m from the subject with a bounce surface increasing the apparent distance (GN loss) to approx 4m.

To do proper umbrella flash set-ups, the HVL-42AM is on the edge of underpowered. It should produce around f8 at ISO 100 with no problem, but if you wanted to use f16 for any reason it might be difficult. It does depend on the brolly, a flat white one is least efficient but gives good light quality, a silver one will gain you a full f-stop (or more) but with harsher light.

David

[883robert]

Hmm, hadn't thought about the extra distance involved in flash to brolly. You're right, with the flash at 1.5 m and bouncing off a silver umbrella at say 0.5 m away, total distance is indeed 2.5 m from the flash head.
I didn't know that the built-in flash normally delivers 1/16 power. Is this 1/16 of the output of the off-camera flash? Or is it 1/16 of the output to illuminate the scene on its own?
In any case it seems the best option for me, if I don't want to spend money, is to set up the off-camera flash to that its output is diffused without losing too much light in the process. Perhaps a flat sheet of thin fabric.
As an aside, another purpose I had for the HVL42 is for architectural interiors. It may prove to be underpowered for that. I intend to position it far from the camera, at the end of a room. Does the end of the room have to be within range of the camera's built-in flash in order for the pre-flash to do its job?
Thanks for all your help, David. I'm learning all the time. My aim is to be able to set up flash lighting by the numbers, ie follow a known plan to give a known result.
kind regards
Robert

[David Kilpatrick]

1/16th power is the lowest output the built in flash is capable of on all cameras with variable manual flash output, and if the figure is not 1/16th, it's close to that. For your architectural interiors, the maximum range for wireless control using the onboard flash is official 5m but in dark conditions, I've had successful triggering up to 10m. If the existing light is bright, the range is reduced, and in very bright conditions (adding flash to an interior with full daylight) may not function at all.

David

[883robert]

Ah, thanks David, you cleared that up for me. I had assumed that the preflash for the purpose of exposure measurement came only from the built-in flash. From your answer I infer that the built-in flash need only be powerful enough to trigger the remote flash, which then emits a preflash for exposure determination. The built-in flash then responds with a flash indicating to the remote flash the correct exposure required from it.
If I've got this wrong, let me know, otherwise thank you for your time and patience.
regards
Robert

[David Kilpatrick]

That's more or less correct - the remote flash provides its own pulse for pre-flash measurement, yes, the built-in flash only controls the sequence.

David