How does my flash work?

[Ricco]

I've recently purchased a flash for my camera, and I'm buggered if I can figure out how it works. Hence I was hoping for some help from cic. Bear with me if they are newby questions. I have tried experimenting a bit but I'm still not sur e if I'm right. Simple questions first, then I deal with the more complex ones.

- a simple question - I assume my flash only fires at one speed and the only variable is the amount of light, is this right? The reason I ask this is because I looked at the strobist.com site and it was speaking about iso, aperture, speed and other factors. This confused me a bit.
- also, if I am in aperture mode, and ettl on the flash, when I get the meter reading I assume this has computed the flashes effect on this. For example, if I am reading 1/100th, then I assume this has already accounted for the effect of the flash?
- first if I deal with ettl in aperture modes.. Say I am in aperture priority mode and have the flash set to ettl. I assume the camera looks at the exposure, calculates how much light the flash needs to add, then adds it. Ok, simple so far (I think).
- now if I switch my camera over to manual mode with ettl, does the flash then automatically compensate for my camera setting? For example,if my camera shows an underexposed scene, then does the flash then try to add in enough light to expose the scene correctly? Likewise, if my manual settings overexposed, then I assume e flash goes on minimum setting.
- so now if I take my flash off camera and set to ettl, does it compensate in the same way as if it was on camera? I.e. does it assume that my flash is the same distance from the camera and hence require the same amount of light to get the right exposure? Following on from this question, if my flash is closer to the subject and ettl, I should then reduce the compensation on the flash to reduce the amount of light?

thanks in advance...

Peter

[Dave Humphries]

Hi Peter,

We're a bit bu**ered without knowing the make and model of flash and camera you are talking about.

Thanks,

[Dave Humphries]

I'll make a start on the general knowledge section

a simple question - I assume my flash only fires at one speed and the only variable is the amount of light, is this right?

In a word; No, it isn't right.

Not exactly sure what you mean, but here's two answers, one will be what you're after

a) The flash will always output the same (instantaneous) brightness of flash, the length of the flash is shortened to 'dim' this when necessary. We're talking in the order of from say 1/5000 to 1/40,000s durations here. Obviously the 'dimming' effect is dependent upon many things; iso, aperture, etc. because of this.

b) It should work, i.e. "fire", at a range of shutter speeds, the highest of these is depenednt upon you camera model and further is modified by whether the flash model has something known as HSS mode which can be switched on. To put numbers on these; we're talking any shutter speed upto say, 1/125 or 1/160 or 1/200 or 1/250, plus any HSS ones available.

I guess we're talking Canon from the terminology, so I'll let someone else answer the rest ..

[Ricco]

Thanks dave, I use a canon 60d and 580exii.

[bambleweeney]

I'm assuming here that Nikon and Canon are basically pretty much the same ...

You can sort of assume that your flash fires at one speed only although this is not exactly true. At high power output the flash speed can go down to 1/200th of a second, at lowest power output it can go up to 1/10000th of a second or faster. Basically only important if you want to freeze high speed motion (in which case you get your flash right up close so that it can go onto its lowest power setting and give you the fastest motion freezing flash possible).

If you're reading a 1/1000th of a second just looking through the viewfinder then this is what the camera is seeing and it won't take flash into the equation until the point the flash fires assuming the flash is on camera. I'm assuming ettl is the Nikon equivalent of iTTl here.

Also at 1/1000th of a second unless your in FP mode or high speed sync mode the flash won't fire as you're camera won't by default synchronise with the flash at faster than 1/200th or 1/250th of a second. Basically shutter speed has little effect on flash exposure at 'normal' (sync and below) speeds and the controlling factor for flash is generally the aperture setting on the camera, setting ISO determines how much light is required for an exposure and so a higher ISO will require less light from the flash, a larger aperture will require less light from the flash and vice versa. The same obviously applies for light in general and not just flash.

Generally speaking you juggle your shutter speed and aperture so that the background or ambient light is at the level you want (and your shutter speed is at sync or less and the aperture is large enough for the flash to have an effect and ....) and then use the flash to light up the subject just how you want it.

In normal ttl mode with the flash on camera the camera will kill the flash when it detects the exposure is correct, when the flash is off camera and being controlled by the camera the flash (for Nikon anyways) will put out a pre flash to calculate the exposure (as far as I'm aware). In manual mode with the flash off camera in iTTl (ettl ?) mode the same thing. If you underexpose in camera the flash will expose the subject properly for you in ettl/iTTL. This depends how you meter the scene, if say you're in manual mode and you underexpose the scene but you spot meter on the face of a person or object near the camera - the face of the person or object will theoretically be well exposed leaving the rest of the scene dark. I say theoretically because obviously the flash won't just illuminate the area you're spot metering but will 'spill' light to some extent to the rest of the scene which will be lit to a various extent depending on mainly the distance betweeen your subject and background.

Essentially your flash will vary the light output in ettl mode to get the correct exposure of the area you've metered, be it closer to or further from the subject. If you meter using full matrix the flash will try to light up the whole of the scene (frame), if you meter in spot mode the flash will fire to expose the area in the frame that you meter off.
If the Canon flash manual is anything like the Nikon manual I sympathise and understand the confusion, I practically learnt mine off by heart and still couldn't make head nor tail of it I swear it's of more use for my lawnmower than my flash.

Basically only a few things to remember.

- Find the shutter sync speed of your camera and make sure when using flash that you set the speed on or below this.

- Make sure you meter for the area that you want exposed correctly (generally means setting the camera to spot metering for portraits) and ettl should take care of the rest. If you put the flash on camera all bets are off - at least for my Nikon but I never use the flash on camera - and thinking about it I rarely use iTTL.

- The closer the flash is to the subject the less power it will need to illuminate it correctly (inverse square law can really kick you in the butt here as you move it further away) and the same applies for your aperture/ISO - wider the aperture, higher the ISO the less light your flash will have to put out.

- Make sure the camera settings match the flash settings - no good having the flash in TTL and the camera in ettl (that one nearly always gets me if I'm in a hurry)

- Make sure that the camera isn't setting exposure compensation for the flash that you've set and forgotten about (another one that often catches me out)

- With the camera in auto mode (Aperture priority for example) exposure compensation will affect the flash too. (On the Nikon anyhow exposure compensation in manual mode only affects the flash)

- The strobist site is a pretty useful resource though I don't believe they have much info on ettl or iTTL modes - mainly manual I think (which is actually a good thing with flash)

Hope that helps a little and doesn't add too much to the confusion.

[Ricco]

Paul - thanks for the info. I think I might need a couple of reads to sink in.

The only way I've been able to "get it" so far is to stick everything on manual - which is good in itself.

So the bit I wasn't 100% sure on is using the ettl / ittl and off camera, does the flash automatically adjust for distance. I must admit I haven't tried it but does the flash fire off a test to get the right lighting levels?

[bambleweeney]

Yep, the flash automatically adjusts. Simple really - more distance equals less light so the flash compensates by upping its output. As far as Nikon goes - and I actually had to test this as I rarely if ever use my flash on iTTL - there is a pre flash which is used to measure exposure and then the flash will fire with the required power to make the exposure. Try it out - take a shot with the flash off camera and if it's anything like Nikon you will see two flashes - first one to measure the exposure and the second to take the shot. Lord knows what happens if you have a red eye setting as in this case theoretically the flash will fire off a few lower power shots to contract your subjects pupils - that and the pre flash should get pretty interesting.
I use my flashes on manual nearly always with a flash meter to get everything spot on - if I'm in a real rush (I'm not a real rush sort of bloke so that's a pretty rare occurence) I'll use iTTL and cross my fingers.

[Geoff F]

The only way I've been able to "get it" so far is to stick everything on manual - which is good in itself.

I had the same problem, Peter, when I went from an old style flash which had one output setting and your camera adjusted the other settings (shutter speed or aperture) to suit.

When I first tried a modern flash with variable output which was calculated automatically by the flash unit I couldn't get much success.

When shooting in Shutter (Tv) or Aperture (Av) mode my camera would decide which were the most suitable settings without flash, or with minimum flash output.

This meant that I was either shooting with the lens fully open or had excessively long shutter speeds. In each case the results were disappointing.

For macro photography in particular, I started shooting in full manual mode and this included setting the flash output manually; which took a bit of trial and error.

Eventually I realised that if I made my camera settings in manual, to suit the scene, the flash made a better job of calculating the exact amount of light required; which it set automatically.

Suddenly my results dramatically improved and I now use that method all the time, although there may still be a bit of initial trial and error in manually deciding upon the level of natural light.

So my normal use is to 'calculate' the best settings (shutter speed, aperture and ISO) to suit the scene and rely on my flash in ETTL mode to supply the correct amount of flash.

Sometimes a little bit of Flash Compensation is required but this simply works in much the same way as normal Exposure compensation.

The limitation is a maximum shutter speed of 1/250 when using standard flash so other settings may sometimes have to be adjusted to allow this. However the flash unit High Speed setting will allow for flash to be used at virtually any shutter speed although the flash coverage may be reduced.

As with many parts of photography, experimentation is required.

[Colin Southern]

Hi Paul,

I'm assuming here that Nikon and Canon are basically pretty much the same ...

There are a few differences ...

If you're reading a 1/1000th of a second just looking through the viewfinder then this is what the camera is seeing and it won't take flash into the equation until the point the flash fires assuming the flash is on camera. I'm assuming ettl is the Nikon equivalent of iTTl here.

Many Canon EOS cameras use a technology called NEVAC that changes the ambient metering whenever a flash is present. When you think about it, this makes sense; if metering is correct for a scene without a flash, then any additional light from a flash will push the image towards over-exposure - so NEVAC drops the basic exposure back about a stop to compensate for this. The easist way to see if it's doing this is to just meter a scene with the flash powered off - meter it again with the flash powered on - and see if there's a difference.

Also at 1/1000th of a second unless your in FP mode or high speed sync mode the flash won't fire as you're camera won't by default synchronise with the flash at faster than 1/200th or 1/250th of a second.

Yes, but it's easier to just leave the flash in HSS (Canon term for FP) all the time, as it reverts to normal mode at x-sync speed or below anyway.

In normal ttl mode with the flash on camera the camera will kill the flash when it detects the exposure is correct, when the flash is off camera and being controlled by the camera the flash (for Nikon anyways) will put out a pre flash to calculate the exposure (as far as I'm aware).

We need to be a little careful about the terminology here - TTL was a technology that measured the light reflecting off the surface of the film, which isn't used used in a digital camera (sorry, not trying to be picky here) - so what we're really talking about is ETTL (or more specifially ETTL II). With ETTL (both varients), the camera doesn't "kill the flash when it detects the exposure is correct", it measures the light in 35 (non 1D series cameras) zones - fires a pre-flash - re-meters those same zones (while the flash is firing) - evaluates how much they've changed - and then instructs the flashes to fire at a pre-set level. By the time the actual flash fires, the camera's mirror is retracted and it can't do any more measuring.

In manual mode with the flash off camera in iTTl (ettl ?) mode the same thing. If you underexpose in camera the flash will expose the subject properly for you in ettl/iTTL.

Sorta / Kinda. If the flash is off camera then for ETTL to work then there needs to be some kind of intelligent link between the camera and flash - either an off-camera shoe cord, or a smart radio link like the new pocket wizards or radio popper. With Canon, if one adjusts aperture / shutterspeed / ISO / or Exposure Compensation, the flash will still try to expose the foreground zone correctly. I understand that with Nikon though, adding Exposure Compensation also reduces the flash power (although I believe that manual mode may be an exception to this). With Canon - unless one is setting the flash power manually - one has to use FEC (Flash Exposure Compensation).

This depends how you meter the scene, if say you're in manual mode and you underexpose the scene but you spot meter on the face of a person or object near the camera - the face of the person or object will theoretically be well exposed leaving the rest of the scene dark.

Um, no - spot metering assumes that you're spot-metering a medium gray, so you have to apply compensation for any variance from a medium gray (normal metering), and I'm pretty sure this doesn't tie in with the flash calculation anyway (ETTL II anyway).

Basically only a few things to remember.

- Find the shutter sync speed of your camera and make sure when using flash that you set the speed on or below this.

I'd suggest just keeping the flash in HSS mode and shooting any speed you like One just has to be aware that once one goes above X-Sync speed then the flash essentially behaves like a constand light source, so changing ISO / Shutterspeed doesn't have the same effect.

- With the camera in auto mode (Aperture priority for example) exposure compensation will affect the flash too. (On the Nikon anyhow exposure compensation in manual mode only affects the flash)

Not for Canon.

[Colin Southern]

Hi Peter,

Have a read throught the bits that interest you here.

NK Guy was also kind enough to send me a copy of his book - which I can heartily recommend as being the "ducks nuts" of learning Canon EOS Flash Photography"

http://photonotes.org/books/mastering-canon-eos-flash/

http://www.amazon.com/Mastering-Cano...9408593&sr=8-1

http://www.amazon.com/Mastering-Cano...9408593&sr=8-2

(the last link is to the Kindle Edition - you can be reading it on your PC monitor or iPad in a few minutes!)

[Ken MT]

This article gives a lot of information on flash development, and mainly Canon improvements such as TTL, ETTL etc

http://photonotes.org/articles/eos-flash/

[Colin Southern]

This article gives a lot of information on flash development, and mainly Canon improvements such as TTL, ETTL etc

http://photonotes.org/articles/eos-flash/

Great minds think alike

[Ricco]

Thankyou everyone for the information - I have a lot of reading to do.

I must admit - I struggled to find information about how stuff like ettl actually works, hence the questions.

Who would have thought flashes would be so complex

[Colin Southern]

Who would have thought flashes would be so complex

Sometimes too smart for their own good, but that's a story for another day!

[Geoff F]

Who would have thought flashes would be so complex

They started out by being very simple, and required a new bulb for each shot.

Then they gradually evolved into something slightly more compact with 'everlasting bulbs'.

After that a committee was formed to decide what would be the next development stage. So . . .

[Ricco]

Colin - I took some of your advice regarding books (just samples at this stage) but one bit of information I gleaned out of what I have read so far I thought useful to share here in case another newb like myself stumbles across this post in the future.....

In general:
- the shutter speed on the camera doesn't affect the flash exposure, only the ambient exposure. Makes sense because the flash duration is a lot shorter than the camera shutter speed.
- the aperture affects the flash exposure. Going from say 4 - 5.6 decreases the light being let into the camera. Hence if you change the aperture without adjusting the flash exposure, the subject will be lit differently.

Probably obvious to those a lot more advanced than myself, but I thought worth sharing none the less.

[Colin Southern]

Colin - I took some of your advice regarding books (just samples at this stage) but one bit of information I gleaned out of what I have read so far I thought useful to share here in case another newb like myself stumbles across this post in the future.....

In general:
- the shutter speed on the camera doesn't affect the flash exposure, only the ambient exposure. Makes sense because the flash duration is a lot shorter than the camera shutter speed.
- the aperture affects the flash exposure. Going from say 4 - 5.6 decreases the light being let into the camera. Hence if you change the aperture without adjusting the flash exposure, the subject will be lit differently.

Probably obvious to those a lot more advanced than myself, but I thought worth sharing none the less.

Hi Peter,

Kinda yes, but be careful to both of those ...

- the shutter speed on the camera doesn't affect the flash exposure, only the ambient exposure. Makes sense because the flash duration is a lot shorter than the camera shutter speed.

The camera has a speed - known as X-Sync speed - which is the fastest speed at which both shutter curtains are fully open at once. Below this speed (eg is x-sync is 1/250th and you're shooting at 1/60th) then yes - the shutter speed has no influence on the flash portion of the exposure - but - if you shoot ABOVE x-sync speed (by using the flashes high speed sync mode) then - because the flash fires for the full duration of both curtain passes - it effectively becomes a constant light source, at which point changing the shutterspeed WILL affect the amount of flash light entering the exposure (ie it'll keep it in the same proportion to the ambient light, which previously it wouldn't have) (which may or may not be a good thing).

- the aperture affects the flash exposure. Going from say 4 - 5.6 decreases the light being let into the camera. Hence if you change the aperture without adjusting the flash exposure, the subject will be lit differently.

Changing the aperture affects the ambient light, but the ratio of flash to ambient stays the same, and the subject won't be lit differently because (assuming you're shooting the camera/flash in Av/ETTL mode) the camera will compensate for - say - a reduced aperture by lowering the shutterspeed, and the ETTL metering will compensate for the reduced aperture by increasing the power of the flash (assuming extra flash power/capacity is available).

This is why some of these "blind faith" rules can bite you in the bum.

[Turgay]

Hi. First of all, I want to thank to all of you who contribute to this topic, it has given to me some answers to my questions about flash photography previously in my mind.

As a newbie, trying to understand the flash exposure, I would like to give an example and then my questions are coming at the end of the post.

As you know, flash exposure has four variables: Aperture value, ISO, distance to subject and the power of flash illuminance.

As a starting point, without flash, I used Av mode (for fixing aperture value) with 2,8 and at the standard exposure, camera gave 1/5 shutter speed. And the result is below. (By the way, photos were taken at home with a yellowish lamp at night and white balance is auto. ISO was at 1600 for all the images.)



Now, I used flash in E-TTL II mode (Canon 580 EX II on EOS 60D) with the same aperture value 2,8 and camera gave the standard exposure value of shutter speed as 1/15. I read the distance value on the flash as above the 9 mt. (My distance to my subject was 70 cm in all cases.) And the result is below:



As indicated by the distance value which was 9 mt when taking this shot, there is an overexposure, so I thought that I should use manual mode in the flash. I used 1/128 in manual mode. With the 1/128 output, I read 6 mt on the distance scale on the flash. It was out of my target distance which was 70 cm. Anyway, I take this shot. Av was 2,8 and the camera gave the shutter as 1/20 for the standard exposure.



After these three shots, my questions are here:

1. At the indoor with a tungsten lamp, can I not take shots with this flash below 6 mt without tripod?
2. In E-TTL II mode, can flash use less power below 1/128?

Additional question: What is the importance of the distance scale readings when exposuring with a flash? Should I always look at the distance scale on the flash when exposuring with a flash? Automatic mode takes picture with 3,2 aperture value and 1/60 at the ISO 400. Distance is above 4 mt (it is so far from my distance to subject, it was 70 cm.) But, the result is more realistic.



I'm a little bit confused now after taking this shot with the full automatic mode

Thanks in advance, and please don't forget that I'm so fresh newbie in the digital photography

[Geoff F]

Several issues here, Turgay.

Firstly, there is no advantage in using a high ISO for shots like this; except in extreme circumstances. You can normally adjust the flash output to work with low ISO settings and prevent possible image noise problems.

I have mentioned previously that when I first got a Speedlite I wasn't really happy with my results until I started making manual camera settings to suit the scene then adjusted the flash output to suit the light.

Av mode can work well where you want to illuminate the foreground with flash but retain some details from the background. Often effective with relatively narrow apertures where you require a greater depth of focus. The downside is that you frequently end up with shutter speeds which are too low for hand held shots. Setting the flash to ETTL is usually the simplest option.

Manual flash mode means balancing the flash output with the other settings and is really something for specialised uses with experienced users. But can be effective when used carefully.

Shooting with Tv allows you to set a shutter speed which is suitable for hand held shots, subject to being within the flash capabilities, ie not faster than 1/250 depending on camera; or using the high speed flash option, where applicable. The downside with this method is that your camera tends to select the widest possible aperture which can reduce your focus depth.

All of which is why I normally find that I get best results by making manual camera settings then adjusting the flash output to suit, but this can require a few test shots.

ETTL is probably the safest flash output setting for average use. One of the great advantages with this option is that it is easy to adjust the flash compensation as required to allow for under or over flash output.

Full auto camera settings, in my opinion, is the worst of all the options as the camera's opinion of what is required usually means a shutter speed of 1/60 and a fully open aperture; which rarely agrees with what I personally consider to be the best alternatives.

Hope this helps a little, but there aren't really any 100% hard and fast rules about using flash. Much of it is about finding a method which suits you, and the current conditions; and reading some of the available flash tutorials is the best initial approach. They can explain things better and in more detail than I can.

[Colin Southern]

1. At the indoor with a tungsten lamp, can I not take shots with this flash below 6 mt without tripod?

I think it would be helpful to remember how the camera "thinks" ...

In Av mode, the camera will set the exposure for the ambient light ... if this results in a shutterspeed so low that camera shake occurs (ge 1/5th) then "so be it"

In Tv mode, you set the shutterspeed (so camera shake need not be a problem), but if the camera can't set an appropriate aperture then your BACKGROUND may be under or over-exposed - the camera will always try to expose the foreground (flash zone) correctly).

If the camera is in Av mode - and meters correctly for a scene - it's assuming that this is without flash. If flash was subsequently added then this would result in over-exposure unless some kind of compensation was made beforehand - this is most likely why your shutterspeed has increased from 1/5th to 1/15th when the flash is turned on.

2. In E-TTL II mode, can flash use less power below 1/128?

No.

Additional question: What is the importance of the distance scale readings when exposuring with a flash? Should I always look at the distance scale on the flash when exposuring with a flash? Automatic mode takes picture with 3,2 aperture value and 1/60 at the ISO 400. Distance is above 4 mt (it is so far from my distance to subject, it was 70 cm.) But, the result is more realistic.

Keep in mind that the distance scale is a RANGE, with maximum and MINIMUM distances. Below the minimum distance, over-exposure is likely to occur.

My strong recommendation would be to get hold of the book I recommended in Post #10; if you get the Kindle version then you can be learning all of this in the next 5 minutes