General Category > General CG Discussion
Impossible to match real photo with 3d camera
Image Complete:
Hello,
I have an issue that drives me crazy... I am trying to match a real photo I took with a 3d camera.
I know what camera was used (mine, canon RP), I know the lens was used (tilt/shift 17mm from canon), I know the spot from which the photo was taken because I took it, but still if I set Focal length to 17mm and film width to 36mm the result is night and day...The problem is not about the shift effect of the lens, the problem is that 17mm at 36mm is much wider than the actual photo.
The 3d camera matches approx. at 26mm focal length. Have a look at the attachments. Any idea why that happens?
Thanks!
romullus:
Try to google sensor size, or crop factor of your camera. Many consumer grade cameras from Canon has smaller sensor (their crop factor is somewhere between 1.5 - 1.6), which means that lens' field of view is cropped and one needs to multiply actual focal length by camera's crop factor to know effective focal length. 17mm times 1.5 is about 26 mm so that's likely the case for your camera model.
Juraj:
Outside of what Romullus said (but it doesn't apply here since 17 T/S Canon is full-frame lens and it really doesn't work adapted well and you mention 'RP' by which you mean the entry-level FX EOS RP which is full-frame camera), there are few additional specifics/complications that only affect Tilt/Shift lens:
1) Tilt/Shift is somewhat unique to lens design, there is no guarantee what 3dsMax does will fully replicate it within same proportions of distortion. There can be small differences in total height of things in frame.
2) Don't use both "Shift" and "Tilt" at same time, they do the same when it comes to perspective distortion (and in reality the differ in how they affect DOF plane but that doesn't matter to us here).
Since you know that Tilt/Shift lens was used, don't use Tilt at all, uncheck the "automatic vertical tilt" as well, so that you don't accidentically introduce another variant if your camera isn't perfectly straight-on looking.
Lastly, most important, sensor ratio is 3:2 (36mm x 24mm) and you have to account that in Focal Length (or Field of view calculations) when you change from horizontal to vertical framing. Use 17mm x 1,5 = 25,5mm.
(Consecutively, 90 degrees of horizontal field of view is only 60 degrees of vertical field of view)
Romullus gave you correct number but for incorrect reason :- ).
MiroM:
The problem is that we calculate FOV from the horizontal size but you shot the image in the portrait mode. Changing the sensor width to 24mm (its physical height but logical width) should fix this.
Image Complete:
Thank you all!
Indeed I should count as sensor width its height as long as I took vertical photos.
Actually, I thought of it while I sleeping (heh...)
Thank you for the extra information too!
Regards,
Christos
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