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In-Camera Exposure Meter and E-TTL Flash

Although I've been taking pictures for a long time, I just got my first DSLR (canon T3i) a few months ago. I purchased a 420EX Canon Speedlite a while later. I have been trying to shoot in manual mode all the time mostly.

I can go through the story of why I am asking this question, but will just ask it: if the Speedlite/flash is providing all or the majority of the light, how do you use the in-camera exposure meter to come up with the right exposure? Or do you?

I was asked to take "candids" at a relative's wedding, primarily after the hired photographer left. The function room had very high ceilings, it was a bit rustic with reddish-brown wood everywhere. There were a lot of windows, the reception started at 5 PM, and it was raining.

I tried taking some under these conditions, and thought I was doing the right thing by centering the exposure meter. I'm embarrassed to write the following: my camera shutter speed was way too slow for hand-holding, and I was telling people "hold still, I don't have a lot of light" before taking the pictures. Also, a lot of these, even with the slightest of movement by the people in them, were blurred.

Later, after 9 PM, when it was just me trying to capture people on the dance floor and general milling around, I just said "the heck with it", set my shutter to 1/125, aperture to F/8, ISO to 1600, and started firing away. Lo and behold they looked pretty good, even though my meter was pegged to "underexposed".

It has really bothered me over the last month that I didn't understand what was going on. After dozens and dozens of test shots, I have come to some conclusions, but I am on this forum to find out if these conclusions are valid:

1) I believe the blurriness in my earlier-in-the-day shots were caused by some camera shake, but mostly "ghosting". I am embarrassed that my shutter speed, in my not understanding what the meter was telling me, was set to 1/8 (I know better now, believe me). Certainly not holding a non-IS lens to that speed. Also, the ghosting was caused not by the subject motion, but that too much ambient light in getting in.
2) True or False: The in-camera exposure meter is only giving me a reading for the ambient light, not what the flash is contributing.
3) True or False: If the Speedlite is the predominantly/only light for the picture, do you go by experience and take the shot? By experience I mean: shutter speed no slower than 1/60 non-IS, ISO at least 800 if not higher, and aperture set to keep everything in focus that you want in focus.

The last statement is the result of a picture I took while trying to figure this all out: I set my camera up and took a picture of a small table with a white vase on top, but the location was a pitch-black hallway in my house. The meter was screaming "waaaay too dark", but when the green "exposure OK" light on the back of my flash lit up, and I looked at a perfectly good picture of this table like it was shot during the day, I was flummoxed.

So, back to my original question/dilemma; What is the exposure meter telling me when flash is the predominant light source, and how do I set exposure in these situations?

Thanks - Dave


  • ZenonZenon Member
    edited October 2014

    I'm just in the middle of creating a flash basics user guide for Canon at another site. When it is done I'll give you the link.     

    First off there is no way to meter a flash exposure. Since the flash has not fired there is no light to be metered. Of course this can be done with a flash meter but you still have to fire the flash first to get a reading.    

    1. Your camera does not care about what the flash is doing and the flash does not care about what the camera is doing. Both have distinct and separate jobs.

    2. Your cameras light meter has nothing to do with flash exposure.    

    Here is how your 430 works. It piggy backs off the camera's exposure system but does not use the cameras light meter. The camera's light meter is only measuring ambient or available light. When you are in ETTL and press the shutter all the way there is a pre- flash before the actual flash. It is so fast you cannot see it. You can test this. Make sure you are on ETTL, look through the viewfinder and take a flash exposure. You will see a flash. That is the pre flash. You won't see the actual flash because the mirror will be flipped up during that event. Put the flash on manual and try it. You won't see a flash because the mirror is flipped up when the flash fires.

    Before I move on lets forget about the flash. Not on your camera right now. You know how your camera's light meter works. If you take a shot of pure white snow it will underexpose and you have add 1 to 2 stops exposure. If you shoot pure black tar it will overexpose and you have to reduce 1 to 2 stops exposure. If you shoot with half the frame with snow and the other half tar your exposure will be pretty much bang on. Your camera light meter averages to 18% grey. This will be important later.              

    Your cameras flash system defaults to evaluative. This is a flash metering mode and has nothing to do with your camera's light meter or metering modes. This includes evaluative, spot, centre weighted, etc.  There are multiple metering zones in your cameras exposure system. First you press the shutter half way to focus and your cameras light meter begins to measure ambient light. When you press the shutter all the way your 430 fires the pre flash. Light from the pre flash is reflected back to the camera. It compares the ambient reading to the light reflected from the pre flash and "isolates" the closest object which is typically your subject/s.

    "Isolates" is important to remember. I will use a bride and groom as an example. A bride in a white dress, a groom in a black tux and the bride and groom together all reflect light back differently during the pre flash. Remember the white snow and black tar example? Exact same thing. That is what the FEC or flash exposure compensation on your flash is for. You can fine tune your flash exposure for your subject/s. 


  • ZenonZenon Member
    edited October 2014
    1. ⅛ is pretty slow. Your flash can freeze objects if the flash overpowers the ambient light but they typically have to be pretty close to the flash. Eventually the flash gets weaker so the ambient light begins to overpower the flash and people in that area will be blurred. Look at the image I inserted at the end of this post. It is about ⅛ shutter. I pre focused and then I swept my camera as I press the shutter all the way. The flash froze the t shirt which was about 6 feet away but got weaker at the xmas tree - about 12 feet away and the ambient over powered the flash.  

    "I mean: shutter speed no slower than 1/60 non-IS, ISO at least 800 if not higher, and aperture set to keep everything in focus that you want in focus". This is a good start but I will post some more info at the very end.
    Even a subject can be blurred when close to the and and when moving so photographs use second curtain sync for this. Here is a good video on it. It can also help you with creative shooting. I'm not sure your 430 has this. 

    Slow shutter speeds is an old photographers trick to bring in more ambient light. As long and the people in the background were not moving too much a little blur was acceptable.         

    2. True

    3. You can go by experience but a good way is to learn how to read your histogram after the flash exposure. ETTL does a pretty good job but is ball park. It gets you close. Whites are your best friend. Get he whites in and the rest falls into place.    

    Here is Neil's take on it.

    The white towel method. Scroll down and see the gent holding the white towel. I attended a flash workshop doing head shots. The instructor had us hold a white sheet of paper in front of the model and we set up the flash exposure.      

  • ZenonZenon Member
    edited October 2014
    So now that you know a little more about how your flash works you are dealing with two light sources - ambient and flash and you need to know how to balance both.   

    Here is a pretty good video on outdoor shooting.

    As for indoor it is the same but typically it is darker. Most photographers like to shoot with the camera on manual because it gives them total control of both the shutter and aperture. What threw me off the first time was the light meter was all the way to the left and I could not wrap my head around that for a few days. Then I realized that the flash would take care of the subject exposure. Just like the outdoor video you can work with the ambient first however depending no how dark it is you may never see the light meter in the middle. Ignore it and trust the process.                    
  • Zenon - thanks very much for the useful info, and answering my "true/false" questions. I have been reading a lot and understand about "dragging the shutter" to balance in some ambient. I understand what the histogram is telling me, so I will make more use of it. From my many, many test shots after the wedding, I understand the flash is freezing the motion of the subject. What I had to figure out, or at least I think I figured out, is the matter of "ghosting", where it's too ambient light caused by the slow shutter speed, as the flash is freezing the subject. I know 1/8 shutter speed was way too slow for what I was doing, and will not make that embarrassing mistake again. I've read that a non-IS lens can be successfully hand-held at 1/60, so that's probably where I would start if I am ever again presented with a similar situation. I would be interested in looking at your finished user guide. Dave
  • Zenon - I just saw your last post, containing "What threw me off the first time was the light meter was all the way to
    the left and I could not wrap my head around that for a few days. Then I
    realized that the flash would take care of the subject exposure." Exactly the same thing I have been going through for the last month. Because of that statement, I am glad you were the first one to respond to my post. You had the same issue as I did.  Dave
  • More on balancing flash and ambient. Basically shutter speed controls ambient exposure and aperture and flash control. Because of ETTL and auto modes there is a little more to this but just for with that for now because if your camera and flash were on manual then that would apply.

    I like Neil's article on this subject. Notice how the background changes by just changing the shutter speed. He calls it dragging the shutter which means a slower shutter speed. Scroll down and see the impact the shutter speed makes.

  • Zenon - thanks for the help. My Canon 420EX Speedlight I believe does not have a manual mode, so I have to stay in E-TTL.

    Also, I shoot in camera manual mode, and am just learning how to use the Canon T3i and 420EX wireless capabilities. I am really trying to nail down my understanding of using flash. I would love to be able to shoot everything with available light, but I know that isn't possible, and I do not want to ever shy away from using a flash.
  • Thanks, Glad I could help. I read that rule of thumb is to try to maintain at least two stops underexposure with your cameras light meter if you can. This depends of course on how dark the venue is and how high you are willing to go with your ISO. If less I don't worry about it. I have shot at 6400 with my FF. Gelling your flash really helps in this situation.  

    If the meter close to is centre then you are getting a nice balance between ambient and flash.  

    So one more thing to add because I lied about your flash metering modes. I don't like to add this until later and people have time to digest the info first but since your a flying with this.

    Again I don't know if your camera has these features.

    Actually there are two more flash exposure modes you can choose from – average and spot metering. Again both have nothing to do with the cameras ambient light meter.    

    Average. This mode can only be accessed in the cameras flash menus, if your camera has it. Older models do not offer this mode. 

    •    The flash must be mounted on the hot shoe and turned on
    •    Find the flash menus in your camera – External Speedlite control 
    •    Locate E-TTL II meter. In this menu you can switch to Average.

    The difference between the two modes. 

    Evaluative. This mode compares the ambient reading to the light reflected from the pre flash and isolates the closest object, which is typically your subject/s.

    Average. This mode also uses the pre flash but it averages the whole scene rather than isolating your subject/s. I will use a bride in a white dress as an example again. Since there are usually some darker areas around the bride the scene is more balanced.

    Some people prefer Average for indoor shooting. Average is not recommended for outdoor as it does work as well. You can experiment and choose which one works best for you. I leave mine on evaluative because it is just one less thing to remember.

    Spot metering. You can spot meter with your flash. When your flash is mounted and turned on press the M-Fn button and the flash will fire. This is not the actual exposure. It calculates the correct exposure within the spot-metering circle in your viewfinder. When you press the shutter the flash will output the correct power. Some people like to meter off skin tones and have their own formulas. Typically like shooting without a flash if you spot meter of Caucasian skin you add 1 to 1 ½ stops. 

    I tried it a few times and found it very irritating. It is not like normal instant pre flash. I always had to tell people that the first flash was just test and it was not the actual shot.


  • Zenon - I will have to look and see, but I believe the external flash menu has both average and evaluative. I have used the camera spot metering many times in certain situations.
    What really helped me be comfortable with my first DSLR was my Canon Powershot S5 IS, which I bought 7 years ago (great little camera, which I still use). It can be used in full manual. Took a seminar at a local college with it 3-4 years ago, and the instructor's goal was to have everyone not shoot "Auto" any more. Really, really helped, although I did fail on many shots until I got the hang of it.
    BTW, I have seen 1-2 of your videos before. Really good and, combined with what Neil is putting out, I should be able to learn a lot.  Dave
  • ZenonZenon Member
    edited October 2014
    For camera spot metering. You are still metering ambient light. So if you metering off your subject you are basically ignoring all the existing light around the subject which is the flashes job. The only way to use it effectively is to meter anything but your actual subject. Typically the background or what we call metering for the sky.    

    Spot metering with your flash via the M-Fn only meters the flash exposure within the spot metering area. I'm only bringing this up because you said - I have used the "camera" spot metering many times in certain situations. Just want to make sure we are on the same page. Camera spot metering has nothing to do with the flash spot exposure metering.      
  • I just noticed something about how I worded this sentence in the previous post and I'm going to change it.  

    So if you metering off your subject you are basically ignoring all the existing light around the subject and it is the flashes job to meter off the subject during the pre flash. This may affect the way you want the background light to look.     
  • Neil vNNeil vN Administrator
    dbruno - your problems you had were all entirely because you didn't consider that there are TWO exposures taking place when you use flash with ambient.

    So even when your ambient is hugely under-exposed, the flash will (if used correctly), pick up the slack and give you correct exposure. Which is what you found out for yourself later in the evening. 

    Please read and re-read this article and all the linked articles in it.

    All the answers to your questions are in there. 

  • Neil, are there still two exposures taking place if the flash is on TTL?
  • TTL has nothing to do with it. It is no different than shooting with the flash on manual. TTL is just a metering system that is designed to help you expose your subject correctly. In manual you are controlling subject exposure. You have control of how much ambient light you want in your image in both modes so yes there are still two exposures.

    Of course if you are shooting in a studio typically the flash is your dominant light source and you don't want any ambient light involved. This again is the same for TTL and Manual flash. 
  • Neil vNNeil vN Administrator
    "Neil, are there still two exposures taking place if the flash is on TTL?"

    Are you adding something to the ambient light? 
  •  "Are you adding something to the light?"

    Sometimes. It was more of a generic question for all flash scenarios. So, yes, it it's fill, you'd be adding light to ambient for sure. If it's studio flash I imagine the flash would be 100 percent of the light despite some ambient light and outside non-fill would be somewhere in between I think. So I am wondering if your question suggests there is one exposure if the flash is total light source and two if it's fill?

    This "two exposures going on" is a difficult concept and I'm sure understanding it goes a long way towards using the flash better and combining it with ambient better. My question stems from Zenon's statement: Your camera does not care about what the flash is doing and the flash does not care about what the camera is doing. Both have distinct and separate jobs.

    If this were so, why use TTL at all? Isn't the point of TTL so that the flash can respond to the camera settings and the light falling on the subject? Or does it only respond (in TTL) to the light falling on the subject and calculating the proper light output for that (depending on whether it's set for main light or fill)? Isn't point of TTL so that flash works in tandem with camera? I attended lecture once where Neil told random people in crowd to call out shutter speed and aperture pairings. And he set those settings on camera without touching flash and light output was the same, his point being (I think) that the flash would respond accordingly. 
  • Neil vNNeil vN Administrator
    two exposures: ambient + flash
  • Neil vNNeil vN Administrator
    Why we consider this as two exposures:
    - ambient light (of whatever source) is continuous
    - flash is instantaneous. A very short duration.

    This is why it makes it easier for is to consider them separately when figuring out exposure and how to balance ambient and flash. Whether flash is dominant or fill, doesn't affect that we consider them as two separate exposures.
  • Thanks Neil. That's a good explanation. I hadn't thought about the ambient being continuous. I was thinking 'How could it be two exposures, it's one photo, one split second that the shutter is fired….. maybe the two exposures are sandwiched together…' Maybe it's semantics like:

    -It's not one sweater, it's yarn and buttons and thread.
    -No, it's one sweater.
    -No, it's yarn and buttons and thread.

    OK, since it's two exposures I guess the issue is how to knit them together properly BEFORE the photo is taken. 

    So I guess the point of pointing out that the ambient is continuous is that it's longer. 
  • The ambient is as long as you want it to be. If you shooting start trails, 10 minutes or much longer before you add the flash to expose your subject/s in the foreground. Or you can shoot with a faster shutter speed and kill the ambient like in a studio set up. Unless you are purposely killing the ambient light you are always adding to it.         
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