Repost: The Light and How to Swing It

Note: This is a repeat of a pre­vi­ous post. Why? Because I’m on a sub-​tropical island and may not have the time to make any posts. With any luck, I’ll bring back some sto­ries and pic­tures for the future.

One of the things that I really liked about the Zack Arias One­Light DVD was a pho­to­graph he showed to illus­trate the Inverse Square Law, or in sim­pler terms, light fall-​off. In it the photo, he marked off the light meter read­ings to show the f/​stop of light as it trav­eled away from its source. It was a very sim­ple and extremely effec­tive demon­stra­tion of how light trav­els for­ward. He also has the same photo in his excel­lent tuto­r­ial on shoot­ing with white seam­less.

Recently, I’ve had dis­cus­sions about the nature of light and soft­ness. To be fair, much of the con­ver­sa­tion was really split­ting hairs regard­ing physics, rather than ulti­mate effect. More specif­i­cally, the dis­cus­sion was whether a dome cap on a flash soft­ens light. A few peo­ple took the posi­tion that it did, and I took the oppo­site posi­tion, that sim­ply dif­fus­ing light at the same size does not soften light — it only reduces the amount of light trav­el­ing for­ward and it spreads light around so it may bounce off other sur­faces (e.g., a ceil­ing or walls). By bounc­ing off a ceil­ing or wall, you change the appar­ent size of the light to your sub­ject, and that is what cre­ates soft­ness. David Hobby has a good expla­na­tion how appar­ent light and close­ness to your sub­ject affects the qual­ity, or soft­ness, of light.

Here’s why we were split­ting hairs on the dis­cus­sion. If you’re in a room and you put a dome cap on your flash, the light spreads and you see a softer result on your sub­ject. To many peo­ple, that’s cause and effect that the dome is what caused the soft­ness. Not quite. The dome is what spread the light to bounce off other sur­faces. The light reflected, or bounced, from a larger sur­face onto your sub­ject is what caused the soft­ness. It’s a sub­tle detail, but one that’s impor­tant to note.

Use that same dome out­doors with noth­ing to reflect, and you’ve not soft­ened the light on your sub­ject at all. The dome still causes the light to spread — seek­ing a sur­face to alter its course. If there is no reflec­tive sur­face, then a smaller por­tion of your light out­put hits your sub­ject. That essen­tially give you the same result as low­er­ing the power of your bare flash.

With that in mind, I decided to mimic Zack’s illus­tra­tion, but for a dif­fer­ent pur­pose. I lugged my cam­era and tri­pod out to the dri­ve­way, placed a flash in front of the garage door, and took some shots to see how the light changed from a bare SB-​800, to the SB-​800 with a dome dif­fuser, and finally to the SB-​800 with a Gary Fong diffuser.

I took all of the shots at ¹⁄₄ power on the SB-​800. The cam­era was set to ISO 200, 1/​80 sec­ond, 60mm. For the first three shots, I used an aper­ture of f/​16. For all of the remain­ing shots, I switched to f/​11. All of the shots are straight out of the cam­era with no post-​processing or tweaking.

Here’s what I found:

Side-​View

SB-​800 Bare: Side View

SB-​800 Dome: Side View

Gary Fong: Side View

The first thing you note is the dis­tance the light trav­els from the flash toward the right. How­ever, note that light trav­els behind the flash when using the two types of dif­fusers. Also, note the light trav­el­ing toward the cam­era is larger when using the domes. That’s not unex­pected, as they’re sup­posed to scat­ter the light. With­out a dome, more light moves for­ward as there’s noth­ing to change its path. Note how the changes in the light source affect reflec­tion from the garage door.

Straight at Garage Door

These shots are about two feet from the door.

SB-​800 Bare: Straight at Garage Door

SB-​800 Dome: Straight at Garage Door

Gary Fong: Straight at Garage Door

In this series, you can see the effect of the light changed by the mod­i­fiers. Remem­ber, the flash remains at con­sis­tent ¹⁄₄ power on man­ual mode. The dif­fer­ence you see with the dome caps is from two aspects. First, the inten­sity of light leav­ing the flash is reduced by the dif­fuser by a stop (per­haps more). Although the flash is at the same power, the dome mate­r­ial cuts the amount of light in half. Sec­ond, the light that does escape the dif­fu­sion mate­r­ial scat­ters in dif­fer­ent direc­tions. The net result is FAR less light trav­el­ing in a straight line. Is the light softer because of the dome, or is it just spread thin­ner around a larger surface?

Straight at Camera

SB-​800 Bare: Straight at Camera

SB-​800 Dome: Straight at Camera

Gary Fong: Straight at Camera

In this series, I pointed the flash head straight at the cam­era. You can’t really see the dis­per­sion of light from this per­spec­tive. All you can see is the rel­a­tive bright­ness of each mod­i­fier. Of the three, the first two appear to be the same size because…well, because they are. It may be a sim­ple thing to note, but the dome dif­fuser does not change the size of the light appar­ent to your sub­ject. All it does is cut down the light trav­el­ing to your sub­ject, both by dif­fu­sion and dis­per­sion. Since the appar­ent light isn’t any larger, the light hit­ting your sub­ject isn’t any soft­ware. There’s just less of it. The Gary Fong dome is slightly larger, but not sig­nif­i­cantly so.

What if we tried to over­come the dif­fer­ence in light lev­els of the domes to com­pare with the bare SB-​800 flash? I decided to increase my ISO to see how the domes appeared:

1 Stop: ISO changed from 200 to 400

SB-​800 Dome: ISO 400

Gary Fong: ISO 400

Both are a bit brighter. The SB-​800 Dome is start­ing to look more like the bare SB-​800 shot. What if we kick up the ISO another stop?

2 Stop: ISO changed from 400 to 800

SB-​800 Dome: ISO 800

Gary Fong: ISO 800

Amaz­ing. We changed the ISO to com­pen­sate for light lost by the dif­fu­sion mate­r­ial of the domes, but it did noth­ing to increase the size of the light. I guess that means that it hasn’t got­ten any softer because the appar­ent size of the light hasn’t changed at all with rela­tion to a subject.

Con­clu­sion

This isn’t to say that domes aren’t use­ful, nor that you can’t use them to mod­ify the light on your sub­ject. It’s merely meant to illus­trate how they mod­ify the light and the impor­tance using them to bounce light off another sur­face, a larger sur­face, to cre­ate a softer light on your sub­ject if you want to elim­i­nate harsh shadows.

What’s that? You’re not com­pletely con­vinced? Of course not. How could you be? All we’ve done is look at the strength and dis­per­sion of light, but we haven’t really looked at the qual­ity of light on a sub­ject. Tune in another time for the con­clu­sion of this topic in a post called The Shadow Knows!