Go Back   Steve's Digicams Forums > Digicam Help > Flash (External)

Reply
 
Thread Tools Search this Thread
Old Jun 22, 2005, 5:09 AM   #1
Senior Member
 
pianoplayer88key's Avatar
 
Join Date: Jan 2004
Posts: 405
Default

Ok, so we're taking a picture of someone's face.

The camera's exposure is set to ISO 12, F/91, and 1/10,000" shutter.

The flash is spot metered on the part of the eye in the middle that's black.

At the time of exposure, the flash fires. That black part of the eye comes out full white (histogram all the way to the right, one pixel wide). (how many stops overexposed would this be?)

Two questions...

Approximately how bright of a flash is required to do that (of course it depends on distance to the subject but let's say you're using something like a 100mm lens for a head-and-shoulders portrait)?

And,

What adverse effects might a flash that bright have on the subject's eyes?
pianoplayer88key is offline   Reply With Quote
Sponsored Links
Old Jun 22, 2005, 6:31 AM   #2
Senior Member
 
geriatric's Avatar
 
Join Date: May 2005
Posts: 326
Default

You have a few extremes there old boy.What camera are you using?.You do not say what flash you are using and is it dedicated.
geriatric is offline   Reply With Quote
Old Jun 22, 2005, 2:31 PM   #3
Senior Member
 
pianoplayer88key's Avatar
 
Join Date: Jan 2004
Posts: 405
Default

I didn't specify any camera or flash. I was just wondering (assuming you're using a dSLR that's capable of those settings) how bright of a flash you'd need if you were using a 100mm lens and were taking a head-and-shoulders portrait of the subject.

Right now, I'm using a Canon S1 IS. There's NO WAY I'd be doing this type of shot with the camera I have right now, and no, I don't have a slave flash (although sometimes at night I've been attempting to take flash shots of subjects 25 to 40 feet away, and they usually come out underexposed and grainy, even though my flash power is set to max, ISO is 400 (explains the grainyness), and my aperture is open as wide as it will go considering the zoom used (F/3.1 or F/2.8).
pianoplayer88key is offline   Reply With Quote
Old Jun 23, 2005, 7:51 AM   #4
NHL
Senior Member
 
NHL's Avatar
 
Join Date: Jun 2002
Location: 39.18776, -77.311353333333
Posts: 11,547
Default

pianoplayer88key wrote:
Quote:
Approximately how bright of a flash is required to do that (of course it depends on distance to the subject but let's say you're using something like a 100mm lens for a head-and-shoulders portrait)?
Whatone needs to know is the distance, not the the focal lenght...

A flash output is indicated by its Guide Number - This is usually specifies at a certain ISO (usually 100) but can easily be derived to other ISO and is available both in feet or meter.Its formula is:GN = distance * f-stop

So for a flash with a GN of 42(m),at 10m -> you'll need to set the aperture @ f/4.2, but at 1m you'll need f/42 otherwise your histogram will be all the way to the right! :G




Now with certain flash, there's also a High-Speed-Sync (HSS) mode where the flash will strobe multiple times at a lower power to follow the shutter curtain @ high speed (i.e. higher than the X-sync of the camera).

-> This will allow you to shootat closer distance with wide aperture without exposing so that you can maximize the DOF of the lens
NHL is offline   Reply With Quote
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 6:11 PM.