update #3: be sure to read Tony Northrup’s comments and answers in the comment section of this post.
update #2: I think Tony Northrup is wrong, Have a look here to grasp the whole story.
update: have a look at mirrorlessrumors.com to get an alternative view of the issue. Says Ale from MR:
There are a couple of logical errors made by Tony Northrup.
First: No one cheats. All company aperture lens info are correct! The Panasonic 12-35mm f/2.8 has a f/2.8 aperture and not a f/5.6 aperture. You don’t have to make the equivalence he says has to be done! Use the Sony A7r and Sony A6000 for the same shoot. To get same result the camera automatically sets on both the same ISO and same lens aperture despite the different sensor size. Just to say that the aperture remains CONSTANT and is not relative! There is no equivalence to make on that!
Please read this why you cannot apply the aperture equivalence you mention on no other than the “depth of field” and “field of view” only:
The focal length and aperture remain constant!!!!
Second: He says Olympus, Panasonic and Sony do cheat. Nope. All companies use the same kind of measuring aperture for all lens formats (medium format, MFT format, APS-C and so on).
This 37 minutes video by Tony Northrup is rather technical and complex, but it is a must see if you want to understand the tricks some camera manufacturer are using to mislead their customers. According to Tony Northrup only Canon, Nikon and Fuji are stating the truth with their crop sensor specifications.
Says Tony Northrup:
Please spread the word to end the cheating: Why you need to apply crop factor to both your ISO and aperture, and how Sony, Olympus, & Panasonic use crop factor to mislead buyers
The folks at iso1200 made a table of contents of the video:
0:00 - Introduction
1:28 - Focal Length & Crop Factor
3:35 - ISO & Crop Factor(2)
15:00 - Aperture & Crop Factor
21:18 - Panasonic misleading consumers
27:10 - Sony misleading consumers
29:53 - Olympus misleading consumers
30:50 - Panasonic misleading consumers (again)
32:01 - Fuji misleading consumers
33:18 - The Good Guys: Canon, Nikon, Fuji, Sigma, Metabones, Voigtlander, and You
36:51 - Summary
That’s what it is about Panasonic’s f/2.8 lens
Real aperture comparison chart
Real ISO comparison chart