Testing ultra-fast flash sync with CCD sensors

04 Jan 2017 14:26

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Several older DSLRs' "CCD" sensors, versus the currently more popular "CMOS" type, enabled flash synchronization at very high "shutter speeds" because these were accomplished by electronically activating and de-activating the entire sensor very quickly during a part of the time the physical shutter was open. In contrast, a focal-plane shutter generally achieves high effective speeds over other sensors (or film) by forming a narrow slit which scans very quickly over each part of the sensor as the entire shutter takes a longer time, 1/250 of a second or so, to cover the entire sensor. Any given part of a very fast subject will be sharp, and the overall subject generally won't move enough during the exposure for noticeable distortion. (An in-lens shutter, common on compact and large-format cameras, completes its entire action within the stated time and can work with flash at any of its speeds, typically with a faster limit than focal-plane shutter's flash sync speeds, but not nearly as high as its highest speeds with the slit trick.)

High sync speed is handy for (1) balancing artificial and natural light at wide apertures and long distances (special extended pulsing "high speed sync" flash modes expend most of their power against parts of a focal-plane shutter that is not open at any given time as a slit scans past) and (2) freezing ultra-fast action, such as hummingbirds and insects, with a single flash pulse. (A single flash, which is quick and peaks over its duration, can work better than high-speed sync mode's highest speeds - especially at lower power, which, in a typical on camera flash, cuts it off early. http://www.scantips.com/speed2.html )

The popular lighting site "Strobist" has extensive discussion of this special capability of several Nikon 6.1MP DSLRs, the original Canon EOS 1D (4.1 MP) and others such as old Olympuses. The Leica M8 and M9 have CCD sensors, but there seems to be no mention of particularly fast flash sync with them. The maximum normally available speed is 1/180s, suggesting that they never use the electronic-shuttering method which gets the Nikons and Canons to a standard 1/500. Use a flash other than one dedicated to the particular camera manufacturer, or insulate the flash's connectors other than the main central one from the camera's with a little cellophane tape. (It's unclear why higher speeds are set to work in manual mode only - the reason could be over-cautious manufacturers worried about automatic flash inaccuracy as faster speeds cut off part of a quicker flash's pulse. But electronic flash is very fast, especially with small on-camera devices, and the pulse is front-loaded - so you can calculate exposure near-normally. Or a full-size Canon flash's LCD display can do it for you.)

Many older on-camera flashes have excessive "trigger voltage" across the hot shoe terminalspossibly just a simple part of the overall flash circuitand can fry a modern electronic camera. http://www.botzilla.com/photo/strobeVolts.html Plus, even some of the safe ones (I tried an 80's or 90's style Promaster) seem to be slower to fire than a modern Canon EX-series, potentially limiting you to 1/2000s or so. Although, some can work fine, such as a Sunpak 680 Super Pro handle-mount flash, and include thyristor non-TTL automation. Physically smaller modern flashes, such as the Canon 90EX versus the 550EX, are not significantly quicker (although a small flash may fire at full power in about the same timeand with about the same poweras a big flash operating at a weaker setting.)

The Nikon D70 and D70s are reputed to work up to their top speed of 1/8000s. The smaller, newer D40 works up to its top speed of 1/4000. (Like the D70 and D70s, its sensor's minimum ISO is 200, making these speeds a stop less effective than expected for enabling wide apertures with sunlight without overexposure. An ND filter may be in order. But, the absolute amount of the speed is what matters for balancing the flash against natural light.) The impressive but clunky, battery-guzzling D1X works up to 1/10,000s, with a faint bluish glow of flash visible up to 1/12,000 or 1/13,000 - not its top speed of 1/16000. It might end its exposure before the flash burst gets started - some report the beginning of a flash is bluer.

The Canon 1D, which is also clunky and battery-guzzling but has an excellent autofocus system and near-full-frame APS-H sensor to use more of the width of a given lens's view, works up to 1/2500, with a faint glow of flash visible up to 1/3200. Perhaps it is triggering the flash more slowly and less directly than the Nikons.

The very highest flash speeds might be made to work by putting the flashes (why not try an automated, or multi-flash setup while at it, noting additional delay for preflashes?) on a camera that is triggered by a Pocket Wizard or other trigger with adjustable delays, before the camera that is actually taking the picture. Awkward: potentially for scientific applications.

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