It seems that the tumblr's exif display the 35mm equivalent focal length instead of actual one... moreover the 35mm equivalent should be 50mm*1.6 = 80mm, and it shows 76mm. I sent them a mail, but they don't answer. Anybody have this issue ?
A screenshot : http://dl.dropbox.com/u/23176032/wrong_exif.jpg
On
I also have this issue. Annoyingly, I shoot with a full frame camera and it still adjusts the focal length so that it's different to the 35mm equivalent.
From what I can tell, this is a Tumblr specific issue - inspecting the EXIF data on the image that I upload, it shows the focal length as 50mm. Tumblr then claims that my focal length is 53mm when I upload it (which makes me think that the alterations are camera specific). Even if I download the image again from Tumblr and view the EXIF of the downloaded image, it displays 50mm.
This leads me to believe that Tumblr probably has camera-specific adjustments that it makes to account for the sensor size. Somewhat irritatingly though, it looks as though it has led to incorrect (35 mm equivalent) focal lengths for both of us.
Tumblr can't be blamed for the EXIF data. It is in the file that you uploaded. Either the camera or any postprocessing software that you used set this value. The camera normally adds two tags, the actual (physical) focal length of the lens, and a corrected focal length that pretends that your camera has a 35mm (full-frame) sensor. The latter is more useful, because that way you can compare focal lengths among cameras and lenses. Point-and-shoot cameras typically have very small sensors and ditto small focal lengths often less than 10mm. From your question it seems like you have a crop-sensor DSLR, with as you say 1.6* smaller sensor than full-frame. Apparently, the camera itself things that its sensor is bigger than that, almost 1.5* smaller than full-frame.
Then there's the post-processing software. Some of the programs out there will adjust the focal length when you crop the image. It will pretend like you zoomed in, and used a longer focal length than you actually did.
To be sure what happens to the EXIF, you can use the wonderful ExifTool by Phil Harvey. It can output the EXIF from your raw camera file, as well as from the file that was edited by your PP software. You can download it for free here. It also allows you to edit the EXIF.