Understanding Flash Metering Modes

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flash-metering-modes

Flash Metering Systems TTL, A-TTL, E-TTL and E-TTL II

Terms used in this article are Canon specific but there are the same or similar terms for Nikon, Sony, Olympus and other camera manufacturers. _J6L0002-Edit-Edit

When you use your camera’s metering system, the meter will measure the reflected light from your subject (see: Metering Modes and How Your Camera Meter Works). This is not the case when you use your camera with a flash, either a pop-up or mounted on your camera’s hot shoe and set to one of the TTL modes. (TTL is an acronym for Through The Lens) Irrespective of which TTL flash mode you choose, the exposure is not based on reading the ambient light, (see: Balancing Flash and Ambient Light with a Light Meter) it is based on the flash output. Measuring flash output can be achieved by either measuring a fixed output pre-flash and evaluating the exposure, or by measuring the flash output as it is fired. This data is then used to calculate the flash output required to expose the scene correctly.

So on to understanding flash metering modes . . .

There are three flash metering modes

TTL or through the lens metering

This is the standard metering mode, typically used when your camera has a pop-up flash or a dedicated external flash heads. The exposure is based on the flash firing. The quantity of the flash output is monitored through the lens by a flash sensor that is mounted below the mirror.

Keep in mind, that when you use a flash the maximum (fastest) shutter speed is the sync speed of your camera (check your manual to check what your camera sync speed is, most are 1/200 or 1/250th). You must never attempt to use faster shutter speeds unless your flash supports High Speed Sync.  Since the shutter speed is fixed at the sync speed or slower (in Aperture priority mode: Av on Canon, A on Nikon) the camera will automatically set it at the sync speed.  In Manual (M) mode you must set the shutter speed to the sync speed or slower otherwise you will get black bands in your image.  This is because the shutter starts closing before the flash has had a chance to fully expose the sensor.

In TTL mode the flash will read the exposure from the light bouncing off your subject and the flash sensor will turn off the flash when it believes the image has been exposed correctly. In this TTL mode changes to the aperture setting will typically affect the exposure of the areas of your image that is lit with just the ambient light.

TTL Flash in modes other than Aperture Priority or Manual:

flash-metering-modes

P – is the Program mode  or Program auto mode.  In this mode the camera manages all the settings. Some cameras have an A or full Auto mode. Canon cameras may also have a CA setting for Creative Auto mode. In these modes the camera sets shutter speed at the sync speed, and the aperture based on the ambient light. When using TTL the flash fires and turns off when the auxiliary sensor tells it to based on the evaluated exposure of the flash.

For TTL the flash power and duration is based on the ISO and the aperture setting. In most cameras the through the lens flash exposure reading is taken from the same area as your focus point. For TTL flash metering there is no pre flash.

A-TTL (Advanced through the lens)

This metering mode performs in the same basic manner as TTL. It will read the exposure through the lens, from the area of focus and trigger the flash to turn off when the appropriate exposure is achieved. The main difference is that this method uses a pre-flash. This pre-flash is used to determine f-stop based on the distance to the subject that reflects the flash output. The pre-flash is fired when the camera’s shutter is half depressed, the camera evaluates the readings and sets the aperture. Now when the shutter is fully depressed the flash fires to correctly expose for the subject. Based on the model of your flash, the pre-flash can be an actual white light flash or an infrared flash.

nikon-flash

Nikon’s latest, the SB910

In all cases the main flash is fired as a pre-flash if the flash head is oriented in any position other than directly at the subject. This pre-flash can be an annoyance. The pre-flash is measured by the flash unit and the evaluated data is communicated back to the camera. If you use your camera in Program or Automatic mode with A-TTL, ambient light exposure and flash based exposure are collectively evaluated and the camera attempts to resolve the lighting to establish the f/stop (aperture) for the best exposure. Since the sensor that evaluates the pre-flash is located on the flash unit instead of the camera’s sensor, using a filter on the camera lens will cause inaccurate readings.  This is because the sensor on the flash is not covered with an equivalent filter.

E-TTL (evaluative through the lens)

As with A-TTL, E-TTL also uses a pre-flash. However, the flash exposure is not measured by the dedicated flash sensor but the main sensor that is used for ambient light exposure readings. This is similar to the exposure calculations and focus locking that occurs before the shutter opens.  This is true for images captured in ambient light when your camera is in Aperture, Shutter Priority or Program mode. E-TTL has far greater exposure accuracy than TTL or A-TTL modes. The pre-flash in E-TTL mode is not triggered when the shutter is half depressed but when the shutter is fully depressed. Since flash durations are very short the pre-flash is not visible to the naked eye. The pre-flash measures the distance and reflectivity, and calculates the appropriate flash output. All this is accomplished just before the shutter actually opens.  The pre-flash is fired immediately before the main flash except when your camera is set for 2nd curtain sync. In addition, if FEL (flash exposure lock) is used, the pre-flash is fired only when FEL is established.

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Canon’s newest speedlite the Canon 600EX-RT

E-TTL II

This is the new standard in Canon EOS systems.  It is a firmware enhancement that uses the camera and not the flash to do all the evaluation. Fortunately it is compatible with older E-TTL flash units.  E-TTL II uses the camera’s evaluative metering zones before and after the pre-flash. In this process, areas with small changes in brightness are then averaged for flash metering. This method prevents the issue where highly reflective materials result in specular highlights. With ETT-L II the  flash metering system is not linked to the selected focus area. This allows focus and recomposition without affecting correct flash exposure. Overall the E-TTL II system allows for much more natural exposures with out the harsh appearance typical of direct flash photography.

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Shiv Verma is a published photographer, educator and technologist and lives in Wrentham Massachusetts. He is an avid wildlife and commercial photographer and conducts photo workshops and tours worldwide. You can check out more of his work. His book "Time-Lapse Imagery" is available in the iBook store Time-lapse Imagery You can check out more of his work on his website.

  • Annoyed

    There are so many weird characters like  appearing in this article.

    Horrible editing that made the article difficult to read.

  • Trevor

    Change your text encoding format on your browser. I was having the same problem until I changed to Unicode

  • Jared Lawson

    Thanks for sharing…understanding your flash is key to the beginning of good lighting. I use my Nikon speedlight in 90% of my portraits, and it is entirely capable of being your only light. California Portrait Photographer

  • Adelina

    Thank you, that worked! 🙂

  • Thank you Trevor – weird how browsers behave.

  • Thank you for your comment Jarred.

  • I was going to say I don’t see them but I was noticing that Safari seems to be the culprit for some reason

  • Keith Starkey

    Great information, Shiv. Thanks very much.

  • I’m seeing the same issues in Chrome – lotsa weird characters throughout the post.

  • cougarscat

    Shiv, kudos for packing a lot of important information into a short article and doing so in easily understandable language.

  • Thank you Keith. Most appreciated.

  • Thank you cougars cat. Most appreciated.

  • Ronny

    Hmmm…I see that others have had no issues understanding this article, but I’m having lots of problems. I’m assuming that the author is not a native English speaker, so I’m trying to figure out what he means in a lot of the sentences.

    Does anyone have a link to another article that explains this topic?

    Thank you

  • Hi – Please let me know what you are having a problem comprehending so I can provide you some guidance.

  • David Rolt

    Thanks! great reading, I generally know how to use the modes as I am a pro, buts is good to know the nuts and bolts of the how behind it all. Dave! Dave’s Photos, Co Kerry, Ireland.

  • Thank you for your comments David. Most appreciated.

  • Shiv the above comment is most likely a spam comment, pretty generic. Unless they reply I wouldn’t worry about it.

  • You are still seeing these Martin? Are you able to take a screen shot and upload as an image for me to see? I am not able to reproduce the problem on my end. Thanks.

  • I’ve change a few possible offending characters. Let me know if you still see any more.

  • I’m still seeing lots of invalid characters in chrome. I’ve uploaded a screenshot high-lighting this at http://martybugs.net/temp/dps-bad-chars.png

  • Thanks for doing that Martin. Very odd! I do not see those and I’ve tried 3 different browsers. The weirder part is, those are just a space, there isn’t even a character there for me to fix/change.

  • I think I got them all. Literally I hit backspace, deleted the offending space and put it back. Now they seem fine. Dunno!

  • Thomas Rutkowski

    Her eyes and teeth are OVERDONE!!

  • The article is not about post processing but about metering modes. Appreciate your feedback.

  • Creative Tangent

    Mr Shiv Verma, good job! Folks, Firefox is always a good browser to enjoy web. Native or not native, info is important. Purpose solved. English is English, it changes from county to county, country to country. Never mind. Keep posting such interesting stuff.

  • Coachz

    I agree with Trevor. Perhaps for an advance photographer it makes sense but for the common Joe? Not a chance.

  • Hi Coachz, Not sure about your comment. You agree with Trevor about the formatting and encoding or a comment I missed about the article. Regards.

  • Coachz

    Maybe the comment was “annoyed’s.” I thought the article was above my understanding and was unclear to my understanding but I find that in general with any instruction written by those in the know. Seems they forget where they were at their entry to the art and write to a more experience crowd. In short, the article was no help to me.

  • Appreciate your feedback – maybe a a more basic article on this topic would work better for you. There are a number of such articles on this DPS website that you may consider exploring.

  • Alan Kearney

    I’m sorry folks, but the text coding is the least of the issues to worry about in this article! Its amazing that it’s been accepted as a “Lesson” when there is SO MUCH incorrect information put forth! Obviously this person doesn’t understand i-TTL, or “balanced” flash / ambient lighting!

  • Krishna

    The article purely technical and doesn’t help beginners like me. Would have been nice if some explanation was given about which occasions need which mode

  • Rangachari Varadarajan

    Hi, this article was too technical in content. Feel it should be made simpler using words that a non-technical person like me will understand.

  • Peter Hintze

    isn´t that a PC/Apple related problem ??

  • Jose Amilcar Bonilla

    Hi can someone explain me the option of 2nd curtain the most cameras have.

  • Hi Jose,
    DPS has a lot of great information. Here is an article that you will find addresses your request:
    https://digital-photography-school.com/slow-sync-flash/

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