Using light meter

Phred

I am using a Pentax 645Z with two 540 flashes.

I'd like to use my Sekonic light meter to set up my shots with both flashes but it is giving me some unusual readings.

I set up the system for say 1/60 f/8 manually by adjusting the flash outputs until I have a good exposure on the camera. But when I trip the flashes and use the meter, it gives me a very low f-stop reading and not the 1/60 f/8. And when I change the power on the flashes using the Cactus, the reading on the meter does not change. Does the power setting on the cactus change the TIME of the flash duration and not true power? That would explain the same readings on the meter.

 I have tried two meters with the same results.

Additionally, when I take the flash to manual and use the Cactus to simply trip the flash, the readings obtained on the meters are correct...and the f-stop changes accordingly with changing the flash power manually. And the readings for a good exposure on the meter match the results of the camera.

So how does the Cactus control the power on the flash? By time or intensity? So how can one use a light meter with the Cactus system? I should not be in HSS with the system set at 1/60

raychan

When shooting in HSS, the flash would have a pre-flash before the main flash. What the light meter measured is a pre-flash, which the result is the same at difficult power level. 

FYI, Cactus uses the light intensity to control the power on the flash. 

Phred

Hi Ray,

Thanks for the response...Yes, the pre-flash makes perfect sense.

I was shooting at 1/60...not in HSS? So should I still get the pre-flash? Or is the P-TTL mode causing the Pre-Flash? Is there any way around the Pre-Flash?

Can the Cactus change the flash power setting if flash is set to Manual?

Antonio

... Can the Cactus change the flash power setting if flash is set to Manual?

No it cannot.
Flash in TTL mode is the only way it could be controlled by the V6 II.

Flash in M mode only listens to one basic trigger signal via the central hot shoe pin.

Class A

So should I still get the pre-flash?

I wish it weren't so, but unfortunately the answer is "yes".

 
 

Or is the P-TTL mode causing the Pre-Flash?

Yes, it does not matter whether the shutter speed exceeds the sync-speed or not, the V6II apparently always drives the flash as if it were part of a P-TTL system (causing the pre-flash).

 
 

Is there any way around the Pre-Flash?

AFAIK, with the V6II your only chance is to set the flash to manual mode (and thus lose remote power control).

You could fire your 540 flashes with the V6 without any pre-flashes. However, you'd have no HSS. This would only work for shutter speeds that do not exceed the sync-speed.

BTW, the RF60x does not issue a pre-flash even when used with the V6II. You still get the unfortunate P-TTL pause before the main flash, but at least there is no pre-flash.

Phred

Interesting on the RF60X's...and no pre-flash.

I have two coming from B&H to try....

So I should be able to meter these...correct?

I can then use the Pentax flashes for background and hair...just dropping power a few stops proportional to the metered RF60s's.

FKD

This use of pre-flash means using a light meter with Cactus 6ii is not going to work for HSS. Period.

You can turn flash units to Manual and the Cactus 6ii will still trigger them but they will not send out pre-flash and you will get meaningful light meter readings.  Just not for over 200 (0r 250 on some).  

is that it in a nut shell?  

Antonio

@FKD you have nailed it.

FKD

this might be deal breaker  

choice is cheaper via Godox X1 or YN622N  0R

Cactus  

or 

bite the bullet and go PW TT5 at about $700 before light meter (two TT5 one mini TT1 AC3 in a baggie set )  controls two off camera   Cactus 6ii would be just over half thzt at $390    Is TTL on a PW system worth twice as much or $310 more???

And that is foreign imported from that strange place called New York