Saturday, 25 May 2013

TTL flash delay

Flash metering has come a long way the recent decades. TTL flash metering for SLR cameras was first introduced by Olympus in the mid 1970's. TTL refers to Through The Lens. The camera measures the amount of light coming onto the film through the lens during the exposure, and cuts off the flash as the exposure is sufficient.

Film based SLR cameras


For film based SLR cameras, this is usually implemented by having a flash light meter in front of the film plane. The amount of light reflected off the film from the flash is metered, and the flash is turned off when there has been a sufficient amount of light for the desired exposure. See the illustration below.


Film based SLR camera with lens

In this illustration, the mirror is raised for exposing the film.

This generally worked well, at least as long as the subject was not too dark or too light, in which case you needed to manually adjust the flash exposure.

Digital SLR cameras (DSLR)


With digital cameras, this does not work well, since the imaging sensor, replacing the film, is not reflective enough. To overcome this problem, most DSLR cameras fire a pre-flash before raising the mirror, and then fire the main flash after raising the sensor and opening the shutter.

The pre-flash is used to determine the amount of flash needed for the exposure. With this method, the TTL flash meter is no longer needed, the camera's ordinary light meter is used. See the illustration.


DLR camera with lens

There are some DSLRs that still measure the amount of light reflected off the sensor chip, and avoid the pre-flash. The Fujifilm S1 and S3 does this.

Mirrorless cameras


As you know, Micro Four Thirds is a mirrorless camera system. So there is no mirror, and no viewfinder prism. The camera also has no light sensor anymore. The imaging sensor is the light sensor.

To find the correct flash exposure, a pre-flash is triggered while the sensor is exposed. Then the camera must make the sensor ready for a second exposure, and fire off the flash with the correct amount of light. This typically takes a bit more time than with a DSLR. The DSLR used the separate light meter for the pre-flash, and could expose the main imaging sensor only once.

Here's a basic illustration of a mirrorless camera with lens. It is much simpler, since there is no mirror, pentaprism, or light meter.


Mirrorless camera with lens

Flash and pre-flash timings


To examine the pre-flash and main flash timings, I have video recorded the cameras using a Panasonic GH1. The recording was done at 50fps (to get the most detailed timing measurement), and at 1/50s exposure, so that I would not miss the flash firing.

Using this setup, I video recorded four cameras doing the flash exposure: The Panasonic GH2 and GH3, Pentax K10D, and the Canon EOS 400D. For all the cameras, I used manual focus when taking the test exposures, so that there would be no autofocus delay. I also set the maximum aperture, to avoid the delay of the camera stopping down the aperture.

During normal indoor lightning


Here are the tests, as recorded by the Panasonic GH1:



And the results:

K10DGH2GH3
Pre-flash delay460 ms120 ms80 ms
Main flash delay580 ms220 ms180 ms

Here, we see clearly that the GH3 improves upon the GH2, however, the results are still fairly similar. The GH3 achieves quicker flash activation, which I believe is because it has a shutter that operates faster. The GH3 also takes more pictures per second during continuous drive mode, another indication that the shutter operation is faster.

During very dim indoor lightning


Here are the tests:



And the results:

400DGH2GH3
1st pre-flash delay340 ms120 ms60 ms
2nd pre-flash delaynonenone160 ms
Main flash delay400 ms220 ms260 ms

The Canon EOS 400D was the slowest, taking 0.4 seconds from the shutter was pressed, until the main flash exposed the image. The GH2 was the fastest here, however, that was just because the GH3 decided to do two pre-flashes. This was probably done for better flash exposure accuracy. I would guess that it normally uses only one single pre-flash, in which case it would have been the fastest in the test.

Avoiding the pre-flash


The pre-flash can concievably be a problem. It can cause your subject to blink, for example. As long as you use TTL flash metering, be it with the on-board flash or with an original external flash, there is no way to avoid this.

However, if you have an external flash, it is very likely that you can overcome this by using the so-called "auto" mode. Auto mode for a flash means that the flash has a light sensor, which measures how much light is reflected off the subject, and shuts of the flash when there is sufficient. So only one single flash is needed.

To be able to do this, you need to tell the flash what aperture and ISO rating you are using. See this article for some examples of how to do this using older, legacy flash units. If, on the other hand, you have an original flash for the Four Thirds or Micro Four Thirds format, then the flash will read the aperture and ISO settings off the camera automatically. See my review of the Panasonic FL360 for an example.

The downside of this method, is if your subject is unusually light or dark. The flash has no way of knowing this, and will give you a wrongly exposed image. If you see the image come out too bright, for example, adjust the aperture rating (on the flash) up to a larger aperture (smaller f-number).

Conclusion


The GH3 has an impressively fast response here, only 0.06s delay from pressing the shutter until the camera fires the first pre-flash, twice as fast as the GH2. And the total delay is around 0.2s. On top of this, you will mostly add the autofocus delay. However, since you have most likely focused during composition anyway, this is just a matter of the camera confirming that the focus is ok. Using a lens with a fast AF motor, this will take a very short time, probably around 0.1s

In the second test, the GH3 did a second pre-flash, probably at a different exposure level than the first. This was done to probe how to best illuminate the subject. As a subject, I had a wall with very little contrast, so a normal subject will probably not require a second pre-flash.

When talking about cameras like this, most enthusiasts would say that you should not use the built in flash at all. The reason is that it is fairly low powered, meaning that you cannot expose people at a long distance when taking pictures indoor. Any distance larger than about two meters might be problematic, depending on the lens you use, of course, the larger aperture the better. Also, the flash is located quite close to the lens, which gives you a quite flat lightning

However, my experience when using the Panasonic GH3, is that it does a good job when photographing people indoor using the flash, both for portrait closeups (less than one meter distance) and groups of people. Of course, using a proper flash, like the Panasonic FL360, or the more recent predecessor Panasonic FL360L will give you much better flash images. But when you travel light and happen to need the built-in flash, don't be afraid to use it.




4 comments:

  1. Very nice. Actually I have the opposite experience with on-board flash.

    I have noth GH1 and Canon 60D. With the EOS1 when I try to use fill-in flash with children, the delay after pressing the shutter button is so long I always miss the shot (not so if no flash in use). But the 60D feels like on-board fill in flash fires instantly. So I have given up on GH1 flash with children.

    I'd be very interested to know if there is a big difference between GH1 and GH2 in this respect.

    Thanks,

    Alex.

    PS Thank you for the blog.

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    Replies
    1. I have previously tested the GH1 vs the GH2 in terms of TTL flash delay, and the GH2 was marginally faster.

      I think the GH3 generally does a better job of exposing flash portraits using the on-camera flash better than the previous cameras. Of course, if you are serious about flash photography, then it is best to get a proper flash! :-)

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  2. Hi Fredrik,

    I mainly use the on-board flash just to lighten the eyes when outdoors - I don't want to carry a large flash around as well, since the reason I like u43 so much is that it is such a small system.

    I haven't bought an external flash for the GH1 because they are pretty expensive if you want to bounce the flash, and because I am not sure it will help with shutter delay. Do you think the flash delay depends on whether the camera is triggering the on-board flash or an external flash?
    Even if it did, I would have thought that any additional delays would be the same for 60D and GH1. So I still cannot understand why I get such a long delay between pressing the shutter button on the GH1 and the camera capturing the shot compared with the 60D. Maybe it is related to the autofocus - perhaps the 60D can confirm focus much faster than the GH1 can, and so avoids 0.1-0.2s of delay there?

    I don't see the same delay when shooting indoors just now. I'll have to try a few more tests...

    Alex.

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    Replies
    1. An external flash has exactly the same TTL delay as the on-board flash. So you don't save any time using this method by getting an external flash.

      One way to avoid the TTL delay, is to use an external flash in Auto mode, though. This could be done using a new, Micro Four Thirds flash, or an older legacy flash. Read about this in the "Avoiding the pre-flash" section in the article above.

      You could try to pre-focus, and then set the camera to manual focus (MF) before taking the flash picture. See if the flash operates quicker then.

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