Focal length, how to measure it?

[tijuana taxi]

I have two zoom lenses that give a different field of view when both are set at 35mm
These lenses are both constant f/2.8 and supposedly at the upper end of zoom lens quality
Exif data says they both are 35mm, highly unlikely given the difference which is plain to see
Do realise that's just the lens via software giving the information, not an actual measurement

I believe this discrepancy is due to something known as "focus breathing"
Not particularly bothered, its what it is, but did wonder how do you know which one is actually 35mm?

 

[Cagey75]

Could it be auto corrections for distortion on cam or in the software?

 

[tijuana taxi]

Could it be auto corrections for distortion on cam or in the software?

Possibly, to be honest I haven't a clue and just happened to notice it by chance.
It was raining again and I messed about with a chart that is supposed to show if a lens is decentred.
Didn't think any of my lenses were, but just having a play, by the way if this thing works they are all fine
Anyway the camera was at the same distance and noticed the images were noticeably different, got me thinking why?

 

[Petrochemist]

It's highly unlikely either are precisely 35mm, manufacturers round focal lengths to convenient numbers when labeling lenses.

How different are their FOVs? Unless it's very drastic the errors you get in any diy measurement will be much larger than the focal length difference. (If you had a proper optical bench I'm sure you'd be a specialist who already knows exactly how to measure focal length by multiple methods.)

The quick & dirty way I use to estimate focal length with a simple unknown lenses is to project an image of the sky onto some paper (a screen) held up behind the lens. adjust the distance till the clouds are focused. With more complex camera lenses this actually gives the flange focal distance (if the lens is set to infinity)

For focal length of more complex lenses (which is measured to the exit pupil - an unknown point along the lens axis) the life size approach is better. You need to focus a suitable subject onto a screen so the image is the same size as the subject (adjusting both lens & screen positions) the total distance from subject to image will be four times the focal length.

 

[Canon Bob]

Focal length is specified with the lens focused at infinity. Most modern lenses show a reduction as they're focused closer due to their method of focusing (IF, OLE).

 

[tijuana taxi]

Thanks both for your explanations, its not a massive difference, just didn't realise it was a variable figure.
Live and learn, not particularly bothered, only set it to what looks right and not a number anyway

 

[Cagey75]

Possibly, to be honest I haven't a clue and just happened to notice it by chance.
It was raining again and I messed about with a chart that is supposed to show if a lens is decentred.
Didn't think any of my lenses were, but just having a play, by the way if this thing works they are all fine
Anyway the camera was at the same distance and noticed the images were noticeably different, got me thinking why?

It was as much me asking as you but thankfully there will be people can explain better for both of us like above

 

[snerkler]

As you say they are zooms maybe they aren't that accurate at the markings and may not be exactly 35mm despite EXIF saying so. I believe EXIF is only info that the lens is communicating and not an exact measurement.