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Flash and ambient exposure explained

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  • Flash and ambient exposure explained

    Photography terminology can at best be confusing in many cases. Some terms have their origins back in the mid – 1800s , but have remained in use even though the media and technology being used has changed significantly. Some just simply do not even give you a clue as to what they mean e.g. ISO literally means “The international Organisation for Standardization”

    The following sketch gives a closer look at the components of the exposure triangles for flash light and ambient light. In discussing these I will attempt to use more common terms and modern language to remove the ambiguity.


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    The first concept to grasp is that here are two different pieces of equipment – Speedlight and camera. Each of these two items have 3 components which effect the overall exposure of the image. We are going to assume that we have set both the speedlight and the camera to manual mode. This way we can concentrate on the actual controls without being confused by computer controlled systems within either unit.

    For the moment forget that you have a speedlight attached to your camera. The image is formed by the light rays passing through the aperture and developing an electrical charge in each pixel of the sensor relative to the intensity of light falling on the pixel. Between the aperture and the sensor is a filter which allows some pixels to be effected by the green light waves, some the blues light waves and the others capture the red light waves. Red, blue and green lightwaves make the visual light which we can see with the human eye.

    In photography it is usaul to refer to a colour, not by name but by the relative components of the Red, Blue and Green light waves which combine to make up that colour. These components are expressed as a relative value between 0 = pure black and 255 = pure white

    Ambient light – is the term which refers to all light from any continuous light source which passes through the lens aperture and contributes to creating the electrical charge on each pixel. When we are outdoors that light source is generally the sun. When we are indoors it may be sun light shining through a window or light from an artificial light within the house, flourescent tubes , incandescant light bulbs or a combination of the above.

    We can measure the light with an exposure meter. The exposure meter in your camera measures reflected light i.e the the light reflected off of our subject / scene through the lens aperture and onto the sensor. The other type of light meter is an incident light meter and that measures the light falling onto the subject / scene. Your camera cannot read incident light, you need a special handheld light meter for this purpose.

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    In the image to the left we can see that we have two priciple areas of light. The dark under exposed area at the bottom of the image and the correctly exposed areas which makes up the reainder of the image.

    When I talk about ambient light exposure I am referring to the exposure settings which we are going to make on the camera so that the lighter area of this scene is correctly exposed.

    This involves setting the aperture to control the amount of light which can pass through the lens. The smaller the aperture (the least amount of light which can pass through) is represented by a higher the “f stop” number.

    The second component is the shutter speed – a rediculous term as it has little to do with speed as in miles per hour. It is the duration or length of time which the shutter is open and the sensor is exposed to the light passing through the aperture.

    Next we come to that innoculous term ISO. When the ligh passes through the aperture and is exposed onto the sensor, the the resultant voltage which is generated is too low to be used as is – it needs to be amplified. When we say that an image has been exposed at an ISO of 200, what we are really saying is that the voltage produced by the sensor has been aplified to a level of 200 units. In reality its effect is not limited to the ambient light only but is applied to the sum of both the ambient light and the flash light because it happens after the sensor has been exposed.

    For the remainder of the discussion, we will treat ISO as a constant and only include Shutter Speed and Aperture in our calculation of the ambient light.

    How do you calculate the exposure values for the ambient light? The same way you always have. Take a meter reading using Matrix or Evaluative meter mode and the adjust the values according to your vision for the image. Higher shutter speed will freeze the action, smaller aperture will increase the depth of field. For each adjustment that we make to either the shutter or aperture values, we must make an equal and opposite adjustment to the other. So if we change the shutter fom 1/100th to 1/200th second we have reduced the length of time that the sensor is exposed to the light source by 1 stop, so we must open the aperature by 1 stop to allow more light to enter.

    Let's go back to our sketch and look at the speedlight triangle. Again we have three components – output, distance and aperture. You will also note that I have circled the aperture components of both triangles. Aperture is the coupling between the flash and ambient exposures. If I calculate that the correct value for the aperture in the ambient light exposure as f8, I must use that value in my calculaion of the speedlight exposure setings, and vice versa..

    But first what are we trying to achieve with the speedlight settings?

    Look at the image above again. We have two distinctly different areas – one correctly exposed and the other under exposed. We only want to change the exposure for the underexposed areas without affecting the other areas. We need to increase the illumination of the underexposed area.

    There are a number of different ways to achieve this objective, but here we are only considering the use of a single speedlight which is mounted to the hot shoe of your camera and both the camera and speedlight are being operated in manual mode except for the speedlight zoom which we will leave in its default setting of auto..

    From our sketch at the top of the previous page, we have 3 components which we can use to affect the amount of light which passes from the speedlight , through the aperture and combines with the ambient light . These components are the output power, the flash to subject distance and the aperture value. Remember the aperture value is fixed by the ambient exposure calculation and we cannot change that. That leaves us with the power output and the distance which we can change.

    Output power or any of its optional titles is a confusing term. The speedlight uses its batteries to build up a charge on a capacitor in the speedlight. When the shutter button on the camera is pressed, the connection between the hot shoe and the speedlight's contacts causes the capacitor to rapidly discharge and apply a very high voltage to the gasfilled flashtube. The flash out put is always of the same intensity – there is no dimmer switch. What happens when we change the output power setting is that we reduce the duration of the flash. The reduction in the output setting is proportional, ½, ¼ etc. but unlike ambient light changing the flash duration has a direct effect on the distance which the flash can cover.

    With respect to flashlight, distance is both a physical and a physics thing. Physical because we can take a tape measure and measure the distance in metres from the speedlight to the subject. But distance also has a effect which can be expressed in physics as the inverse square law for light. This means that the amount of change for a 1 stop adjustment on the speedlight is not the same as a 1 stop adjustment on the camera. But it doesn't matter.

    Let's back up a bit – our objective is to produce a light output from the speed light, which when passed through the aperture of the lens, will produce a correctly exposed subject.

    The output power is simply a measure of the duration of the flash relative to the flash duration at full power. What is important to know is that the effect of any change is proportional, predictable and repeatable. It also has a definable relationship to the distance that the flash can cover.

    Back in the 'good old days' when lenses had distance scales, aperture scales and depth of field scales, speed lights such as the Nikon sb-22 had a chart on the back which gave a correlation between the distance to the subject and the power setting of the out put. Life was easier then we could just look up the chart on the back and press the appropriate button. Today almost all that information is hidden from us – the latest Nikon lenses do not provide much in the way of information for us to make appropriate calculations. However, they are electronically smart and have computer chips in the lenses, camera body and speedlight which in auto or semi auto modes allow the different components to pass information between themselves. That is fine if you are happy to shoot in auto mode, all the work is done for you.

    Let's look back at our sketch, we know the aperture (we worked that out when we arrived at the ambient exposure); we know the distance ( we can measure it with a tape measure or make an educated guess), but how do we determine the power output when we are in manual mode?

    Well there are 4 options:
    1. Use a special hand held exposure meter that will measure both ambient and flash light. The best option,but very expensive.
    2. Use a scientfic calculator or spreadsheet. Frankly a scientific calculator might be OK in a studio, but I cannot imagine using it outdoors nor can I see you carting around your laptop to do excel spreadsheets.
    3. 'Suck it and see' – take a test shot and adjust the output settings until you get the right exposure. Somehow, I don't think I could rely on a bird to hold a perfect pose on a branch while I kept flashing away trying to find the correct settings.
    4. Send me a private message through this forum and I will send you an electronic version of my spreadsheets – Its absolutely free! Or you could go to the trouble of making your own. Once you have the spreadsheet you can download it onto your iPhone or smart phone and you have it with you wherever you are.

    If are using the spreadsheet, all you have to do is find the intersection of the row which relates to the aperture value and the column which contains the distance, then read the output value in row 7 of that column. After using it a few times you will become familiar with the relationships and will not need to refer to your spreadsheet as often.

    The distance that the speedlight will illuminate infront of and behind the distance from the speedlight to subject distance is really very small. e.g with a 200 mm lens and a speedlight to subject distance of 15m at full powerand with an aperture of f11, the range of the flash is only from about 14m -16m. This means that in most cases shadows caused by the flash are not going to be that noticeable.

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    This image was taken using the above techniques.





    Cheers, Bob
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