Flash questions

[Actor]

• To get aperture you divide the guide number of your flash by the camera to subject distance. That's if the flash is on the camera. If the flash is off camera, and at a significantly different distance, what distance do you use: flash to subject or camera to subject? Or is there some other formula for this case?
• When using flash for fill is it better to use the flash on-camera or off-camera?
• How can you get two flash units to go off at the same time? Is it possible to use some sort of Y for the flash cable that connects to the PC? (Here PC means Photo Contact, not Personal Computer.)
• How far off the camera does a flash have to be to avoid red eye?

 

[Buckster]

To get aperture you divide the guide number of your flash by the camera to subject distance. That's if the flash is on the camera. If the flash is off camera, and at a significantly different distance, what distance do you use: flash to subject or camera to subject? Or is there some other formula for this case?

Flash to subject. This is about dealing with the inverse square law effectively. Wherever you place the camera, the light on the subject remains what it is as long as the flash and the subject don't move, so the f stop doesn't move either.

When using flash for fill is it better to use the flash on-camera or off-camera?

In my opinion, it's always better to get the flash off the camera if possible.

How can you get two flash units to go off at the same time? Is it possible to use some sort of Y for the flash cable that connects to the PC? (Here PC means Photo Contact, not Personal Computer.)

Y cables exist but are expensive and limited in range. What you want to do is get a set of radio triggers. You can get the inexpensive Cactus triggers or Yongnuo triggers, or you can get expensive Radio Poppers or Pocket Wizards. Either way, you'll be much happier with them than with any cable system.

How far off the camera does a flash have to be to avoid red eye?

Far enough that the reflection of the light on the back of the eyes' retina doesn't bounce back into the camera. Light travels in straight lines and bounces off things like billiard balls, so picture that in your head to start with. A pre-flash works well also to close down the pupil, which makes the colorful iris bigger as well, and that's usually a good thing for portraits.

 

[Garbz]

I agree with all of that except for the last point. There is nothing about small pupils that looks good in portraits in my opinion. The ability to get large pupils also seems a great motivation for most of the off camera flashing guides I have seen around online.

Think of it this way; would this scene from Shrek 2 be quite the same if the pupils were large?:

The eyes are the window to the soul, don't halfclose the window.

All this is in my opinion only.

 

[Buckster]

I suppose it depends on the mood of the photo. I tend to be drawn more to bright, colorful eyes, with lots of iris detail, than to black ones - in most cases.

 

[Dwig]

• How far off the camera does a flash have to be to avoid red eye?

Its not a matter of distance. It a matter of the angle between the subject-camera line and the subject-flash line, and its somewhat variable. Things that affect it are:

1. Size of the pupil - this is the apparent size as seen from the camera. Red-eye is less likely if the subject is not looking directly into the camera.
2. Color of the iris - Its more pronounced in people with blue eyes
3. Skin tone- Its more pronounced in people with fair skin
4. Hair color - its more pronounced in people with blond hair
5. Gender - it more pronounced in females
6. Age - its more pronounced in younger subjects

If you are shooting pics of swarthy old men with dark hair and brown eyes you'll have little problem and the flash doesn't need to be at a great angle. If you shoot fair skinned blond baby girls with blue eyes plan on doing some retouching in PP.

 

[KmH]

• How far off the camera does a flash have to be to avoid red eye?

The flash doesn't have to be off the camera to avoid red eye. It just has to be high enough above or low enough below the lens axis to prevent the flash from reflecting off a persons retina and onto the image sensor. Most shoe mounted speedlights are high enough to prevent red eye.

Fill flash, by technical definition, is always on the lens axis. If you have light from a direction off the lens axis, that is also at a lower light ratio than the main light, it is a second light.

Secondary lights are also known as kickers, hair lights, rim lights, etc.

 

[Patrice]

You don't really need radio controlled remote triggers either. You can fire nearly any old box standard flash with an inexpensive photosensitive trigger. These have their limitations but if you are the only person taking photos and you are indoors in a modestly sized space then they should work just fine. Not pro gear but fun and inexpensive amateur gear.

 

[KmH]

You don't really need radio controlled remote triggers either. You can fire nearly any old box standard flash with an inexpensive photosensitive trigger. These have their limitations but if you are the only person taking photos and you are indoors in a modestly sized space then they should work just fine. Not pro gear but fun and inexpensive amateur gear.

Inexpensive eBay radio triggers. This set has a transmitter that goes on the camera hot shoe and 2 receivers that can out of sight from the camera position.

They also have single receiver kits.