Front curtain sync, 1/20 second shutter
Rear curtain sync, 1/20 second shutter
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 near the start of the shutter duration The flash finishes quick, and freezes the motion, and then the slow shutter remains open longer, and can blur due to the continuous ambient light, So the ambient blur appears later (out in front of where the flash fired), appearing to lead the motion. Not a natural look.
A distraction: The D90 and D7000 and the later Nikon models are now instead naming Front Curtain Sync mode to be called Fill Flash mode. No actual change, it is still Front Curtain sync (everything but Rear Curtain Sync is.) The mode affects sync for any flash unit, but Fill Flash comes from the internal flash always doing TTL BL mode (flash is balanced with ambient, unless Spot Metering.) So this menu is not actually selecting Fill Flash, all the other menu choices do TTL BL mode too (if internal flash.) It is just using another name now, maybe it is less techie sounding. Specifically, either name (called Front Curtain Sync or Fill Flash) is just a default menu choice which specifically means: "None of the Above". It simply does not select any other menu choice.
Rear Curtain Sync waits to fire the flash until almost the end of the shutter duration, so that the ambient shutter blur occurs first, which then appears to follow the subject (appears back where it previously used to be, before the flash), which looks more natural to the motion. Only point of Rear curtain sync is to be used with a too-slow shutter speed, to intentionally make the blur trail visible. Rear Curtain Sync simply will not matter at any normal fast shutter speed (if no blur trail.)
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.
Front curtain sync, 1/20 second shutter
Rear curtain sync, 1/20 second shutter
Said again: The shutter duration is represented by the longer blur trail of the motion. The instant the flash occurred is represented by the sharper location of the subject at that instant.
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), 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 Curtain sync only determines if a blur trail follows the subject instead of leading the subject. There is really no point of using Rear Curtain sync except with the very slow shutter speeds that can cause blur trails.
Rear Sync will also ignore the 1/60 second minimum shutter speed menu limit with flash (in camera A or P modes.) Rear Curtain Sync mode is intended for slow shutter speeds. Same thing said another way:
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 (because the flash is faster than the shutter speed), so the flash exposure was unchanged, but the ambient exposure was decimated.
Front curtain sync, 1/200 second shutter (motion and flash same as before.)
Shutting out the ambient prevents any blur from shutter speed.
Saying it again, as it is important, and slightly tricky. The 10x faster shutter should make a blur trail that is 1/10 as long. The tape dispenser is 3.2 inches long, and the blur trail above seems about that same length too. So we would expect the blur trail to be reduced to about 0.3 inches long then (still as wide as a pencil), which would still be a blurred picture in this close view. Which would be obviously true if we also opened the aperture the same three stops to create an "equivalent exposure" of the ambient. But we didn't open the aperture. The flash saw the same aperture and still provided the same exposure (not affected by shutter speed.) But the ambient became over three stops more dim, due to faster shutter speed. Light that is too dim to be visible adds no visible blur. The picture is sharper and the background shadow is darker, NOT because the 1/200 second shutter was able to freeze anything, but only because the 1/200 second shutter kept the continuous ambient light off of it (no addition of two sources) - letting the speedlight do its work.
The third picture is all the same, the same flash, same room, same motion, just without any contribution from the continuous ambient light, due to the faster 1/200 second shutter speed shutting it out. It seems very important to realize this distinction - it is not that the 1/200 second is fast and stopped the motion (1/200 would still be insufficient speed to do that when up this close.) The flash stopped the motion - 1/32 power is 1/17800 second duration. What happened was that the ambient was made to be very dark and invisible at 1/200 second exposure, when it simply didn't matter any more. No visible blurring because it was dark. This is a biggie to realize and know, very usable.
And it is easy to do indoors with dim ambient, but much harder in bright sun, because the maximum shutter sync speed has limited range. Cameras vary a little, but 1/200 second is the ballpark of as fast as the shutter can go with flash.
Auto FP: Note that so-called High Speed Sync (FP mode) cannot help do this, since the HSS flash itself becomes continuous light then too, same as the sun. Rear Curtain Sync makes no sense for FP mode, and no possible contribution. Continuous light has no motion stopping ability anyway. FP shutter speed could be faster than 1/200, but nothing like the speedlight already does, and a fast shutter speed will attenuate both HSS flash and sun the same (both are continuous.) etc, etc.
White Balance of room lights with Color Filters
If shooting flash indoors with mixed ambient lighting, you have to pick one of the two choices about white balance... We 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 ghastly 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. If you are using high ISO (or Auto ISO) to balance indoor lighting with flash, then keep reading below, you definitely need a filter on your flash, to be able to balance color too.
Below are a few Rosco color filters from the (near free) Roscolux Swatch Book - and there are other choices too. The significance is 1) these are sized to fit a speedlight as is (attach with masking tape), 2) they are near free, and 3) a vast variety of choices are provided. Descriptions and spectrums of these swatch books are Roscolux and Cinegel. These Rosco Swatch books may be fully free from your local camera store if you ask about them... they are samples. 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 color filters are polycarbonate or polyester plastic today, but they are still commonly called gel filters, or gels, from when gelatin was used.
Note: the printing below is on a small white paper separator that comes between each clear filter.
Any one specific room lamp can be one of several shades of these colors, so these conversion filters come in several shades too. Selection may require some testing to match your lights (but close is better than none.) These orange filters are named CTO (Color Temperature Orange) to match flash to various incandescent lamps, or PlusGreen to match flash to various fluorescent lamps. The CTS set (Color Temperature Straw) are popularly used instead of CTO, being slightly less red, 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, but there are differing colors of these.
These filter 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.
Fluorescent lamps have a non-continuous spectrum, hard to match completely. Sources say color is:
Fluorescent lights are continuously on, except with the old magnetic ballast, they flicker with the AC line frequency (twice per cycle of 60 times a second in North America, 50 Hz in many other places.) This is a big problem for shutter speed duration, which can randomly sample partial cycles and see varying color shot to shot (often randomly seeing a more brown cast.) Setting the shutter speed to match the AC line frequency, either to 1/60 second, or double at 1/120 second (North America) helps to see only complete cycles for more stable color. Slower multiples of 1/120 second work OK (1/40 second or 1/30 second), but faster shutters are unlikely to work. But modern electronic ballast (in 1990's) has changed this situation greatly today, then converting fluorescent operation to about 20,000 Hz (but is less used in large or old ceiling fixtures, but common in the smaller systems, like CFL.)
You can determine if your fluorescents flicker this way: For this, don't use Auto WB. Intentionally use a shutter speed faster than 1/120 second that does not match. You are seeking flicker, so 1/150 or 1/200 second would be good, or rather, bad. You can aim the camera directly at the lamp for this, for brighter light to achieve faster shutter. Take a few shots, all the same. If results vary dramatically (some are brown), that's flicker. If all of several results are the same, no flicker. If all the same, but color is off, that's just White Balance.
But regarding White Balance... Even if it is called "Daylight", no fluorescent has a continuous color spectrum, which makes matching White Balance be a special problem, an exact match is never possible. Fluorescent lamps have a CRI rating (Color Rendering Index. See the spectrum test on this link, shown by single slit diffraction - discrete lines vs. continuous spectrum.) A high rating (CRI 80+) has better color - not perfect, but better, often nearly acceptable, but a lower CRI is poor for color photography. Perhaps most colors look right, but the colors not in its spectrum won't look right. In contrast, incandescent lamps do have a continuous spectrum, and they are the definition of theoretical maximum CRI 100. Incandescent may be orange, but it can easily be matched, all of the spectrum is present.
Nikon furnishes a few color filter sets, see Nikon SJ Speedlight Flash Color Filter Sets. The red filter on bottom flash is from SJ-1. 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, like 20x24 inches for around $6, 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 with a slight bow, so it is 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 a more elegant mounting if desired - or not, masking tape works fine too.
Auto White Balance can be best for a hot shoe flash, but not with these colored filters (unless they are part of the filter system for the SB-700, SB-900 flash systems, as instructed) (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 - but only works in TTL BL mode), 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 background 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 very differently on newer Nikon camera models (D300S and later) than on older models (D300 and older.) Newer camera models boost ISO based on the ambient light availability, so that an indoor flash always finds itself working into high ISO (you can always simply turn Auto ISO off.) Older camera models in Auto ISO mode assume we do not need high ISO since we are providing flash, and they remain at low ISO until it is seen that the flash will need more power than is available, and only then ISO is boosted.
The rest here was written about the the older way, D300 and older.
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. :)
Copyright © 2009-2012 by Wayne Fulton - All rights are reserved.
