GESO - Balancing flash with available light

[P. J. Alling]

In Av mode, Pentax camers use the FL of the lens to pick the lowest 
"useable" shutter speed using the 1/FL formula, so when you mount a lens 
that reports it's FL you'll get the rule of thumb shutter speed, 
adjusted for the difference in size for the APS-C format vs the 35mm 
format,  (if you mount an A or earlier lens you'll get maximum X sync). 
I'd assume that you've set the zoom to somewhere between 70 and 100mm 
based on the shutter speed that the camera is providing.

George Sinos wrote:
> Yes, I probably wasn't very clear.  As the aperture changes the flash
> will compensate.  I should probably rephrase that comment and move it
> to the intro.
>
> I shot a series in Av mode, running the aperture from f/2.8 through
> f/11. In Av mode, with the flash on,  the camera seems to like 1/125
> sec shutter speed.  I could clearly see the flash change it's power
> output from shot to shot.  In this series all of the backgrounds are
> about the same, but you can see the depth of field change.
>
> In Tv mode I ran the series from 1/4 sec to 1/180. At 1/4 sec the
> camera chose f/3.5.  It chose f/2.8 at all other shutter speeds.  The
> results were similar to the manual mode series.  In the end I used
> manual so I could control as many variables as possible.
>
> GS
> <http://georgesphotos.net>
>
> On Sun, Jun 1, 2008 at 11:45 AM, Bob W <pdml at web-options.com> wrote:
>   
>> That's quite instructive. In the 3rd picture, though, you write "Flash
>> adjusts output to keep the foreground subject correctly light". The
>> flash doesn't need to adjust the output - for the same distance and
>> aperture the flash output should remain constant, and TTL isn't
>> needed.
>>
>> Bob
>>
>>     
>>> -----Original Message-----
>>> From: pdml-bounces at pdml.net [mailto:pdml-bounces at pdml.net] On
>>> Behalf Of George Sinos
>>> Sent: 01 June 2008 17:14
>>> To: Pentax-Discuss Mail List
>>> Subject: GESO - Balancing flash with available light
>>>
>>> Balancing flash with available light.
>>>
>>> This is a gallery of a few shots put together for a class drill.
>>>       
>> The
>>     
>>> idea is to light the foreground subject with the flash and the
>>> background with available light.  The lightness/darkness of the
>>> background is controlled by changing the shutter speed. The aperture
>>> is held constant for all photos. The P-TTL flash keeps the subject
>>>       
>> lit
>>     
>>> at the desired level.
>>>
>>> For the Pentax folks here's a little additional info.
>>>
>>> The Flash is a 540 set in the wireless mode.  It's set to +1/2 stop
>>> flash exposure compensation.  It's zoomed in as far as possible to
>>> narrow help keep light from spilling onto the background.  There are
>>> only seven images in the gallery, but I did a lot of experimenting
>>>       
>> and
>>     
>>> fired the shutter just short of 100 times.  The flash is very close
>>>       
>> to
>>     
>>> the subject so it probably didn't need to use much energy.  Even
>>>       
>> when
>>     
>>> I was experimenting with very small apertures requiring noticeably
>>> more powerful flashes the flash recycled almost immediately.  The AA
>>> alkaline batteries seemed to work OK for this application.
>>>
>>> The lens is the DA* 50-135mm f/2.8.  The focus point is the black
>>> queen in the back row.
>>>
>>> The camera is the K20d, flash set to wireless control mode.  Manual
>>> mode for this series.  Though I had set the white balance to
>>>       
>> daylight,
>>     
>>> it didn't make any difference.  I had custom setting at "WB auto
>>>       
>> when
>>     
>>> flash attached."  I've since set it to "unchanged" so it will use
>>>       
>> the
>>     
>>> WB as set on the camera.  No matter, I was shooting raw files.
>>>
>>> Here's the link
>>>
>>> <http://Georges.smugmug.com/gallery/5066768_nv8WE/1/304882093_iUGLx>
>>>
>>> Oh, the opening image documenting the setup was shot with a Canon
>>>       
>> G9.
>>     
>>> This was time well spent, if only to learn about the custom settings
>>> on the camera and flash that interact.  I've only had the K20 and
>>>       
>> 540
>>     
>>> for a short time.
>>>
>>> GS
>>> <http://www.georgesphotos.net>
>>>
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>>     
>
>   


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