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Re: [OM] IMG: Mother's Day Orchid

Subject: Re: [OM] IMG: Mother's Day Orchid
From: Jez Cunningham <jez@xxxxxxxxxxxxxx>
Date: Thu, 17 May 2012 20:48:55 +0200
Yes, it could be microseconds, but forget nanoseconds!

On 17/05/2012, Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx> wrote:
> Yes, there's a minimum flash duration but I think the camera and flash
> circuits are faster than you think.  I have a little Vivitar 550FD which
> does TTL on Minolta, Pentax and Olympus.  At ISO 100 it specs minimum
> distance at 2.1 feet at f/5.6 or smaller, 3 feet at f/4 and 6 feet at
> f/2.  The T-32 specs 0.5 feet at f/22, 1.1 feet at f/11, 1.6 feet at
> f/8, 2.2 feet at f/5.6 and (a bit slower than the lowly 550FD) 6.5 feet
> at f/2.
>
> Using auto mode rather than TTL gives approximately the same minimum
> distances.  The 550FD only has options for f/2 and f/4 giving minimum
> distances of 6 and 3 feet respectively.  The T-32's auto mode options of
> f/4, f/5.6 and f/8 give minimum distances of 3.2 feet, 2.3 feet and 1.7
> feet.
>
> Real comparison of these flash units for minimum distance is difficult
> since the 550FD puts out much less light than the T-32 which means it
> has to be used at larger apertures.  I remember trying to use the 550FD
> on the 5D one time.  I don't recall the aperture but it was probably in
> auto mode at f/2 and it failed to quench in time to prevent overexposure
> from about 4 feet.  Now I see why.  But auto mode at f/4 should have
> worked.
>
> Chuck Norcutt
>
>
> On 5/17/2012 7:40 AM, Jez Cunningham wrote:
>> As you say, the response time of the quench circuit in the flash unit
>> will be measured in hundreds of microseconds, whereas the speed of
>> light is orders of magnitude faster (and is thus irrelevant).
>> That means that there's a minimum flash duration that may still be
>> giving too much light for the subject (at the selected aperture)
>> Jez
>>
>> On 17/05/2012, Chuck Norcutt<chucknorcutt@xxxxxxxxxxxxxxxx>  wrote:
>>    But if you get too close with auto flash control the electronics may
>> not have enough time
>>> to respond.  Remember one foot to the target and another foot back to
>>> the TTL flash sensor is only about 2 nanoseconds.  I have no idea what
>>> the minimum distance spec is for the built-in on the E-510.
>>>
>>> Chuck Norcutt
>>>
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