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Using flash as the main light source and for low light photography

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  • Using flash as the main light source and for low light photography

    In the previous discussion, we loooked at using our speedlight as the secondary light source. In those examples we used it as a fill flash and balanced the ouput level of the speedlight to control the contrast in our image. Effectively we used the flash to brighten the shadows.

    In this discussion we will look at the situation where we will use the flash as the main light source and make adjustments to the ambient light while holding the speedlight output levels constant. This is the typical approach in low light photography or where the main subject is in deep shadow while the background is in bright sunlight.

    For the first part of this thread, I will discuss my approach. Image 1 is a typical situation where the main subject is in deep shade and the background in full sunlight.
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    The fundemental of all photography is the same – we start with the ambient exposure. In this case I am going to measuring the exposure of the background and will then utilise the i-TTL (e-TTL) capability of the camera and speedlight to balance the ambient light with the flash light.

    For image 1, a reflected light reading was taken from the grey colorbond fence giving an exposure reading of 1/250 sec @f5.6 and ISO = 100. Obviously the orchid in the foreground is considerably under exposed.

    I said earlier that we would hold the speedlight settings constant and let the I-TTL function do its thing. How does this work?

    When the speedlight is in i-TTL mode, the camera will cause the speedlight to emit a series of pre-flashes. These are processed by the camera and ,with the ambient light exposure readings, a flash duration time is established. When the shutter button is fully pressed the camera will control when the flash is switched on (ensuring that the shutter blinds are fully opened) and then switch off the flash when the amount of light required has fallen on the sensor. All this happens in less than 1/1000 of a second.

    For this to work, the output flash level should be set to the maximum output (1/1) and the zoom level set to match the focal length of the camera lens (preferably in manual mode) – we do not change these settings.

    OK – we have the 'correct' exposure reading for the ambient light and have set up the speedlight, let's take a photo. Chances are you are going to get a result similar to image 2 - “Not happy Jane!!”

    The orchid is not bad, but the background in really over exposed.

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    Image 2 1/250 f5.6 100ISO

    Based on the advice of those who understand the inner workings of i-TTl better than me, we have to reduce the exposure value of the ambient light by 2 – 3 stops for this technique to work. OK lets try 1/250 @ f14 ISO 100 – that is a reduction of 2 2/3 stops.

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    Image 3

    I am pretty happy with the balance between the orchid and the background and although there is a couple of shadows cast by the key light, but I don't think it is a problem. The sun would have been high overhead and although the plant is in shade, some shadows would be present. However it still looks like a 'flash photo'.

    Later in the afternoon just at the commencement of the golden hour I decided to do another shot. This is the result and I think I like it better than the earlier image, but it still looks like a 'flash photo'.

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    Insert image 4

    In the case of Image 5, the ambient light reading was taken with an incident light meter and adjusted for a 2 2/3 stop decrease by closing down the aperture. Note the house on the other side of the street is still very well exposed. Better but not brilliant.

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    Insert Image Image 5

    Let us consider an image in which the ambient light is already very dark. Are we still going to reduce the ambient light by 2 – 3 stops? Yes.

    First let's look at an image where I didn't.

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    Image 6 # Turkey

    The turkey is close to being well exposed but the background is close to 3 stops under exposed. I don't have the exif data and the image was taken about 6 years ago. I suspect that it was taken on the D100 using a SB-22 and probably in shutter priority mode (that was the default 'go to' method back then).

    Now, if we follow the process recommended by the internet experts, what I should have done is to take an ambient reading for a shutter speed of 1/200 second and then closed down the aperture by 2 2/3rd stops. This might have require reducing the shutter speed and or changing the ISO to obtain a suitable ambient exposure setting. The objective should be to duplicate the exposure I achieved for the turkey, but to lighten the background to a level at which point the shadows from the flash are not overly visible. Fantastic if the subject is stationary, but try doing it when your subject is running across you from left to right!

    The rule is that the flash is our key or main light source: our task is to balance the ambient light to blend with the key light. We do this by adjusting the shutter speed and or ISO. The slower the shutter speed, the longer the shutter is open and more ambient light will fall on the sensor. More light falling on the sensor; the lighter the background is going to be. Conversally, shortening the shutter speed darkens the image. However when adjusting the shutter speed you need to keep below the camera sysn speed.

    Now the astute reader might suggest that I am contradicting myself. What I have been saying all along is that the first task is to determine the ambient light exposure. But in the above paragraph, I said our task is to balance the ambient light to the key light or flash light. Surely this suggests that we set the speedlight settings first – and that is correct. But the flash settings are a constant 1/1 output power and approriate zoom to match the camera lens. The ambient exposure setting is the variable. It is set on the camera and then the i-TTL function will attemp to establish a balanced image.

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    Image 7

    In Image 7 the ambient exposure was established from a refective reading of the ground mulch using the spot meter on the D800 and again adjusted for a 2 2/3 stop reduction. Compare this image with the Iris image in the Fill Light post where the background was much darker, even though the iris and statue are less than 500mm apart. The difference is due to the manner in which the ambient light is assessed and our vision for the image.

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    Image 8 Iris

    Now I find the above procedure does work pretty well in most cases, I just don't trust i-TTL to do the job in all cases. Also, I want the freedom to decide what the final image should look like. I am particularly concerned that even when the balance is OK, in many cases it looks like a flash photo – the subject tends to get to be too bright.

    Now there is a better option. Manual control of the flash output and the camera exposure settings. Its simple if you have a flash /ambient light meter- but I understand that that would put you in the minority.

    So how?

    Out with the iPhone and bringup the spreadsheet for the lens that you are using. Now set the output to full power (1/1) and the zoom to suit the focal length of the lens you are using. That's the key light set. Now to balance the ambient light with our key light. Column C ontains the speedllight to subject distance – read down the column until you find the correct distance and read the aperture setting in column A which relates to that distance. Now to the camera take a matrix (evaluative) reading using aperture priority with the aperture value set to the value you read from the spreadsheet. This will give you ambient exposure of the scene. Since we are talking about low light levels, it is unlikely that the shutter speed will exceed the camera sync speed. If it does there are 2 options ISO or move the speedlight closer to the subject. We will discuss this further in the next post. So let's see it in practice. Its a dreary overcast day and the ambient light is 1/200 at f5.6.

    Step by step instructions. If you don't have the spreadsheets and accompanying documentation – send me a PM with your email address – it is free.

    Step 1 – calculate (measure, educated guess) the speedlight to subject distance.

    Step 2 – read down column C on the spreadsheet and locate the row with a distance closest to the distance from step 1.

    Step 3 – Read the aperture value in column A of that row

    Step 4 – with the camera in aperture priority mode, set the aperture to the value read from the spreadsheet and take an ambient light reading of the overall scene.

    Step 5 – Change the camera mode from aperture priority to manual - if you are comfortable with using manual exposure you could use manual mode to get the ambient light and this step will not be necessary.

    The resulting image:

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    image 9

    So what is the difference between the fill light method and the low light method?

    With fill flash we take an ambient light reading and use the aperture to determine the flash output. In low light flash we we use the flash output and distance to calculate the aperture which is used to determine the ambient light exposure.

    Cheers Bob.
    Last edited by oldgreybeard; 26th August 2017, 04:33 PM.

  • #2
    Just checking if edits worked


    • Grumpy John
      Grumpy John commented
      Editing a comment
      If everything looks okay to you Bob I'll delete the Test thread.

    • oldgreybeard
      oldgreybeard commented
      Editing a comment
      Everything looks Ok John.

  • #3
    What do I do if the scene is too bright? That is the flash has over powered the scene resulting in an over exposed or harshly lit image.

    In the examples above, when I have used the speedlight as the Key or main lightsource, I have used the full output flash level. If this is too strong, there are 4 commonly used ways to resolve the issue.
    1. Reduce the speedlight out put level.If you are using my spreadsheet, instead of using column C which is the 1/1 column, use column D for a 1 stop reduction, column E for 2 stop reduction, etc. Simply read down that column and find the distance and then read the aperture value from column A. Calculate the ambient aperture in the usual way.
    2. Decrease the ISO (if possible). Changing the ISO will affect both the speedlight exposure and the ambient exposure
    3. Move the speedlight further away from the subject. Unless you are prepared for the subject size to be reduced in the image, moving the camera further away from the subject is probably not an option. The requires us to remove the speedlight from the camera and use a flash cord or a wireless connection to couple the flash and the camera controls.
    4. Fit a modifier to the speedlight or use bounce flash techniques to reduce the intensity of the flash.

    Off camera flash, speed;light modifiers and wireless flash using the camera's inbuilt commander system will be covered in the next segment.



    • #4
      Bob, does the flash output increments match stops, or fractions of a stop. Eg. Is 1/4 power one stop less than 1/2 power or 1/1 ( full power ) ? Or is there too many variables for it to be that simple.
      Cheers, Brad.


      • Ironwood
        Ironwood commented
        Editing a comment
        Thanks John, I'll do a bit of reading.

      • oldgreybeard
        oldgreybeard commented
        Editing a comment
        Brad is not quite that simple. Light falls off in accordance with the inverse square law of light. If you double the distance - the fall off of the light is 1/2 sqaured or 1/4. Therefore each fraction of the power output is reduced by the reciprical of the square square root of 2 which is equal to .707. So if You take a guide number of 60 at full power, the appropriate distance (GN) for 1/2 power is 60 *.707 = 42.4.
        Similarly when dealing with flash exposure, 1 f stop is also subject to the inverse square law for light. GN 60 @f2.8 will be 42.4 when the fstop is closed to f4.
        Open up the spreasheet and you should be able to follow this. Follow accross row 7. A reduction in distance from 60m to 30m is a change in output power of 1/1 to 1/4.

        The maths is 60 *.707 = 42.4 42.4 * .707 = 30.

        I will go into this in more detail in the next segment when we look at off-camera flash. It takes a bit to get our heads around the fact that we have to calculate 2 exposures: ambient and flash and to a large extent, they obey 2 different rules. Ambient _ shutter speed - aperture - ISO : Your speedlight only uses aperture, distance and power
        I hope to post the next segment next week, but if you need more information please post .
        Last edited by oldgreybeard; 28th August 2017, 09:17 PM. Reason: fixed typo in formula

      • Ironwood
        Ironwood commented
        Editing a comment
        Thanks Bob. I read a few pages suggested by Johns search and gathered there was a lot more to it.
        The formulas and calculations on one of the pages did my head in.

    • #5
      Brad, try to think in terms of ambient light as a continuous light source e.g the sun or an incandescent light. If you set a shutter speed of 1 second, the light will be the same for the whole second. The light entering the camera is contolled by the size of the aperture and the shutter speed. However with flash, even if the shutter speed is 1 second, the flash duration is very brief typically less than 1/1000th second. For the same aperture ,the amount of flash light entering the camera is controlled by the Output power and the distance of the flash from the subject. Changing the output value doesnot change the amount of light which is produced by the speedlight - only the lenght of the flash

      Many commentaries on the internet refer to differences in stops as we understand them as part of the ambient light exposure triange to also refer to halving or doubling of output power , e.g.1/1 to 1/2 as being 1 stop. This really increases the confusion because ambient light has a 1:1 relationship, where as flash is subject to the inverse square law and the 1 ;1 relationship of ambient light does not exist.

      In ambient light 1 stop decrease represents a decreases the light by 1/2 (.5) . In flash exposure a 1stop reduction in light output is a reduction of light intering the camera of .707.

      If you haven't looked at the spreadsheets yet, I urge you to do so as this may make the relationship between "stops" in flash exposure a bit clearer. Look at diagonal lines and note that a relationship does exist . I deliberately try not to get too bogged down with the physics because in the long run, it is not necessary to understand it.

      Try to just think of the relationship:
      Aperture is common to both ambient and flash- it is the key to setting the power output on the one hand and the shutter speed on the other.
      Last edited by oldgreybeard; 28th August 2017, 10:19 PM.


      • Ironwood
        Ironwood commented
        Editing a comment
        Your explanation there helps Bob, thanks.

        I will have to print out your spreadsheets so I can have a better look at them. I have opened them in Excel a few times, but I think if I have hardcopies it might be better.

    • #6
      Brad, I have uploaded a more detailed explanation and put it as a separate topic
      Flash and ambient exposure explained


      • #7
        A great effort Bob. I have placed your spreadsheets on my website so others may download them. They are all in a ZIP file and available HERE.
        This is what you will see.
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        Last edited by Isac; 13th September 2017, 10:56 AM.


        • #8
          Thank you for your effort in making these spreadsheets available to all viewers



          • Isac
            Isac commented
            Editing a comment
            You're very welcome Bob - it is a pleasure. It took longer than I expected, but I've been super busy getting a 2018 calendar ready for print, which involved a lot of editing and design - but all done now and everyone is happy with the project. It's for Kaarakin Black Cockatoo Conservation Centre of which I am a volunteer.
            Heaps of photos to work through which were submitted by the volunteers (and a couple of my own as well).
            Last edited by Isac; 13th September 2017, 03:05 PM.