[KPhotoAlbum] Numbers - Re: "F Number" vs. "Aperture Value"

Robert Krawitz rlk at alum.mit.edu
Tue Dec 3 00:13:45 GMT 2019

On Mon, 02 Dec 2019 23:47:35 +0100, Johannes Zarl-Zierl wrote:
> As an addendum, here is a rough translation table based on my exif
> database. Taking a closer look at the invalid values reveals that
> not all cameras store the APEX value at all:

ApertureValue is log2(FNumber^2).  Technically speaking it should
probably be log2(TNumber^2) because it reflects light exposure, but
still camera lenses are rated using the F number (which is a geometric
relationship) rather than T number (which is photometric).  For a
perfectly transmissive lens with a perfectly round aperture the T
number will equal the F number, but for real lenses the T number will
be bigger, reflecting losses within the lens.

(As an aside, motion picture lenses -- the lenses that professional
cinematographers mount on their Arri, Red, Black Magic, Canon Cinema,
etc. bodies -- are both rated and calibrated in T stops.  That's so
that if the director wants to switch from a wide angle shot to tighter
framing that the light exposure will be identical.  Cinema lenses are
generally very expensive; they're built to very tight tolerances.  A
70-200 T3.1 might have a similar optical formula to a 70-200 F/2.8,
but the construction is quite different.  Usually all lenses in a
series are designed so that the aperture or "iris", focus, and if
applicable zoom rings are identical in diameter, width, positioning,
and grip surface, and the grip surface is usually geared.  They're not
autofocus at all and normally have an extremely long focus rotational
throw.  That's because the person who does the focusing, called the
"focus puller", can perform extremely precise focus actions to realize
the director's artistic vision.)

I don't think the ApertureValue has any use on its own; it duplicates
FNumber in a way that photographers don't use.  Combined with
ShutterValue you'd get something useful (constant exposure), although
I'm not sure how many people would want to filter on it.

I don't really understand why ApertureValue and ShutterValue are
included at all, since they're derived from F number and shutter
speed.  But regardless, there's no good reason for KPA to offer
ApertureValue, especially as it's not implemented correctly.

> sqlite> select count(*) from exif;
> 30492
> sqlite> select count(*) from exif where Exif_Photo_ApertureValue < 0.1;
> 24395
> sqlite> select count(*) from exif where Exif_Photo_FNumber < 0.1;
> 4135
Robert Krawitz                                     <rlk at alum.mit.edu>

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