EXIF data and Aperture 2

[Magnusvb]

Hello

Aperture 2 is supposed to support lens model metadata. Well, for my images taken with Dynax 7D it is supported (almost), but with my A700 the Exif field is just empty. Is this "normal" or is it just my camera / lenses / aperture? All my images are converted with the v2 converter.

Regards, Magnus

[David Kilpatrick]

The lens data is is two-figure number code provided by the lens. Some lenses - for example the new 70-300mm G SSM - don't appear to be recognised by programs. However, I have found that Aperture does recognise focal lengths even with this (DxO Optics Pro doesn't recognise the 70-300mm at all - not even the focal length used). It is interesting to note that while the EXIF from a processed pic does not seem to include lens identity, at 150mm an aperture of f4.64 was recorded (116/25) but the camera indicates that the maximum aperture at this setting is 5.6 - in fact, 5.6 from 135mm onwards. This is a big discrepancy between the reported aperture to the camera, and the reported aperture in EXIF data. Now I have something else to investigate.

David

[01af]

When dealing with f-stop data in EXIF fields, be sure not to confuse "aperture" and "aperture value."

Consider the international aperture scale---f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, and so on. Now number those f-stops consecutively, starting with No. 0 for f/1, No. 1 for f/1.4, and so on. These index numbers are the "aperture values" (a.k.a. AV) of the f-stops. Similar concepts exist for shutter speeds (TV), ISO speeds (SV), and brightness (BV). Use the formula

Code: Select all

k = 2 ^ (AV / 2)

to compute the f-stop number k from the aperture value AV. Use the formula

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AV = 2 * ld(k)

(with "ld" meaning the logarithmus dualis, or the logarithm to the base of two) to compute the aperture value of the f-stop number k. For accurate results, use the mathematically correct f-stop numbers rather than the common rounded numbers. For example, f/2.8 really means f/2.82843 (= 2 * sqrt(2)); f/5.6 really is f/5.65685 (= 4 * sqrt(2)), and so on.

It is interesting to note that while the EXIF from a processed pic does not seem to include lens identity, at 150 mm an aperture of f4.64 was recorded (116/25) ...

I guess it wasn't f/4.64 but AV = 4.64 which means f/5. At a focal length just short of 150 mm, that would be correct, wouldn't it? Or at least, the discrepancy is not as high as it seems when interpreting 4.64 as an f-stop number.

-- Olaf

[David Kilpatrick]

Well, the lens reports f5 at 85mm to 130mm, and a definite f5.6 on the camera at any point past 130mm.

David

[Magnusvb]

For my lenses (two Sigmas and one Sony) the aperture seems to be right (except for f2.8 which became f3.0). But using Dynax 7D the lens model is also reported (but not quit right): Minolta AF 28-80 (D) – it is actually a Sigma 24-70.

Using A700 that field is empty.

Regards, Magnus

[01af]

For my lenses (two Sigmas and one Sony) the aperture seems to be right (except for f/2.8 which became f/3.0).

Once again: are you sure you didn't confuse aperture and aperture value? Because AV 3.0 corresponds to f/2.8 exactly. In the range between f/1.4 and f/5.6, the numerical values of f-stop numbers and aperture values are fairly similar to each other and thus, easily confused. And for f/2 and f/4, the aperture values even are equal to their f-stop numbers.

That said, in some cases nominal and actual maximum aperture may differ slightly indeed. Still, the EXIF data usually report the nominal values, not the actual. So I'd be surprised if your EXIF data really reported f/3 for the maximum aperture of your 1:2.8 lenses ... particularly when this happens with more than one lens.

By the way, some EXIF data readers automatically translate aperture values into f-stop numbers in the data display; others don't. This behaviour sometimes adds to the confusion when using different programs to look at the EXIF data. Furthemore, when tranlating the names of the EXIF data fields from English into other languages, the translators usually have no clue what they're translating so they often get the translation wrong. For example, in German language, "aperture value" always gets literally but falsely translated into "Blendenwert" while "Blendenleitwert" would be the correct term.

-- Olaf

[Magnusvb]

You are right Olaf. I compared the Exif data in Aperture, Preview and Sony IDC.

Aperture have two fields for this: "Aperture" and "Max aperture value". The same fields in Preview and Sony IDC is: "F number" and "Max aperture". Both Preview and IDC reports "Max aperture" as 2.8 – and Aperture report 3.0.

The Exif handling is Apertures weakest part.

Regards, Magnus