Shooting wrote in post #17976178
I was watching a wedding video by Joe Buissink and he said he likes to shoot in manual with flash to bounce but dials down the ISO to be -1 below the meter so the flash will not overpower the ambient light and he showed doing that on the camera and dial down the iso but the aperture and shutter stays the same, he keeps the on camera flash on ettl and really cuts the power of the flash to just a hint of flash because he bounces all the time. He was shooting with a 5D Mark II back then and I love his work right out of camera. So I was going to try it on mine but I can't figure out how to be looking thru the viewfinder to dial the iso up or down to be a -1 below the metered reading. He can dial up the iso and the needle (I call it) moves up or down the middle position for exposure compensation but it affects the flash output - don't know if I'm explaining it correctly. He meters manually and zeros the needle on the middle mark as being correct, he then while looking thru the view finder dials the iso until the needle is -1 below the middle mark. He dials it to a -2 outdoors for fill. He said that is the quickest to get to where you don't want to blow out the ambient, you want to keep the ambient and give just enough light to light the people.
So I started out writing out this answer to the above:
If you are dialing in your exposure, I assume with the camera set to an ISO value that is at least ISO 200, so that it is at whatever the "correct" level is should be, and then reducing the ISO by one stop, why do you still need to see either the shutter speed/aperture combination, or the metering level. In manual exposure mode, once you have set the shutter speed and aperture, making a change to the ISO value won't affect them. Also In manual exposure mode I can tell you exactly what the effect of reducing the ISO by one stop will have on the metering indication, it will indicate that the exposure is going to be one stop lower, which means the indication will move one stop to the left of where it was to start with. So why not just dial your settings to put the exposure indication in that position to start with. Then there will be no need to touch the ISO. At least that is what you are suggesting you want to do.
Then I realised that what you wanted to do was to just reduce the output from the flash system so that it gives you less of a fill effect. At least that is what you are implying in this next post.
Shooting wrote in post #17976590
I want to be able to set my aperture and shutter speed and then underexpose the flash by -1 stop. Main objective is to cut down on the flash power so it will not over ride the ambient as it lights the person(s). When Joe does it, the more he makes the ISO a - the less power the flash puts out when he bounces, so much so that when he bounced off a wall on the left you never saw the flash, it was so faint that it touched the people perfectly and lit them up so subtly and left the ambient alone, great result. Easier to do that in the view finder than messing with flash power, etc. while shooting a wedding and doing some formal poses - great short cut. He uses a 5D Mark III now and still a 580ex II.
To do this when using an ETTL flash all you need to do is reduce the FEC value. Set the FEC to -2 and the flash will be at two stops below ambient, set it to -1 and it will be at one stop below. This will work if you are shooting in either Manual or Aperture Priority exposure modes. Providing that you can get an exposure for ambient that is within your flash sync range. Although you could use HSS if your flash supports it.
On an additional note, if you want the background to be relatively darker than the flash (which is what the first half of this reply does), then use some negative EC, or set the exposure so that the meter indication is over to the left, and then use some positive FEC to being the subject up with the flash. With Canon ETTL II flash systems the cameras EC, or simply setting the exposure in manual to where you want it controls the relative level (brightness) of the ambient exposure. Changing the FEC changes the relative level (brightness) of the flash exposure.
Also remember that when using ETTL II, which is an auto exposure mode for the flash system, if you reduce the ISO of the camera, the flash system will simply INCREASE the flash output to compensate and maintain the same level of brightness from the flash part of the system in the resulting image.
Can I suggest that you set up a scene so that you can try some experiments. First set up your exposure in manual for the ambient, so that it is "normal" and then shoot some shots with the FEC set from -2 to +2 in one stop steps. The reduce the exposure for the ambient by one stop. Move the meter indication one stop to the left. Now repeat the shots using the FEC. In the second test you should see the ambient exposure on the background be darker by one stop, while the flash exposure changes with the FEC in both cases. The try the same test in Av, but instead use the EC control to add -ve EC to bring the background down in brightness, or +ve EC to brighten the background. Again the FEC control will do the same for the flash exposure.
Finally if in manual exposure mode, and with a flash with a constant power output, which would actually require a manual rather than ETTL II flash, reducing the ISO by one stop will simply reduce the brightness of the whole image by one stop.