Flash metering question

El Kiwi · December 30, 2006

Hi folks,

I've been reading about my flash recently, in an attempt to finally understand it - who would have thought it would be so complicated? I've been helped by the fact that I pretty much only shoot in Manual, so I can easily balance ambient light with flash. The bit I still don't quite understand is how the camera determines how much light to use from the flash - I understand the mechanics, ETTL preflash and all that, just not how it decides how light the scene should be. I assume the camera uses the flash to make the scene (or the metered part of it) middle grey - is this correct?

I shoot Manual on the camera because it allows me to meter different parts of a scene and decide on the exposure that I want, and the camera never second guesses me. But suddenly with the flash it seems like there's no way for me to control this (except using manual flash I guess). So say I have a low-key scene where I want it to be darker than middle grey. I meter the scene, decide on my exposure, and the camera meter is telling me that the scene will be underexposed by a stop. I assume that without any FEC the camera will use enough flash to make the scene up to middle grey, correct? If I dial in -1 on the FEC, then I should get the scene I want?

Thanks for any and all advice!

Scott_Quier · December 30, 2006

El Kiwi wrote:

Hi folks,

I've been reading about my flash recently, in an attempt to finally understand it - who would have thought it would be so complicated? I've been helped by the fact that I pretty much only shoot in Manual, so I can easily balance ambient light with flash. The bit I still don't quite understand is how the camera determines how much light to use from the flash - I understand the mechanics, ETTL preflash and all that, just not how it decides how light the scene should be. I assume the camera uses the flash to make the scene (or the metered part of it) middle grey - is this correct?

I shoot Manual on the camera because it allows me to meter different parts of a scene and decide on the exposure that I want, and the camera never second guesses me. But suddenly with the flash it seems like there's no way for me to control this (except using manual flash I guess). So say I have a low-key scene where I want it to be darker than middle grey. I meter the scene, decide on my exposure, and the camera meter is telling me that the scene will be underexposed by a stop. I assume that without any FEC the camera will use enough flash to make the scene up to middle grey, correct? If I dial in -1 on the FEC, then I should get the scene I want?

Thanks for any and all advice!

The way you describe your technique, it's almost as if the flash is acting to fill rather than be the primary light source.

The meter is telling you that it has evaluated the scene, done it's averaging (or whatever it does) and has decided that you are not allowing enough exposure. It's second guessing you here. Setting the camera to manual as you have is your way of ignoring what the camera is telling you.

I'm not sure what the flash is going to do with the camera set to manual. My guess is that it's going to talk to the camera and then do it's best to bring the light level up to what the camera thinks is needed for that situation. With respect to getting the scene you want, there is no way anyone can tell you what -1 FEC will do. We don't know how much scene you are attempting to light with your flash or how far away your subject.

Photons are Phree! Take the shot, evaluate it and determine what settings, if any, need to be modified. Make the changes and shoot again. Repeat as needed until you get the shot you want or until you determine that you can't get there from here. This is how you learn.

Looking forward to seeing the results, especially as much of what I have just written is guess work. I would love to learn from your experience.

pathfinder · December 30, 2006

Kiwi,

I have answered this question several times and the answers are not short, so I am going to refer you to the best links on the web re: the EOS system flashes. You cannnot decipher the results UNLESS you know the philosophy behind the various modes - The exposures are different when you shoot in Av or Tv, P, or Manual and you need to understand what the sytem is doing or it will be very confusing - Lots of folks complain about the exppsure with the EOS system and it is because they do not understand the basic plan behind the results.

The EOS System flashes work great and are emminently flexible and controllable, but their manuals do a poor job of explaining how to get them to do your bidding. I use the EOS flashes frequently since I read these three links and figured out what the underlying philosphy re: expsoure is. Do a search here on dgrin using "pathfinder and flash" and you should find some of the threads I have posted here also.

There are three links

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

http://photonotes.org/articles/eos-flash/index2.html

http://photonotes.org/articles/eos-flash/index3.html

There is also a great video put out by Blue Crane and availablr from B&H that is very helpful

http://www.bhphotovideo.com/bnh/controller/home?O=658&A=details&Q=&sku=414626&is=REG&addedTroughType=categoryNavigation

Read the links, and if you still are having problems, PM me and I'll help you sort it out.

El Kiwi · December 30, 2006

Thanks for the responses, guys. Pathfinder, I have actually read those links but I probably need to read them again, it's pretty heavy going the first time. And I've read many of your posts on here about the flash, I have to say your patience amazes me - you really help to explain a difficult subject. I shall buy you a beer on the Shootout!

I think I probably just need to practise some more as well as reading all the theory, my head hurts after all the flash stuff I've read recently. It's like exposure - once you understand it it's not complicated, but the difference with exposure is I didn't expect it to be easy!

El Kiwi · December 30, 2006

Scott_Quier wrote:

With respect to getting the scene you want, there is no way anyone can tell you what -1 FEC will do. We don't know how much scene you are attempting to light with your flash or how far away your subject.

Sorry, reading back over this, I didn't explain it particularly well. I'm not thinking of a scene in particular, I was thinking out loud about the process that (I assume) the camera uses to determine how much flash is required. Since I use manual, if the camera's meter is telling me that I'm underexposing by one shot, I know the metered part of the scene is going to be one stop darker than middle grey. My suspicion is that if I don't apply FEC then the camera will use enough light from the flash to bring the metered part of the scene back to middle grey. I'm going to test this.

Scott_Quier wrote:

Photons are Phree! Take the shot, evaluate it and determine what settings, if any, need to be modified. Make the changes and shoot again. Repeat as needed until you get the shot you want or until you determine that you can't get there from here. This is how you learn.

This is also what I need to do!

pathfinder · December 30, 2006

When you shoot in Av or Tv, the camera ASSUMES that you want to illuminate the forground with flash, and use ambient lighting for the background. Hence, in Av mode you might find yourself with a 5 sec shutter speed if the background is very dark. In Progam mode (P) the assumption is that this is a hand held shot, hence the shutter speed will not go below 1/60th sec. No need for a tripod in P mode, but defintely can happen in Av with flash in ETTL.

When you set the camera to Manual Mode, you are free to set the shutter speed and aperture ( you are not able to set the shutter speed faster than 1/200th or the High Speed Synch speed - unless you enable High Speed Synch on the strobe ) and the strobe in ETTL mode, the strobe will attempt to illuminate the focused forground properly - and yes, I suspect properly asssumes a neutral gray target.

I really like shooting in manual mode with the strobe in ETTL, because the aperture controls DOF, the strobe exposes the forground properly, and the amount of ambient exposure and color balance is a function of the shutter speed which you get to chose. A very high shutter speed can drive the background to black, which with macro lenses is a pretty cool thing.

This is what I was referring to when talked about the philosophy of the strobe, and what causes folks to get frustrated with the EOS flash system. They cannot choose the shutter speed in P mode ( despite usually being free to choose the shutter speed in P mode ) and very long shutter speeds in Av mode with flash take folks by surprise. They just did not read enough to gather the underlying philosophy. It really works very nicely once you understand what the assumptions are

Thanks for the nice comments, El Kiwi - I like Corona. Harry has taught me well!

I look forward to meeting you at the Glacier shootout.

El Kiwi · December 30, 2006

I shall bring at least one Corona, in that case! I look forward to meeting you (and others!) as well.

I just conducted an experiment, in the name of science. When I was learning about exposure, the guy teaching me had us all take photos of walls at various exposure values so we could see how it varied. Here's my improved version with flash.

I set my camera up on a tripod, about 2.5 metres from a blank, relatively evenly lit wall. The camera (350D) is in manual mode. I took 3 photos without flash, one exposed according to the meter (1/40 sec) and two underexposed by one and two stops, 1/80 and 1/160 respectively. The "correctly" exposed one on the left is thus middle grey according to my camera:

BTW these are imported into Lightroom, and directly converted to B+W and exported, nothing else.

I then took nine with the flash bounced off the back wall. 3 each at the same ambient exposures as the first 3, then 3 with FEC -1, and 3 with FEC -2. I expected the camera to produce more or less the same image for each set of 3, compensating more with the flash as I used less ambient light, and for each set to be more or less one stop underexposed compared to the previous one. But there's a fair amount of variation within each set of 3, and the sets are clearly not separated by a stop.

I figured that this variation within the sets might be because the flash (430EX) was simply not powerful enough to bounce enough light from the back wall, so I took another 9 with the flash pointing straight at the wall. I expected each set of 3 of these to be pretty much evenly exposed (again, more flash compensation for less ambient), and each set to be underexposed by roughly a stop from the previous one:

These are clearly all evenly exposed, the part that confuses me here is that the FEC appears to have had no effect whatsoever. It's pretty clear that the issue with the bounced light is just a lack of power on the flash, and it's also pretty clear that 1 FEC doesn't equate to one stop of exposure difference in the final image (or even really a noticeable difference in this case, the 9 are all pretty similar). Also curiously, this is overexposed from middle grey by at least one stop.

I decided to try the bouncing again, this time off the ceiling. I have a nice low ceiling, mostly white but with some exposed wood beams. But basically a pretty nice reflector:

Again, some variation within the sets, and not very consistent separation by a single stop.

Of course, all this is terribly academic, I need to go practise some more in real situations! My basic conclusion from all this is that flash exposure is considerably more of a black art than standard exposure. I guess it just takes a lot of experience to know what's required in each circumstance.

El Kiwi · December 30, 2006

Scott_Quier wrote:

The way you describe your technique, it's almost as if the flash is acting to fill rather than be the primary light source.

One thing jumped out at me after reading your comment when I re-read the article that Pathfinder pointed me to - I quote: "Unlike certain other camera systems (particularly Nikon), Canon EOS cameras always default to fill flash mode when the camera is in Tv, Av and M modes" - which I guess pretty much everyone here uses. And thinking about it, I'm not sure the fill flash idea is so useful, it's really the ratio of flash to ambient that matters.

pathfinder · December 30, 2006

Av and Tv definitely assume that you want fill flash, unlike P mode.

But that is not true, in my opinion, for Manual Mode ( in the camera Manual Mode )

When shooting in Manual Mode the exposure will be determined by the aperture and the time to quench of the flash tube for proper exposure. The shutter speed will control the flash to ambient balance. Very short shutter speeds will lead to black backgrounds ( this can be good or bad )

Outside in sunlight you will need to turn on High Speed Synch to use Manual Mode with flash Out of doors. When I shoot macro in bright sunlight, I can drive backgrounds to black by using a small aperture and a very fast shutter speed with proper exposure of the forground controlled by the flash in ETTL.

So I do not believe that flash defaults to fill in Manual Mode at all.

Interesting experiment that you performed. Not sure I can explain all the variations of exposure that you demonstrated.

I am not seeing that variation when using the 580ex in ETTL with the camera in Manual Mode, with the flash triggered remotely via an ST-E2 IR trigger here

LiquidAir · December 31, 2006

You got me curious because I have seen some of the same things with FEC. I just did a similar wall test with a couple differences: the room was pitch black and I set the apeture to f/22. I haven't done any real measurements, but chimping the histogram gave me a result similar to what you would expect: -1 FEC is one stop underexposed and -2 FEC is two stops.

I set the camera to f/22 to be sure that I was not hitting the minimum flash power which can sometimes cause overexposed shots. When you pointed the flash directly at the wall this might have been an issue. I have had occasional issues with overexposed shots where I used direct flash for fill. I have been meaning to test to see if stopping down solved that problem.

I did the experiment in a dark room to make sure the ambient light is not having any effect. My theory is that the flash power is determined from the metering zone with the largest light difference when the pre-flash goes off, but on a white wall there are no shadows so the ambient light may confuse the meter. A good test of this would be to shade some of the wall from the ambient light to see if the flash metering works better.

Another thing to be aware of is that newer Canon lenses provide focus distance information to help with flash metering. Since the AF will often fail on a white wall I got to wondering was your lens focused on the wall when you took the shots or did you switch the lens to manual?

El Kiwi · December 31, 2006

pathfinder wrote:

When shooting in Manual Mode the exposure will be determined by the aperture and the time to quench of the flash tube for proper exposure. The shutter speed will control the flash to ambient balance. Very short shutter speeds will lead to black backgrounds ( this can be good or bad )

I think the main thing I learned is that there's considerably more variation in flash exposure than normal exposure. Normal exposure is pretty straightforward - listen to the meter, get middle grey. Underexpose by a stop, get an image that's twice as dark. The flash, and the camera's interpretation of "correct" flash exposure seems to add a fair amount of unpredictability, which I guess is where the experience comes in! And like I said, these are pretty academic conditions, the flash system is after all designed to work with real scenes. I suspect that rules of thumb like yours here are much more useful.

El Kiwi · December 31, 2006

LiquidAir wrote:

Another thing to be aware of is that newer Canon lenses provide focus distance information to help with flash metering. Since the AF will often fail on a white wall I got to wondering was your lens focused on the wall when you took the shots or did you switch the lens to manual?

Hey, interesting variation. I'll test that if I get time this evening, I can't get my lounge pitch black till then. And yes, I focused on a postcard on the wall then set the lens to manual focus.

El Kiwi · December 31, 2006

More pedantic tests. Following LiquidAir's idea that maybe the flash was incapable of emitting a sufficiently *small* amount of light to correctly expose, I tried again with f/22, 1/125, ISO 100, flash forward. The room wasn't completely dark but at those settings was so underexposed that it might as well have been. 3 shots, FEC 0, -1, -2:

This is almost identical to the first test I did with only ambient light: each shot is separated by about a stop, the first one is correctly exposed as middle grey. So with no interference from ambient light, the camera and flash actually do a great job.

So it looks like flash overexposure in small spaces especially could cause a lot of problems. A lot of people complain that the Stofen cup loses a lot of flash power, but maybe this is actually a good thing. Returning to the previous settings, flash bounced off the ceiling with the Stofen:

...and bounced off the back wall with the Stofen:

Thinking about what I expected previously from these 9-grid tests, it wasn't actually correct. The first row should all be middle grey since the FEC is set to 0. The camera will use more flash to compensate for the lack of ambient. The second row should go from middle grey (since in the first column the ambient is correctly exposed and theoretically the camera actually shouldn't need the flash at all) to underexposed by a stop (ambient underexposed by 2, FEC at -1 so the flash will provide one stop of fill). The third row should go from middle grey (ambient correctly exposed) to underexposed by 2 stops: FEC at -2, ambient underexposed by 2 stops so again the flash is theoretically not required.

Looking over all these tests, the only one that really comes close is the last one, Stofen cup bounced off the back wall. The others are all lighter than that. So it seems that in a small space, the Stofen cup is actually really useful, it essentially dampens the flash enough that the camera can control the exposure correctly. In all the other cases the camera seems to overexpose, which is probably the minimum flash power LiquidAir talked about. And my flash is just a 430, the 580 would probably suffer even more from this. This would also explain why you didn't see this variation in your Christmas shots Pathfinder, since your space was much larger than mine and your subjects probably a lot further from the flash.

jdryan3 · December 31, 2006

I just wanted to thank you El Kiwi for taking the time to do the tests.

Easy to follow along. pathfinder & LiquidAir have both done a nice job of fleshing the discussion (pathfinder - your Christmas Day photos are what I would like to get from using flash).

Along the lines of the lens issues, could the camera be a factor? I know that when I was using my Rebel 300 and I was doing available low light shooting on a tripod I had certain setting. But my 5D, I have found out, almost always wants me to add 2/3 to 1 stop - I just set it up that way rather than chimp and it usually works out. Just a thought and Happy New Year to you all.

pathfinder · December 31, 2006

I think you are onto something LiquidAir - shooting a white wall may confuse several issues, including focus which contribues distance information to the mix.

I would suggest shooting an open newspaper - that is a mix of black and white that has contrast to allow focusing, and more closely matches a neutral gray target. Just a thought.....

Regarding my Christmas photos, all were shot at a distance greater than 5-6 feet, except some of my sister who was sitting directly adjacent to the flash at a distance of less than 3 feet. She WAS overexposed as a result, so I would be careful about shooting closer than three or four feet with the 580+ foam diffuser and an IR trigger anyway.... I shot those images at ISO 200 or 400 to be sure that I would be able to adequately illuminate anywhere in the room. If I had backed off to ISO 100, the flash might have behaved better at closer distances. Maybe...

Regarding differences in FEC on a Rebel versus a 5D, I cannot really comment as I have not shot with a Rebel very much. I did not dial in much -FEC on my shots - the exif is availabe in the galleries for those who are interested.

LiquidAir · December 31, 2006

El Kiwi wrote:

And my flash is just a 430, the 580 would probably suffer even more from this.

I have both a 580 and a 430. The 580 has on more stop worth of power control than the 430 does (8 vs. 7). I did some quick tests on each with my flash meter and found that the peak power of the 580 is about 2/3 stop brighter than the 430 and the minimum power of the 580 is about 1/3 stop dimmer than the 430.

Flash metering question

Comments