Flash and metering


TPF Noob!
May 31, 2012
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Hi guys, sorry for a noob of a question ( I have many of them in future I'm sure)
So i have a Canon T2i
Just got a 420EX II flash for pics indoors

One thing i dot get is the exposure meter when using a flash
Say is a dark inside shot. If i have settings at something like 1/250 f8 iso100, then when i look through the view and half press the metering obviously detects a hell of a underexposed shot and warns me to that effect
If however i turn on the flash and take the shot with the same settings I still get the exposure bar warning me of under exposure, but when I actually take the shot and the flash fires the exposure is great! So why does my exposure meter not take into account the fact the flash will fire?

Sorry I can reword if needed
Thanks !
Because it can't. Simply put: The exposure meter can only deal with the light that it exists when you press the button. When you have the flash activated (In TTL mode), the camera detects the return from the flash and automatically adjusts flash output and/or exposure to deliver an appropriately exposed scene.
Well, to put it simply, it's not a flash meter. It's metering ambient light. You may want to read up on TTL (through-the-lens) flash metering. I don't think I can explain it very well.
Here is the step, pretty much... I hope you understand it.

1. The camera meters the ambient. This has nothing to do with the flash. It is just giving you an idea how much the light surrounding you will affect your exposure. So if you are way underexposed and the flash doesnt fire, you have almost black photo.
2. Assuming your flash is set to auto, right before the shutter opens, the flash shoots a very low power light. The flash will take reading trough the lens (TTL) on how much that little power of light change your exposure compared to the reading ambient reading (no. 1).
3. From the data gathered, the flash decides how much power it should use to give you a proper exposure. It is not always correct. Sometimes you need to do flash compensation. It wont be consistent either depending on how reflective your subject is. The only way to get it consistent is if you use manual power so it shoots the same flash power all the time.

I hope that makes sense.
wow quick response guy! thanks
Ok so the flash is set on ETTL
I aim at my subject (its still showing that im under exposing)
I press the Flash Exposure Lock button
The flash prefires, the viewfinder on the camera shoes FEL for a second but Im not sure what its acutally done?
Has it adjusted the power on the flash to make it the correct exposure?
Thanks everyone
You dont have to do Flash Exposure Lock button. Just fire away. FEL will just lock the flash power if I am not mistaken. If you keep shooting without a long pause, the flash keeps firing at the same power if you have FEL on I believe. I dont really use it so I am not 100% sure what I am saying is accurate.
ohhhh is that why there is a flash right at the begining of the shot, even if i set it on rear curtain and have a long exposure, that initial flash is just the TTL calculating the required flash before the shutter even opens!
Also, if you are way underexposed, your flash will work extra hard and maybe at full power. It will take a while to recharge. If you are super duper underexposed, your flash cant put out enough flash power to correct the exposure so it will shoot at full power AND still underexposed. You are the photographer so it is up to you on how much underexposed you need to be depending on what you are trying to achieve.
ohhhh is that why there is a flash right at the begining of the shot, even if i set it on rear curtain and have a long exposure, that initial flash is just the TTL calculating the required flash before the shutter even opens!

yes. Most of the time if you dont set it to rear curtain, you wont even notice the preflash.
outstanding! thank you very much for taking the time to explain that to me! i really appreciate it.
This is a great site, ill definitely be sticking around!
Flash is different than natural light photography. In theory, when you use flash correctly, you don't necessarily have to meter. I'll explain.

The hot-shoe flash (which works a bit differently than a studio flash) is rated based on an industry standard called the "guide number". The guide number isn't actually the amount of light it produces... it's the amount of light that is delivered to the subject. The reason they're not one and the same is because the bulb and reflector inside the flash are motorized and allow the flash to spread the light wider or focus it tighter. The flash communicates with the camera, which communicates with the lens. It knows the focal length of the lens. Therefore it knows how much it needs to focus the beam (studio flashes don't have motorized heads & reflectors that dynamically focus the beam.)

Your Canon 430EX II has a guide number of "43" meters (note that the model is 430 but the guide number is 43... the 580 has a guide number of 58, the 270 has a guide number of 27 -- and these are all measured in "meters" -- not feet (and you have to be careful... sometimes a flash guide number is measured in feet, but Canon always uses meters.)

That means: your flash is capable of delivering enough light for a proper exposure assuming the following baseline: ISO is set to 100, and f-stop is set to f/1.0. If you are using ISO 100 AND f/1.0, then the flash can deliver enough light for a proper exposure at a distance of 43 meters (141 feet).

The ISO 100 part is no problem. But the f/1.0 ... nobody makes an f/1.0 lens (Canon used to make an 50mm f/1.0 lens ... they haven't made that lens in quite a while.) So you might be wondering: why use f/1.0 when nobody has an f/1.0 lens? It's because f/1.0 makes the math VERY easy to convert to whatever you really are using.

The formula is this: divide your guide number (43 meters or 141 feet) by the f-stop that you are using (e.g. if you are using f/5.6 then you divide 141' by 5.6... and you arrive at about 25'.)

Note that this is enough light to create a proper exposure where ALL of the light comes from the flash and NONE of the light is ambient light from the room. In theory, the ambient light in the room is dim enough that it's inconsequential and won't impact the exposure. If the location happens to be very bright then it alters the conditions... because you've basically got "flash" which is momentary light combined with ambient which is a continuous light source.)

Since you have a 430EX II, you have a choice: You can shoot using Canon's E-TTL or E-TTL II protocol... OR you can use manual.

E-TTL and E-TTL II are very intelligent (there are some subtle differences). When using this mode, your flash actually always fires twice -- but the flashes are so close together that you probably only think it fired once (it's VERY fast). The first flash is a pre-flash. The camera shutter isn't open when the pre-flash fires. It fires this at 1/32nd power (that's the default but you can change it.) The camera and flash "talk" to each other. The camera meters the scene without any flash, then meters the scene with the pre-flash (at 1/32nd power) and notes the difference using Canon's matrix metering. It's specifically looking for areas of the scene where the light appears to have not changed much at all (which would indicate an ambient light source in the room -- such as a table lamp in the scene) and it also looks for areas where the difference is VERY dramatic (such as a the reflection in a mirror), and finally it also looks for areas where the difference in lighting is expected. It actually does take into account what the lens reports as the focused distance to the subject (assuming you're using a Canon brand lens that reports this information.)

Once it performs all the metering (via the pre-flash comparison to ambient vs. flash) it THEN calculates exactly how much power should be delivered for the *real* flash when the shutter is open.

The other choice is manual. This merely requires that you know how guide numbers work, AND you know the "inverse square" rule. The simple version is that, when firing at full power, you know the flash will deliver enough light to a subject 141' (let's just say 140 feet because it'll make the math a LOT easier -- as 140' is exactly 100 times the square root of 2 (which we round off to simply 1.4)). That means that at full power, your flash can cover the 140 foot distance at f/1, at f/1.4 it can only cover 100 feet (140 Ă· 1.4 = 100). At f/2 it covers 70 feet, at f/2.8 it covers 50', etc. HOWEVER... if you start bumping up the ISO from 100 -> 200 -> 400, etc. then you get to multiply the distance by 1.4 for each time that you double the ISO.

This "sounds" complicated and you may have to re-read it three or four times to digest, but once it clicks ... you realize that using manual flash is actually pretty easy.
Tim, NO! Don't tell them all that! Next thing you'll know people will be taking properly exposed pictures with flash, and know what they are doing! And then what are we going to do for entertainment value???

(J/K - very good explanation. Do we get to call you Professer?)
Thanks TCampbell for that great explaination. Thanks for your time to educate those of us who need it.
I was playing with a new light modifier last night with a meter. I suggest the Sekonic 358.

And my girl Jersey last night proved no matter what the light meter said at different settings, the Octabox makes some soft light =)

Pic of meter:

What a great thread, bookmarking this for when in ready to get a flash.

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