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Basics of Flash Photography
Four Fundamentals we must know

A few extra details, extremely useful to know for some cases, but perhaps less essential to routine flash operation.

Rear Curtain Sync

Rear Curtain Sync can be useful when a slow shutter speed and significant continuous ambient light (room light or sunlight) can be expected to blur the motion, and you want to see that blur. The very fast flash duration will stop the motion, however a slow shutter speed can blur it anyway (if there is significant continuous ambient light to do it).

Front curtain sync is the normal flash mode, with the flash being triggered at the start of the shutter duration. The flash finishes quick, and then the shutter remains open longer, so the ambient blur appears later (out where it isn't yet), appearing to lead the motion. Rear curtain sync waits to fire the flash until almost the end of the shutter duration, so that the shutter blur occurs first, which appears to follow the subject (back where it used to be), which looks more natural.

The three pictures below are a tape dispenser swinging on a string (the motion is to your right in all cases). The ambient room light is morning window light, but not direct sun. D300 settings were ISO 200, 50mm, 1/20 second f/4.5, with a hot shoe manual flash at four feet. Frankly, 1/20 second seems abysmal to stop motion, but this is indoors, and I am tring to show blur here, blur caused by the ambient light. The SB-800 flash is at 1/32 power, which has a flash duration of 1/17800 second (spec chart in rear of flash manual). The flash certainly stops the motion of the swinging tape dispenser fine (note its shadow), but the 1/20 second shutter still allows the continuous ambient room window light to blur it severely anyway. There are clearly two separate exposures in a flash picture. The flash is very fast, but the much slower shutter continues the continuous ambient exposure much longer after the flash has finished.

Some people imagine that the delayed result of rear curtain sync causes a sharp stopped image superimposed on top of the blurred image (so is sharper), which may appear true of the leading edge, but the opposite is true of the trailing edge. So while there definitely are two separate exposures, and the flash does freeze the action when it triggers (and the continuous light continues blurring it), my imagination assumes the effect is not "on top" of anything. Each pixel can only contain the one total accumulated pixel exposure value, regardless of when. The only difference is when the flash occurs. Rear sync certainly does not hurt anything, however it only makes a difference with slow shutter speeds (and it is helpful with slow shutter speeds). Rear Sync will also ignore the 1/60 second minimum shutter speed menu limit with flash in A or P modes (rear sync is for slow shutter speeds).

Be that as it may, note that the point of the previous page is that if we had wanted to eliminate the blurring, we would eliminate the continuous ambient light by using maximum shutter sync speed (around 1/200 second) instead of a very slow 1/20 second. That fast shutter keeps out the continuous light, which is generally more true indoors, but it is more difficult outdoors in sunlight. Sure the shutter is set to be 1/10 the duration, which is faster, but also the continuous ambient light is now weakened to be over three stops more dim. The blur is simply too dark to be seen. The shutter speed does not affect the flash. The flash is faster than the shutter speed. The tape dispenser is 3.2 inches long. Since the blur trail above seems about that same length too, then the 10x faster shutter should make a blur trail that is only 0.3 inches long, which would still be a blurred picture. But light that is too dim to be visible makes no visible blur trails, and the three stops can do that indoors. The third picture is all the same, the same flash, same room, same motion, just less the continous ambient light due to the faster 1/200 second shutter speed shutting it out. It is not so much that the 1/200 second is fast and stopped the motion. It is more that the ambient was dim, and was very dark and invisible at 1/200 second exposure, it simply didnt matter any more.

White Balance of room lights with Color Filters

If shooting flash indoors with mixed ambient lighting, you have two main choices about white balance... You cannot mix two colors of lights (flash with incandescent or fluorescent), so do you want the room lighting to contribute into your picture, or do you want to minimize the room lighting white balance problem?

If you don't want the continuous room light messing with the white balance in your flash picture (easy way - subject of previous page), then simply use the maximum shutter sync speed to keep it out, so Flash White Balance can be used. Faster shutter speed does not affect or reduce the flash, but it does affect and reduce the continuous ambient. 1/200 second or so will really knock it down in most cases indoors. A smaller diameter aperture (f/5.6 or f/8 instead of f/4) will increase this effect (this ambient reduction at 1/200 second), but that will require more flash power, which must be available. Studio lighting will always use maximum shutter sync speed to keep the continuous ambient out of the controlled environment. Even the 150 watt modeling lights in the close flash units cannot survive f/8 at 1/200 second (we verify that the same picture comes out totally black without the flashes).

If you do want the room light to contribute in your picture (assuming it is bright enough), then incandescent lights are orange, and fluorescent lights may be green, while the flash is daylight (white). Sometimes we like a mild orange warming, but more can be gastly too. You cannot process with two White Balance values for mixed lighting, you have to choose one. Meaning, we have to make the light colors match. We can use colored filters on the flash to convert the flash to be orange or green too, and then set camera white balance for incandescent or fluorescent, to match the ambient lighting.

Below are a few Rosco color filters from the (near free) Rosco Swatch Book - and there are other choices too. The significance is 1) these fit on a speedlight as is, 2) they are near free, and 3) a vast variety of choices are provided. These swatch book samples include almost every filter (the stack is about two inches thick), and are suitable for taping on a speedlight - just not directly against the flash lens (see below). The Rosco Cinegel Swatch book is "only" 1 3/8 inch thick, but it contains these same standard filters too. B&H says these are 2.75x1.75 inches, but mine are 3.25x1.5 inches. The rosco.com site has some techinfo. The color filters are polycarbonate or polyester plastic today, but they are still commonly called gel filters, or gels, from when gelatin was used.

Room lamps can be one of several shades of these colors, so these conversion filters come in several shades too. Selection will require some testing to match your lights (but close is much better than none). These orange filters are named CTO (Color Temperature Orange) to match various incandescent lamps, or PlusGreen to match various fluorescent lamps. The CTS (Color Temperature Straw) can be used for CTO too, popular, sometimes better for human skin. Also, CTB (Color Temperature Blue) filters can go on the lights themselves to convert tungsten to match daylight (similar to the blue flash bulbs of yesterday). The terms tungsten and incandescent mean the same thing regarding white balance.

These densities can be stacked, for example, stacking 1/2 CTO + 1/4 CTO makes 3/4 CTO, which is why they are named this way. These filters reduce the effective flash power somewhat, perhaps 0.3 EV to 1.0 EV.

Nikon furnishes a few color filters with the SB-800/SB-900 flashes, and more are also available as the Nikon SJ-1 Set. Fits the SB-600 too. These are convenient, they have the large flap that inserts into the flash head, and then simply fold down over it. My best guess is that the Nikon filter numbers approximately match Rosco this way:

This match is close, but we don't know the color of our lamps anyway, lights vary too.

Rosco also packages 55 of these filters into a Strobist Kit. It includes a few multiple copies of the important colors.

These filters come in larger sheet sizes, but the Rosco Swatch Book offers many choices to see and try, suitable for speedlights. The CTO filter from the Rosco Swatch book is shown here at top on a SB-800 speedlight, simply using masking tape to attach the filter to the flash. But not touching the lens, so it does not melt. Shown at bottom is a Nikon Red filter, which has been a little too close to a full power flash at time or two. Doesn't affect its light, but just a slight air spacing is surely better (bow it slightly, as shown on CTO Rosco at top). After you figure out what filters you may need, then you can do something more elegant if desired - or not, this works too.

Auto White Balance can be best for hot shoe flash, BUT NOT WITH THESE COLORED FILTERS (and assuming not mixed light sources). Of course we select incandescent or fluorescent White Balance for this concept of matching the ambient light color.

Changing subject now, no filter: Flash tubes are generally the same as daylight color, but speedlight color varies significantly with the power level used, more red at high power (far or bounce), and more blue at low power (close or direct). In the Nikon and Canon systems, the speedlight knows the power it actually used, and if on the hot shoe, and if Auto White Balance is selected (allowing WB value to be changed), it can report an actual color temperature to the camera (called "Flash Color Temperature Information"). This is a very special deal (hot shoe flash totally changes meaning of Auto White Balance), and this information can be used for a more accurate White Balance temperature. Whereas, if you instead set Flash White Balance, that is what you get, but it is a constant correction for any picture. Still close maybe, but the speedlight flash color varies with power level.

However (just my opinion), otherwise, other than with the system's hot shoe speedlight flash, Auto White Balance is a mixed bag... it tries to analyze the picture results, and shifts the three RGB channels to have equal brighness levels. That may or may not be correct, it depends on chance. So then (without any Flash Color Temperature Information reported), Flash White Balance seems better, at least it knows the light should be generic flash color (which is same as daylight).

Colored backgrounds can be implemented with these colored filters too. We can create red, green, blue, yellow or fuschia backgrounds, with a filter on the flash that is illuminating your studio background. The filtered light will change the color of the background, perhaps to better compliment your subjects clothing color. A medium gray backgound is great for this. If the background is white, you can only use minimum power level (more power just makes the background more white), and your main light may wash it out unless it is way back, and you probably will get only pastel colors. If your background is black, you can get intense shades, but it requires very high power level to turn black bright. A middle gray background is a very good compromise, very practical. Keep its distance back far enough so there is no risk of colored light reflection onto your subject.

Auto ISO works with hot shoe TTL flash too (but not with Remote flash - not in Commander mode and not in Manual flash mode), but auto ISO in every case does not trigger until whatever automatic adjustable settings reach their final limit. For example, camera mode S has hit the limit at the lens widest aperture before Auto ISO triggers. Camera mode A has to limit at the shutter speed in the Auto ISO settings (this intermediate menu limit prevents it having to first reach the actual 30 second lower shutter limit. The shutter speed will still go lower if necessary, after maximum ISO is reached. In camera mode M with no flash, Auto ISO triggers immediately, at any change when the selected aperture and shutter speed are not sufficient to give correct exposure, but with the TTL flash enabled, the TTL flash must limit out at max power before Auto ISO changes. In any mode when TTL flash is used, the flash has to reach its maximum power limit before Auto ISO triggers. Auto ISO always only operates at the extreme of whatever available limit exists.

All this is not 100% true, as the Nikon Auto ISO does seem to trigger early in many cases with flash. And another confusing quirk for Nikon flash is that the Auto ISO shown in the viewfinder never shows this ISO increase for flash. With flash, the viewfinder always shows the Base ISO, probably always ISO 200 or 100, with no hint Auto ISO will trigger. Probably because this ISO value is not known until after the TTL preflash. However, the final Exif data shown in the rear LCD image result statistics will show the higher ISO value (in red text) when it has triggered. I suppose my own straight-laced notion is that if we depend on Auto ISO, we are not in control, and we must be willing to accept surprises if not paying attention. :)

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