"Sensor Limit" on Quantum's Qflash

Discussion in 'Lighting Equipment' started by anwarul_alam, Jan 26, 2011.

  1. Hi All,

    Just wanted to know if anybody here knew exactly how the Sensor Limit feature worked on Qflashes by Quantum. I know it's supposed to limit the distance/range the sensor eye on the flash can "see", thereby preventing the flash from trying to light a far away background, which would otherwise overexpose the subject in the foreground. What I want to know is the technical explanation of exactly how the flash manages to put this limit on the sensor's viewing/working distance.

    Besides just being curious about the technical operation of this feature, I also want to know about this for another reason that I'll try to articulate below:

    This is how Quantum defines the Sensor Limit feature somewhere on their Website:

    "Many flash units, when in automatic or TTL mode, will cause severe overexposure when the subject matter is far from the center of the frame, and the background is very far from the main subject. The unique sensor limit setting of the Quantum Qflash will prevent overexposure when the photographer dials in the approximate flash-to-subject distance, effectively telling the flash to disregard the distant background."

    I can understand why this is the case, since the sensor, with its narrow angle of view (Usually something like 30 Degrees), is looking at the background and not the subject that's away from the center of the image area being captured. What I don't understand is how limited the sensor's viewing distance tells it to measure light from the off-center subject instead. It's not really looking at the subject anyway. And if the subject and background are both within the viewing angle of the sensor, then how does limiting the distance make it ignore the background? I'm sure it does, but I just want to know how that's achieved.
    Also, the example image that Quantum provides at http://www.graphics.qtm.com/content/3233/329/600/329_1002.jpg does not really have the subject off-centered. The subject (The couple) is right at the center, yet it gets blown out by overexposure due to not using Sensor Limit, while using the feature exposes the subject correctly. Perhaps someone could use this example and describe exactly what is happening with the sensor eye in each case? That would be much appreciated.

    Thank You.
  2. In Nikon's flash technology, the lens focus distance information would be used. Also many sensors in the camera across the frame of the picture would contribute information, not only the central narrow sensor beam. The pre-flash illumination would measure multi-point light values. Balanced flash and ambient light illumination patern could be selected, etc. .. there is more to it...
    That was how Nikon would do it.
    I am waiting for someone to tell us how Quantum willl do it... it would be really hard to explain without use of a cooperating camera that commands compatible flash.. performs some testing, etc...
    Let's call it "Quantum Magic"...
  3. Let's call it "Quantum Magic"...​
    Or maybe even, "Quantum Physics"
  4. The Simple explanation...
    The sensor in the flash is a light meter in effect
    The capacitors store the charge for the flash tube
    this is dumped when the flash is triggered by the camera
    when the sensor sees that enough light has bounced back from the subject a thyristor acts as a on off switch and turns off the voltage from the capacitor
    This is why when the subject is close the recycle time is shorter for the next flash, some of the voltage was saved.
    This is as simple as I can think to explain it. I am sure some will find problems with this, but in effect this is what is happening with a (Thyristor controlled automatic flash unit).
    Some of the latter dedicated flash units used the cameras meter to do the same function.
  5. @Frank: You mention sensors in the camera, so I guess that's how Nikon would do it for TTL. When Non-TTL Auto is used and only the sensor eye on the flash itself is relied on, that's where Nikon would do it differently, in that they'd only rely on whatever reflected light the sensor would see, right? And that's all good, except that Quantum is now "choosing" how far to see, which is what puzzles me. Anyway, I agree with you that experimentation would really help and I mean to try to get hold of a Qflash to try some test shots with. I'll share the gleaned wisdom here if I can get the tests done :)
    @Simon: Every magic has an explanation and that's what I'm after :) I don't want to simply call it magic in this case. I've been losing sleep over this :)
    @Russ: What you explained is how a sensor normally works, Yes. And the subject distance, like you said, determines the duration of flash, thereby amount of flash, which in turn dictates recycle time. All that's good. What Quantum does differently with the Qflash sensor is this: Say a person is at 10' and she is in an open space with the nearest background (Say, storefront) is 30' from her. According to Quantum, the sensor on the flash (not in the camera) will usually try to properly expose the storefront, resulting in overexposing the person. Sensor Limit then lets the camera user tell the sensor to "not look beyond 10'", and the result is a properly exposed person. How that is achieved is what I'm after. How does the sensor know how not to look beyond 10' if the storefront is indeed within its view? I'm an engineer and either I'm not a good one or there's something that's truly unique about what's going on with the sensor. Either way, I can't sleep until I find out :) The pitfalls of being an engineer who also loves photography.
  6. @Simon: I see you're the one who called it "Quantum Physics". Sorry I misread earlier. I like that better than Magic :) Physics, I understand. Magic, not so much, until it's explained as Physics :)
  7. Yes, I was talking about the iTTL/CLS in Nikon. In thyristor automatic mode, Nikon flashes loose all the smarts, and behave like an ordinary old thyristor flash.
    For completeness I should mention that Nikon also has a SU4 device and some flashes have that mode, that extends the thyristor automation for many flashes, the non-TTL way.
    I hate to be sceptical, but there were many enthusiastic claims out there, some even defying laws of physics. So, let's say things are possible...sometimes, or most of the time.
    Like they say "The proof is in the pudding". Quantum are great flashes, if used in experienced hands.
  8. @Frank: I'm glad you said that about the enthusiastic claims. I have never used Qflashes, but have heard from good sources about their merits, so I respect Quantum and am willing to believe they have achieved what they claim with Sensor Limit, but I wonder if they describe it well or are trying to make it easy for consumers to understand, resulting in a description that doesn't quite match the actual operation. I'll continue to wait for an explanation. Thus far, everything I know of Physics and Optics and simple Math and Geometry has failed to figure out how they're achieving that Sensor Limit thing. Thanks for the information on SU4, though. I used to think SU4 was for TTL use; didn't know it was for thyristor auto.
  9. Frequently we get simlified descriptions of something in cases where full technical explanations are either too long or too difficult to digest. I think this is OK, as long as you test it first. Getting Quantum flashes, I would not hesitate, even if the "Sensor Limit" is a simplifies explanation.
    The SU4 latest incarnation mode in CLS flashe is compatble with Nikon CLS, and commanded by a CLS commander, also operates in the iTTL mode, where the flash output from SU4 connected flash units is determined during pre-flash testing, and is set to a fixed value for the exposure time, like in the old maual mode. In CLS commanded mode the SU4 thyristor operation can adjust the flash output during preflash, but not alter during exposure.
    However, older film FP flashes are not compatible with the new FP CLS.
  10. Sorry I did not know that you are an engineer. Forget there is no explain any thing to you then....
  11. Hahaha :) Na, I'm very open to explanations, as long as every detail is explained to the point that I can't ask any more questions :)
  12. Maybe you could ask the tech people at Quantum?... t
  13. @Tom: Precisely what I've done already :) I hope the response I get is not something like, "This is our proprietary technology that we cannot share." :) I know they can't share their trade secret and I don't want them to either, but I want to know the basic principle that allows them to achieve what they claim. I will share the wisdom I receive from them, if indeed they enlighten me.
  14. Hello Folks,

    Quantum responded to my inquiry about Sensor Limit on Qflashes and they nicely explained how the feature worked. When a certain distance is set as the Sensor Limit on a Qflash, it doesn't really set a limit on how far the sensor can "see". Instead, what happens is that no matter how dark the sensor finds the frame it views to be, the flash does not try and illuminate it with more light than it thinks a subject should need within that distance per the Inverse Square Law given the aperture and the ISO, both of which the flash is aware of through the hotshoe.

    An example is always a good way to explain, so here's an example:

    Imagine a wedding couple with a background behind them that's far away and appears dark. If the sensor's 25-Degree view angle can see both the subject and the background, it can end up metering the overall frame as dark enough to need so much light that the couple ends up overexposed. It's as if the flash is trying to illuminate the distant background. The Sensor Limit tells the flash that no matter how dark the frame the sensor sees appears to be, it should not output more light than the maximum determined by the Inverse Square Law. If the sensor, however, measures the frame to need an amount of light within that maximum amount, it will output just the needed amount and cut the light.

    Hope this helps everybody here as it has helped me.
  15. You will obviously need to set manually what Ws your QFlash is using (200/400 on the X versions), what reflector is in place on the flash, and whether there are any diffusers on that reflector as well :)
  16. @Ian: I didn't know that. Thanks for the enlightenment. You have a QFlash then, I presume?
  17. Hi Anwarul, yes I have several, I use the whole system.

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