What is 26mm f8 on aps c in full frame?

26mm on APS-C gives the view of 39mm on full frame. For the same depth of field, f/8 there behaves like about f/12 on full frame. Exposure doesn't change: f/8 gives the same brightness on any sensor.

The same view on every common format:

• Medium format (44 x 33): 49mm at about f/15.2 for the same depth of field
• Full frame: 39mm at about f/12 for the same depth of field
• APS-C (yours): 26mm at about f/8 for the same depth of field
• APS-C (Canon): 24mm at about f/7.5 for the same depth of field
• Micro Four Thirds: 20mm at about f/6 for the same depth of field
• 1-inch: 14mm at about f/4.4 for the same depth of field
• 1/1.3-inch: 11mm at about f/3.3 for the same depth of field
• 1/1.7-inch: 8.5mm at about f/2.6 for the same depth of field
• 1/2.3-inch: 7mm at about f/2.1 for the same depth of field

Crop factors are by the sensor diagonal, rounded as they're usually quoted; a format with a different shape (4:3 against 3:2) frames a little differently across the width. More in the focal length converter.

Computed from the sensor sizes, not written by a model.