Help with Flash Exposure Compensation please

[precious]

Hello all,

I have some very basic questions about flash. I'm hoping someone can help me out.

I'm shooting indoors, an informal get-together of sort. The light is so low, w/o flash I would have to open the lens wide (f1.8 on 50mm lens). This gives me enough shutter speed to hand-hold the camera, but has a much greater DOF, so if people are in different planes, one of them is in focus and the other one isn't. (A couple for example).

So, how can I use flash to overcome this? I have a Canon Xsi (on-camera flash) from what I read online, it has flash exposure compensation to reduce the flash power. So, when I experimented yesterday and noticed how the pictures were better lit when the flash fired at -2 compensation.

But what I don't understand is:

(1) Does turning ON flash have any effect on exposure? I mean, can I increase shutter speed if Flash were on, vs. not?

(2) When I press shutter half-way, and have the flash open, during the auto-focus process, the flash fires in bursts at full-power which causes inconvenience to the people.

How can I get around it? Is the exposure calculated differently if the flash were while focussing vs. not?

(3) If I were to try to AE lock, I notice on viewfinder, FEL lights up. This is same as flash compensation?

Thankyou in advance.

[OsmosisStudios]

Originally Posted by precious

(1) Does turning ON flash have any effect on exposure? I mean, can I increase shutter speed if Flash were on, vs. not?

(2) When I press shutter half-way, and have the flash open, during the auto-focus process, the flash fires in bursts at full-power which causes inconvenience to the people.

How can I get around it? Is the exposure calculated differently if the flash were while focussing vs. not?

(3) If I were to try to AE lock, I notice on viewfinder, FEL lights up. This is same as flash compensation?

Thankyou in advance.

1: Yes. You can increase the shutterspeed to a preset "Sync Speed", which is generally 1/200 or 1/250. The thing is, when using flash, the shutterspeed affects the AMBIENT light while the APERTURE controls the flash output.

2: That's how some cameras (namely the Canon Rebels) use their Autofocus assist. What it means is that there isn't enough light in the room for the camera to focus. There's no way to turn this off, but you can try using a flashlight on your subject. if you use a speedlight, the speedlight has a red-beam IR that it uses instead of flashing.

3: AE lock and FEL are both LOCKS. What they do is they lock the exposure (in automatic and semi-automatic) modes at the current setting. Useful for focus/recomposing.

Flash exposure compensation is a way of "tricking" the flash into being stronger or weaker. The camera calculates the flash power necessary for each shot, but you can tell the it how to adjust that output to your liking. -2 sounds like a lot though.

If you're using the pop-up flash, you may find the power lacking for large groups.

[inkista]

Originally Posted by precious

(1) Does turning ON flash have any effect on exposure? I mean, can I increase shutter speed if Flash were on, vs. not?

This is where flash photography differs hugely from straight-up ambient photography. You're actually dealing with two separate exposures combined in one image. Flash doesn't simply add light to the ambient in the way you think it does.

The burst from the flash is fast. REALLY fast. Faster than your X-sync shutter speed. So, you can adjust the shutter speed all you want (well, up to the X-Sync speed) and the amount of light coming from the flash is going to be the same.

This is NOT true with the ISO and aperture, however. The ISO and the aperture will both affect the amount of light you're getting from the flash illumination.

(2) When I press shutter half-way, and have the flash open, during the auto-focus process, the flash fires in bursts at full-power which causes inconvenience to the people.

How can I get around it? Is the exposure calculated differently if the flash were while focussing vs. not?

The flash may be doing one of two things. It may be firing the preflash for e-TTL (in which case, you're stuck with it--but it's not all bad, as this is how the camera determines what power output to set on the flash), or it could be firing the AF-assist lamp to help you autofocus in the dark (this you can turn off in the camera's custom functions [CFn.III-7]).

[precious]

Osmosis, Inkista, thankyou for your detailed replies.

I think I understood some of it. I will try some experimentation tonite.

[precious]

Originally Posted by OsmosisStudios

3: AE lock and FEL are both LOCKS. What they do is they lock the exposure (in automatic and semi-automatic) modes at the current setting. Useful for focus/recomposing.

Thanks, I noticed when I press AE lock button and have flash open, it says FEL. Two locks in one button got me confused.

Originally Posted by inkista

The burst from the flash is fast. REALLY fast. Faster than your X-sync shutter speed. So, you can adjust the shutter speed all you want (well, up to the X-Sync speed) and the amount of light coming from the flash is going to be the same.

This is NOT true with the ISO and aperture, however. The ISO and the aperture will both affect the amount of light you're getting from the flash illumination.

The flash may be doing one of two things. It may be firing the preflash for e-TTL (in which case, you're stuck with it--but it's not all bad, as this is how the camera determines what power output to set on the flash), or it could be firing the AF-assist lamp to help you autofocus in the dark (this you can turn off in the camera's custom functions [CFn.III-7]).

Right, I noticed the X-sync for my cam is 1/200. How does Aperture/ISO control flash output? In manual mode, I can fix shutterspeed to 1/200, select Aperture and ISO, and the camera would determine flash power based on the settings? It would lead to foreground being illuminated by flash and the background would be black?

Thanks, the [CFn.III-7]) did the trick. When I turned it off, the pre-flash did not occur. I wish I can determine the pre-flash power as well, so it doesn't blind people.

[inkista]

Originally Posted by precious

Right, I noticed the X-sync for my cam is 1/200. How does Aperture/ISO control flash output?

Same way it controls ambient. The higher your iso, the more the flash registers. The larger your aperture, the more the flash registers. It doesn't directly set the power level of the flash via the camera the way you're thinking.

I repeat: there is no single "correct" set of exposure settings. There's a range of ways you can balance the flash against the ambient, and still have the overall resulting image seem correct--all the way from flash completely overpowering the ambient, leaving a black background to ambient being the majority of the light in the image, with flash serving as fill to lift the shadows.

The first thing you need to know is that with Canon cameras, the CAMERA mode will change how the flash behaves. Shooting with the camera in M mode gives you full control. Shooting in Av or Tv, the default of the camera is to use the flash as a fill unless you've changed the custom functions for how the flash should behave in Av. Essentially, the camera will assume you want most of the light to come from the ambient, and the shutter speed in Av will be set to pretty close to what it would be without the flash. In P or green-box auto, the assumption is that flash is the main source of illumination and if the background's going to be black, so be it.

When we talk about shooting with the flash in Manual, howver, we're talking about the flash mode, not the camera mode. With the flash, you have a choice of going with eTTL, in which case the camera's AE system and metering is controlling the flash power, or shooting in Manual in which case you can set the power directly on the flash back (or through the camera back) to full power down to 1/128th power (if you have a 580EX, 1/64th if you have a 430EX).

In manual mode, I can fix shutterspeed to 1/200, select Aperture and ISO, and the camera would determine flash power based on the settings?

No. If the flash is in eTTL, the camera sends out a preflash of a pre-determined brightness, meters it, and then adjust the flash power to get that burst where it thinks it ought to be. You can adjust an offset to this with FEC (flash exposure compensation). If the flash is in Manual, whatever you dial in is the power being used.

The spread of the flash is determined by the focal length of the lens attached to the camera.

For the basics on balancing flash against the ambient, I recommend "Dragging the Shutter on Planet Neil, and this lesson and then this one from the Strobist's Lighting 102.

I would also recommend that if you don't want to be blasting people in the face with your light, you may want to consider the black foamie thing to flag off direct light from the camera.

[tadthephoto]

Re your point 2, the flash fires to provide enough light for the lens autofocus to work. (Many autofocus systems, Canon's incuded, look for contrasts in the scene, low light means insufficient contrast, so the ambient light is supplemented by the flash). I think you will find that by switching the lens to manual focus the autofocus beam will not fire. You will then have to focus manually, but if you choose your aperture to give you a deep depth of focus you should not have to refocus for every shot. (What I do is initally set the focus with autofocus on, then switch the lens to manual focus for the rest of the shoot).

[precious]

Originally Posted by inkista

When we talk about shooting with the flash in Manual, howver, we're talking about the flash mode, not the camera mode. With the flash, you have a choice of going with eTTL, in which case the camera's AE system and metering is controlling the flash power, or shooting in Manual in which case you can set the power directly on the flash back (or through the camera back) to full power down to 1/128th power (if you have a 580EX, 1/64th if you have a 430EX).


No. If the flash is in eTTL, the camera sends out a preflash of a pre-determined brightness, meters it, and then adjust the flash power to get that burst where it thinks it ought to be. You can adjust an offset to this with FEC (flash exposure compensation). If the flash is in Manual, whatever you dial in is the power being used.

inkista, thankyou for the detailed explanation and the links to the tutorials.

Canon Xsi does not give me much control on flash (like how it should be in Av.) Using C.Fn 7, I turned off the pre-flash. Now, when I switch to Manual mode, I don't see a pre-flash either. I will have to figure out how to control the camera using the basics you mentioned.

Appreciate the help

tadthephoto, the auto-manual focus shift idea is cool. I will try that out, thanks.