Flash Exposure Question

[Spinners]

Quick simple question:

how is setting your flash exposure on a neutral colored portion of your shot to adjust flash exposure different from just stepping down your flash exposure manually?

when is using one better than the other.

[Spinners]

No answers.. im beginning to feel like i asked a really stupid question.. or noone understands me.. :(

[daveh]

I'm afraid I don't understand so I'll tell you what I do. I first set a flash setting (by guestimation), fire it with my flash meter, then adjust the flash to give me the aperture I want.

Does that help?

[RichardtheSane]

The way I see it when you set your flash exposure on a neutral coloured portion of the shot you will get a more accurate starting exposure from the flash (assuming it is a ttl/ettl dedicated flashgun) and then when you step it down you will get the result you are aiming to achieve.

But then I may not have understood what you aim to achieve so I may be totally wrong

[Spinners]

Let me ask my question this way..

what does the camera do differently in each respect.

when you step down your flash in manual, is the flash adjusting to what it would shoot a 1/2 step lower? how is it calculating a step? What is a step? does it correspond to a shutter speed or aperature setting?

when you flash meter a portion of your shot, in manual.. what does the camera do the adjust the flash.. it cant change aperature or shutter, so does it just adjust its brightness?

i was just curious as to the relationship between the settings, and the changes the camera makes..

does this make sense.. :(

[Yance]

Typically the exposure meter in the camera will average a scene to be 18% gray or neutral. Not all scenes will average out exactly if you are shooting in a snowy field or in a darker environment. If your scene is not average then you may want to meter off a gray card or make some exposure compensation. With flash you really can't meter off a gray card unless you have a hand-held meter, so you have to dial down your flash to prevent it from overpowering your scene if it is dark. Either that or use spot metering which is specifically tied to a focus point.

[daveh]

spinners wrote:
does this make sense.. :(

Not entirely.

Earlier I assumed you were talking about a studio flash since you had another thread on those. Now I think you might be talking about a dedicated flash or built-in flash. For studio flashes it's very simple since the only connection between the camera and the flash is a signal to fire. With dedicated flashes it varies based on what system you're using and what mode you've got it in. There are so many options with dedicated flashes that you'll need to get specific about all the mode settings. Do you have the flash yet? Have you read the manual? If not it may be hard to pin down the exact question.

[mattchase]

Flash exposures can be confusing...especially if you throw in some ambient light as well. This is one of those areas of photography where the no - fun technical things they tried to teach you in school actually become fairly important. I will try to give some answers / examples to your question first, and will go into some other flash exposure info later. Hopefully it won't bore you to death.

I believe with a TTL style flash (though don't quote me on this! ;), the flash will output at a given power until the flash, in combination with the camera, detects that enough light has bounced back through the lens to properly expose the subject based on 18% grey, for whatever aperture the camera is set to. Right...
So if you have your flash compensation set to minus 2/3rds, then the flash will output until it (and the camera) has detected 2/3rds less light bouncing back through the lens for a proper 18% grey exposure.

Shutter speed doesn't really apply to flash exposure, except that for normal flash usage you need to keep it at or under the camera's sync speed ("normal usage", I will hit on an exception later). A flash fires very fast, most of them in the 1000ths of a second range, so to control how much light hits the sensor you have to change the aperture - a larger aperture letting more light in, a smaller aperture letting less light in - or change the flash output through exposure compensation. For example, imagine you have a portrait subject sitting 10 feet from you in a dimly lit room (a 40watt lamp over there somewhere), and lets say the proper exposure for the ambient light in the room is f3.5 at 1/30th second - perfect "mood" lighting (this example works best if you imagine an attractive person of the opposite sex as your subject). But since you are using a flash and ignoring the ambient light, we will keep our exposure settings well above the ambient light exposure. That way, without the flash your picture would turn out black, eliminating the ambient light from the equation. So with a TTL flash (set to TTL) and the camera on full manual, if you take one shot at f16 and 1/200th of a second, and another at f16 and 1/60th of a second, the flash will output the same despite the change in shutter speed, and the two exposures should (in theory) be the same or very similar.

Ok, I admit it, this example has one flaw. Afterall, who in their right mind would shoot a portrait at f16?? (I know, there's always that one time when...)

This (the part above about shutter not affecting your flash exposure) also holds true with manual or non dedicated flash units. The difference with those is that you need to set the power levels correctly on the flash unit to match your chosen aperture first, but once you have them matched then you can change your shutter and it won't affect your flash exposure. A Vivitar 285 has what would be called a caveman version of TTL, that being it's built in thyristor (don't laugh, that caveman thyristor is damn accurate!). With the thyristor set to the Blue position, it will output enough light to expose a subject at about f8 at up to about 15 feet based on 18% grey and ISO 100. The thyristor is smart enough to adjust the flash output, so that if a subject is 5 feet away it will output less, and if a subject is 12 feet away it will output more, always trying to give you f8. The thyristor also has other settings that correspond to other apertures. A studio power pack works by you setting the wattage (or rhiostat, if it has one)((I think I spelled rhiostat wrong)) which will give you X amount of light (aperture) for X amount of distance (feet). It won't, however, compensate for a subject that moves forwards and backwards like the Vivitar does.

That last part was a bit simplified, in that with a power pack / head combo, you would often be using a bounce card, umbrella, spun, or other accessories that would change the actual output you would get (think of those accessories as a manual form of exposure compensation).

Enough already, shut up!! Sheesh.

Sorry. I get carried away sometimes.

[Spinners]

I am referring to my 420EX flash.. i own it, and have used it alot. I am getting really good shots, however i want to understand why my shots come out good or bad. Some reasons are obvious to me. Over/under exposed, blurry etc.. but some.. i think could be better, had i used a different exposure technique.

let me try this example. lets say im taking a shot of a bride. shes all white, standing dancing with her new hubby, he has his vest off, so they both will reflect alot of light. Its obvious that i will need to adjust my flash exposure to compensate. How is this different than just choosing a darker color to set my flash exposure on then recomposing.. does that make sense?

[robertwgross]

Interesting example you made. Shooting a bride with a white dress dancing with groom in a black tuxedo can be difficult. If the camera's meter swings onto the white, you will get one meter setting. If you move ever so slightly and the camera's meter swings onto the black, you will get a completely different meter setting. Evaluative metering might be good, but it is not perfect, and it can be fooled.

---Bob Gross---

[Spinners]

All of this information is great.. however, noone has answered my question. or maybe you all did and i just dont know it yet..LOL.

Let me try this..

Is stepping down my flash the equivalent of metering a different shade on a subject.. Lets take the bride example again. I could just step down my flash 1/2.. this works well, or should i meter something nuetral and not step down.. becuase if i meter something nuetral theres a good chance the whites will come out ok. i think?

i hope im not driving you all nuts.. but this one thing really bothers me. it seems like i can acheive the same thing two different ways.

[hmhm]

If you're trying to gain an understanding of how E-TTL metering works, you'll find some information here:

http://photonotes.org/articles/eos-flash/

Using a 420EX with E-TTL, the camera always determines the power of the flash itself. Your setting of the exposure in M/Av/Tv mode is primarily to allow you control of depth of field and the balance between flash and ambient lighting (e.g. to control the lighting of the background). The camera will always apply whatever flash power it thinks it needs to properly expose the "foreground subject" given the aperture you've chosen.

You need to be particularly careful about the "meter, then recompose the shot", as this has _very_ different implications when using E-TTL. Make sure you understand what "FEL" does, or you'll be in for a rough time.
-harry

[tonygamble]

I'll repost a message I put yesterday on to the CIS Photoforum. It may help.

There are lots of people expressing disappointment at the variations in exposure they are getting
with ETTL and the 220, 550 flashes.

What very few people know is that the ETTL metering is based on only the light being seen by the
active focus sensor when a shot is taken.

Imagine a situation of a landscape shot of a person standing in front of a dark curtain. If you focus
on the person and keep them in the middle of the frame your ETTL reading will be based on the light
coming back from the person. But you may decide to focus on the person and pan the camera
sideways so that the figure is now at the edge of the frame. Your focus sensor will now be facing
the dark curtain - and your ETTL metering will be from the dark curtain also. Clearly you will get a
very over-exposed flash shot.

The way round this problem is to use CF4-1 which turns the * button from an exposure lock button
into a focus lock button.

The reason why this is the way round is because if you press the * button for focus and then
release it you have no 'active sensor' on which ETTL can work. What the ETTL does, without an
active sensor, is to take the average of all the sensors (45 in the case of the 1Ds).

So you now have two ways of using the ETTL. If you focus with the * button and keep it held down
whilst you take your shot your ETTL reading will come from the active sensor. If you focus with the *
and then release it before taking the shot your ETTL metering will be based on the average of all 45
sensors clustered around the centre of the frame.

The first time round this all sounds very complicated - but, once understood it adds extra power to
ETTL.

I now plan to get into the habit of using CF4-1 all the time now for flash. I might not use it for
ambient as it is not as fast as letting the shutter button do the focus finding.

I use several 550 flashes for location portraits as I like their wireless ratio control facility. But I was
getting some very odd exposures - and it will be because I had been focussing on the client with the
half position of the shutter button and then re-framing. If I reframed on to a window the flash would
have underexposed. If I reframed on to a dark wall they would over-expose.


Tony

[Yance]

You have to understand that the distribution and output of light is always going to be constant. You have to always make a choice what part of the image is going to be at a proper exposure. If you have extremes of tonalities in a picture there will always be something either too dark or too bright. You can't have a black tux properly exposed next to a white dress. It isn't physically possible, unless you only point the flash at the black tux and block most of the light from hitting the dress. Usually the face matters most for being properly exposed.

Recognize the tonalities in a scene before you take any pictures and try to figure how to light them. Maybe using the flash on the camera won't work. Get an off camera shoe cord and hold it at a different angle. Do a manual override of the flash zoom setting to maximum and point it only at the faces while using spot metering. Get the STE-2 and have someone hold the flash for you several feet away. There are many ways to approach problem scenes if you recognize them beforehand.