>Firstly, I know that a 1.4x TC will effectively reduce the light
>falling on the film by 1 stop ( and a 2x TC by 2 stops). Many people
>say "it reduces the aperture" but is this technically correct? Does
>it increase your depth of field to that which would be obtained at
>the smaller aperture? ie a 250 F2 used at F4 has y amount of depth
>of field when focussed at say 40 feet
>If I add a 2x TC to get effectively a 500F4, and leave it set at F4
>on the lens, do I still get y depth of field? Or do I get the depth
>of field as if I am using the lens set at F8?
If you double the focal length (e.g. from 250mm to 500mm), you quarter
the depth of field at a fixed object distance. If you don't turn the
aperture ring, you increase the F-number by a factor of two (you
reduce the effective aperture from 2.0 to 4.0). This increases the
depth of field by a factor of 2. The net result is, the 2x
teleconverter reduces the depth of field by a factor of two.
You can look at that from a different point of view: Depth of field
is based on the circle of confusion. Circle of confusion is hopefully
not a new name for all the readers of this posting <g>, no it means,
that objects, which are not in focus create a picture which is an
overlap of millions of small discs. The picture will be acceptably
sharp, when the discs are not to big. What acceptable is, is a matter
of definition. A usual definition is: A print with a diagonal of
25cm/10inches viewed from 25cm/10inches looks sharp, when the
circles of confusion in the print are under 1/175 inch or 0.15 mm.
This corresponds to a 1/1000 inch or 0.025 mm on the negative. To
have an object within the depth of field means, to have an acceptable
amount of unsharpness (is that an existing word?). Now - let's use a
teleconverter. To use a teleconverter gives the same result (only
more grain) as enlarging the central part of the negative and
cropping it down back to 24x36 mm. The parts in the picture which are
out of focus, are out of focus by the same amount in the
"teleconverter" picture as in the "enlarged negative" picture.
Is that a contradiction to the statement, that you halve the depth of
field with a 2x converter? No, because the 2x enlargement of the
negative enlarges also the circle of confusion by a factor of two.
That means, the enlarged negative has a two times reduced depth of
field, exactley like the teleconverter negative. Objects at the
beginning and at the end of the depth of field which were
sharp (with the 250 mm lens) are now unsharp/fuzzy (with the
teleconverter, or in the enlarged and cropped negative) because the
circle of confusion increased from < 0.0025 mm by a factor of two to
> 0.0025. Confused?
Christoph
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Christoph Hertzler hertzler@xxxxxxxxxx
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