I am a predominantly a wildlife photographer, and I have both the 7DII and 90D. I much prefer the 7DII over the 90D for the tracking, which is clearly superior to the 90D - I, like many wildlife specialists, was gobsmacked that Canon didn't include the tracking from the 7DII into the 90D, it would have made it a brilliant camera. Absolutely, the 90D has more pixels and is newer, but if you can't consistently focus on a subject, especially a moving one, they mean nothing. Furthermore, more pixels does not mean better photos if you don't use them - most websites (including this one) demand significant downsizing of images to post in any case, and considering these are crop sensors, a significantly greater number introduces the risk of greater noise in low light.
I use the 90D for other purposes where its greater MP size has an advantage. I have used the 7DII with a variety of lenses: Canon EF 70-300 (IS USM MkII & L versions), Canon EF 100-400 MkII (with and without the MkIII 1.4x extender), Sigma 150-600 and 60-600 units and get images from it I am happy to live with. The following images were all taken the with the 7DII, but had to be significantly downsized to fit on this site. All are taken in available light, hand-held.
As far a tracking goes, catching a raptor in full flight as it is directly opposite is a good test.
The image can be cropped to concentrate on the subject where one cannot get close.
The image below of a NZ Tui was taken in extremely dim light, but the sensor handled it with no issues.
This is in very dim light, complicated by speckles of brighter light, and cropped.
Another test of autofocus is the ability to keep the eye in focus as the target is moving directly towards the photographer, as in this shot of the Australasian gannet
For low light shooting the 7DII is the better choice. It is a 20MP camera and that relatively moderate resolution makes it pretty good at higher ISOs. The following is a test shot I did with one of my two 7DII at ISO 16000:
Note that's ISO 16000.... NOT ISO 1600!
Yes, there is noise in the image, as you can see in the above crop. However IMO it's pretty well handled. That image was shot by RAW at ISO 16000, using EF 100-400mm Mk II zoomed to 158mm and f/5 aperture, hand held at 1/125 shutter speed. The only lighting was from a window approx. 8 feet away and a 60 watt bulb even farther away. Image was converted to JPEG in Lightroom 6.14 with nothing more than a slight boost in contrast and the default level of noise reduction.
I don't have 90D to compare high ISO performance directly. But it is the highest resolution APS-C camera anyone makes. With 32.5MP it has the most crowded sensor of any interchangeable camera I'm aware of. The next closest APS-C camera is 26MP. If a full frame sensor were made with the same pixel density, it would be 83MP... The most any FF camera has right now is 61MP. If a medium format sensor were made with the same pixel density it would be 140MP... The highest resolution MF camera currently is 102MP.
Actually, considering how high resolution it is the 90D appears to do very well keeping noise under control. But you should expect to see some with it.
There are other reasons to consider a 7D Mark II for wildlife photography. It has a unique 65-point AF system that's one of the best and highly customizable for different situations (there's an approx. 50 page user manual for the AF system alone). The camera is also well sealed for weather and dust resistance. And it has a high durability rated shutter (200,000 actuations, compared to 120,000 for the 90D). 7DII also has two memory card slots (one Compact Flash, the other SD)... the 90D has a single slot (SD). 7DII can optionally be fitted with BG-E16 battery grip. 90D uses the same BG-E14 that its 80D and 70D predecessors used.
Both AF systems are rated to work as low as -3EV (bright moonlight). Both also are "f/8 capable", meaning they can focus an f/5.6 lens with a 1.4X teleconverter, or an f/4 lens with a 2X (other cameras may be "f/5.6 limited", unable to autofocus those combos). Image below was shot with EF 100-400mm Mk II lens and EF 1.4X II teleconverter at 560mm and f/8, on 7D Mark II at ISO 1600, 1/800 shutter, hand held.
The 7D Mark II can only focus an f/8 combo like that with its center AF point. The 90D has more AF points capable of f/8 focusing... but only with certain lens and teleconverter combinations. One of those is the EF 100-400mm Mk II lens with EF 1.4X III, where up to 27 of the 90D's 45 AF points can focus. There are several other lens/TC combos that allow more than one of those points, see the 90D user manual for details.
The 90D has a fully articulated rear LCD that's an active Touch Screen. The 7DII's screen is fixed and it's not a Touch Screen.
7DII has a bigger buffer for continuous shooting... It's rated to handle 31 RAW files and unlimited JPEGs (until the memory card is full). In comparison, the 90D is rated to handle 25 RAW or 58 JPEGs. And, to be honest, I'm not sure how big a deal this is... I try to avoid long continuous bursts because too much of that just means more time sitting at a computer editing a big pile of images! Still, there are times when the fast frame rates come in handy...
Reportedly the 90D is more power efficient. Canon claims it can get 1300 shots on a battery charge, while the 7DII is only rated to do 670 shots (with an earlier, slightly lower capacity battery). However, these are both pretty conservative numbers. I've often gotten upwards of 1200 or 1300 shots per battery charge out of my 7DIIs and still had power to spare. I have not compared this with 90D, but imagine it out-performs its rating too.
Depending upon your budget, today you might want to consider the Canon R7 or R10 for wildlife photography. The R10 is 24MP and costs $979 (body only), while the R7 is 32.5MP and costs $1499 (body only). These mirrorless cameras have fantastic AF systems, capable of things we only dreamed about with our DSLRs. They also take frame rates to whole new levels. Where the 90D and 7DII both can shoot at 10 frames/sec., the R10 and R7 mechanical shutters can do 15 frames/sec. (Both also have even faster electronic shutters, but rolling shutter effects may limit their usefulness with fast moving wildlife.) The R7 also has in-body image stabilization that combines with IS in lenses to offer 6, 7 or even 8 stops worth of assistance! The R10 doesn't have IBIS.
There are other pluses... and some minuses... to the mirrorless cameras. While there are a whole new line of RF lenses being offered for the R-series cameras, a $100 adapter allows any EF and EF-S lenses you might already have to be used on them too. People who have done often report that the EF/EF-S lenses work just as well or even better than the did on their DSLR.
Have fun shopping!
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7DII (x2), 7D(x2) some other cameras, various lenses & accessories
FLICKR & ZENFOLIO
