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[OM] Re: New Sigma lenses for 4/3

Subject: [OM] Re: New Sigma lenses for 4/3
From: Moose <olymoose@xxxxxxxxxxxxxxx>
Date: Sat, 25 Feb 2006 17:08:49 -0800
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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