# which focal length for tracking footage solves

I already know that I prefer to use the actual focal length over the effective focal length, I use a 50mm prime with a T3i(1.6 aps-c crop sensor)…using 80mm(50x1.6) to me never looks or feels right, but many times a solve will give a focal length somewhere around 75-80 I tend to reject those in favor of forcing lengths around 50mm…am I doing the right thing or is it a matter of preference or should it be always try to get close to actual FL or approximate FL.

If you render your 3d elements with blender’s camera set at the wrong fl, you MIGHT get away with it, but usually you will notice a difference in perspective between the original video and the 3d in the composited video so it is best to use the correct fl. However, the correct fl all depends on what sensor width you are using…

Crop factor is a way to determine how a lens will work with a particular sensor width compared to a 35mm camera with a 36mm ‘sensor’

If you are using the preset for the t3i (600d) it sets the sw to 22.3mm which automatically accounts for the crop factor (50mm lens on a camera with a sensor width of 22.3 = 80mm on a 35mm camera with a sw of 36mm (commonly known as the 35mm equivalent) Try this: Set your sw = 22.3, then set your fl =50… now set the sw =36 and watch what happens to the fl -> 80.717 (35mm equiv) This means that if you are setting fl = 80mm on a sw of 22.3 you are actually applying the crop factor twice (ending up with a 35mm equiv of 127mm).

I recently got to briefly play with a t1i (18-55mm lens) and did some calibration tests* on it and found that the calibrated focal length was almost always** very close to the expected fl and blender’s best refined fl:

thumbnail exif 18mm (18x1.6) = 28.8: .2685 (refined k1, k2)
I calibrated it to 30.17: .2682 (refined k1, k2)
blender final (refined fl, k1, k2) 29.461: .2662

thumbnail exif 35mm (35x1.6) = 56: .3972 (refined k1, k2)
I calibrated it to 58.47: .3949 (refined k1, k2)
blender final (refined fl, k1, k2) 58.247: .3949

way too much camera shake motion blur on this one…
thumbnail exif 55mm (55x1.6) = 88 1.1832
I calibrated it to 88.01: 1.1833 (refined k1, k2)
blender final (refined fl, k1, k2) 98.954: 1.1823 <-- wrong fl! & k1, k2 are much too high **

*my calibration method uses 35mm equiv values so all fl values are with sw = 36mm

**Be careful…Refine fl, k1, k2 can sometimes give you an artificially low solve with a totally wrong fl and distortion values…which will mess up your efforts to properly composite the 3d into your video

ok cool thanks for the info plenty to digest…so even if it didn’t quite look write in my eyes, the first solve which gave 74.165 FL is possibly better than the solve that returned 50.416 FL? …also I used a tripod but had raised it’s crank shaft to it’s highest level and was slowly lowering it during filming so I didn’t use tripod mode for the solve.

Sorry, I guess I was way too confusing… It’s the ratio between the sw & fl that matters so if you are using the preset and your sensor width is set to 22.3, use 50.416 fl… if your sensor width is 36, use 81.389 fl… if the sensor width is still set at the default 35, it works out to 79.128 fl… all 3 sets of values will solve the same

ok cool thanks for clearing that up