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Re: [OM] Oly supplimentary lenses

Subject: Re: [OM] Oly supplimentary lenses
From: "John Petrush" <jpetrush@xxxxxxxxx>
Date: Sun, 10 May 1998 16:15:57 -0400
C.H.Ling <chling@xxxxxxxxxxxxxx> responded to what

Doug Nowlin <wa5ohb@xxxxxxxxxxxxx> wrote:
>>
>> I found many years ago that distance measuring is NOT reliable. The old
>> method for flashbulbs used a guide number based on conditions of an
average
>> room that reflected the flash off the surrounding surfaces. I tried this
in
>> a cavern where virtually no light is reflected, and I hardly got an image
on
>> the film. This is one reason I bought an Auto Strobanar as soon as they
came
>> out. For cameras that use OTF control, distance measurement is a good way
to
>> set the aperture. It will also work well under average indoor conditions.
>>A  flashmeter is probably the most foolproof method, but it requires
>> calculations for macro use.
>>
>> Doug
>
>Hi Doug,
>
>May be you are right, I have not idea on the very old flashbulbs, but for
>the two T32, one F280 and a T20 that I have, they are all over 1 stop below
>the claimed value when working in a normal room with no ambient light, at 1
>meter away from the object. But I always take this into account during
>calculation. For indoor and outdoor flash photography where I needed to
>balance the ambient light, I always calculate the aperture or the flash
>power required by checking with the object distance (I use T32 Wide/ND
>adapter set to reduce the flash power if required).  This is the best
>method I found, the TTL flash only work when your object is very large or
>the background is very close to your object but unfortunately most of the
>cases are not, especially for shooting people outdoor.
>
>I agree that in macro mode the TTL flash come more close to the ideal. But
>with different objects, for example different color of flowers, you have to
>set some +/- compensation to reduce the error. If using incident flash
>meter and you can place the meter at the same level as your object, I don't
>see why you have to do calculations, unless you want some special effect or
>simply you are doing photomicrography where there is no room for the meter.
>
>C.H.Ling

You need to calculate an exposure factor for extensions longer than the
focal length of the taking lens or you will always get under-exposure.  This
is because as the bellows length increases, the intensity of the light
reaching the film decreases (inversely proportional to the square of the
distance).  At extreme extensions, reciprocity failure may also contribute
to under-exposure.  All this assumes no additional filters to compensate
for.

The formula to calculate the exposure factor is very simple.  It is the
bellows length squared, divided by the lens focal length squared.  So if I
have a 50mm lens and 100mm of bellows (or other means) extension, then the
exposure factor is 100 squared divided by 50 squared, or 10000 / 2500 = 4.
An exposure factor of 4x is two stops more exposure than would be indicated
by a flash meter positioned at the subject.  So if your flash meter
indicated 1/60 sec at f/11, you would need to adjust your lens to f/5.6 to
get the right amount of light on the film.

A simpler way is to remember the 50 percent rule:  for every 50 percent
increase of extension over focal length, increase exposure by one stop.



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