Softness issue with Canon 100-400 F/4.5-5.6L IS II USM

[Seanac]

I picked up the Canon 100-400 F/4.5-5.6L IS II USM recently and found that many of my images are now soft.

I use this lens with the canon 90D body; and I upgraded from the sigma 100-400

I shoot aviation, and on many occasions half of the plane is soft and the other half is sharp. For instance, the nose is very sharp but then the tail is soft. I am shooting at roughly 220mm and am 1/4 mile from the airplane, so it’s not like a crazy depth of field issue. 
I’m using AI servo, and have tried many different autofocus point modes. With my old lens I used auto AF points, but due to my issues with the new lens I have tried every AF point mode and they all seem to have the same issue. I’m also shooting F/8, ISO100/200 and with a fast enough shutter that I used to have no issues with (1/800). Some shots are fully sharp, but a GREAT majority are sharp in the nose and soft in the tail. 

Really frustrated because I have missed many shots due to this, and am curious why this may be? I’m considering selling the lens and going back to my Sigma. Would love some input

[K_ozdragon]

So in one of the photos, you're at 182mm, 100 ISO, & 1/640 shutter speed. This is causing motion blur. I'm assuming you're near the planes landing at that focal distance & subject size in the frame. The plane is moving pretty fast. You would need a minimum of 1\1000 for a sharp photo. Don't be afraid to bump up your ISO. Capturing motion is exactly what the ISO is good for. A tiny amount of grain is better than photos that aren't sharp due to motion blur.

You might be able to capture these at those speeds while panning, but unless you're highly skilled at it, you're going to get blur that looks like sharpness issues. The plane isn't centered in the frame, so if it was a pan shot, your tracking is off. Bump up your shutter speed using a higher ISO. That should fix your problem.

Also, be sure you're on servo mode for continuous focus. If that isn't on, there's no way you're capturing a sharp photo on a moving object. I figure you know this, but it never hurts to be sure.

[Waddizzle]

Are you using a UV filter?  That particular lens doesn’t perform well with one on a DSLR. 

[ebiggs1]

Even though this thread is 2 months old I will suggest you are the problem. You are all over the place with your AF.

I can almost guarantee Ai-servo is the culprit, use One shot and preferably the center focus point.

"Are you using a UV filter?  That particular lens doesn’t perform well with one on a DSLR. "

Absolutely ditch any filter you attached to the lens. ALways use a lens hood for this type work. This lens is not really great with IQ and is hampered more if you put any filter on it.

" I’m considering selling the lens and going back to my Sigma."

You can certainly do that the Siggy has better IQ. I had two copies of the 100-400mm (1st and 2nd model) and could not like either so I sold them both. If you want Aa 400mm lens check out the ef 400mm f5.6L prime it is a fantastic lens and very sharp.

[Seanac]

Thanks, I would like to ask; how do you know my focus is all over the place? I feel like I throw away most shots, even nearly a year later. Is there a hidden technique I don’t know of that I didn’t need to use with my sigma? Would you recommend getting the 90D and the 100-400L II professionally calibrated? Or is it somehow user error that I’m not aware of? I am not using a UV filter and I do use a lens hood. I have never had to use one shot rather than ai servo, but I can try it out.