EXIF:MaxApertureValue?

[zetton]

Hi,

exiftool -exif:maxaperturevalue=3.5 a.jpeg

result in 3.614709851551957

Why?

[Phil Harvey]

I don't want to point the finger at anyone in particular, but posts like this are very common:

result in 3.614709851551957

It is because you weren't using ExifTool to read back the value, and your other software is displaying the value differently.

See the EXIF tag name documentation for a hint about what is going on in this case.

- Phil

[zetton]

Thanks!
actually,
exiftool -exif:maxaperturevalue a.jpeg
returns 3.5 !

but my Mac OS X's Aperture and Preview show it as 3.61...

OK, Max Aperture seems not so important tag, I'll leave it blank.

[zetton]

Hi,
I've read the document you mentioned, and I understand the symptom.
This tag's value is stored as APEX value ( x as Fnumber = root(2)**x ).
Then I checked the MaxApertureValue's of my lenses and find many lenses have wrong value! wow!!

[Phil Harvey]

Interesting.  Where these written by a camera?  If so, what model?  Note that this value will usually change with focal length for zoom lenses unless the long and short focal lengths both have the same aperture. (This tends to be the convention, although the EXIF specification does not mention how zoom lenses with variable apertures should be treated.)

- Phil

[zetton]

Hi,

I picked up some (almost all of my lens' that have EXIF).

I understand what you said (about zoom lens case). but here I didn't list focal length for zoom lens' , sorry. most case wide or tele- side.

I collected data with

exiftool -lens -maxaperturevalue *jpg

and I also read "Preview" EXIF values of Mac OS X 10.8.2.

All, but 50mm are Micro Four Thirds (m4/3) lens', 50mm is Four Thirds.

======== P1010738.jpg
Lens                            : OLYMPUS M.12mm F2.0
Max Aperture Value              : 2.0
"Preview" :2
======== P1020019.jpg
Lens                            : LUMIX G VARIO 45-200/F4.0-5.6
Max Aperture Value              : 5.6
"Preview" :4.973
======== P1040225.jpg
Lens                            : OLYMPUS M.9-18mm F4.0-5.6
Max Aperture Value              : 4.0
"Preview" :4
======== P1070634.jpg
Lens                            : OLYMPUS DIGITAL 50mm Lens
Max Aperture Value              : 2.0
"Preview" :2
======== P2020819.jpg
Lens                            : LUMIX G 20/F1.7
Max Aperture Value              : 1.7
"Preview" :1.531
======== P2020858.jpg
Lens                            : OLYMPUS M.60mm F2.8 Macro
Max Aperture Value              : 2.8
"Preview" :2.973
======== P2030881.jpg
Lens                            : LUMIX G VARIO 12-35/F2.8
Max Aperture Value              : 2.8
"Preview" :3
======== P2161543.jpg
Lens                            : Samyang 7.5mm F3.5 UMC FISH-EYE MFT
Max Aperture Value              : 3.5
"Preview" :3.615
======== P6021355.jpg
Lens                            : Reflex 300mm F6.3 MF Macro
Max Aperture Value              : 6.3
"Preview" :5.309
======== P6101578.jpg
Lens                            : OLYMPUS M.12-50mm F3.5-6.3
Max Aperture Value              : 3.5
"Preview" :3.613
======== PB020354.jpg
Lens                            : OLYMPUS M.9-18mm F4.0-5.6
Max Aperture Value              : 4.0
   12 image files read
"Preview" :4

[zetton]

uhm,
Does exiftool displays Fnumber for MaxApertureValue?

What does it means, for example:

exiftool -maxaperturevalue=3.5

does that mean APEX is 3.5 or Fnumber is 3.5?

at least, I do exiftool -maxaperturevalue=x, exiftool -maxaperturevalue returns x

If exiftool argument is Fnumber, Preview's value seems as APEX unit

[Phil Harvey]

Right.  Preview is displaying the APEX value.

ExifTool converts many values to standard units for convenience.  This makes it much easier when copying information.  In this case, the standard units are the F number.

- Phil

[zetton]

totally, I see.

another reason why I confused.

For example, I thought F/6.3 means aprox. number of 5 1/3 stop ( (root 2)^5.3333...) = F6.350,
so I thought APEX value in EXIF should be 5.333.
but in fact, the APEX value in the real EXIF is 5.309 ( little less than 5.333 = 5 1/3 ) and the F number is exactly 6.3!
why I thought F number is approx. number, because for example, F/3.5 is displayed (on the lens F stop) both in 1/2 step lens and 1/3 step lens (between F/2.8 and F/4.0)
in 1/2 series, 2.8 3.5 4.0
in 1/3 series, 2.8 3.2 3.5 4.0
so I thought 3.5 in 1/2 series and 3.5 in 1/3 series were different but approx. number.
                                                                                                                                          (my English OK?)

[Phil Harvey]

Yes, these values may be rounded off by both the camera and ExifTool.  You can use the -n option to avoid the rounding in Exiftool.

- Phil