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07-22-2014, 02:25 PM   #1
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f-numbers for SMC 135 2.5 and 85 1.8

This may be a mute point, but I would like to know what the half stops between two lenses are. SMC 85-K 1.8: there is a half stop between 1.8 and 2.8, does anyone know what f-number that would be ... my guess is f2.


And on the 135-K 2.5 lens there is a half stop between 2.5 and 4 and my guess is that it is either f3.5 or 2.8.


I realize that this is not going to affect my metering that much, but I kind of would like to know just the same.


Thanks for whoever answers this,


Cory

07-22-2014, 02:46 PM   #2
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f numbers hop by double alternately.


1.4.....2.8.....5.6.....11.....22
.......2.......4........8......16.......32


The natural progression is therefore f1.4 f2 f2.8 f4 f5.6


I see f1.8 as a half stop oddity that manufacturers used to give a slightly wider aperture than f2 would have.


Your conclusion is I believe a reasonable one
07-22-2014, 02:51 PM   #3
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1.8 should be a third of a stop below 2, and 2.5 a third below 2.8 (full stops being 1.4, 2.0, 2.8 and 4.0). As such, you really can't set a half stop away. I believe if you've set your camera to half stop increments, the aperture will first move a 1/3 rd the nearest full stop, then move in half stop increments. If you could move in 1/6th increments you could get there (1/6th from the full stop, or about 2.1 or 2.2 for the first, and about 3.0 for the second).
07-22-2014, 02:52 PM   #4
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QuoteOriginally posted by The dog walker Quote
This may be a mute point, but I would like to know what the half stops between two lenses are. SMC 85-K 1.8: there is a half stop between 1.8 and 2.8, does anyone know what f-number that would be ... my guess is f2.


And on the 135-K 2.5 lens there is a half stop between 2.5 and 4 and my guess is that it is either f3.5 or 2.8.


I realize that this is not going to affect my metering that much, but I kind of would like to know just the same.


Thanks for whoever answers this,


Cory
For the K85/1.8 the unmarked click is f2.4. (So it's f1.8, f2.4 then f2.8)

For the K135/2.5 the unmarked click is f3.4. (So it's f2.5, f3.4 then f4)

Pentax never used unmarked clicks as full stops.

Phil.

07-22-2014, 03:01 PM   #5
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QuoteOriginally posted by gofour3 Quote
For the K85/1.8 the unmarked click is f2.4. (So it's f1.8, f2.4 then f2.8)

For the K135/2.5 the unmarked click is f3.4. (So it's f2.5, f3.4 then f4)

Pentax never used unmarked clicks as full stops.

Phil.
After posting this thread I found your other remarks regarding the similar question, thanks Phil.
07-22-2014, 03:15 PM   #6
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Strictly, a full stop is a factor of √2=2^(1/2) in f-number, half a stop is 2^(1/4)=1.1892, and a third of a stop is 2^(1/6)=1.1225. So, if you really do start at f1.8, the half-stop progression is 1.8, 2.14, 2.55,3.03, 3.6, ... (to 2 decimal places), and the one-third stop progression is 1.8, 2.02, 2.27, 2.55, 2.86, 3.21, 3.6, ... On the other hand, the neater and more likely 1/3 progression is 2^(n/6) (n=-1,0,1,2,...) which gives 1.78,2,2.24,2.52, 2.82, ... That is, 1.8 is really 1.78 and is a 1/3 stop down from 2. So, half way from 1.78 to 2.82 is 2.24 on this logarithmic scale.
07-22-2014, 03:20 PM   #7
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07-22-2014, 04:17 PM   #8
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From a Pentax lens guide:

https://www.pentaxforums.com/forums/attachments/10-pentax-slr-lens-discussion...2269.jpg?stc=1

Phil.

07-22-2014, 04:51 PM   #9
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QuoteOriginally posted by The dog walker Quote
This may be a mute point, but I would like to know what the half stops between two lenses are. SMC 85-K 1.8: there is a half stop between 1.8 and 2.8, does anyone know what f-number that would be ... my guess is f2.


And on the 135-K 2.5 lens there is a half stop between 2.5 and 4 and my guess is that it is either f3.5 or 2.8.


I realize that this is not going to affect my metering that much, but I kind of would like to know just the same.


Thanks for whoever answers this,


Cory
Every pentax lens I own even those which start at 1/2 stops is the same, first and last clicks are full stops, between these tow every other click is a 1/2 stop
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