On the Canon website I compared the specs for the 77D with those for the 80D, I found that:
The 77D has a maximum shutter speed of 1/4000 vs 1/8000 for the 80D. I find 1/8000 occasionally useful when shooting an f/1.4 lens in daylight.
The 77D has a slightly smaller sensor, 22.3x14.9 vs 22.5x15.0mm for the 80D. This is probably not an important difference.
The 77D has lower viewfinder magnification, 0.82x with a 50mm lens vs 0.95x for the 80D. To me this is moderately important.
The 77D has slightly lower viewfinder coverage, 95% vs 100% for the 80D (according to imagesource.com.)
The 77D has expanded ISO to 51200, vs 25600 for the 80D, (according to imagesource.com - I could not find anything about this on the Canon website.)
The only things that would tempt me to buy the 77D instead of the 80D are the higher ISO (if this is true) and the lower price. But I would find the control layout difficult when switching back and forth from the 70D to the 77D. I don't think I will buying either one soon, but if I do, it will probably be the 80D.
Canon 80D, 60D, Canon 10-22EFs, 15-85EFS IS, Sigma 100-400, Sigma 135/1.8ART, Sigma 30mm f/1.4DC, Canon 60mm EFs Macro, Rokinon 8mm fisheye, 550EX flash, Olympus TG6 underwater P&S
Postprocessing: DxOLabs 5, DxO Viewpoint 3, Paint Shop Pro 2021
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