550ex Flash Factors

Discussion in 'Canon EOS' started by roger_k, Jun 24, 2004.

  1. Hi there,

    Just got my 550ex flash and I was messing around with it last night.
    I am new to the 'external flash' world. So my major question is,
    Does shutter speed play a big part in flash exposure. I put my
    camera in M mode and set the shutter to 15,30,60,125 etc...I shot at
    f4.0 with my 17-40mm on my Canon 10D. I checked the histogram after
    each shot and I didn't notice any difference. Also the look of the
    picture was the same in each shot. I also experimented with changing
    the apeture with a shutter speed of 60. When I closed down the lens,
    the exposure would get brighter the lower i went. I know there's alot
    to learn, but am I on the right track... Also I took some really nice
    photographs when I bounced the flash of my ceiling and increased
    flash compesation by 1 stop. The pictures came out really bad when
    the flash was pointing to the subject. It made the background very
    dark. Maybe tonite, i'll post the pics
     
  2. You got it about right, the shutter speed plays no part in flash exposure (apart from it being open long enough).

    http://photonotes.org/articles/eos-flash/
    is the way to go.
     
  3. To answer to your major question, the duration of electronic flash bursts are very brief; considerably shorter than the fastest shutter speed of your camera.

    Since exposure is a combination of flash and ambient light of the scene, the selected shutter speed of your camera will affect the exposure of the ambient light level but not the flash. Obviously, in your example, the ambient light level was very low, so altering the shutter speed did not affect the background exposure because you did not choose an appropriate shutter speed for the background; e.g., slower than 1/15th second, perhaps several seconds.

    The E-TTL system is strongly biased on the reflectance of the subject under the active focusing point(s). So it is importance to choose a neutral or average toned subject and not recompose the picture as the camera determines exposure at the time the shutter button is fully pressed and before the mirror rises.

    Unlike shutter speed, aperture will affect the flash exposure. However, the camera will adjust the flash power and duration in order to obtain the proper exposure for your subject. You should have obtained equivalent exposures for the various apertures you selected.

    You should also acquaint yourself with how Canon Speedlites work in the different exposure modes in order to utilize the advantages and appropriateness of each mode for various situations.

    Check out http://photonotes.org/articles/eos-flash/
     
  4. The way many photographers handle a Canon SLR is to point the camera directly at the subject, half press the shutter to lock focus plus auto-exposure, then re-compose and shoot. It's a working style that gives quick and reasonably predictable results without flash, but as soon as you put flash into the equation it's a recipe for frustratingly inconsistent results.

    There's at least two strategies for flash that are better suited to the 550EX and E-TTL. If you've got the time and want a perfect flash exposure then use the FEL button, assuming your camera has one. Alternatively, use spot metering and set the active spot/focus point such that you don't have to re-compose after focusing.

    Another thing to remember is that Canon bodies have a default setting that tries to adjust the flash output to balance it with the ambient light for a more natural look. Great in theory but it adds another layer of unpredictability, maybe the camera's idea of natural doesn't agree with yours. Some bodies allow you to use a custom function to remove the camera's urge to show such initiative! Maybe that's another area for you to explore.
     
  5. Gary's response is very insightful. However, I think there are a couple of points that are a little confusing.

    First of all, it's important to realize that the E in E-TTL means Evaluative, so even if you set spot metering, the camera will determine the flash exposure using the evaluative metering system. Besides, many people have difficulty with the system because some EOS bodies use excessive exposure biasing based on the active focusing point. I would assume spot metering, if it worked in this case, would only exacerbate the situation. FEL, on the other hand does, in effect, use spot metering (if your camera has it) to preset the exposure so that you can recompose the picture.

    Also, the "default" mode that Gary referred to, is the EOS feature that comes into play for flash fill modes. It is called "Automatic Fill Flash Reduction Control". It uses the principle that excessive flash power in fill modes will result in overly bright, unnaturally lit subjects, so it reduces the fill flash proportionately, as the ambient brightness increases. In my experience, it works very well. However, many prefer to disable the function with the appropriate custom function and apply their own compensation. This has some pitfalls as you must be able to predict how your set compensation will affect the picture based on the ambient light level and you have to remember to reset the compensation when conditions change. It also complicates the process when you are in a hurry to capture an image.
     
  6. The reason your exposures were getting brighter is because, unless you have your 550 set on "High Speed Sync", it will only sync up to 1/200. More than likely you were hitting shutter speeds above 1/200 when you opened up the aperature. To prevent this, simply set your 550 on High Speed Sync. When trying to expose the background, shoot in AV/eval metering and adjust your ISO appropriatly to keep the shutter speed up.
     
  7. Roger stated that his shutter speed was set at 1/60 when he varied the aperture.
     
  8. Like Bill said - go to PhotoNotes.org and you'll learn about how flash works and how Canon ETTL works. It's a long read but outstanding. Then it will all make sense.
     
  9. Well I understand about the shutter speed now. Attached is 2 pictures I took right now. Here are the specs: Lens: 28-135IS @ 135mm, f8.0, 1/60, Manual, 550ex pointed on left of the subject. I bounced the light off the ceiling on the second photo that's why it's softer. The first photo, i pointed the flash right on the phone... My question is this, even though I chose the f stop and shutter out of the blue. I think I did well with exposure. I am confused. In manual mode on the camera, can I use whatever fstop and shutter combination I want and the flash will expose correctly? This was pretty easy to do. FYI, when I shoot flash, I try to keep my shutter speed between 30 and 200... I just remember reading that somewhere. Any how, I chose my fstop and shutter without any thinking and it seems the flash did all the work... Am I correct , or am I confused?
    008eGi-18509584.jpg
     
  10. Notice in my post above the pictures attached. I have another question. What if I am shooting a portrait with the speedlight in the same configuration. The speedlight directly to the left of the subject. I want that dramatic look with light only on one side of the person's face. The problem is when I point the flash directly to the subject, it is harsh and ugly as you can see in the first photo. I want the soft look like in the second picture, but only to the left side of the subject and the nice soft feel to it (to create the one sided dramatic effect). Can it be done. Here's what I'm thinking, Can I bounce off a reflector or white paper plate. The flash would still be in the left side, but I would point the flash into a reflector. Would this solve my issue. I am coming up with these solutions in my head, so I don't know if I'm in the right direction......
     
  11. The answer is yes to both of your questions. In manual mode, the camera will expose your subject correctly for the settings you choose.

    Also, using a reflector will soften and spread the light.

    However, the only way you are going to learn how to control the light and get the effects you apparently want is to get a book on portrait lighting. However, there are only a limited number of effects you can achieve with only one light.
     
  12. >>I want the soft look like in the second picture, but only to the left side of the subject and the nice soft feel to it 008fMO-18537284.jpg
     

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