manual flash / TTL flash

December 26, 2007 @ 3:42 am · Filed under flash, flash photography, technique

This posting is going to seem a little dry, and overly techy … but it pertains to questions that I am often asked.  So hang in there and let’s go over some of the basics and see how it all fits together.

There are two distinct ways in which flash is handled.  Manual flash or Auto / TTL flash.
(For the purposes of the explanation here, Auto and TTL flash can be grouped together wrt D-SLRs.)

.

.

Manual flash

Firstly, flash could simply be a constant amount of light that is emitted from the flashgun.  In the case of manual flash, there is NO control by the flashgun or camera, over the intensity or duration of the pulse of light from the flash unit.

This is manual flash.  Photographers can control the output of their flashguns by adjusting the settings in fractions of the maximum possible output, eg, ¼ power, 1/16th power.   It should be obvious that the absolute value of ¼ power will vary from flashgun to flash gun, as each model and make of flashgun has a different maximum power.  Manual flash exposure is most easily measured by a handheld flashmeter.

With manual flash,  four things control the exposure metering of our subject:
 - the actual output level from the flashgun, (ie the ratio of the full power),
 - distance from our light source to the subject,
 - aperture,
 - ISO.

Any of these four things can be used to control the amount of light falling on your subject.

.

.

TTL flash

The second way to control flash exposure, is as an automatically controlled flash burst. This flash output can either be controlled by the flashgun itself (usually called Auto mode), or by the camera in conjunction with the camera’s metering system (usually called TTL flash).  When the flash is controlled by the camera, as then measured as the amount of light coming through the camera’s lens, it is called Through-The-Lens flash metering, (hence, TTL flash.)

For most purposes in understanding the basics of flash exposure with D-SLRs, there is little difference between Auto flash and TTL flash.  So, as mentioned earlier, for simplicity’s sake here, we could group Auto flash and TTL flash together.  (There are differences with pre-digital cameras and older flashguns, in how Auto flashguns interface with the cameras, and then the explanation is slightly more complex.)

Anyway, with  TTL flash, the flash output is varied and controlled by the camera’s metering system. This means that for a certain range, our chosen aperture or ISO, or distance to our subject, does not influence our TTL flash exposure.

This is such a crucial point to understand about TTL flash, that I want to mention it again for emphasis. 

With TTL flash, our chosen aperture or ISO (within a certain usable range), doesn’t affect our exposure - and in a sense becomes transparent to our exposure metering. What does affect our exposure, is the reflectivity of our subject, and how large our subject appears within our frame.

This is something that is difficult to comprehend at first, but is easily verifiable with your D-SLR.  There, at home, you can photograph any subject in your immediate surroundings using a TTL capable flashgun.  You can change your aperture from f4 to f5.6 to f8 … and your exposures should look the same.  Similarly if you changed your ISO, your TTL flash exposures should look the same.  The reason for this is that your camera and flashgun’s TTL flash metering system takes care of the basic flash exposure … and can do this within a certain range of chosen apertures and ISO settings.  Your flashgun will emit more or less light as required for correct exposure, dependent on your settings … but your exposures should appear similar.

So, looking back at the four things which affect manual flash, you’ll notice that none of these, not distance, nor aperure, nor ISO setting, seem to have an effect on our flash exposure. (Within reasonable range of course.) 

And it is in this, that the huge difference arises beween approaching metering for manual flash, and TTL flash metering.

The only way to control TTL flash metering, is with flash exposure compensation.  You could control manual flash exposure with any of the four variables mentioned earlier on, but with TTL flash, you would have to change your flash exposure compensation.

.

.

In these differences, the individual strengths of using manual flash, or using TTL flash, can be seen.  And this will most likely affect which of the two kinds of flash lighting we end up using. 

With manual flash, since the flash is a specific level, our subject’s reflectivity or our choice of composition (ie, how we frame our subject) has absolutely no impact on our metering.

This is a hugely important aspect of manual flash.  Once we have our lights set up at a specific distance, and determined our aperture and ISO … then the subject’s reflectivity (ie, how much lighter tones or darker tones there are), have NO effect on our flash exposure.

However, with TTL flash, the subject’s reflectivity / tonality, and our choice of composition, WILL affect our flash exposure.   And hence, we often need to control our flash exposure with the flash exposure compensation. 

More on Flash exposure compensation

.

.

The concepts explained in this section are so essential to our further understanding in flash photography, that I would strongly suggest re-reading this section until it makes sense.  Also check other webpages or books on this topic.  It is also important to your own understanding of flash photography, to try and figure this out with your own camera and flashgun. 

In  thoroughly grasping of the differences between manual flash and TTL flash, our approach to balancing flash with available light will become so much easier.  But that is subject matter for a future series of topics. :)

20 Comments »

  1. Dan Said,

    December 26, 2007 @ 10:51 am

    Yeah, it’s a dry topic, but an important one!

    Thanks to your blog and comments, I actually decided to shoot with flash for a change, and what a difference it’s made for indoor photos! I finally got mine mailed to me just in time to try it out for family photos through all the gatherings over Christmas.

    I’m not so great a reading manuals and then keeping all of that book knowledge in my head to try and apply it later, so I took the usual route of just trying everything. So far I’ve used TTL flash, but other than adjusting +/- exposure settings on the Canon 580EX, I’ve been able to do little else. I’m hoping you’ll go over the mechanics of how flash and distance throw along with dragging the shutter and angling bounces to best like the subject come into play whether with manual or TTL settings.

    Even though I haven’t had a chance to try it out, perhaps you can go over just how to lift shadows in outdoor settings? Yes, I know you’re not a fan of saying X-settings work for X-situation, but understanding how the flash works to bounce or directly light the subject would be important to me here as well. I prefer full-manual on my camera, so why not with the flash as well?

    Thanks again for your excellent instructive dialogue. Its been a big help in approaching new subject matter, and the excellent examples round out the learning. Where’s your PayPal link? Your gratis work is totally worth a bit of monetray appreciation.

    Cheers!

  2. Neil Said,

    December 26, 2007 @ 7:41 pm

    Hi there Dan ..

    Thank you for the kind words. :)

    I definitely intend on expanding on these topics and the questions that I receive … so check back frequently, or set your RSS feed for this blog. There will be more.

    I’ve now added a paypal link to the ‘Contact Me‘ page.

    thanks

    Neil.

  3. Hugh Said,

    December 26, 2007 @ 8:10 pm

    Hi Neil

    Yet another excellent post. Could you also give me an indication of situations when one would choose manual flash over TTL and why?

    Thanks in advance
    Hugh

  4. Neil Said,

    December 26, 2007 @ 8:30 pm

    Hugh …

    As a general rule, I would use manual flash when my subject is (fairly) static in relation to my light-source. This would be the case for either when my flash is on my camera, and I am static … or when I am using off-camera flash which is mounted on a light-stand or similar.

    Then the distance from the light-source to my subject remains the same, and manual flash is the easiest way to get consistent exposures.

    But when *I* am constantly moving around (with my flash on my camera) or my subject is moving around a lot .. then TTL flash is the easiest to use. This is because the camera’s flash metering technology brings us quite close to the correct exposure. (We might have to adjust it though with flash exposure compensation.)

    I hope this helps make the decision process a little clearer.

    Neil.

  5. Peter Said,

    December 26, 2007 @ 9:09 pm

    This is brilliant. I did not appreciate the differences between manual and TTL flash, and definitely did not understand how TTL would compensate for the changes in ISO, Aperture and Distance. But now you’ve said it, it makes so much sense.

    Thanks again for another brilliant article that helps us lessor mortals to reach a higher level of understanding flash.

    Cheers

    Peter

  6. planet neil - tangents » juggling those three balls … Said,

    January 3, 2008 @ 6:26 pm

    [...] To recap quickly from the previous posting where I went over the differences between manual flash, and TTL flash. [...]

  7. Kafka Said,

    January 7, 2008 @ 9:52 am

    Hi, Neil,

    Since you use mostly manual exposure metering mode and always adjust such settings once you step into a new environment, what you may need to change thereafter in the flashgun’s manual mode to get your expected results is still only one of the four following factors, isn’t it?

    - the actual output level from the flashgun,
    - aperture,
    - ISO,
    - distance from our light source to the subject, which can be more or less the same if you can keep the distance between you and the subject. This doesn’t matter much while the flash is used as a fill flash, and anyway you would need to adjust something if the flash is used as a main light source. Isn’t it?

    I’ve learned a lot from your blog and here I am not challenging you at all. I am just not understanding very well. On the one hand, you suggest using manual metering to achieve consistent exposure and better learning curve, while on the other hand, adjusting flash exposure compensation seems to bring back all the concerns about tonality, reflectivity and composition which we’ve just overcome with manual exposure metering.

    Kafka

  8. Richie Said,

    January 10, 2008 @ 9:05 am

    Hi there, thanks for quiet few things actually one was repairing my own hot shoe thanks to you!!! Bravo
    Quick question. I shoot premieres and my Canon580II Brand New, being used with a Canon 1ds is over exopsing all the time, at times by around 3 stops!!! AAhhhhhhhhhhhhh
    It does not happen with my other flash Canon580 not the II.
    Any know settings to be changed or maybe faults with the new 580II ????
    Few people said to me that it is not compatable with the 1ds!!!!!!!!!!!!!!!
    Thanks
    http://www.chicproduction.com

  9. Hugh Said,

    January 12, 2008 @ 9:58 pm

    Neil, thanks for the clarification. Much appreciated. The fog is clearing…!

    Hugh

  10. severoon Said,

    January 17, 2008 @ 5:33 pm

    I’ve searched long and hard for an explanation on why M + TTL is better than Av + FEC + TTL flash. I’ve discussed it with many talented photographers whose results I wish to have for myself, and none of them has been able to crystallize an explanation as succinctly as you do here. It seems sometimes that the world is full of experts that cannot externalize the knowledge they have simply and clearly.

    Since reading your posts on using flash properly, I’ve switched to M + TTL and started to achieve much better results. The improvement in my work due to your site is as significant as when I moved from direct undiffuse flash (way back when I started photography) to bouncing and/or diffusing. Or, what I thought was bounce–I didn’t realize that the bit of direct still hitting the subject was oftentimes keeping me from achieving the result I was after. Now, when I approach a situation, I imagine where I’d put a softbox and how big and bright it would be, and aim my half-snooted flash right there.

    I was at a complete loss to move forward before, now I can move intelligently in a positive direction. Thanks!

    P.S. Have you considered writing a book? I searched the web and the bookstore both for the info on this page and came up empty.

  11. planet neil - tangents » “Dragging the Shutter” revisited Said,

    February 29, 2008 @ 8:31 pm

    [...] explained on these two previous pages ..  - Manual flash vs TTL flash,  - juggling the three controls .. TTL flash exposure will follow your chosen aperture and ISO. [...]

  12. Neil Said,

    March 11, 2008 @ 10:41 pm

    Severoon …

    I actually do have a book contract, and am in the process of writing one.
    Hopefully it will be out by the end of the year.
    I will most definitely announce it here on this blog when it does come out.

    thanks

    Neil.

  13. mad Said,

    March 26, 2008 @ 1:32 am

    thanks neil- this is the best site yet!

  14. planet neil - tangents » so, what are your settings? Said,

    March 27, 2008 @ 3:31 am

    [...] were then 4 factors controlling the flash exposure.  This topic was covered in two recent posts: manual vs TTL flash // juggling shutter speed, aperture, ISO.   (And if anyone is interested in the specific [...]

  15. Yap Tsok Sam Said,

    May 15, 2008 @ 10:08 am

    First of all thanks for sharing. I’ve just ventured into flash photography after reading your blogs. very informative and definitely a must read site for any photographers who wish to learn flash photography.
    However, i’ve tried the experiment you suggest above that with TTL flash, our chosen aperture and ISO does not affect our exposures. I shot using manual exposures and change the aperture from 4, 5.6 and 6, but every shot i got is slightly different in exposure. Doesn’t that means my exposures got affected evrytime i change the aperture? Do i need to use direct flash or bounce for this experiment? Also, when i meter an object using in-camera meter(manual exposure), do i need to keep the exposure indicator in the view finder at the center to get proper exposure? hope you can help me to clarify this, please….. thanks.

  16. Neil Said,

    May 15, 2008 @ 10:49 pm

    HI there …

    The difference in exposure that you’re seeing now is because your ambient light exposure is now changing as you vary your aperture.

    This isn’t a negative … it’s just an indication that you now have a LOT more flexibility in mixing flash with ambient light, and doing so on the fly.

    Regarding your other questions … Yes and no. It all depends. Centering the needle of your exposure meter in your viewfinder does not necessarily mean you have correct exposure. It all depends on the tonality of your scene. Zeroing the needle will give you the same results though as any of the automatic metering modes.

    Read these series of articles to see if this all makes more sense:
    http://planetneil.com/tangents/exposure-metering/

    Neil vN

  17. Chris C Said,

    July 3, 2008 @ 1:17 am

    Hi Neil.

    Ive always thought the same regarding TTL flash (iso and aperture not influencing the exposure but a few months ago i happened onto a problem which totally baffled me)

    I tried to do this. I set up the room to be totally dark meaning i shot very late at night, windows closed and no light whatsoever. I put a white pillow up with stripes for focus locking.

    i put my camera(30D) to shutter of 1/200 and Aperture of F4. I was around 6 feet away from the pillow. I shot at iso 100 and then shifted to iso 1600. The ISO 1600 was overexposed and i cannot explain why. This is shooting with the flash gun (580 EX version 1) direct with no diffusers.

    The funny thing is, if i bounce the flash, the exposure will be the same, if i pull down the diffuser, the exposure will be the same. If i remove the flash gun and use the onboard flash the exposure is very near each other.

    In this case, the ambient did not affect my expposure because i shot a iso 1600 frame to check if any ambient was contaminating my exposure and the frame came out all black.

    can you please shed some light on this, ive been trying to wrap my head around this for like 2 months now.

    I sent in the flash for checkup and canon did not find anything wrong with it.

    Thanks Neil. Hoping you can finally give me peace of mind regarding the matter.

  18. Neil Said,

    July 3, 2008 @ 3:14 am

    Chris,

    Your problem is that you worked outside the range of what your flashgun is capable of. Not only do flashguns have a maximum range, but they also have a minimum range, within which the circuitry just can’t switch off in time to control your flash exposure.

    At 1600 ISO, and f4, your 580EX will give you a mimimum of around 3 meters .. which is approx 10 ft. Since you were about 6 ft away, you were simply too close for the flash to control your exposure.

    So I have two pieces of good news for you.
    a. There is nothing wrong with your flashgun.
    b. You’re going to spend some more time with your 580EX manual. ;)

    Neil vN

  19. Chris C Said,

    July 3, 2008 @ 4:59 am

    simple yet profound hehehehe…..thanks so much Neil, i know youre used to the compliments by now but it really is true. Youve made us all better photographers. Thanks so much for helping us, your time is gold and you still help those in need. KUDOS to you!

  20. Chris C Said,

    July 3, 2008 @ 5:00 am

    oh and PS, FYI….i did read the manual like 3x now, my brain just chose to ignore the parts that actually explained the reason hehehehe…… ;)

RSS feed for comments on this post · TrackBack URI

Leave a Comment