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DPChallenge Forums >> Hardware and Software >> Canon 400D - metering in Av/Tv/M with flash
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02/15/2008 01:57:50 AM · #1
Hi,

I bought a 400D recently. Enjoying the camera very much. However, in Av/Tv modes, when I pop up the flash, the camera still meters as if there would be no flash, resulting in much slower shutter speeds or much bigger apertures than needed. Therefore, I always have to go to the manual mode, and then I still have to guess the shutter speed/aperture combination, since the metering system always says I totally under-expose (it doesn't take the flash into account).
Is it supposed to be this way? Do I have some problem with the camera? Am I doing something wrong?

P.S.: I'm talking about indoors photography during daytime, the metering system suggests shooting at 1/10sec with F/4.5 (around 35mm), while I override it to ~1/80sec with a flash and get great results.
02/15/2008 02:37:31 AM · #2
Keep in mind with your flash shutter speed will control the amount of ambient light and aperture will determine flash exposure.

So you probably want to go manual when shooting with flash anyway so you have proper control over both of those things.
02/15/2008 02:51:29 AM · #3
Hm, I never thought about it this way - I'm too accustomed to my previous non-SLR camera setting the shutter speed automatically to 1/40 when shooting with flash.
02/15/2008 05:26:53 AM · #4
Well, there are 3 choices to get around this problem.

1) Go manual if you really want full control over everything. Set the shutter to 1/200 if you want to just go with flash, or set the shutter slower if you want some ambient light. 1/60 will often let a little ambient light while still being hand holdable. Set the shutter slower if you want to drag it for some warmth from ambient tungsten light.

2) Flick over to P for quick flash shots. I use this a lot, because I get lazy, and I can't always think quick enough to muck around with Manual mode for casual shooting, especially when I have people waiting to be shot.

3) There is a custom function to automatically limit the shutter speed to 1/200 in Av mode when the flash is used. This will let you set the aperture just like normal in Av mode. I tried this a few times, but once I realised what was going on, I am actually more comfortable leaving this custom function off, and allowing Av mode to drag the shutter. You just have to be aware of what's going to happen, and use solution 1) or 2) when needed.

02/15/2008 05:41:11 AM · #5
I find I get the best results in Manual Mode.

Set your prefered Aperture and Shutter and use a hotshoe flash. Set to TTL, Point at the cieling and pump the flash exposure comp up a touch.

Other than that, if your flash has a manual settings option, set everything manual for consistent results with no colour balance changes.
02/15/2008 09:55:12 AM · #6
Thanks for the replies.
Is there a way, however, to make the metering system compensate for flash usage?
02/15/2008 11:23:26 AM · #7
Originally posted by chrono_trigger:

Thanks for the replies.
Is there a way, however, to make the metering system compensate for flash usage?


Well, you got to look at it this way. The camera doesn't know, until you press the shutter button and it fires a pre-flash how much light is going to be reflected back into the lens.

Your camera's built-in light meter reads ambient light only. If you want to meter flash exposure, your going to have to purchase a flash meter.
02/15/2008 02:30:56 PM · #8
Originally posted by fotomann_forever:

Originally posted by chrono_trigger:

Thanks for the replies.
Is there a way, however, to make the metering system compensate for flash usage?


Well, you got to look at it this way. The camera doesn't know, until you press the shutter button and it fires a pre-flash how much light is going to be reflected back into the lens.

Your camera's built-in light meter reads ambient light only. If you want to meter flash exposure, your going to have to purchase a flash meter.


Or use the P mode, in which the camera assumes the flash to be the main source of light. Any other modes and the camera uses the flash for fill only. I'm not sure if the FEL (flash exposure lock) button will give you the same metering effect...
02/15/2008 02:49:14 PM · #9
In Av and Tv modes the camera is choosing the exposure based on the AMBIENT light. In Av mode it may slow the shutter to 30 seconds (yes, that long!). Tv mode will go to the widest aperture and then flash in your viewfinder to let you know it's reached a limit.
Try upping the ISO to 800 or higher. I've read that 800 is what canon designed the flash to work at for optimum metering. It will also reduce how much flash you need, so your batteries will last longer and recycle time will be less.

With flash I shoot in M mode - ISO 800 (or more), 1/40th and F4 as a starting point. With luck your lens is one that reports the focus distance back to the brains and your subject distance is factored into the flash exposure.
02/16/2008 01:54:16 AM · #10
Ok, thanks :)
02/16/2008 06:16:08 AM · #11
I think part of your confusion is that you're assuming that your flash outputs the same amount of light for every shot. This simply isn't true with TTL metering.

What's actually going on is this:
1. You set the aperture where you want it. You vary the shutter speed (so long as you're slower than your flash sync speed) to get the exposure where you want it.
2. You focus and press the shutter.
3. Your flash sends a preflash and determines how much power it needs to properly expose.
4. The shutter opens and your flash fires at the power level determined above.

All you have to worry about is that your flash is powerful enough to expose with the setting you've provided. Remember, slow shutter speeds won't necessarily induce blur as the longest your flash will fire is usually somewhere around 1/900 second which is plenty fast enough to freeze the motion.

I'm a Nikon shooter so I'm not sure how it works with Canon, but my flash will alert me when it can't produce enough power to give proper exposure and actually tell me how underexposed the image is (-0.7 or something). This enables me to correct the problem by either opening my aperture or slowing my shutter speed. Do remember that if it's somewhat dark, slowing your shutter speed may not help much.

Message edited by author 2008-02-16 06:17:18.
02/16/2008 09:15:28 AM · #12
This may help a bit.
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