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Old Mar 24, 2007, 10:39 PM   #11
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JimC wrote:
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... That's because the sensors used in digital cameras are too reflective to meter from like you could to with off the film flash metering. So, the flash systems were redesigned for digital. ...
Jim, I've seen that statement several times from several people so it is likely true, but it seems very strange to me. Couldn't they just put a filter ahead of the flash meter sensor to cut down the light? Dealing with to much light is usually much easier than dealing with not enough.
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Old Mar 25, 2007, 8:04 AM   #12
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Bill:

I've heard that off and on for a long time, so I pretty much accepted it as being true.

But, perhaps I'm just contributing to an urban legend. Your question prompted me to do some searching, and I found some comments from an engineer that once said that the problem is caused because sensors are not reflective enough to accurately meter from during a flash exposure.

So, the "too reflective" comments may be incorrrect.

Perhaps it's really all a big conspiracy to get everyone to buy new flash systems. lol But, it's likely that there are some serious issues trying to meter from a digital camera sensor. Otherwise, the camera manufacturers wouldn't have gone to the lengths they did to try and get around it with complex systems requiring flash upgrades.

Some digital cameras take the same approach that an older Auto Flash would take and have a separate built in sensor that measures reflected light during an exposure to eliminate the need for a preflash. I've had a couple of NIkon models that worked this way (for example, a Coolpix 950 I still have uses a built in sensor), and I've got a little Konica KD-510z that has a built in sensor, too. There are some more that were made that way, too (but, manufacturers no longer attempt to do it this way with most newer models).

Some dSLR models still tried to meter from a sensor during an exposure. For example, the Fuji S1 and S2 use this approach, but not the newer S3. But, Ive seen comments that this impacted flash accuracy of the older models (trying to meter from the sensor during the exposure). Of course, flash accuracy using a preflash system can leave a bit to be desired sometimes, too (depending on the manufacturer/camera model and the sophistication of the algoirhms and lens designs that report focus distance). It's also my understanding that some of the first (no pun intended) Pentax dSLR models don't require one. But, I have not researched it at length to try and determine flash accuracy differences between models that do or do not require a preflash. Most digital cameras use one now.


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Old Mar 27, 2007, 6:51 AM   #13
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Another reason I've read for the pre-flash is for digital camera to adjust their auto-white balance according to the temperature of the flash - My older Minolta Dimage A1/A2 surely did this as well as many cameras that meter from their sensor with live-view...

However on a dSLR this would make even more sense as the pre-flash is measured off the mirror, which is a true reflection. With the main flash firing however, wouldn't the color be off since the measurements are taken from reflection of the sensor which does not happen to be white, since the mirror is now out of the way?

OTF from the old day only measured flash intensity and wouldn't care much about WB...



JimC wrote:
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It's also my understanding that some of the first (no pun intended) Pentax dSLR models don't require one.
Some of the first Nikon digital cameras did this too - This was an intermediate attempt to re-use an existing (film) flash system until they can ready their own 'digital' flash which requires the extra circuitry to fire quick bursts in a row... for the pre-flash
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Old Mar 27, 2007, 9:35 AM   #14
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NHL wrote:
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Another reason I've read for the pre-flash is for digital camera to adjust their auto-white balance according to the temperature of the flash - My older Minolta Dimage A1/A2 surely did this as well as many cameras that meter from their sensor with live-view...
What I find interesting is that my little Konica KD-510z has the best white balance of any camera I've ever used, with or without flash, even changing the shutter speed to slower or faster values to let in more or less ambient light as desired at wider apertures indoors . I never bother to adjust the colors at all for photos I take with it (they're close enough without any further tweaking for my tastes).

Now, it's not perfect (slightly warmer in some lighting, slight green tint in some flourescent lighting). But, it's the best one I've ever used overall, especially with mixed lighting, even leaving it on Auto WB all the time, with or without flash.

Yet, it doesn't even use a preflash (it's got a separate built in sensor that measures the reflected light during an exposure). The algorithms must be pretty good at figuring out how much the flash will be contributing to the exposure without it (and it does appear to take focus distance into consideration, too -- based on tests I've done with it, in addition to looking at the reflected light from the extra sensor. ).

This little Konica pocket camera runs circles around a camera like my Konica Minolta Maxxum 5D for both exposure accuracy and white balance. I think it may even be better with Dynamic Range. I spent days working with Dave Coffin and another developer trying to get better Dynamic Range from raw files this camera produces (it's got a hidden raw mode), and I couldn't surpass the camera produced jpeg images, even with custom modifications to the dcraw.c code ( or even get close for that matter). lol

Yes, it's got the advantage of using the same sensor for metering and recording. But, it doesn't even use a preflash.


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