Focal length question

**Brock** · 9th February 2009

Somebody posted a photograph up on another forum of a shot taken with a Sigma 15-30 lens but the focal length was 31mm.

Can somebody explain this as I have never come across this before and the lad that took the shot did not know either.

**BertRoot** · 9th February 2009

fuck knows mate. May be that the lens is fucked and actually goes as far as 31mm. Maybe down to the crop factor?

**wakadoo** · 9th February 2009

do lenses come with a tolerance....

maybe its like +/- 5% (eg) on a lense so enabling the 31..

just a thought.

**Brock** · 10th February 2009

Originally Posted by kry0genik

EXIF data become corrupted?
Quick Google about suggests that some EXIF data can be corrupted when using some image editors.

He said it's the same in the original photo.

**greaseweasel** · 10th February 2009

Just one of those things I guess. For the sake of 1mm I wouldn't worry about it

**neo2810** · 10th February 2009

Not come across that before...

**ka$h** · 10th February 2009

Originally Posted by greaseweasel

For the sake of 1mm I wouldn't worry about it

1mm can make all the difference to some people

**GedR** · 26th March 2009

Could have something to do with the 1.5x conversion factor of a lens on a DX format camera. In this case the 15-30 would be 23-45 on a 35mm. Maybe the software reading the file is converting the focal length to the 35mm equivelant.

GedR