TTL on 420EX

[mc_nguyen]

Hi,

I have a Canon 420EX. If I set the camera setting to Manual mode, then if I increase the f-

stop by +1 but leave the shutter speed as is, the picture is darker by 1 stop. I was under the

impression that the Canon 420EX will use its TTL sensor to compensate for the f-stop

increase by pumping out more power to provide the lighting. But that doesn't seem the case.

Would anyone know?

Thanks

MC

[a_maraschky]

The f/stop controls only the rate at which light is let in. So if you make the hole smaller, less light will be let in.

 

The flash doesn't compensate for this because you have it in manual. TTL is a seperate mode. Flash power is controlled by the f/stop, guide number, and distance. Shutterspeed has nothing to do with it.

[a_maraschky]

Oh, I thought you said manual on the flash. Sorry about that. The bit I said about TTL being a different mode is wrong. I forgot The 420EX doesn't have manual.

[Ken Katz]

When you use flash, you have both the impact of the flash exposure and the ambient light exposure. Assuming the framing was the same and subject was within the effective range of the flash, the flash exposure should be the same. What you may be seeing is that the ambient exposure has decreased and the image is darker.

[chrisjb]

Hi, you dont mention the camera, if using a film body like T90 or other with TTL, it should adjust to the manual settings unless the ISO is to low and you`ve exeeded the flash range. if digital it may depend on the focal point placement as the flash is metered from a nutral tone. which is why some as we do always use FEL for flash.You may like to say which body you are using.

[PuppyDigs]

<P><A HREF="http://photonotes.org/articles/eos-flash/" TARGET="_blank">EOS

Flash Basics</A></P>

[mc_nguyen]

Hi ChrisJB,

I'm using a Canon 20D.

Thanks

[quicksilver1]

http://photonotes.org/articles/eos-flash/index2.html#manualexposureflash - In Particular. Thanks.

[quicksilver1]

WHen you use flash, the flash metering takes place for the subject(where you focussed) and that remained the same in your case.

 

What you see in the viewfinder is the ambient metering and that is for the background. By keeping the shutter speed the same, and increasing the f-stop, you underexposed your background. Hence, the background is darker by 1 stop.

 

Just my 2 cents.

Thanks.