Auto flash and daylight balance fill

BetterSense · May 27, 2009

I don't use flashes hardly ever. I was wondering if I have an old camera like an Agfa Isolette with a PC socket, and I get a PC cable, I can hook up my SB-28 or other auto flash unit to this old camera, and the flash will automatically stop flashing when proper exposure has been reached, if I have set it the same as the camera. Furthermore, since the lens syncs at any aperture, I can use whatever camera settings I want then, basically, right? In that case, how do I choose? I've always been accustomed to being stuck on the 1/60 sync speed of my 35mm cameras.

Also, I'm not completely sure what the point is of daylight fill flash. I think it's because when it's really bright out, people's faces show harsh shadows, and using the fill flash helps to fill it in and give them a more even lighting. I think modern cameras can do all this automagically. However I'm not sure how you would go about doing this if you were using an old camera and incompatible flash.

Konical · May 27, 2009

Good Evening, BetterSense,

With between-the-lens shutters, fill flash is fairly simple. With an auto-sensing flash unit, set the unit for a somewhat larger aperture setting (smaller f-stop number) than the required f-stop for the situation. The flash will, therefore, put out less light than necessary for your camera lens setting, giving a bit of shadow relief but not enough light for a flash-only exposure. Example: Say that conditions require an f-stop of f11/f16 with your chosen shutter speed; adjust the flash sensor setting so that the unit puts out light for about f5.6/f8. It's not foolproof for all situations, but it tends to work more often than not.

Konical

Nick Zentena · May 28, 2009

For me I make less mistakes if I set the flash film speed higher. So if you're using 100 ISO I set the flash to 200. This way I just match the apertures and don't need to think that much-)

2F/2F · May 28, 2009

I have no idea how auto flash works on that model. Doesn't the flash need to talk to a Nikon camera for it to work? Or is there a sensor on the flash itself, like on third-party units (e.g. Sunpak)?

IMO it would be 10x easier to just set your flash manually. Via a GN/distance/aperture chart or flash meter. Shoot some test rolls starting at a 1:1 subject to background ratio, and then trying 2:1, 1:3, 3:1, 1:3, etc. Then you get a feel for what ratios look what ways, and you know how to set your flash once a reading is taken (or the chart read) for 1:1.

The point of fill flash is to give you more control over your lighting than by just using the ambient light alone. There are a million ways to use it, but it all boils down to more control. Generally I use it to give me more control over subject to background lighting ratios, and sometimes just to change the way a subject is "modeled".

BetterSense · May 28, 2009

It has a sensor on it, so I can use it in auto-mode with any of my cameras as long as I set the settings on the flash to match the camera. I suppose this works only because I have the flash pretty close to optical axis of the lens.

This is where I'm confused. It does make sense that you could do the ratio method with manual flash and estimated distances. But I don't understand why auto mode should work for fill flash at all.

People say, "look at what camera settings you are using, and set the flash to 2 stops less than that, on auto mode". What doesn't make any sense to me, is that on auto-mode, the flash is supposed to fire until the sensor sees enough light to expose the film properly given the settings that you have programmed into it. If your normal camera settings are say EV 12, and you set the flash as if the camera setting were EV14, then why should the flash fire at all? It should be already seeing 2 stops more light than necessary.

Steve Smith · May 28, 2009

BetterSense said:
But I don't understand why auto mode should work for fill flash at all.

Auto mode works because you set the flash to under-expose one or two stops either by telling it your aperture is one or two stops more open than it really is or by telling it that the film you are using is one or two stops faster than it actually is. (your camera aperture and shutter speeds must be correct for ambient exposure for whatever film speed you are using).

Luckily the flash doesn't know that you are telling it lies and dutifully tries to expose to the best of its abilities for the settings you have given it.

As long as the distances involved do not require more power than the flash can produce then the auto sensor will shut the flash off when it thinks sufficient light has been delivered.

BetterSense said:
If your normal camera settings are say EV 12, and you set the flash as if the camera setting were EV14, then why should the flash fire at all? It should be already seeing 2 stops more light than necessary.

Because if your flash is just working as a standard, non-intelligent flash i.e. connected to your Isolette via the sync. lead, it doesn't know or care what the camera settings are. it will just do as it is told.

However, if you use the flash with a compatible Nikon camera which supports TTL flash metering, then the flash and camera can communicate with each other via the extra contacts on the hot shoe connection and depending on the mode chosen, can work out the correct power between them.

Steve.

removed account4 · May 28, 2009

BetterSense said:
I don't use flashes hardly ever. I was wondering if I have an old camera like an Agfa Isolette with a PC socket, and I get a PC cable, I can hook up my SB-28 or other auto flash unit to this old camera, and the flash will automatically stop flashing when proper exposure has been reached, if I have set it the same as the camera. Furthermore, since the lens syncs at any aperture, I can use whatever camera settings I want then, basically, right? In that case, how do I choose? I've always been accustomed to being stuck on the 1/60 sync speed of my 35mm cameras.

Also, I'm not completely sure what the point is of daylight fill flash. I think it's because when it's really bright out, people's faces show harsh shadows, and using the fill flash helps to fill it in and give them a more even lighting. I think modern cameras can do all this automagically. However I'm not sure how you would go about doing this if you were using an old camera and incompatible flash.

i am not sure what kind of shutter is in your camera,
if it is a 5 leaf shutter ( like a copal &C ) not a 2curtain shutter ( like a 35mm ),
your flash will sync at all speeds. with 35mm cameras &C you can always
set the shutter slower than the X, and drag it ... your hands
are never tied as tight as it seems.

if you want to try an experiment so you will have a better idea
and not rely on the flash's cut/off -- auto mode,
you can also take a meter reading ( if you have an ambient meter )
to figure out what the sun light is, and then burst your flash to see
if it overpowers or matches or is less than your sunlight. if you need more
light you can adjust your flash, or fstop or shutter speed accordingly.

good luck!
john

BetterSense · May 28, 2009

As long as the distances involved do not require more power than the flash can produce then the auto sensor will shut the flash off when it thinks sufficient light has been delivered.

But hasn't sufficient light been delivered before the flash even fires? I understand that for daylight fill flash, your film will be more-or-less exposed even without the flash, and the flash is for f oreground fill of shadows. I thought the sensor on the front of the flash measured the amount of light coming back at the camera, and shut the flash off when sufficient light has been delivered according to the flash's settings, like you have said. But why should the flash fire at all in that case? I mean, you have specifically told the flash that your film needs 2 stops less exposure than it really does. If the ambient light is enough to expose your film at its real settings, then it should be 2 stops more light than the flash thinks your film needs. Thus, the flash shouldn't fire at all!

The only way I could see it working is if the flash's auto-sensor could somehow distinguish the flash's light from other sources of light.

It makes perfect sense to me if you are using manual flash, because you can just use your flash chart, and fudge it down a couple stops. But I don't see how you can defeat the auto-flash's feed back mechanism.

Steve Smith · May 28, 2009

BetterSense said:
But hasn't sufficient light been delivered before the flash even fires?

Yes but the flash doesn't know that. You could equally be taking a photograph in a coal cellar.

BetterSense said:
The only way I could see it working is if the flash's auto-sensor could somehow distinguish the flash's light from other sources of light.

The sensor can distinguish between the flash light and the ambient light because the flash light is significantly brighter than ambient. It just doesn't last very long.

The flash only lasts for a tiny fraction of the time the shutter is open. This is why shutter speed does not affect the flash exposure.

Steve.

Sirius Glass · May 28, 2009

BetterSense,

Just try a few shoots. It works! Really!

This sensor is looking for its own light - a very bright but short period light and ignores the rest.

Glasshopper, trust in the force you must!

Steve

BetterSense · May 28, 2009

I have figured it out. I wasn't, as Doc Brown would say, thinking fourth-dimensionally.

The flash's sensor evidently only reads during the time that the flash is "flashing". During this small amount of time, the flash's intensity is so much greater than the ambient light that the ambient light is approximately ignorable. All you practical people probably understood that just fine; I just needed it put in physics language. I even made myself graphs:

c6h6o3 · May 28, 2009

What I do is take an incident light reading, and stop the lens down 1/2 stop from what the meter says. I then set the flash (which in my case is a TTL metering arrangement, but it shouldn't matter as long as your flash has a sensor you can set to a film speed) to double the film speed. I'm not sure why this works, but I get perfectly balanced exposures this way.

Example: Portra 400 VC film, the meter says f11 at whatever shutter speed I've selected (leaf shutter in my camera). So I set the lens halfway between f11 and f16. Set the flash to a film speed of 800. Take the pictures. You'll get fully exposed, open shadows and bright specular highlights without blowing anything out.

Auto flash and daylight balance fill

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