Help me learn manual flash

Started Mar 5, 2012 | Discussions thread
Suntan
Veteran MemberPosts: 5,116
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Re: It has been my experience...
In reply to CaseySRT, Mar 6, 2012

Fair enough. Anyway, on to flashes. When set to manual mode, they just pop out a set amount of light. Pop after pop. Personally, I wouldn’t bother with learning all the intricacies of the guide number for your SB 700 right now and trying to calculate the distances based on formulas or tables. At least not initially. It’s overly confusing and counter intuitive to just learning how the thing works.

Just take in the basics. If you set the flash to 1/1 you are getting “full power” from the flash. If you set it to ½ you are getting “half power” from it. If you set it to ¼ you’re at “quarter power”, etc. To the flash all it knows is that it is supposed to use full, half or quarter power every time you press the trigger.

Now at the camera level things get more complicated. Where “full power” was properly exposing the image, now that it is down to “half power” (half as much light) you need to compensate with either aperture or ISO. As you know, opening the iris of the lens from, say f5.6 to f4 will offset the fact that you are getting half as much light. Or, jumping the ISO from 200 to 400 will make the same offset. (Don’t worry about the shutter speed right now, your SB700 and 603 trigger will sync at 1/250, but just set shutter to 1/200 for convenience and leave it there for the time being.) “Quarter power” needs a two stop bump a the camera (f2.8 or ISO 800 in this example) and so on.

But what about zoom head setting and placing the flash distance.

Say the flash is set to ½ power. If the head is zoomed all the way out, that ½ power is being spread all over a broad area (picture a big wall all being lit up) so it should be obvious that by spreading that certain amount of light all over that big wall, there is less light available (less intensity) to light up just one specific brick in that wall. Now imagine you zoom the head in (keeping the SB700 in the same spot) so that your ½ power is all being focused on just that one brick. Obviously the brick will be getting a lot more light shined on it. So the zoom setting will affect the intensity of the light hitting a subject even if you don’t change the power level of the flash. So be aware that zoom can affect light levels. As for what to do with it, you’re best off to set the zoom setting such that you get even illumination across the scene and leaving it there for the duration of your shooting. (Taking a step or two back and shooting the whole room with a WA lens can be really good in giving a person an idea of what exactly the flash is doing to a scene.)

What about distance? Most everyone will tell you that light falls off with the square of distance and that you’ll make baby Newton cry if you question this or don’t properly take the time to calculate these things out with a tape measure or some such. But the thing is, this only applies for non-collimated point sources of light (think a bare light bulb sitting a long distance away from you.) Highly focused light sources or light sources fitted into large modifiers (like a softbox or umbrella) and placed closely to the subject do not act like point sources. [For anyone that doesn’t believe this, place a large softbox 1’ from a model’s face and expose for it, now move it to 1.4’ from the face and see if you are really down an entire stop.]

Also, a little sidebar about the notion of exposing purely based on using distance and guide number alone to set the “proper” amount of light. This is basically the idea that a camera can be made to take a properly exposed picture based purely on the amount of light placed at the subject’s location. The thing is, cameras take pictures based on the amount of light that is reflected off the subject and back towards the lens. If you think you can always get spot on, “correct” light levels by distance and calculations alone, try this other test: Nab that bright orange/yellow reflective construction vest from your brother-in-law (those ones that always blind you with three rows of bright reflective stripping when you drive past a construction area at night with your headlights on) and place it right next to a folded bit of black velvet fabric. Now take your calculated “proper” exposure. Which one is properly exposed? Do you see the detail in the black velvet fabric such that it looks well exposed, or do you see a completely uniform black blob with no noticeable amount of detail or texture? Would you call that properly exposed? Now look at that construction vest in the picture, do you see a well exposed vest that looks like what you would see in a sales brochure, or do you see three blinding strips of light streaking across your image?

So how is a guy supposed to set the flash distance? “Trial and error until you get a feel for it” is my suggestion for a beginner. Really flash distance should be set based on a number of legitimate reasons, but none of them is intensity imo. However those reasons are somewhat beyond the level of the beginner course, so just pick a good distance to begin with and go from there.

Set the flash to ¼ power, place it about 6 or 7 feet back from your model (you already have your shutter set to 1/200 sec right?) and set your aperture to f4 for starters. Take a few pictures and see what you get. If everything is way too bright, drop your aperture to f5.6 or even f8 and try again. Learn to read your histogram and learn what it is telling you.

Most importantly, once you get something dialed in so it looks about right, try making one change at a time and see what happens. Move that flash another 3 feet back and see what happens, move it forward, see what happens, move it back 3 feet and try and compensate for it by adjusting the aperture. Go back to your default settings and this time adjust the power of the flash up or down, or adjust ISO. Learn what happens.

Get comfortable with that and then look to move to more advanced stuff.

-Suntan

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