[alamo5000]

Why the background is so bad on the third pic compared to the others.

As the others have already said... it has to do with the lighting, colors, brightness and the fact that there is a lot of white reflective stuff (snow) doesn't help.

If you look at the background on the second vs the third shot look at how bright the background is in the third...

There isn't just one answer to your question. I think it's a combination of things that cause that 'nervous' bokeh...

[Another dyemention]

So if I used say F4, would that fix the background? According to the DOF chart, it seems logical since the BG wouldn't be so OOF.

[drypenn]

So if I used say F4, would that fix the background? According to the DOF chart, it seems logical since the BG wouldn't be so OOF.

Nope. It still would be busy, and nervous. But at f/4 they would be more distinctly busy and nervous.

[alamo5000]

So if I used say F4, would that fix the background? According to the DOF chart, it seems logical since the BG wouldn't be so OOF.

In my opinion you would have better overall photos (of these) if you stopped down to f5.6 or something like that... or more... but the problem is not your aperture. It's your lighting. You have to learn (along with the rest of everyone) to recognize these types of contrasts 'in the wild' and how the camera might react to them.

The problem is lighting not lens.

Edit: Let me amend that statement a little bit... lighting on a light background....

Thin little twiggies on a light background like that with lighting like that... there is the result.

[Another dyemention]

Thanks for clarifying that. I was just about to ask what you meant. And I agree, wasn't a good choice for a BG.
So much stuff to learn.

[drypenn]

Thanks for clarifying that. I was just about to ask what you meant. And I agree, wasn't a good choice for a BG.
So much stuff to learn.

Yup, that's the thing with snapshots.

If we really want a photograph worthy of at least a few minutes in post processing, we should be mindful of the lighting, the foreground, background and of course, the composition.

[Na Horuk]

When I take photos of forests, landscapes, nature, I try to keep the aperture between f6.3 and f8. That way the DoF is large, and bokeh is not a problem.
Bokeh depends a lot on the background and light. The third photo might look better if it were more underexposed, or if the angle of the camera were different. Tiny details, branches can give an odd bokeh at certain distances. In such situations, positioning is the only thing you can do, apart from changing camera settings or just looking for a better frame.

[KDAFA]

Yes, focal length, aperture, and distance from subject is what gives different apparent DoF. DoF is most shallow at nearest focus, smallest f-number, and highest focal length.
Keep in mind that bokeh is not the same as DoF. Bokeh is the character of the blur. Other than the previously mentioned factors, bokeh is also determined by lens design and background (well-lit or dark, with lot of detail or little, point light sources,..).
This is why using f1.8 takes some skill. You need to know what kind of result you want, and then use the appropriate aperture. f1.8 is not useful for landscapes, architecture, most nature photos, documentation, macro, and product photography. f1.8 is good for portraits, moody empty photos, subject isolation, and so on. Photography is a balance act, each setting has effects, and you need to select the three settings, as well as place yourself and take aim, in the way that will give results that you want to see. Good luck.

Thank you for the clear and good explanation. Although I'm not the OP, I benefited from reading this.

[emalvick]

The first two images look ok. As others have stated the third image is too busy in the background; too bright and too many twigs and things. Changing the aperture would help but probably not fully since a lot of the distraction appears to be items that are close enough to the camera that changing the aperture might only make them sharper and they might stay a distraction. Although it could look good, too.

I'm not very good at getting compositions quite the way I envision them when I'm in similar environments either. The hardest thing sometimes is recognizing that something is causing a distraction while you are taking a shot. I sometimes don't notice bad bokeh or items in the way until I get back to a computer, at which point it is too late to do anything about it.

[Na Horuk]

Thank you for the clear and good explanation. Although I'm not the OP, I benefited from reading this.

Thanks for saying that ! I try to give replies that are clear, and contain the "whole" information, so anyone can learn from it - or correct and add to it!