Basic Flash (Light Meter) Question

[silentknell]

Hi, I'm fairly new to using external flash, and completely new to using a light meter, and I have a question hoping that someone here can answer it.

My goal is to take a portrait, with the subject at the correct exposure with flash, and the background 2 stops underexposed.

I'd like to keep my shutter speed no faster than 1/160th in order to keep from using HSS. I have a ND16 filter on my camera to keep it at fairly low shutter speed, and wide aperture in bright sunlight.

So how do I use my light meter to make this happen?

From what I've read, I meter for the background first by putting the light meter into ambient mode. And since I know I want to have the final output at 1/160, I put it in shutter priority.

But then what? Before I use my flash meter in flash mode on the subject, what adjustments do I need to make, given that 1. I want the background underexposed, and 2. that I have a 4-stop ND filter on my camera?

Thanks for your help!

(Edit: Also, when you test for your flash exposure, what is the usual method in testing the flash (without a cord)? Do you just press the "test" button on the flash, or do you usually start at a certain power output, like 1/4 or 1/8?)

 

[silentknell]

Wow, absolutely no one, huh. Maybe I phrased the question in a way that people didn't quite understand what I was asking? Or maybe no one uses a light meter any more?

Well, after reading online a bit, the ND16 filter part may have been solved. It seems that on the light meter, one can just lower the ISO by 4 stops in order to compensate for the 4-stops lowered on the camera. So if I wanted to use ISO200 on my camera with the filter, then use ISO12 on the light meter to meter for the ambient light.

Also, since I want to underexpose the background by 2 stops, I'm guessing I can initially meter in shutter priority by having the shutter speed at 1/40. Then once it tells me what the aperture is, plug that into the camera, as well as ISO200 and 1/160 (so that I can avoid having to go into HSS).

Then I guess meter for the flash on the subject with ISO12, 1/40 shutter speed, and then adjust the power output or the distance of the flash until the aperture is the same as the reading I had for the ambient light.

Does that sound about right? Would really like some input from some of the more experienced here.

Also, I just noticed that the optical triggering system of Olympus and the Sekonic's cordless flash meter reading don't really go well together due to the pre-flash. Is there a way around that, or do I need to get a cord. And if I need a cord, what cord would I need? I've just been changing the power output in slave mode, pressing the test button on the flash, then once I figure out the power/distance, then placing it back in the RC mode, and plugging the power in the camera what I needed in the slave mode. Seems overly complicated.

 

[OzRay]

The best thing to do is to take an incident light reading off the subject and then simply set the background flash units manually to a higher setting (assuming that you have a flash meter). You haven't really described what flash units/lights you have, so it's difficult to give accurate advice. Ideally, you would be using flash for all of your lighting, in order to keep the colour accurate and not mix incandescent/halogen with flash, else you will need coloured cells on your flash to compensate. You should also be able to use flash without any ND filters, as they should be fully adjustable. If all else fails, with a digital camera, all you need to do is fire off a few test shots to determine the correct flash exposure for the subject and then repeat for the background, and you should be right in the ballpark.

 

[Rudy]

It's really pretty easy in principle.
Your background exposure is determined by the camera settings only, so do whatever is necessary to underexpose by 2 f-stops while keeping your shutter speed at or below your sync speed. I would recommend manual mode unless the background lighting changes quickly.
Then set your key light to a best guess power level and set your flash meter to your camera settings. Pop the flash and get the reading. Depending on the meter it will either tell you how far off your exposure was or the correct f-stop to use.
Make the appropriate adjustment on the power level of the flash to zero it in and that's it.

Details:
The meter does not know about the ND filter. You can make the adjustment via the ISO setting as you mentioned or simply make the 4 f-stop adjustments in your head. For really accurate exposures you will also want to make an adjustment for the T value of the lens. Lenses with many elements or poor coating transmit less light than their f-stop value would seem to suggest. This is not an issue when using TTL metering, but again your external meter has no idea what lens you are using.
Telling the meter your shutter speed allows it to do a flash plus ambient reading which will be more accurate in terms of achieved exposure, but can mess with power adjustments. For example if the meter reads your exposure as being 1 f-stop too high then dialing down the flash by that amount might not be enough if there is a significant amount of ambient. In the extreme you could turn off the flash completely and still be overexposed.
RC mode issues:
If you want to use the cordless sync of the Seconics then you will have to use simple slave mode, none of the fancy modes will work. The Olympus system uses preflash communication which will start the Seconics trigger and will count towards the measured exposure even though the camera does not yet expose. In dumb mode all slaves (and the meter) will fire when the first flash happens. Radio triggers are usually easier to use than the light based control Olympus uses.
Rudy

 

[silentknell]

Sorry, OzRay, from your post, I can see that I clearly did not give much information about the setting I had in mind. First of all, I have an E-M5, along with FL50R and a FL36R, and a Sekonic L-358. The setting I had in mind was just using one flash in bright outdoors, the subject in light shade, with much brighter background. Which is the reason I bought a ND filter to keep the shutter speed down below the sync speed. But I appreciate the input. When you said, "simply set the background flash units manually to a higher setting" are you referring to lights behind the subjects, like a rim or hair lighting?

Thanks Rudy. When you stated, "set your key light to a best guess power level" I'm guessing that only multiple experiences and experiments will help me learn what power level is the best at certain distances? The other things you mentioned is still a bit too complicated for me to grasp (as I'm not familiar with T-value) but I thank you for adding that bit.

I haven't read much about the slave mode on the Olympus flashes. In RC mode, the pre-flash seems to trigger the Sekonic, and the actual flash doesn't register because of it, so the light meter tells me that I am underexposed. In slave mode, do they all flash at the same time without pre-flashing? what sort of radio trigger would you recommend, and can you still use them with the Olympus flashes despite them being used in dumb mode?

I appreciate both of you for taking the time to answer these what I assume to be elementary questions. And I'm sure I'll have even more questions as I experiment with them even more, or add more flashes.

 

[OzRay]

The Olympus flash units can be remotely adjusted for power, but the Sekonic is most likely triggered by the optical pulse. I'd use one Olympus for fill flash on the subject and another for side light etc and the Sekonic for background lighting.

Radio flash controllers do work, but you have no control other than manual adjustment. Personally, the Olympus wireless is great for what you appear to want to do, but you have to learn how to use them effectively.

I have a Minolta light/flash meter, but with digital cameras, it's so easy to see your results and adjust accordingly, that I don't bother with it much at all. I've actually found it easier and faster to set up the lighting just using the camera. The ISO (ASA) reading on my meter don't really reflect what the camera will record, so that's one reason I tend not to use it.

 

[alex66]

Ok simple thing to get what you want is the Aperture is adjusted for the flash and the shutter for the ambient. So if you want to set the back ground 2 stops under you raise the shutter speed by 2 stops over the ambient reading, but in brighter light you run into under powered flash as an issue. There are a lot of guides to this sort of thing on the web, you could look at the strobist site for example. I have tried this with moderate success but only at dusk or dull days, flash units powered by AA batteries on the whole don't have the power. My solution was to mount a load of old flash guns that I had lying around with optical slaves to make a larger unit, it was very 'Heath Robinson' but works. If I was doing it seriously, over just having a go to see if, I would buy a portable high powered battery flash set (more like Studio flashes) so I had the power.

 

[OzRay]

If you really want to defeat the sun, you need to do this: http://strobist.blogspot.com.au/2008/05/joe-mcnally-desert-shoot.html.

 

[alex66]

If you really want to defeat the sun, you need to do this: http://strobist.blogspot.com.au/2008/05/joe-mcnally-desert-shoot.html.

Thats the sort of thing, though 7 top of the range flashes is a fair amount of cash. I think you could pick up a portable studio light set up for less, learn to do it manually and you save a fortune, its not too hard with instant previews on cameras now.