Sony a6000 review
https://youtu.be/j8D_PNMz6I8

Photographing in low light tips
https://youtu.be/jYZs75Gxr-g

The Sigma 100-400 is a fantastic lens; the Olympus 100-400 is a modified old DSLR version of this lens, and the L-Mount Leica 100-400  even uses the identical lenses inside its custom housing. Thus, its optical formula is good enough for Leica to be branded as Leica at a 6500€ price tag (2400€ realistic street price). Olympus and Leica have better weather sealing, though the optics are good and trusted by other brands as well.  

However, I think the Sony 70-350 4.5-6.3 G OSS is an even better beginner-friendly option. It is a bit shorter, 525mm full-frame equivalent instead of 600mm, but this is not a big difference in the field. The biggest benefit is that it is significantly smaller and half the weight of the Sigma, but optically comparable and even cheaper. The A6700 has a relatively small body, and it is way more comfortable with a lighter lens to hold. A lens you have no issue carrying around because it is light enough can be beneficial, and it is also easier to hold for a long time without a monopod. When you eventually upgrade to full frame, the 400mm can be a bit too short; thus, you have to upgrade to the 200-600 or even the 400-800 anyway, and you can just sell it together with the A6700 as a "ready-to-use starter kit".  The Sony 70-350 is a native Sony lens; therefore, no artificial limitations on AF speed and fps. And, as it is around 105mm full-frame equivalent on the short end, it is actually more suitable for a Safari or close-up of big animals, as no lens change is needed then to get it captured in full size. As a G lens, it has light weather sealing and even a hold-focus button.
If you really want significantly more reach, go directly with the Sony 200-600.

A6700 + 70-350 + 16-55 2.8 + 11mm 1.8 and you have a lightweight, compact, and travel-friendly kit you can fit in any small bag, and even with a second battery, lens filter, and camera strap, it is below 2 Kg all together! And you paid less than some people spend on their full-frame body alone, and you can not only do wildlife, but also all kinds of photography.

Alternatives from other brands:
Canon R7 with 100-400 (similar problem as Sony 100-400, as it is f8 at 400mm) or old Canon EF 100-400 L II with adapter, or RF 100-500 if money is not an issue. A more professional body (better grip etc.) with similar AF speed and accuracy.
Panasonic S5 II with Panasonic 100-500 or exactly the same 100-400 (may be a bit short on full frame), full frame, inferior AF, better noise, good weather sealing. For wildlife only, the other options on this list are actually better.
OM System OM1 (used) with 100-400, superior weather sealing, most professional body of all previously mentioned cameras, more noise, less DoF, crazy fast, best viewfinder of all mentioned cameras.
Panasonic G9 II with either 100-400 or 200mm 2.8 with included 1,4 TC (it is actually quite affordable used (below 1000€), possibly the sharpest and fastest lens in this list, also compatible with OM1), superior weather sealing, DGO sensor (APS-C-sensor-like dynamic range), excellent stabilization.
Fuji X-T5 with 100-400 XF or exactly the same Sigma 100-400 with X-Mount. AF is slightly inferior to Sony; the rest of the Fuji body is better, though.
Nikon Z5 II with the Tamron 150-500.

The A6700 doesn´t support pre-capture, but I consider it one of the most useful features for a wildlife camera. I guess, eventually a successor of the A6700 will get it together with a stacked sensor; then it will become an almost perfect wildlife camera, but we have to consider what is available today. From the list above, all except the Nikon support pre-capture, but only the Micro Four Thirds cameras do it on steroids. I think the others are still sufficient for beginners. It makes it so much easier to capture a spontaneous movement of an animal and results in way more interesting pictures.

I think, for a person who starts at point 0, the aforementioned A6700 kit will do fine for all-purpose and travel photography.
However, I would go with the Panasonic G9 II as a portable, beginner-friendly wildlife camera that also has crazy good video features and is still portable with the 200mm 2.8 +1,4 TC, a set that costs used almost the same as the A6700 with 70-350 new. But I guess I am special in this regard.

For wild life the a6400 has a big flaw, with subject set to human it can fluidly track objects with the 4d tracking or if it finds a face or eye it will focus on that, however in animal mode it doesn't do the general 4d object tracking so if you want to shoot animals moving and then get shots better in focus when they stop you have to change modes.

If you try to set a custom button to change modes even though there are only two modes there is no hold option or single press toggle, you have to press it and then click up/down and wait for the animation to move then you can focus with the new mode, not ideal if say you have a dog running towards you and you want to do object focus and then eye focus when it gets closer. You also can't set a button to track. There is custom function called tracking on if you set that to a button and try to use it with animal mode it just says nope, you gotta by a more expensive camera. To be clear I'm not expecting animal body tracking just general 4d object tracking which sees oh hey subject moved from 15 to 14m lets pre focus to 13m for the next shot, this should have been in cameras for ages but it's only just here now, with my a6300 if my dog ran towards me and I tried to take a photo 99% eneded up with focus on the back because of no 4d tracking.

I did find a work around for this, you can set a custom button to recall custom hold and only have focus area ticked and on tracking (and optionally you can have it focus mode and af on if you dont have them set already), I've got the af /ael switch button set to hold m1 with tracking wide then switch down it does hold m2 with tracking spot, with this setup I can have the camera set to animal mode which will do normal non 4d tracking or spot an animal eye and then if I press the custom buttons I can do the 4d tracking focus in wide or spot selection for a general object like a moving animal or a human eye.

I wish it was the other way around so it would do 4d/human with half shutter press and custom button for animal eye because pressing the custom button is fairly quick to start tracking though not as actually setting it to human mode and half pressing the shutter, but I believe this is the best you can do on this camera just because of bs firmware.

Compared to older cameras without any animal or object tracking this might seem a bit spoiled but it just drives me crazy when good hardware is significantly heald back by one stupid line of code.

I generally understand 25mm as one times, which makes conversion easy. 35mm is approximately 1.5 times, 50mm is 2 times, 85mm is approximately 3 times, 100mm is 4 times, and 240mm is 10 times. Additionally, 16mm is approximately 0.6 times. It's quite easy to calculate.

How about Nikon Z50 ii paired with Tamron 150-500 lens? That's even cheaper alternative!

I was considering getting into wildlife photography, and your detailed explanation about the Sony a6400 paired with the Sigma 100-400 was incredibly helpful!
I was debating my budget, and the information was exactly what I needed.

Just got this lens for the a6400, I'm loving it so much, amazing setup!

0:52 "one times what" pretty sure phone manufacturers use their main camera (usually 23-24mm) as the base for focal length. So for a phone with main camera at 23mm focal length, its 2x is 46mm, 5x is 165mm. It's not a stupid measurement, in fact it's intuitive for people who don't know yet "23mm" means how wide or "165mm" means how far.

I am using this lens for a year now paired with my a6700, and despite i like that kit a lot, i am having some autofocus issues with birds in flight, especially in long distances... I already tested different autofocus settings, but it looks like the lens always misfocus a little in that kind of situation, even with the subject being properly recognized and tracked by the camera 😕 Is anyone else having the same problem? Suggestions on how to solve that?

Literally the best video for this set up great job explaining everything. I currently have the a6400 with the sony 55-210 and the lense is ok not that great in my opinion but i paid $100 usd for it so, but i wanted to upgrade to the a7iv really like the ergonomics thats about it i dont think i need 2 card slots hdmi none of that i found the a6700 for about 1,521 usd with the kit lense brand new and this sigma lense for $950 used here in colombia for a total of $2471 and if i do get the a7iv comes with the 28-70 kit lense for  $1927 used and id still have to buy the zoom lense for 950 thats about $2877 usd just debating if i shld go full frame for the extra $400 dollars or just upgrade to the 6700 keep the 6400 and the 55 210 for my wife. What would you do?

PSA keep in mind when buying used cameras if you are importing into the uk anything over 135 pounds even if used will have about 20% tax to be paid, I just bought an a6400 and forgot about this so the price went from 411 to 503 which I wouldn't have bought if I knew. Also keep this in mind when selling, to see what others are selling for either just check for your country or when looking at options from other countreis take into account import fees so you don't sell your camera for 20% less than you can.

Pity the Sigma isn't compatible with a Fuji teleconverter.

Id argue that Sony 70-350 could be even better for a beginner, as its even more compact and a lot cheaper. The Sigma is a reasonable step up from that!

Thanks, that's really helpful and also the comments from others.  Where is the best place to buy used cameras and lenses?

I bought a sigma 100-400 for my nikon because i was told by hobbyists it was known for its clarity.
Best purchase ever.

When I'm not having to walk to far, I take my a7iv and 150-600 lens. When there's more walking involved, I take my a6700 and 100-400 lens. When I just need a lot of reach, I take my a6700 and the 150-600 lens, an effective 225-900.

How about tamron 70 300 with  6400

Just recently got into real wildlife photography and picked up a Nikon D7000 pretty cheap used. For the lens I went with the Sigma 150-500mm. It's been fun but man is it heavy...the chunky DSLR sure doesn't help. Great video!

I use this setup, it's plenty good to have some fun, now that I know a bit more I kind of wish I'd go with Tamron 150-500, more reach works on FF as well, and have way better minimum focusing distance

Imagine after sony buys Tamron, that they take the 18-400, and upgrade it to drive by wire for the mirrorless age.