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<blockquote data-quote="B0B" data-source="post: 2636005" data-attributes="member: 303455">apertures are in steps of the square root of two (as double the size of hole in the aperture admits 1,4 times the light)so, teach has confused themself by being partly mindful of that.there are often third stop intervals (Zeiss do this) but most lenses go in half stops, but again obeying the root two principle;so f2,8&nbsp; is one stop (double the light transmission)&nbsp; from f4,0 (though many makers engrave f4,5 instead)and the half stop between is not (2,8+4)/2 = 3.4, but is normally given as f3,5 (more or less by tradition).This you seem to have grasped.&nbsp; The point is, the lens ring may not be that accurate, the iris hole isn&#039;t a true circle (except some old Zeiss ones) and if you move the meter on tenth degree the reading may change half a stop anyway.So in this the Teach is right, forget the math, third stop accuracy is irrelevant normally.Grab the closest to the reading to one significant decimal - so 2,5&nbsp; and 2,7 and 2,9 are all f2,82,99 is getting my lens fingers twitching to set the half stop (f3,5) but that&#039;s cos I use slide film and underexposing slightly is good, as indeed it is for digi pictures taken in RAW.The third decimal is beyond the difference between batches of film or variations in sensors, probably beyond the effect of a finger mark on the lens!My Gossen actually shews proper stop and third stop numbers but a second display gives the &#039;nudge&#039; value small figures as well, which some scientist might one day need (not me though)</blockquote>

[QUOTE="B0B, post: 2636005, member: 303455"] apertures are in steps of the square root of two (as double the size of hole in the aperture admits 1,4 times the light) so, teach has confused themself by being partly mindful of that. there are often third stop intervals (Zeiss do this) but most lenses go in half stops, but again obeying the root two principle; so f2,8 is one stop (double the light transmission) from f4,0 (though many makers engrave f4,5 instead)and the half stop between is not (2,8+4)/2 = 3.4, but is normally given as f3,5 (more or less by tradition). This you seem to have grasped. The point is, the lens ring may not be that accurate, the iris hole isn't a true circle (except some old Zeiss ones) and if you move the meter on tenth degree the reading may change half a stop anyway. So in this the Teach is right, forget the math, third stop accuracy is irrelevant normally. Grab the closest to the reading to one significant decimal - so 2,5 and 2,7 and 2,9 are all f2,8 2,99 is getting my lens fingers twitching to set the half stop (f3,5) but that's cos I use slide film and underexposing slightly is good, as indeed it is for digi pictures taken in RAW. The third decimal is beyond the difference between batches of film or variations in sensors, probably beyond the effect of a finger mark on the lens! My Gossen actually shews proper stop and third stop numbers but a second display gives the 'nudge' value small figures as well, which some scientist might one day need (not me though) [/QUOTE]

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