Still struggling with on-camera flash

Discussion in 'Beginner Questions' started by justinweiss, May 17, 2009.

  1. I still don't have the hang of my SB-900 flash. I have read a lot of suggestions here on photo.net as well as on other websites and even books, but I still find myself unsure of how to set up my D700 to get good photos with flash, reliably. Right now I am using trial and error; sometimes it works and I get a good shot. And sometimes it doesn't.
    What I am looking for is a set of steps I can follow to get good exposures, instead of a discussion of theory that doesn't help me when I am actually trying to get a shot. For example, when I am NOT using flash, I know what to do:
    (1) Set ISO
    (2) Set either aperture or shutter speed to suit the subject
    (3) Adjust the other variable (shutter speed or aperture) for a correct exposure
    (4) Take the shot
    Now, of course there are lots of reasons why you might alter that set of steps to get particular effects in specific situations. And I also left out a lot of other settings and modes to check, like metering, autofocus, exposure, white balance (not needed in RAW mode, right?) etc. But at the same time, that is a fundamental set of steps that will get you a good exposure.
    What I am looking for is a similar set of steps for using flash. There could be more than one set of steps depending on the situation you're shooting in (indoors, outdoors, motion, still subjects, etc.), but ultimately I am looking for a "recipe" to get good shots with flash, much like Scott Kelby has "How to get this kind of shot" instructions in his digital photography books. Once I know these recipes, I will be able to experiment with changing them and trying my own things, but I would like to get the basics down first.
    The "recipe" or checklist I have put together so far is for indoor shooting, like in someone's living room, where there is ambient light from windows and/or room lights, and it looks like this:
    (1)Set ISO around 400, give or take. (This would not be high enough for a correct exposure without flash.)
    (2) Set aperture for desired DOF (so for a simple portrait, I might choose F8 or F11)
    (3) Set shutter speed somewhere around 1/250 to 1/60. (Again, the camera is indicating severe underexposure at this point.)
    (4) Set flash to TTL mode (not BL) and normal firing mode (i.e., not slow or rear curtain, etc.)
    (5) Angle flash head to bounce off ceiling
    (6) Take the shot
    The above steps leave out any fiddling with the flash output level; assume it's at the normal "zero" level, but it can be raised or lowered. Just another variable to worry about, I guess!
    Anyway, sometimes these steps work; sometimes they don't. The most common problems with my shots are excessive noise (from too high an ISO?), motion blur (from too slow a shutter speed?), and insufficient DOF (from too wide-open an aperture), all things I did because I was worried about not having enough ambient light in the picture.
    The specific thing I'm having trouble with is that I don't see any clear way to make sure I am getting steps (1) and (3) correct in the above list. I may know what aperture I want, but without the exposure meter to guide me, how can I know that my choice of ISO and shutter speed will generate a good exposure with the flash included? Is this really just a matter of experimenting over and over until you instinctively know what works, or is there a system or checklist you can follow, at least until you get the hang of it? Thanks.
     
  2. Hi Justin,
    Many photographers are familiar with fill flash - i.e subject has the sun to their back and normal exposure would leave the subject dark. In this case, daylight is the key light and the flash acts to fill the shadows.
    In a living room situation as you describe, the same applies but in reverse. Your flash becomes the key light and the ambient is the fill. This is a concept that some find difficult to understand as they have always looked at flash as a fill.
    So look at it like this. First, work out the correct exposure for the scene WITHOUT flash. Now, once you have that, dial down that exposure by a couple of stops (you can dial down via changing ISO, Shutter Speed or Aperture - or a combination of all three). Obviously you understand the consequnces of change each of three parameters. In particular, your shutter speed has to be within the camera flash sync speed, and not so slow as to cause camera shake. Aperture and ISO affect DoF and noise. It's clear you understand this (though I would say f8 or f11 for an indoor portrait seems way too slow for me - I would be aiming for f4).
    Ok, so now your left with an exposure that is under exposed by 2 stops. The scene is a bit dark but there should be some detail in the shadows. The ambient light in the room at this exposure is now providing FILL (it's lifting the shadows but not much more). Now you need to dial in +2 stops of flash to provide KEY or main light.
    How to do this? Well there are several methods. You can use TTL and hope the camera gets it right but remember that TTL will be influenced by the scene it is reading (a white shirt vs a black shirt will cause the TTL reading to change and the flash output to vary). For more accuracy you can use TTL with a grey card, meter off that and use the flash lock function - this should give you spot on exposures but who carries a grey card with them at a informal party? In truth, with digital, the easiest way is to fire a shot with the flash set to manual / check the display and adjust accordingly. The bottom line though is once you have your exposure for the ambient fill light, how much flash you add starts to become a matter of taste and style - providing it's between 1.5 and 2.5 stops, the exposure won't be wrong.
    Have a look here: http://strobist.blogspot.com/2007/09/lighting-102-33-balancing-flashambient.html
    Best of luck,
    Paul
     
  3. One rule to remember: Flash is almost instant, at least in comparison to the shutter speed. This leads to two conclusions: 1) as you slow your shutter down, you will let in more ambient light, but it will not brighten up the flash exposure. 2) if you're getting motion blur, it's from ambient light, not from the flash.
    Without flash, you have three variables to play with: ISO, aperture, and shutter speed. You seem to have a handle on that. With flash, you have all of those, plus you have flash power to play with. A key to understand is that aperture, ISO, and flash power affect the flash exposure, while aperture, ISO, and shutter speed affect the ambient exposure. Flash power doesn't affect ambient exposure, and shutter speed doesn't affect flash (at as long as you're not using FP sync). Every flash picture can be considered a "double exposure", combining ambient light and flash light. You can use the shutter speed and flash power to affect the balance between flash and ambient light.
    If you use TTL, you're leaving the flash power up to the camera to decide. There are many times when that's absolutely the right thing to do, but you may learn a bit more by starting out setting your flash power manually, and experimenting with repeated trials.
     
  4. The Strobist lesson Paul gave you the URL to is perfect for answering your question. Now, do the exercise exactly as the Strobist tells you to (and put the camera on a tripod to do it).
    Besides, once you do the exercise a few times, you'll know the right exposure for the family couch. That's experience, and it will make your job easier the next time you decide to do a portrait of the Heir Apparent, Wife, or both. Or any other subject for that matter.
    One more thing. Do you play pool? Because the angle you set the flash head for bounce flash is critical. Set it too high or low, and the light misses your subject. Are you using the little card thingie that sits atop the flash? And once you set it, if you move forward or back, it may need resetting.
    Do the exercise and let us know how it goes.
     
  5. Thanks very much to all of you. I will do the Strobist exercise as soon as possible (which may not be tonight!) and post the results.
     
  6. That exercise works, and gives good results, but its thinking is backwards. There is another way. It is faster and easier. This is the method I use for manual flash with any film or digital camera. Tripod optional, but common; needed for below 1/30 in film.
    With the flashes and subject in place:
    1. Set ISO to any speed you desire. (100 is common; use whatever speed the emulsion is, if using film.)
    2. Focus on the subject. Note the distance to subject.
    3. Use the distance to subject to set aperture for flash.
    Guide number (the power of the flash) = distance * aperture (solve for this aperture with simple math).
    To use ambient lighting also, the aperture you solve for will be the widest opening. Use up to 2 stops smaller
    to include ambient. Guide numbers for flashes are usually noted for 100 ASA. If you use another speed, you will find that the guide number for those speeds will go up accordingly. Interestingly, the guide numbers will usually double with every two doublings of film speed. For example, ASA 25, 100 and 400 will end up being doubles of the predecessor (GNs 50, 100, 200, for example on my Vivitar 285HV; GNs vary by flash design).
    4. Set camera to synch speed or slower.
    To light the scene with only flash, use the synch speed.
    To light the scene with flash and ambient, use slower than the synch speed. Often, this will be near 1/8
    or 1/4 of a second. Using this slower speed is sometimes called "dragging the shutter."
    5. Make the picture.
    If multiple flashes are used, power settings on the flash unit or variances of flash to subject distance can be used to control how much light they throw on the subject. Generally speaking, the strongest flash will drive the exposure; often in those cases, the strongest flash will be set at a 3:1 ratio against the weaker flash.
    If multiple flashes are used of more balanced intensity 1:1, for example, then double the amount of required light has hit the subject, and you may find yourself wanting to close up one stop. Use the rules of flash intensity to solve for how many stops to close up to compensate for sets of flash that are more than two. It could be used to solve for four flash units working together.
    Each bouncing of a flash will cut its power in half; note this as you predict its power before you fire. A flash bounced from card to wall behind you would be two bounces, for example. Flash meter against a gray card for test. These rules will get you to within 80% solution on the first test fire.
    *****
    So, for the checklist outlined in the OP, I would say that you are having difficulty predicting your exposure for strobe because you are setting aperture for desired depth of field. This is a little different. It can be done. However, the way to do this, successfully, on the first shot, would be to run through the calculations as above; but, adjusting the intensity of the flash to permit it to fall on the subject in a way that lets you keep the desired aperture for DOF purposes. It's another layer to the logic problem.
    You can increase the power on the flashes or move them closer to subject if the aperture you solved for is too small (High f/number) for the desired depth of field. Or, you can decrease the power or move them farther away if the aperture you solved for is too large (low f/number) for the desired depth of field.
    Yet, keep in mind that aperture drives strobe exposures because the strobe will fire in about 1/1000 or faster. The shutter sets the longest limit that the image can receive ambient; it will not control the majority of the light that strikes the film or sensor. Instead, the swiftness and intensity of the flash will do that.
    Keep these ideas in mind, and you can use any camera, any flash or lamp, in any quantity, with or without ambient, anytime you want. The number of test shots you will have to chimp will be far fewer than with the method on strobist; although, that one, too, works.
     
  7. How long to drag the shutter can be worked out in advance by understanding that it's a problem of proportional influence, balancing "flash only" against "ambient only." Meter to determine what an ambient only exposure would be, and then split the difference between that shutter time at the set aperture and your synch time. That point would be 50% ambient influence. Split again, before and after, to determine shutter durations for 25% ambient influence and 75% ambient influence. Using this method, you can predict what the shutter drag will be, if you know how much you want ambient to contribute to the photo, relative to the flash setup, through imagining the picture in your mind.
    Method's a little longer to explain, but "easy-peasy" in execution. Benefit is that it will work with any, any, equipment.
     
  8. Unfortunately, you have to deal with theory to fully understand flash, particularly balancing flash with ambient. The strobist exercise is good if it helps you understand the theory. You can also read the articles at planetneil.com and search for 'dragging the shutter'. There is no single checklist that will work in every situation.
    As for your issues.
    1) Noise may or may not be from too high an ISO. Any kind of underexposure will emphasize noise, particularly if you pull the exposure up in post production.
    2) Basically, if your ambient exposure is less than 2 stops underexposed in relation to the f stop/flash exposure you have set, you introduce the possibility of motion blur (hand holding shake or subject motion). The above is a guideline only and is dependent upon how fast a subject might be moving, and whether your flash is underexposing.
    3) Insufficient DOF has nothing to do with flash. Either you have it or you don't, and DOF is not just dependent upon f stop, but also on focal length and subject distance. Be sure you aren't expecting miracles. There are limits.
    Choice of shutter speed is part of dragging the shutter, which is dealt with in the strobist exercise and in the planetneil article on on camera flash, under techniques. Again, you must understand that theory first or you will forever be using a canned formula that may or may not work.
    ISO is a different story, particularly if you bounce the flash. At f11 and low ISOs like 100 and 200, your flash may or may not be able to provide enough light. Even ISO 400 you may not have enough light if the ceiling is high and/or the room is large. This is something you CAN get a feel for with experience.
     

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