Flash and exposure...

[SmOcKxY]

Hey guys..sorry to bring this up again..basically i've read through the canon flash bible and tried my best to understand whatever is written..i have some questions still and i hope u guys can help me out..fyi i'm holding a 50D and 430 EX II flash unit..

1) It mentions that on eTTL mode..the flash unit will fire a preflash to determine the correct flash exposure..however i never saw this before..whether the flash is pointing direct or swiveled upwards..all it does is a red AF light shining when the shutter is half pressed that's it. Am i missing something here?

2) It also says that even if i used a diffuser or bounce card, i do not have to compensate for the power loss because the flash unit will automatically compensate for it with the preflash (connected to my 1st question)..is this really the case? I just have to meter as per without flash and the electronics will do the rest of the calculations..will this be true if i swivel the flash upwards with a lambency diffuser?

Hmm..unless i misread something..i can't find the answers to my above questions..can anyone help out? I'll go scour the bible again to see if i missed something!

 

[GRbenji]

The preflash is too short and fast for the human eye to see. And yes, ettl will adjust the flash power automatically. Less light from preflash falls on subject if bounced, hence ettl will signal flash unit to raise power in order to expose subject correctly.

 

[daredevil123]

Hey guys..sorry to bring this up again..basically i've read through the canon flash bible and tried my best to understand whatever is written..i have some questions still and i hope u guys can help me out..fyi i'm holding a 50D and 430 EX II flash unit..

1) It mentions that on eTTL mode..the flash unit will fire a preflash to determine the correct flash exposure..however i never saw this before..whether the flash is pointing direct or swiveled upwards..all it does is a red AF light shining when the shutter is half pressed that's it. Am i missing something here?

2) It also says that even if i used a diffuser or bounce card, i do not have to compensate for the power loss because the flash unit will automatically compensate for it with the preflash (connected to my 1st question)..is this really the case? I just have to meter as per without flash and the electronics will do the rest of the calculations..will this be true if i swivel the flash upwards with a lambency diffuser?

Hmm..unless i misread something..i can't find the answers to my above questions..can anyone help out? I'll go scour the bible again to see if i missed something!

The preflash is fired when you take the shot. The pre flash goes out, followed by the actual flash, in a split second. If you do not already know, if you do not have AE-L activated, metering of the scene is taken a split second before the shutter opens. The metering taken during half press is for reference purpose.

 

[GRbenji]

The metering taken during half press is for reference purpose.

Bro, this metering is for ambient lighting. This metering allows one to correctly expose the background. There are 2 separate meterings, ambient and flash.

 

[SmOcKxY]

Wow ok..thanks guys for the prompt response! Feel like an idiot now...haha!

Thot the preflash is like those on the PnS camera to eliminate red-eye..haha! Considering shutter speed at 1/80 that preflash must be damn fast! Possible to catch it with human eye?

Thanks alot anyway!

 

[ziploc]

When using flash, keep in mind that the subject distance plays a very important part. Due to the flash sync speed (which usually limits from 1/30 - 1/250s or narrower depending on cameras), there is a range of aperture values, given a certain distance, where the flash will be able to adjust it's power. If you use Av mode, since you're fixing the aperture and the shutter speed has a very narrow range, the distance now becomes the factor. If you're on eTTL mode, this distance range is displayed on the LCD of your flash. You'll have to place your subject within that distance range for it to expose correctly. Too near and it is overexposed; too far and it's underexposed.

 

[ziploc]

Wow ok..thanks guys for the prompt response! Feel like an idiot now...haha!

Thot the preflash is like those on the PnS camera to eliminate red-eye..haha! Considering shutter speed at 1/80 that preflash must be damn fast! Possible to catch it with human eye?

Thanks alot anyway!

If you've used your main flash to trigger auxilary flash, you will notice the "non compatile" flash will be falsely triggered by the preflash, and fired at the wrong timing.

 

[SmOcKxY]

When using flash, keep in mind that the subject distance plays a very important part. Due to the flash sync speed (which usually limits from 1/30 - 1/250s or narrower depending on cameras), there is a range of aperture values, given a certain distance, where the flash will be able to adjust it's power. If you use Av mode, since you're fixing the aperture and the shutter speed has a very narrow range, the distance now becomes the factor. If you're on eTTL mode, this distance range is displayed on the LCD of your flash. You'll have to place your subject within that distance range for it to expose correctly. Too near and it is overexposed; too far and it's underexposed.

Thanks for the tips! Yeah i think i'd have to practise this!