Ambient vs. flash & a mixture of both
I’ve already written a short blog on metering and exposure, but I thought it was worth discussing the differences between exposing for ambient light, exposing for flash and finally mixing both types of light sources.
The best place to start is with ambient light, also called available light, continuous light or indeed hot light. This light source is one that is present at all time during our exposure, and ideally a constant power throughout it, although not necessarily the case. The most obvious example of this sort of light source is natural light. But other examples exist, for instance the lights in our homes, be they incandescent or fluorescent, are continuous light sources. Some studio photographers also use continuous lights (called hot lights, because they’re usually high power and on for long periods of time, hence they get hot). Strictly speaking these last few examples aren’t available light sources, as they’re added to what ever available light there is at the time, but they share the important property of being “constant” for the duration of the exposure.
For the purposes of this blog, I’m going to focus on ambient/available light and we’ll assume we’re outdoors, in a field with no way of diffusing or modifying the light. Hence we have no control over the power of the light and no way of softening it.
A bucket of water
A common analogy used in explaining exposure is that of a bucket which needs to be filled with water. The bucket represents our sensor (or film) and the water represents light. The goal is always to get a correctly exposed image, or a fully filled bucket.
Our exposure controls
With ambient light, we have three primary controls which allow us to control exposure:
- ISO: ISO dictates how sensitive the film/sensor is to light. The lower the number, the less sensitive it is and the more light it needs in order to achieve a “correct” exposure. The higher it is, the less light is needed. To bring this back to the bucket analogy, the simplest way to look at this is that ISO is the size of the bucket. A larger ISO number means we have a smaller bucket, which obviously needs less water to fill it in comparison to a smaller one. Using whole stop increments, typical ISO values include 100, 200, 400, 800, 1600, 3200, 6400. There are values, both before 100 and after 6400
- Shutter Speed: Shutter speed dictates how long our sensor/film is exposed to light. The shorter the duration (quicker the shutter speed) the less light is absorbed by the sensor and similarly, the longer the duration (the slower the shutter speed), the more light that is absorbed by the sensor. When applying this to the bucket analogy, the simplest way to look at this is to equate light to a water tap. The longer you leave the tap on, the more water flows into the bucket. Similarly if you only leave it on for a short time, only a small amount of water will flow into the bucket. Using whole stop increments shutter speeds would include …1/15 1/30 1/60 1/125 1/250 1/500 1/1000 1/2000 1/4000 1/8000
- Aperture: Our last exposure control is that of aperture. Aperture is a diaphragm that sits in front of the sensor/film, which opens up or closes down (never completely closed), controlling the amount of light hitting the sensor. In terms of he bucket, aperture equates to the flow of the water from the tap; how much we turn it on. If we have our aperture wide open, we have the tap turned on full and there is a torrent of water from the tap. If we close down (referred to as stopping down) the aperture, it’s equivalent to turning the tap off to just the point of a drip. Using whole stop increments, aperture values include f:1 f:1.4 f:2 f2.8 f:4 f:5.6 f:8 f:11 f:16 f:22 f:32…
As it’s been mentioned a few times already, I think it’s worth a quick explanation at this point. A stop is measure used by photographers to describe either twice or half the light, depending on whether you’re increasing or decreasing the amount of light in your exposure. For instance, going from a shutter speed of 1/250 to 1/125, your shutter will be opened for twice as long as it was previously, hence letting in twice the light. You have increased the exposure by 1 stop. Similarly if we changed our aperture from f8 to f11, we have closed down the diaphragm, known as stopping down, allowing 1/2 the light in, decreasing the exposure by 1 stop. The same concept applies to ISO.
Multiple correct exposures
By combining ISO with aperture and shutter speed, our goal is to always fill the bucket of water to the brim and achieve “correct” exposure. It’s worth noting at this stage, that each of the controls has certain properties that will change the look and feel of your photograph, for instance the ISO figure you use will have an effect on the noise or grain in an image. The shutter speed can effect things like camera shake, and how subject movement is captured (frozen vs. blurred) and finally your choice of aperture decides the depth of field of your image, i.e the distance of the closest object to you that will be in sharp focus, to the furthest object which is in focus. These are very much artistic choices, but the point is, that you can have multiple combinations of ISO, shutter speed and aperture, all of which fill the bucket, but all of which result in completely different photographs.
For the moment, ignoring how we got this reading, lets say to obtain a correct reading, or to fill the bucket to the brim, we need IS0100 @ f:5.6 1/250. Lets keep the bucket size (ISO) fixed for the moment and look at how shutter speed and aperture interact with each other. If we were to leave the tap on for twice the time, changing our shutter to 1/125 our bucket would overflow. As such, we need to counteract this by stopping down the lens by one stop, going from f:5.6 to f:8, reducing the flow of water from the tap, bringing out bucket back to being full only to the brim. The following combinations are also equivalent exposures:
ISO100 @ f4 1/500
ISO100 @ f5.6 1/250
ISO100 @ f8 1/125
ISO200 @ f5.6 1/500
ISO200 @ f8 1/250
The various combinations are virtually endless. The important thing for the photographer is firstly to be able to achieve a correct exposure, but also to understand the various effects aperture, ISO and shutter speed have on an image, and to choose the best combination to obtain the shot they want.
Under and over exposure
Ok, so we know by now that by changing ISO, aperture and shutter speed, we can fill the bucket. But what happens if we have too much water and the bucket over flows (over exposure) or indeed if there’s not enough water to fill the bucket entirely (under exposure)?
I think over exposure is an easier concept to deal with first. Once a bucket is full of water, any other water that tries to enter the bucket simply floods away; this water is lost for ever to the bucket. Similarly with light, if you over expose a shot, the detail is lost for ever. Over exposure, first of all effects the brightest/highlight detail and as the exposure gets gradually more over exposed, it then begins to effect mid-tone and finally high light detail, until such a point that all detail is completely lost and your shot ends up white. Obviously this would be a case of a dramatically over exposed image. What occurs more frequently, is to have areas or parts of your of your image over exposed.
Under exposed images occur where not enough light was present to expose the film/sensor. Referring back to the bucket analogy, this would be equivalent of not filling the bucket to brim. How under exposed the image is, really depends on the size of the bucket and the amount of water present. Unlike over exposure, where detail is lost for ever, image detail is captured and retained in an underexposed shot. The issue is how much detail is there versus how much noise would there be in the photo should we try and enhance and brighten the detail.
Noise in shadow detail
The diagram below displays 8 different stops/tones. Each stop is twice as bright as the one before it, going from complete black all the way through to complete white. Note, pure white is simply a line at the very end of the diagram. The stop is displayed as grey, because anything before complete pure white will have some detail/tone. The nature of digital sensors is that noise resides primarily in the shadow detail (the lower end of the diagram).
However, the diagram above is a little misleading in that it shows eight equally sized bins for each tone. This is not the case in real life! I’m going to get into bits here and decimal encoding, so if you’re unfamiliar with any of these concepts, I’d suggest you check out my bits and bytes blog, before reading on.
By starting with a single bit, the MSB (most significant digit) we describe the first bin. As it is a single bit, it can be assigned a value from 0 through to 1, which has a range of 1, storing values from 0 through to 1. Introducing a second bit, we can now represent 4 values and can store values from 0-3. So with the first bit, we can count up to 1 (range of one) and with a second bit, we can count up to 3 (two more values). The second bin is twice the size of the first! Let us now add a third bit. This enables us to record 8 values, counting from 0 through to 7. This bin now holds information from 4-7, which is a size of 4, again twice the size of the previous bin. The moral of the story is that with every additional bit, the next bin holds twice the amount of detail as the last.
1 bin/bit: size of bin is 1 representing range 0-1
2 bins/bits: size of bin is 2 representing range of 2-3
3 bins/bits: size of bin is 4 representing range of 4-7
4 bins/bits: size of bin is 8 representing range 8-15
5 bins/bits: size of bin is 16 representing range 16-31
6 bins/bits: size of bin is 32 representing range 32-63
7 bins/bits: size of bin is 64 representing range 64-127
8 bins/bits: size of bin is 128 representing range 128-255
This is more elegantly shown in the diagram below, where the size of each bin is drawn proportionally to the bins beside it. What should be noticeable is that the highlight bin (from 128 through to 255) holds half of all the detail in the exposure. If we assume the bottom three bins are the shadow detail, these only contain 8 values, a very small amount of the overall detail (255 is total). The 4 midtone bins, from 8 through to 127, hold almost as much as the single high light bin.
Making the bucket smaller
This is a very important concept to understand, particularly when it is applied to the fact that noise occurs primarily in shadow detail. In this case, noise only resides in up to 8 bins out of 256, which is less than 3% of the overall range.
Now let’s imagine we have an underexposed shot or indeed we want to boost the ISO. Effectively what we’re dong, is making the bucket smaller or removing one of the bins. With a smaller bin, it’s easier to fill the bucket, in the case of increasing ISO, or indeed removing the high lights bin in the case of an underexposed shot, since there is no detail/information in that bin anyway.
This has implications in terms of noise though. Before, we had noise in 8 bins out of the total 256. However, since we have removed a bin, it is now in 8 bins out of a new total 128, which is now approx 6% of the range. Make the bucket smaller again, by either increasing the ISO or underexposing the image by at least two stops and we’ll be in a situation where noise will make up around 12% of the range!
The dynamic range of a scene is the measured difference between the darkest and brightest parts. On a sunny day, this could easily be above 10 stops. Now remember, a single stop is twice the light. So a two stop difference is 4 times the light. Hence a 10 stop difference is where the highlights are over a 1000 times brighter than the shadows! One of the limitations we often face is where the dynamic range of our sensor/film is less than that of the scene we’re going to try and capture! It simply isn’t possible to record 10 stops of dynamic range if we’re only capable of capturing 8. In this case, the photographer needs to make the decision as to what detail is captured. For instance in the above case, the photographer could choose to ignore two stops of highlights and record the first 8 stops (exposing for shadows), or alternatively, ignore the fist few stops of shadow detail and expose for the highlights, recording the last 8 stops.
Exposing for ambient light; putting it all together
After reading all that, no doubt a lot of you will be tempted to put your camera in some sort of automatic mode and click away. In one way, I can’t blame you. There is a lot to consider. But, once you get your head around the 3 controls of ISO, aperture and shutter speed, it starts to become second nature.
Always try and ensure you get a good exposure. A lot of photographers don’t worry too much about exposure due to the latitude we have in post-processing for tweaking exposure. This is definitely a huge advantage, but there is no substitute for a correct exposure, obtained in camera. Measure the dynamic range in your scene. Know the dynamic range of your camera and expose accordingly for each scene! Fill the bucket, but don’t over fill it!
A lot of what I talk about now has already been covered in my flash 101 – The technical bits. A lot of the finer technical details are explained in more depth there, so it maybe worth your while checking that out before moving on from here?
Imagine this scenario: a completely dark room, with no ambient light all. If we used the highest ISO and the widest aperture possible, what sort of shutter speed would be needed in order to obtain an exposure? The answer is: it doesn’t make a difference. If we held the shutter open for 1/8000 of a second, or indeed 30 seconds, the exposure would still be completely black. Shutter speed had no effect on the exposure.
A flash is a burst or strobe of light that happens for a very short period (thousands of a second usually). Regardless of what shutter speed we used above, if we introduce flash into the equation, things change. Taking the example of a 30 second exposure, we’ll fire the flash at the halfway stage of the exposure (15 seconds). Effectively what we have is 15 seconds of complete darkness, where there is no ambient and no flash, then one strobe of flash, followed by another 15 seconds of darkness. The entire exposure happens in a short bust, the period before and after it have no effect on the exposure. Imagine now we shorten the shutter speed down to 4 seconds, as shown below, and once again only fire the flash half way through the exposure. As before, the periods prior to and following on from the strobe have no effect on the exposure. This points to one very interesting and indeed useful fact; shutter speed has no effect on exposure when using flash!
This needs to be quantified a little, but before I do, make sure you read back over the last two paragraphs and are comfortable with what I just said. The two stipulations on this are that firstly, the shutter speed can not be faster than the maximum sync speed, usually 1/250. Secondly, it is important that at what ever shutter speed, aperture and ISO combination you’re using, the exposure is more or less blank in terms of ambient light. If this is not the case, you’re actually in the domain of mixing ambient and flash, which we’ll deal with later. A very simple check to determine how much ambient light is in an exposure; is to leave your camera settings fixed, but turn off your flash and then take a picture. If the result is more or less completely black then ambient light is so low it won’t affect your image. If however, there is detail or indeed the result is “correctly” exposed, then ambient light is playing a huge part in the overall image. This is why studio photography is quite nice; we often have the ability to shut out any ambient light and use just flash for our exposures, making our jobs considerably easier.
So how much light can a flash produce? There are lots of different ways of characterising flash power. When it comes to speed lights (hot shoe flashes), they are often characterised using guide numbers. If you’re using studio heads, another characterisation used is simple wattage. Apart from saying, bigger is generally better, the actual power of your heads isn’t that important. More powerful flashes generally give you more creative options. In terms of being able to light larger spaces, being able to have the lights a considerable distance from your subject and still produce a large amount of light or indeed being able to over power ambient in the case of mixing flash with ambient. Most of the shots in my galleries were lit using a single Nikon SB800 speed light, which has a guide number of 125 @ ISO100. At the end of the day, what ever flash we have, it’s maximum power is all we have to work with, so let’s start there.
What ever flash you’re using, believe it or not, it always outputs the same power of light, regardless of what setting you use. So even I dial down its “power” output, the brightness of the light produced is identical to what it would have been on the previous setting. Another interesting fact However, we generally require the ability to control the light output from our flashes, so this could be a pretty serious limitation! The flash head does this, not by controlling the brightness of the light, but by controlling the duration it is on for! A flash that produces light for 1 second continuously will be one stop brighter in terms of exposure, as one that is only on for ½ a second. The same principle applies, even if the flash periods are 1/1000 and 1/2000 – they are still a stop apart in terms of exposure.
From an exposure point of view, effectively we are changing the “power” of the flash, so why am I being so pedantic? Well a flash of a long duration versus a flash of a shorter duration can affect the look of certain shots. If for example, I wanted to absolutely freeze motion in an image, then a shorter flash duration would be more useful i.e. I would use a lower flash power.
Flash Distance & Fall Off
The most important thing to realise about flash is that it is only capable of lighting one distance correctly. Anything closer to the flash than that distance and your shot will start to become over exposed. Anything further away receives less light and it begins to become under exposed. How quickly this transition in exposure occurs is called light drop off or fall off and is directly related to the distance of the flash to the subject.
Flash is what is considered a point light source. As light leaves the flash it begins to disperse, with less light present at each unit of distance as you move away from the flash. How quickly it loses power is called the inverse square rule. Without going into too much detail here, a good rule of thumb is that the closer the subject is to your flash, the quicker the light falls away. The further away you are, the less the light fall off.
Another impact distance has on a light source, is that it changes its apparent size. This is very important when considering the quality (harsh vs. soft) of the light. If a light appears bigger to a subject, the light it produces is softer. On the flip side, if a light appears smaller, the light becomes harsher. This effect should definitely be considered when you position your lights relative to your subject.
Flash – Putting it all together
With ambient light, we had 3 basic controls for changing exposure. These were ISO, aperture and shutter speed. Whilst there are hundreds of possible combinations of the same to achieve a correct exposure, each one affects the image in different ways. With flash photography, we no longer have shutter speed as an exposure control. But we do have two new controls, being flash power/duration and flash distance. Being able to balance all the variables is really important and understanding how each one affects the image is critical.
Imagine we arbitrarily place down one flash and position a model 10” in front of it. Our ISO is set to 400, shutter speed is set to 1/125, aperture is f16 and flash is on ½ power (these are figures plucked out of the air). We take the image and examine the result on the LCD on our camera. The shot looks over exposed, buy 1 stop – so what can we do?
- Change shutter speed: No difference at all! This is a pure flash exposure remember.
- Decrease ISO: This makes the sensor less sensitive to light and means less noise in the image.
- Change the aperture: We could stop down the lens, having an affect on DOF and possibly leading to softness from diffraction.
- Move the flash away: Reduces the amount of light hitting the subject, but reduces drop off, leading to flatter looking light and possibly harsher light as the light appears smaller.
- Reducing flash power: We could dial down the power on the flash, reducing it’s duration, which increases it’s ability to freeze subject movement
With the exception of shutter speed, all of the above will allow you to change the exposure, but each also changes the image. It’s a case of choosing and controlling the variables you want. For me, flash power is often the obvious choice, particularly more so if you have an assistant with you that can change the flash for you without you needing to move away from the camera.
Mixing flash with ambient
When we’ve talked about flash up until this point, we focused on the scenario where there has been no ambient light present. But now we’ll move into the domain where we want to mix flash with ambient light. When doing this, we have 3 basic options. The first is where our main light will be ambient and we are filling shadows with flash. The second choice is where we are going to perfectly balance flash with ambient light and thirdly we can overpower ambient light with flash, using flash as the primary light source and use ambient as fill.
Now we have 5 exposure controls available to us: ISO, shutter speed, aperture, flash power and flash distance. The table below hopefully clarifies, which controls affect which part of the exposure.
For instance, if I change ISO and/or aperture, I affect both the ambient and the flash parts of the exposure. So I will be changing the over all exposure of the image, more so than the mix of the light sources. Shutter speed allows me let more or less ambient light into the shot, but has no effect on flash and similarly flash power and flash distance, allow me to control how much flash will effect the exposure, without having an effect on ambient. Controlling the mix of flash power, flash distance and shutter speed allows us change the ratios between both sources to achieve the image we want.