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[OM] Compairing Lens-speeds

Subject: [OM] Compairing Lens-speeds
From: frieder.faig@xxxxxxxxxxxxxxxxxxxxxxxxx
Date: Fri, 29 Sep 2000 18:14:21 +0200
Hallo,

I´ve made some toughts about the speed difference from Lenses, I want to share. 
Purpose: Question was: Difference 50 mm F /1.2 to  F1.4 or F/1.8

Now I calculated the difference in of F-stops with:

LV (F) = Log( F^2) /Log (2)

(LV is not really light value, but the difference in light values when shutter 
speed remains)

With this Formula it is possible to calculate a decimal equivalent for uneven 
f-stops.
Now the official f-stop series is:

1       0.00
1.2     0.53
1.4     0.97
1.414   1.00  !
1.8     1.70
2       2.00
2.8     2.97
3.5     3.61
4       4.00
4.5     4.34
5       4.64
5.6     4.97
6.3     5.31
6.8     5.53
8       6.00

This would indicate that F1.2 is about a half F-stop faster than F1.4, which is 
0.7 F-stops faster than F1.8. 

Now it is very unlikly that the Zuiko lenses exact match this academic values.

But I found the meassured values from moder photography which are
in the web at: 
http://brashear.phys.appstate.edu/lhawkins/photo/mp-zuiko-tests.txt 
Thanks to R.Lee Hawkins.

lens    actual 
        speed   LV:

REF     1       0.00
1.2/50  1.26    0.67
1.4/50 SC       1.37    0.91
1.4/40MC        1.44    1.05
1.8/50  1.86    1.79 

The SC and MC-Version of Zuiko 50mm F/1.4 do slightly differ in Speed.
0.16 matches 1/6 EV which is half the difference which is relevant.
 All tested F1.8 lenses are about the same value.

So this would mean:  
The Zuiko 50mm/F1.2 is ~1/4 -F-stop faster than the SC-50 F/1.4
The Zuiko 50mm/F1.2 is ~0.4 -F-stop faster than the MC-50 F/1.4
The Zuiko 50mm/F1.2 is ~1.1 -F-stop faster than the 50 F/1.8

Now I meassured with my OM2sp the same wall with the 1.4/50 SC and
the 1.8/50 MC. The difference is about 1/3~1/2 F-stop only. Hmmm.
Seems the lever from one lens isn`t too accurate, or the vingetting problem??
Now l`m curious about to see the test slides!

Frieder Faig

PS: The values of other MP-tested Zuiko`s:

lens    actual 
        speed   LV:

1.2/50  1.26    0.67
1.4/50 SC       1.37    0.91
1.4/40MC        1.44    1.05
1.8/50  1.86    1.79

2.0/21  1.91    1.87
2.0/35  2       2.00
2.0/180 2.01    2.01
2.0/40  2.03    2.04
2.0/90M 2.04    2.06
2.0/100 2.05    2.07
3.5/21  2.58    2.73
2.8/135 2.79    2.96
2.8/35  2.81    2.98
2.8/100 2.81    2.98
2.8/180 2.9     3.07
2.8/24  2.93    3.10
3.5/28  3.5     3.61
3.5/18  3.51    3.62
3.5/50M 3.64    3.73
4/200   4.01    4.01
8/500   8.27    6.10




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