Originally posted by Mystix
In your first example when u metered for the scene first (this is without flash rite?)
After that for the fill flash, u said to go 1-1.5 stops less with the aperture for the flash exposure. So am i correct to say that
1. u metered with flash in manual mode, then apply 1-1.5 stops down with the metering....Or
2. u metered with flash in manual mode, then applied 1-1.5 down in flash exposure compensation?
i always get confused with these 2 ....are they the same? with ETTL would the flash juz increase the output for 1. thus defeat the purpose?
I used a flash without TTL ability. That is, my flash only has aperture auto and manual. Let me explain what this 2 methods are before explaining the rationale behind my first example.
In aperture auto, I simply set the flash to a particular aperture (let's say f/5.6). Using the flash in this method and within the appropriate camera-subject distance, the flash will fire a certain amount of light until the sensor on the FLASH unit detects that this amount is enough, then the flash will stop firing. Now, with this method, I have to set my LENS aperture to f/5.6 so as to receive the "correct" amount of light. If I want to get 1 or 2 stops less light onto the film, I would then set my LENS aperture to f/8 or f/11. You can do it the other way, that is set the FLASH to f/4 or f/2.8 and keep the LENS aperture fixed to f/5.6.
The main disadvantage of aperture auto method, to me, is that if the background is very bright (like afternoon sky). The light from the sky will fool the sensor on the FLASH into thinking that enough light has been fired from the flash. Thus, the flash will stop its light output before the appropriate amount has been fired. Resulting in an underexposed subject. The reverse is true when using this method in the dark. The sensor on the FLASH will think that not enough light is fired, so it will fire more light than appropriate, resulting in an over-exposed subject.
In manual mode, I estimate the distance. Let's say the parrot was 2 metres away from me. Then I will set the power of the flash to a distance of 2 metres for my given LENS aperture (say f/5.6). Using this method the sensor on the FLASH is not working but the flash will always fire an exact amount of light that is appropriate for a subject 2 metres away and for a LENS aperture of f/5.6. So if you shoot with this setting on the FLASH and f/5.6 on your LENS the exposure will be "correct" for anything that is 2 metres away. Now if you want to compensate, you then simply set the LENS aperture to f/8 or f/11 for -1 or -2 stops respectively. The flash compensation (on the FLASH unit) does not work in my case because the light output is fixed and the light sensor is not working.
Now for the parrot example. My first consideration was that I have to use a shutte speed fast enough so as to freeze the movement of the parrot. And I know that my top flash sync speed is 1/125. So I set my camera at that shutter speed. Now I wanted to blur the background so I opened the LENS aperture to f/4. Remember now, my camera and lens setting is 1/125 at f/4. So at this setting the picture will normally be underexposed in the afternoon shade. I then estimated the camera-subject distance to be 2 metres, so I used the manual method, setting the FLASH to fire an amount of light that is appropriate for 2 metres and at f/4. So now everything that is at 2 metres away from me will be appropriately exposed but anything beyond will be underexposed. Therefore, the parrot was lighted up while the background (which is more than 2 metres away) was dark.
So remember, when using the FLASH unit in manual mode, the light sensor on the flash unit is not working. TTL is also not in play here. In manual mode, the light fired by the flash is FIXED, you will have to compensate by either changing the aperture on your LENS or by moving further or closer to your subject. The reason I prefer manual mode (and it is strictly a personal preference) over aperture auto or TTL is that the light output is always correct for a subject that is a certain distance away regardless of whether the background is dark or bright. I therefore have one less factor to consider when using flash in manual mode. For me when using flash my personal rule-of-thumb is: use shutter speed to get the appropriate background exposure, use lens aperture to get the appropriate flashed subject exposure. If however the background cannot be appropriately exposed because of limitations in shutter speed (like the parrot example) then bo-chup the background, just whack the subject and compose it in my frame as much as possible.
andrew
p/s it's too long to type about TTL system and the principle behind it. suggest you continue reading. if you want, e-mail me privately and i'll try to answer.