Photography 102 - flash guns

[smilodon]

The second essay I wrote for my friend, reproduced here. I'm less happy with this one. It's far too long and wordy and I'm not sure how clear it is in it's descriptions, but here goes anyway.
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Flash photography is a dark art isn't it?

To a degree photography is easy. Many modern cameras are so full of auto settings they almost take the picture themselves. Auto-focus, auto-exposure, auto-white balance... just point the camera at the subject, press the shutter and chances are you'll get a fairly decent result. It's a fact that this made me something of a lazy photographer in the past. I sort of expected to get a good shot without really understanding why I got it. Thank you Nikon another good photo in the bag.

However when I first strapped on a flashgun (strobe) and delved into the world of flash photography it was less than a stunning success. I got rubbish results and didn't really have any idea what was going on or why I wasn't getting the shots I wanted. So I threw the strobe back in the camera bag where it sat unused for years. I was definitely a 'found light' photographer and not a 'brought light' one.

More recently I've had to go back to strobes as part of my job and have actually managed to work out what they can and cannot do and why the can or cannot do it. So here's my introduction to flash photography. It's actually a lot more straight forward than I thought.

First we'll have a quick look at how a modern flash gun works. Then we'll look at the four cornerstones that IMHO make up flash photography. Yes there are only four and two of them are pretty simple and straight forward. So actually there are only two concepts that require any real effort to master. Cool eh.

How they work

Modern flashguns that you connect to your camera via the hot shoe fitting come in many shapes and sizes with a variety of different features. However they have some basic settings in common. They will most likely come with a 'manual mode' and a 'TTL' (through the lens) mode. Now different camera manufactures will add different names for the various versions of TTL they offer. For example Nikon started with TTL for film cameras, then moved to D-TTL when they released their digital range and have recently re-branded the name to i-TTL (intelligent through the lens metering). The names are not really important just the concept of TTL and that it pre-meters a scene before taking the shot. Manual has its place but is usually confined to photo shoots where you have time to set up your shot, check the results and try again. For the majority of photography with a flash gun where you're looking to get the shot first time you'll be in TTL mode. Consider it to be the normal mode with the others only being used when specific requirements arise.

So we've set  TTL mode on our flashgun which is hot shoe'd to our camera. Obviously we're in  a dark environment where we need a flash gun to get a shot. We're in A or Av mode (aperture priority) We've set an aperture setting we like (see the essay on camera modes). The shutter speed is not relevant as it's the power of the flash combined with the aperture that will decide the exposure, so don't get your camera into shutter priority when using flash, it won't work properly. As long as the shutter opens long enough for the flash to fire, we're good to go. Most modern camera and flash gun combos will set this shutter speed themselves, although I tend to set mine manually to 1/60 second. When we point the camera at our subject and press the shutter the flashgun needs to take an exposure reading, make adjustments and let the camera take the shot. First the flash needs to know how much light to fire in order to properly expose the shot. So to do this it fires a very low powered pulse or series of pulses of light. It then takes a light reading from the middle part of the frame. This function will depend on the camera and flash combo you are using but on most modern camera's the flash will read the exposure wherever the active focus point is positioned, except when using spot metering. In this instance it will read the exposure from the metered area. It then works out how much real power it needs in order to correctly light the scene. The shutter opens, the flash fires at this new appropriate power and the shutter closes. All things considered we should have our shot. This whole process takes only a fraction of a second. So fast that the two flash pulses appear to be just one single flash. And that is about it for flash guns. Some have other sexy features but basically this is what a modern TTL flash gun does. Simple.



The four cornerstones of flash photography

Being thick I've had to break flash photography down into four bits to help me understand what is happening when I use a flash gun. These 'bits' are

Strength â€" how much light the flash produces and what that light does when it leaves the flash head.
Shape â€" how the shape of the light affects the shot, as a strong direct beam or a soft diffuse spread.
Colour â€" the colour of flash light and how it mixes with other light sources.
Combination â€" how flash light can be balanced with existing light to compliment and enhance your photographs.

Strength

A flash gun can only produce a fixed and finite amount of light with each exposure. It can fire at different power outputs across different exposures but each time we press the shutter only a single fixed amount of light is produced. Seems obvious. But there is a consequence to this single instantaneous burst of light. When the flash gun is fired all the light is concentrated on the face of the flash gun lens. As it moves out into the scene the light spreads out, so any given point gets less light the further away from the flash gun it gets. A one square foot subject positioned one yard from the flash gun with get almost all the light produced. Move that one square foot subject ten yards from the flash gun and it will get only a small percentage of the light. This is a property of all light sources. Press your face against the glass of a lighthouse and you'll burn off your retina's. Sit on a ship twenty miles away and that same lighthouse is a small flash of light on the horizon.

When a flash gun fires it has calculated the quantity light it needs to produce so that when that light reaches the subject it will be at exactly the right strength to properly expose the image (from the pre-flash remember.) For example if a subject needs a light value of say 20 units and it's ten yards from the camera the flash might fire at 80 units of power. As the light streams away from the flash gun it will weaken. Soon it's down to 40 units, and at ten yards it's dropped down to 20 units which is bang on what we want. This is TTL in action. The numbers are not accurate but the principle stands. The consequence of this is effect is that anything that is closer to the camera than the main subject will get more light. Add a subject at five yards and it will get maybe 40 units of light and most likely will be over exposed. Conversely a subject way back at from the camera and flash gun will only get a few units of light and will probably not appear in our final image at all. We see this all the time especially when we use small compact cameras with weak LED flash. The little built in flash bulb can reach and illuminate the person who is stood close to the camera but the people waving in the background are lost in an inky black darkness.


A fairy in a coal cellar?

So the fundamental concept here is that a flash gun can only correctly illuminate one subject at a set distance. Anything closer to the camera than the subject will probably be over exposed and anything further away will be under exposed to the point where it will not appear at all. If you have two subjects, one at 10 yards and one twenty yards then you need two light sources, Don't expect a flash gun to correctly light through a scene from front to back. It won't.

Also a flash gun can only increase it's output so far. When it fires at 100% that's all you get, there's no more. If your subject is too far away so that 100% still cannot illuminate them, you're out of options, other than moving the camera and the subject closer together. Every flash gun gas a GN (guide number) The larger the number the more power the flash gun can produce and the further it can throw it's light. But even the biggest flash guns have a limit. Learn what yours is and you won't be asking for the impossible.

Shape

A property of light is that small light sources make for harsh directed light and big light sources make for soft diffuse light. Take the sun as an example. In a cloudless sky the sun brightly lights our world and produces harsh, sharp and angular shadows. However on an overcast day the sunlight is scattered through the clouds and appears to be coming down from every angle. Shadows disappear and the light takes on a creamy soft texture. This is generally why photographers love cloudy days for outdoor portrait photography but stay at home on sunny days. The soft light on a dull day is wonderful for portraits while the harsh sunlight from a sunny day is most often not.

Flash gun heads have a small surface area (a few square inches) but produce a lot of light, so like the sun on a bright day they produce a harsh angular light. Sometimes this form of light is what we want but often it isn't. Generally we want a softer more flattering light. To get this we need a bigger light source. How do we make the flash gun head produce a bigger softer light? Easy. First we'll look at the free option, then the cheap option and then the not cheap option.

The free option. Almost all hot shoe mounted flash guns (not flash built into the camera) can be tilted up and down and turned left and right. So we can point the flash head in different directions. If we're inside we have the option of pointing the flash head up at 45 degrees. When the flash gun fires the light will fire upwards, hit the ceiling and bounce back down onto our subject. There are two consequences of this. First the light no longer comes from the same plane as the camera lens. Flash light that fires directly into the subject from the same direction as the camera often doesn't look very good, especially with portraits. If that light is coming from above and is offset it makes for a generally better looking shot. Bouncing light off the ceiling means the light is now coming from above and so as a result it often looks more flattering. Also the light will defuse and spread as it reflects off of the ceiling so it becomes softer which is what we want. It's a simple technique. We can also bounce from walls as well, which can work in a similar way to ceiling bounce. However there are two possible problems we need to think about. First we need a ceiling and that ceiling needs to be within 6 -10 ft of our camera to work (depending on the power of our flash). A vaulted cathedral ceiling won't hack it. Second the flash light will pick up and colour present in the ceiling. If our ceiling is white we're fine but if we have a green ceiling then the reflected flash light will pick up that green colour and tint our subject green, which probably won't be good. So bear that in mind.

The cheap option. Most medium to high end flash guns will come with a dome diffuser. We can also buy 3rd party domes for cheap flash guns that do not come with their own. These domes clip over the head of the flash gun. They are opaque and will soften and spread the light. They don't make a huge difference to the lights shape and being opaque will cut down on the maximum power output of the flash output, but the are great for taking the harsh edge of the flash light and I use them almost as standard. For a few quid they're a must have IMHO.

The expensive option (but not very). If we don't have a usable ceiling we can actually bring our own. Several companies make attachments for flash guns that mimic bouncing light from a ceiling. I use kit made by Lumiquest specifically the pocket bounce.



This neat little attachment fits on the flash head and mimics the ceiling bounce effect. Currently you can get this from Amazon for £12.00

Moving on from the pocket bounce we can add mini soft boxes, and various other bits of kit that diffuse and soften our light. Obviously there is a maximum size we can go with things that physically attach to our flash guns.


The biggest bit of kit I own is the Speedlite Pro No.6 which has a 29cm diameter face



A soft light that wraps around your subject can be very flattering.

Of course sometimes we want to go the other way and make our light more directed and sharper. For this we can buy snoots or honeycomb’s.


Snoots are simply tubes that fit onto the flash head and direct the light onto the subject. They give a strong dramatic look to the shot.


Honeycomb's are just that, a lattice of squares that fits onto the flash head and also directs the light forward but in a less harsh way that gives a different look to the light.


The effects of harsh direct flash light. Depending on the subject this can work quite well.

We don't need to go and spend money though. I've made a snoot on location from a bit of rolled up black paper and sticky tape. I've also used bits of white card held above the flash gun as a ceiling bounce card. Once a friend even fired a flash through his spread fingers to mimic a honeycomb. Paper, card and sticky tape will do the job in many cases. So shaping and controlling flash light is pretty straight forward and low cost. As with everything it's all about getting out and having a go. Try different techniques with paper and card and see what the results are. Then if you think it's worth the investment jump on Amazon (through the dMw affiliate link of course) and buy what you think you can use. Just remember that vanilla flash light out of the gun is probably going to be harsh and a bit 'in your face'.... literally.

Colour

I've written this bit twice now and without a quick and dirty explanation of white balance it doesn't really work. If you are cool with white balance then jump down to the next section, there's nothing for you here.

All light is coloured. Sun light contains all the colours of the spectrum, something we learnt in physics with prisms and whatnot. However most light has a colour tint to it. Sun light in the morning or evening has a warm red tint as it heads towards the horizon. Household tungsten lights are orange, cloudy days are blue, and fluorescent tubes are often a sickly green colour. Each 'colour' of light is defined by it's colour temperature measured in degrees kelvin. Daylight is about 5600K, a cloudy day is closer to 7000K while a household bulb is nearer to 2800K. All sorts of different colours for all sorts of different light sources. However our brains have the wonderful ability to cancel out these colour casts and we still see white as white even when it's showing a definite colour tint. The newspaper we read in the warm late afternoon sun is actually now pink rather than white, but we still see white in our brain. Unfortunately when we look at a photograph of someone reading a newspaper in evening sunlight we don't correct the colour and the paper looks pink. Although the image is actually faithfully reproducing the light it's not as we would 'see' the scene if we were there. So we use 'white balance correction' to fix these colour shifts. Most cameras have an auto white balance setting which does this for us. Or we can set the white balance ourselves from a number of pre sets. Some cameras will let us dial in the actual colour setting we want in kelvins. either way the camera adds colour to the image which cancels out the colour cast and brings the image back to a 'natural' look.

Flash guns are all set to the same light temperature as sunlight i.e. 5600K. If you shoot in sunlight and use a flash (more on this later) everything is at 5600K and the shot looks even. However if you use flash inside where there are already household tungsten lights then the flash gun will produce light at 5600k and the house lights will produce light at 2800K. The camera cannot correct for this as there are two colours of light but we can only have one white balance setting. Not good. Generally the camera will work with the flash gun and set a normal daylight white balance. (Some cameras have a dedicated flash white balance setting but it's all the same in the end). This daylight/flash white balance leaves the tungsten lights alone with the result that they show as being very orange and over warm. We see this a lot when flash is used in doors and the room looks all orangey.


Natural skin tones from a flash gun, but check the big orange light behind the models head and how it makes the background look red. Who didn't add a gel to their flash gun?

Usually we can't do anything about the ambient light. We cannot change the colour of the light fittings or the evening sun, but we can change the colour of the flash. If we make the flash light the same colour as the ambient light we're shooting in then all our light is one colour and we can use white balance to correct it all. Hurrah. The way we do this is to use gels. Gels are little strips of coloured film we can put in front of the flash head. When the light comes out from the flash it fires through the gel and picks up whatever colour we have chosen. So an orange gel will turn the flash light orange to match the tungsten lights. Other colours correct for other light sources. It really is as simple as that. I use flash gels http://flashgels.co.uk/shop/index.php?main_page=product_info&cPath=9&products_id=84  and http://www.lumiquest.com/products/fxtra.htm  but you can get them from anywhere and they're much the same wherever they come from. Add a gel and you're good to go.

Combination

We've spoken about gels and using flash with other light sources but we need to look a little closer at this. Flash isn't just for when it's dark and we need light. It's also for when it's light but we need more light. Normal TTL looks at the middle section of our frame and exposes for what it sees there. It completely ignores the outer areas of the frame. Depending on the amount of ambient light available we may or may not see the background. But TTL just worries about the main subject, making sure it gets an accurate exposure. However we can balance the flash light with the ambient light to create a consistent look throughout our image. We use this mostly in daylight with back lit subjects. Not all flash / camera combos will have this setting but if you buy a branded flash i.e. a Nikon flash on a Nikon on a Nikon body or likewise canon on Canon you'll most likely have the feature. Nikon call it i-TTL-BL, Canon refer to E-TTL II and for Sony it's simply fill flash.  Now bear in mind that I am a Nikon user and don't regularly shoot with Canon, Sony, Pentax or Olympus. I also shoot with professional gear and so while I've checked to make sure most camera manufacturers offer this form of flash photography I cannot guarantee your gear will be able to do this, apologies if that is the case.

We use fill flash, or whatever our camera maker decides to call it this week, when we find the background is brighter than the subject. We'll see this either outside on sunny days, when we sit subjects in front of brightly lit windows or against white walls. The usual result of this kind of shot is that our subject often looks under exposed, dull and 'shadowy'. Not a good look. To fix this we can use flash light to fill in the shadows and brighten the face. It also adds nice catch lights into the subjects eyes, which always greatly improves the look of any portrait shot. Using flash 'balances the light on the subject with the ambient light from the surrounding scene, hence the name balanced fill light.

Imagine a scene where our subject is sitting inside but in front of a brightly lit window. To get this shot to work we need to add light to the subjects face so that it nearly matches the brightness of the scene through the window. A little darker than the ambient light is good and gives a more natural look. So we put our camera into manual mode and using the cameras light meter we set, as an example, f/4 as our aperture and 1/100th off a second for shutter speed. When we push the shutter the following occurs. First  the focus system becomes active, we get good focus and camera meters the whole scene for light. Now the cool bit happens. Unlike in basic TTL mode where the ambient light is ignored, this light reading from the camera is sent to the flash gun. Armed with this information the flash gun  fires the pre-flashes we discussed earlier. The metering system inside the flash gun measures the light that gets reflected back but concentrates only on the light coming from the centre of the frame. For this reason it's always best when the subject is centred. We can crop for better composition later. The flash metering system now compares the ambient light reading it got from the cameras metering system with this new reading from it's own pre-flashes, and  does some secret magical stuff to work out how much flash light is needed to make the subject about as bright as the background. It is balancing the two together, hence the name balanced fill flash. Finally the shutter opens, the flash fires at the power level determined above, and the shutter closes. Job done. The camera concentrates on making sure the background is properly exposed and ignores completely what the flash is doing. In fact the camera doesn't actually know the flash is attached. The flash doesn't care what the camera is doing either. It simply takes the ambient light reading from the cameras light meter and adjusts the flash output to make the subject the ame brightness as the ambient background. One caveat is that if the subject is exactly as bright as the background they tend to look a little 'odd'. It's better as mentioned if they come out a tiny bit darker. Most flash guns have a button where we can adjust the flash up or down a bit. If we set this to minus 1 or minus 1.7 we'll get a more natural look.



Final thoughts

There are lots of additional features we can find in flash guns and there are certainly more uses for them than described here. One of the biggest recent advances is using flash guns off camera and remotely firing them in sync with the shutter and we haven't even touched on that. But hopefully this brief (or not so brief) essay has at least given a broad overview of flash photography and what we can realistically expect to get from it. There are far more clever people who have written far more comprehensively about digital flash. One of the best resource is the Strobist. This site is becoming the de-facto go to place for anything to do with shooting with one or more flash guns.

So, as always, remember 'digital film' is free. Go shoot lots and see what happens.

[smilodon]

I just realised that I made a mistake in the post. Old flash gun/camera combos that were pre 2003 (Nikon D100) would always meter for flash from the middle of the frame. However modern systems will probably meter from where ever the active focus point is located. Some also take into consideration how far the camera is from the focus point. Nikon D lenses can send distance information back to the camera metering system which can also affect how the flash guns decides on it's power output. So the exact details of how flash metering works is far from an exact science, dependent on your specific camera flash set up and is way beyond my ability to describe in detail.