Chuck Norcutt wrote:
>Since the definition of the "standard" lens is focal length equal to the
>diagonal of the film (43mm) the 21/2 would make for a better fit to the
>"standard" lens definition if you have one.
>
>
I'm not sure I follow that one, Chuck. For those who consider 50mm to be
"normal" for 35mm. 21mm would definitely be wider on 4/3.
Using the diagonal rule, the AOV of 50 mm on 24x36mm format is 46.8° and
the fl with that angle on 4/3 is 26 mm.
Since the formats have different aspect ratios, I think of the width as
a better measure of equivalence.
Using the width rule, the AOV of 50 mm on 24x36mm format is 39.6° and
the fl with that angle on 4/3 is 25 mm.
So, for those who consider 50mm to be "normal" for 35mm. 24mm would be
the closest to eq. on 4/3, just slightly wider than 50mm on film.
For those who consider 35mm to be their "normal" fl on 35mm film, 18mm
would be a close eq. for 4/3.
21mm on 4.3 is eq. to 42mm on 4/3. Since the width of 4/3 is exactly
half of 35m film, the eq fls are always in a 2:1 relationship.
Parenthetically, all this precision is strictly theoretical. Lens tests
usually list the actual measured fl of the test lens. Differences of up
to 5% in either direction are considered to be within spec. and greater
variances aren't uncommon. So a "50 mm" lens may be from about 47.5 to
52.5 mm in fl without being considered anything more than normal sample
variation. And a 24 mm may be from 22.8 to 25.2 mm.
Just for amusement, here are the measured fls of the lenses tested by
Modern Photo that Brian gathered for us:
Lens meas. fl.
18/3.5 18.76
21/2 20.15
40/2 41.33
50/1.2 52.48
50.1.4 50.94
50/1.8 51.87
90/2 90.44
100/2 100.65
180/2 180.57
So in this case, the 18 & 21 were more like 19 & 20 mm lenses, and only
4° apart in AOV. Nothing much should be read into theses numbers except
that there is sample variation. Another selection of lenses would
undoubedly have different actual fls.
"How can I tell what the actual focal lengths of my lenses are?" You
can't without specialized optical equipment. Don't worry, be happy, take
pictures, enjoy.
Moose
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