Sure. I'll play, and without any personal attack on Tony Northrup. But I will say he often misses the mark with his videos and he seems to care more about views than presenting useful, objective information (those are observations of Tony as a professional, not personal attacks).

The main problem with his "testing" of full-frame vs. crop lenses is lack of standardization. He compares images on a crop sensor with images on a full-frame sensor, making any objective comparison of lens image quality absolutely impossible. Both sensors have roughly 24 million pixels, but one sensor is much larger, so its pixels are much larger. While newer technology has made pixel size less a factor than it once was, smaller pixels have an inherently lower s/n ratio and produce a lower-quality image.

Had Tony put on his thinking cap, he'd have used the same camera but in crop mode for his tests. This is so obvious that I can't help but think Tony did this intentionally to support his mistaken notion. He is the type of guy to deceive us if he thinks it'll make him a buck. Now that was personal, but otherwise how'd I do?

Yo Tom,

The best answer to that "photographer" would have been "I'm using this lens because no one makes a 'crop-only' EF-S 100-400mm lens". In fact, most telephotos are FF capable. There are very few that are "crop only". No crop only lens I can think of for Canon longer than 250mm.

And there is ABSOLUTELY no truth to that BS about not using FF lenses on crop cameras.

In fact, there can be some ADVANTAGES to using FF lenses on crop cameras. Most lenses are sharpest in the center and gradually become less sharp as you move toward the corners of the image. So a crop sensor camera will potentially only be using the BEST part of a FF lens, cropping away the weakest parts of the images the lens makes.

Also, with telephotos in particular, a crop sensor camera will act like a "free teleconverter". What I mean by that is the crop of your 80D is like putting a 1.6X teleconverter on your 100-400mm, which will behave as if it were a 160-640mm on a full frame camera.... except there's no loss of light the way there would be with an actual teleconverter. (They don't exist, but if they did a 1.6X teleconverter would "cost" approx. 1.5 stops of light lost.) While some combinations of lens and TC work very well, teleconverters also might cause some compromise of image quality and autofocus performance, which you can avoid by instead using the lens on a crop camera.

In fact, speaking of teleconverters, that's another consideration. Say you wanted to use one on your 100-400mm.... No problem. Canon makes some great 1.4X TCs (I use version II and it's very good, but their III is even better). However, until very recently no one has made a teleconverter that can be used with an EF-S lens. Partly the reason for this is that there are almost no EF-S lenses it makes sense to use with a teleconverter.... Maybe the EF-S 55-250mm or the EF-S 18-135mm. Today Kenko's "HD" teleconverters are usable with both EF and EF-S lenses. Though I still see little reason to ever do so and wouldn't expect particularly good results using those lenses with teleconverters. The image attached below was shot with Canon EF 1.4X II on EF 100-400mm II lens on APS-C format 7D Mark II camera (the most enlarged version of that image may appear over-sharpened on a computer monitor... this image was sized and sharpened for printing, so is higher resolution than usual displayed online).

Where "crop only" lenses are most needed are at the wide angle end of things. When DSLRs were first being introduced, nearly all of them were "croppers" (arguably, the first "mainstream" full frame camera for the masses was the Canon 5D introduced in 2005), but the lenses we had for use on our APS-C cameras were all carried over from film SLRs. Aside from fisheye lenses with heavy distortions and a few very expensive 14mm and 15mm primes, the typical wide angle from the film ("full frame") era was 20, 18 or 17mm at the widest. Those simply weren't very wide on APS-C cameras.

Lens makers saw that need and soon filled it with non-fisheye "crop only" lenses that went as wide as 12, 11 or even 10mm. The earliest of those weren't all that great, but became much better in subsequent versions. Canon's EF-S 10-22mm is superb, though it was pretty pricey. That's changed, too, though. Crop lenses also can be smaller, lighter and less expensive... In fact, Canon themselves really turned things upside down when they introduced their EF-S 10-18mm IS STM for under $300. That lens is not only one of the lightest and smallest ultrawides, it also was the first of that zoom type to have image stabilization. A bit plasticky, but capable of making excellent images, it's price might have been the most revolutionary thing about it. This forced other lens makers to reduce their prices and/or develop competitive products Nikon soon intro'd their own AF-P 10-20mm with VR for around $300 (though for some reason Nikon continues to offer the two most wildly overpriced ultrawides: AF-S 10-24mm for $900 and AF-S 12-24mm for $1100!). Sigma's 10-20mm f/3.5 now sells for $500 to $440 in Canon & Nikon mounts... But isn't discounted in Sony mount, where there's no competition and it sells for $650.

One of the coolest things about your 80D is that it can use BOTH Canon EF (full frame) and EF-S (crop only) lenses equally well. You can fit it with any of the 130 million Canon EF/EF-S lenses ever made the last 30+ years and can choose among the almost 90 different EF and EF-S lenses Canon currently has in production. The only other manufacturer with similar lens choices is Nikon. There are also a lot of very good third party lenses made to fit your Canon (and for Nikon F-mount cameras).

P.S. Full frame cameras require full frame lenses. Canon EF-S lenses can't even be fitted to Canon FF cameras. So, the guy who questioned you using that lens on your camera actually has it exactly backwards. However, yes, some crop lenses can be fitted to some FF cameras and the cameras can be set to crop the image or it can be cropped in post-processing.... but doing that usually ends up with far less resolution than your 80D offers. To crop in that manner, the FF camera needs to have 50MP or more resolution.