Same Focal Length / NOT same angle of view ?? Panasonic 14mm, Olympus 12-40

this one has got me stumped.

In shooting an image comparison between the Panasonic 14mm and the Olympus 12-40 Pro, (The Olympus focal length set at 14mm) my resulting images yielded different perspectives, although both photographs were taken using the SAME focal length. (14mm)

As I understood it, so long as all other factors being equal (e.g. distance of camera from subject, same aperture setting , and lenses are produced for the same format (in this case MFT), ) , the angle of view in the final image should look somewhat identical regardless of the fact that one lens is prime lens and one is a zoom. Is that not the case?

any insights would be greatly appreciated.

Panasonic 14mm perspective.

Olympus 12-40 2.8 with focal length set at 14mm (angle of view is reduced) not the same as the Panasonic 14mm.

Looks to me like one of them isn't really at 14mm. This usually occurs on zooms much more than primes (almost unheard of afaik), and especially digital zooms with no lens barrel FL indication, as the internal electromechanical sensor that states what position the lens is actually at can be improperly or poorly calibrated, and thus read erroneously. There was recently a thread about such a lens that was off a few mm on the short end.

If you are absolutely positively sure that you used both lenses at the same distance and the 12-40 was at 14 then one of the lenses is off ...most likely the 12-40.

Heres a question - did you put the camera on a tripod and simply change the lenses? Because if you did the length of the 12-40 would put it much closer than the 14mm.

doccy wrote:

Just in case... make sure you don't have a sensor crop setting on (it is called ETC in the panasonic... maybe something else in Olympus) or a digital zoom.

I checked that on his image first thing. There's no DTC enabled in the apparently longer/closer image.

DTC shows up as 200/100

Any error in setting and reporting focal length is magnified for objects very close to the lens (and even effected by lens length).  See attached for the difference between  crops of the back fence and the front plant from your images (Panasonic lens images on top).  While there is a difference in the fence FOV, it is actually not that much.

A millimeter of zoom is actually a fairly wide range at the lower focal lengths.  You can test and see that as you slowly rotate the zoom ring, the value on the screen will remain, say, 16mm, but you'll have changed your FoV noticeably.  So it's probably a combination of you turning toward the end of the 14mm range on the 12-40mm lens (just about at 15mm), combined with some calibration errors.

Guy Parsons wrote:

kdouglas7 wrote:

thanks Guy!

I'm chocking it up to the effect of the 'nodal point' , as you mentioned.

After some more tests, I realized that by moving the camera (w/zoom lens attached) back by a few increments I was able to achieve the same FOV as the 14mm prime lens.

'nodal point' ... good to know!

I may have been giving misleading info after doing a bit more reading.

Maybe the nodal point is not the answer but the front entrance pupil. Anyway, whatever it is called I find it when needed (for panoramas) by rotating the camera on a tripod head and observing the background in relation to a vertical stick or string held close to the lens.

Shift the camera back and forwards in respect to the point of rotation until the stick/string always obscures exactly the same part of the background. It may change position with zooming and focusing the lens.

I always called that the nodal point but now I think my terminology is wrong.

Have a read of this page http://www.johnhpanos.com/epcalib.htm to see my confusion and maybe get the right names.

I note your further reference, but I just wanted to say that where the sensor sees one 'nodal point' (i.e. where the 'rays' come from) is not the same as where the 'scene's rays' apparently converge to!

All theory for amateurs is based on a 'thin lens' that assumes there's a point in the centre of the construction through which all light rays pass. Useful. but just ain't so with modern complex designs. As you pointed out correctly that can be resolved.

Mike

If I look to the main subject, which is bigger, and the background, which has the SAME size/AOV, I would guess that the Olympus lens was not at the same distance to the plant as the 14mm lens.

As the Olymous lens is much longer than the pancake your effect is easily explained by distance from the lens to the plant.

So there is NO AOV digfference but a distance difference?

Christof

According to Panny's tech spec of 14 f/2.5, its AoV is 75°.

According to a standard AoV calculator, the standard AoV of the 14mm of a M43 lens should be 75.4°.

That is to say AoV of Panny's 14 is slightly short of standard 14 of M43 format.