Exposure factors

[grant_lupton1]

Assume I have a light reading of f8 at 1/125, an adjusted exposure for

a factor of 2 would be f8 at 1/60; a factor of 4 would be f8 at 1/30.

 

However, a factor of 3.1 would be f8 at 1/45. What exposure would I

use here - 1/30 (over-exposed) or 1/60 (under-exposed)?

 

1/30 would equal a factor of 4, 1/60 a factor of 2 - c 33% difference

either way. Is this 33% relative, absolute, significant (or rubbish)?

 

Alternatively, what about a smaller aperture, eg f9.5, f11, etc?

 

If f8 to f11 is one stop, is the increment from f8 to f9.5 a 1/3 of a

stop, f8 to f10 a 2/3 of a stop? (F11 follows.)

 

Finally, if 1 stop is (+/-) 2x the exposure (eg 1/125 to 1/250), what

is 1/2 a stop exposure? For example, my meter shows f8 at 1/250, f9 at

1/200, f10 at 1/150 and f11 at 1/125.

 

Grant.

[dougmiles]

Hi Grant -- I'll wager the lack of responses here is because you stated your own answers whilst asking the question. :-)

 

With very rare exception, setting shutter speed manually goes in one-stop increments only; you generally cannot get accurate speeds between marked positions. On the other hand, aperture settings are continuously variable, with handy clicks at full- and half-stop positions. I think the only 1/3-stop clicks are found on the new Carl Zeiss lenses for the M-Leica mount.

 

A 1/3-stop error in exposure is unlikely to be very noticeable in print film, moreso for slides.

[grant_lupton1]

Doug, Thank you for the info, and my apologies for not responding sooner. I'm still grappling with this.

 

Grant.