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Old Sep 28, 2010, 6:29 AM   #1
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Default EXIF focal length question

I downloaded the EXIF viewer for Firefox and have been viewing the data looking at the specifics of some of the shots here.

I clicked on one of my own and the focal length did not report as I expected. I was using the Tamron 17-50 f/2.8 Di II.

The EXIF shows

"Focal Length in 35mm Film = 75"

The Di II model Tamron is supposed to be for the APS-C bodies so why would it show a focal length be + x1.5?

Am I misunderstanding something?

Here is a link to the image

http://forums.steves-digicams.com/me...-teething.html

Steve
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Old Sep 28, 2010, 6:42 AM   #2
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Steve,

The exif will house 2 values. When I look at the exif from that image I see focal length = 50mm. Focal length in 35mm = 75mm. 50mm * 1.5 = 75mm.

Focal length is a physical property of the lens. A 50mm lens is 50mm on any camera. So a 50mm lens designed for aps-c is still a 50mm lens. That keeps everything in a standard measurement. That way you ALWAYS know to apply 1.5 to determine field-of-view (FOV) equivelent focal length. Not only would it be more confusing otherwise, it would be incorrect. Whether I use a 50mm 1.4 lens on a canon full frame, aps-h (1.3x) or aps-c (1.6) camera body - it's still a 50mm lens. It's that other value "35mm equivelent" that takes into account the sensor size the lens is mounted on.
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Old Sep 28, 2010, 6:47 AM   #3
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my exif reader shows 50mm
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Old Sep 28, 2010, 11:21 AM   #4
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Exif viewers do differ.

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Old Sep 28, 2010, 12:56 PM   #5
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The EXIF Specification lists two "Focal Length" Tags. "FocalLength", and "FocalLengthIn35mmFilm".

The "FocalLength" Tag is defined as follows: "The actual focal length of the lens, in mm. Conversion is not made to the focal length of a 35 mm film camera."

The "FocalLengthIn35mmFilm" Tag is defined as follows: "This tag indicates the equivalent focal length assuming a 35mm film camera, in mm. A value of 0 means the focal length is unknown. Note that this tag differs from the FocalLength tag."

So the EXIF data in every image lists two focal lengths. One is the actual focal length that the lens was set at when the image was captured. The other is the 35mm equivalent focal length, as determined by the actual focal length and the size of the image sensor, and is to provide a common method for comparing the angles of view of cameras with different size sensors.
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Old Sep 28, 2010, 2:29 PM   #6
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Thank you for the replies.

I did not think about the viewer getting it wrong or just making assumptions. The viewer is seeing the focal length in mm correctly and knows I use a body with a crop factor but just assumes the lens is for full frame.

Being APS-C specific this lens would not yield its FOV specification on a full frame sensor. It should be wider but with the corners clipped.

None of this really makes any difference in the real world but it bothers me when the math does not seem correct.

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Old Sep 28, 2010, 3:02 PM   #7
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The focal length of a lens is a physical property of that lens, and it doesn't change based on the size of the sensor it projects an image onto.

What does change when you change the size of the sensor, is the angle of view. The 35mm equivalent focal length is a common way to relate the angle of view obtained with one size image sensor with the angle of view obtained with another.

The EXIF data contains the correct focal length, and it contains the correct angle of view, as expressed in terms of the 35mm equivalent focal length. True, it would be more correct to simply describe the angle of view in degrees, minutes, and seconds, but for the time being, we're stuck with the 35mm equivalent focal length.

BTW, Field of View is a term that describes the current extent of possible observation, and is used in many different contexts. But in Photography, the term Angle of View is used instead.
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Old Sep 28, 2010, 6:02 PM   #8
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I get it now. I spend time looking through various types of field glasses so the only real common denominator is their field of view at a given distance. Same thing for the most part but it is always better to use the correct term for the gear.

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