so my Sigma 18-35mm 1.8 lens is the wrong type of lens to use to achieve the below?
No . . . ish.
Depth of Focus 'zone', the amount of front to back distance in 'acceptable' focus in front and behind the distance you focus on, is proportional to, first the aperture; bigger aperture/lower f-no, smaller that zone will be; Focus Distance; further away you focus, the Deeper the DoF will be.
Focal length.... gets a bit more complicated, in theory it doesn't make any difference... but it does. Longer lenses have a close focus distance further away from the camera, and a longer range of 'critical' focus between thier nearest focus distance and the 'hyper-focal' distance, where everything beyond is in 'acceptable focus'.
So, for a given focus distance, you will tend to have a DoF zone pulled closer to the camera, because of the closer focus distance.... but then because the lens gives a larger 'magnification' to get a person sized subject in the frame, you will tend to move further away from them and set a longer focus distance than using a wider lens, so you get back a bit of DoF... but you will get more 'effect' of shallow DoF, because for the same effective framing, you will likely have to increase the Focus range less than a longer lens reduces closest focus distance, so pulling the DoF zone shorter and closer to the camera.
BUT... to answer your more specific questions; this is NOT a question of equipment, its a question of know-how.... and as far as the red-dots go..... well? They are putting the focus distance on an arbitery 'subject' the electrickery measures between the camera and a solid subject.....
Make the leap from expecting the camera to serve up the effect you want, and apply old fashioned craft... and rather than trying to get 'Shallow-Focus' to chuck back-grounds out of the DoF zone, you can start exploiting 'Selective Focus' to do the same..... and get far more, in far wider range of situations, and do it without the sort of 'gear' folk rave about as 'essential' to deliver shallow focus 'on demand'
GO MANUAL...... focus, not exposure!
The Red-Sots need a solid 'target' to get a bead on to do their focus range calculations, then set that specific focus distance for you in the electrickery.... BUT... you want to chuck the background 'oof' but keep your subject in the DoF zone.... chasing shallow focus with ever wider apertures and longer lenses, you will get to a point where the DoF zone is so shallow, you can focus on a subjects eyes, and chuck their ears and nose oof.... yet, the back-ground isn't all that out of focus still.
I will get told off for repeating the old rule of thumb that the DoF is aprox 1/3 ahead of the focus setting, 2/3 behind, by widgetal pedants... who are technically correct.. but still.... that's the old rule of thumb. If I pick up one of my old manual focus lenses, it has a handy focus scale, lots of modern widgetals often don't, but more still, it has a pair of DoF brackets to indicate on the focus scale where the DoF zone will fall, for any aperture setting... neat hugh! Didgital adherants have to faff with a Smurph-Phone to work this out!
BUT... fire the red-dots.... now, I DONT focus on my Subject!
Picking up my trusty olf Ziess 50mm, 'cos its to hand.... If I set focus to 10ft which is about right to frame a standing person completely, DoF marks tell me that at f8, my DoF will be from aprox 7ft to 15 ft... so the back-ground has to be at least five foot behind my subject before it even starts going oof, and is probably not very oof even ten foot behind them.... if I open up the aperture to f4, DoF is from aprox 9ft to 12.. and still the back-ground has to be quite a long way behind my subject to start oof-ing....
BUT... 'Selective Focus' rather than simply 'shallow-focus'... subject is 10ft away.... at f8 I have DoF zone of approx 7ft to 15ft.... and the back-ground isn't going very oof very fast.... because the majority of the DoF zone is behind the Focus Setting... which is the bit I DONT want in focus! Bit in front? There's nothing there to be in or out of focus! SO... being 'crafty', I can pull the focus forwards.. focus at perhaps 8ft focus range.... NOW the DoF zone shrinks, just like I had used a bigger aperture..... WITHOUT setting a wider aperture.....I still have more DoF but most of it is now 'wasted' in that region infront of my subject where there';s nothing to be 'in' or 'out' of focus.... and I am getting the back-ground going 'oof' earlier, and probably a lot earlier than it would with a wider a aperture..... make sense?
There's only one niggle..... there's nothing infront of the subject for the red-dots to get a lock on.... so I HAVE to focus manually, because the camera's electrickery would get confused!
NOW... we get into the question of 'Bokah'... which is almost utterly subjective, and isn't about how much 'OoF' fuzziness you get, but how pretty it is... and this is entirtely dependent on the subject not the equipment.
If you have sun filtering through trees; you get little halos of light in the OoF zone, where each high-light diffuses into the shaddow around it. If you have a beige concrete wall... there's no high-lights and shaddows, so nothing to blurr, and you dont get any 'bokah'.... so there's even more to the craft, this is entirely in the scene, and no lens or focus scheme or camera setting will change whats out there infront of the lens; you either have a back-ground that's condusive to the 'effect'... or you dont..... and up to you to find it, exploit it, and make as much or as little of it as you will... BUT its in the composition to begin with; not in the kit, and you can only enhance it with the equipment, not make it. And you will only enhance it a little expecting the kit to do it for you using wide-aperture and or long lenses for 'shallow-focus', to make the most of it, you STILL need to apply more craft and exploit 'selective focus'... WHICH is where asking about red-dots and focus schemes is missing the point, and making life harder for yourself not easier, because red dots will only focus on something that is physically there.... and to utilise shallow-focus you need to focus on something that ISN'T there.
Look at your photo.... and look at the back-ground. There's lattice trellis in the back-ground, and shrubs, there's not an awful lot there conducive to making a pleasant OoF bokah back ground.
Look at the example photo you'd like to emulate; The Background is far more busy, with a chap in very busy Hawaiian print shirt for starters! But street signs, shop lamps, car headlights, theres a lot more detail in there and a lot more contrast between high-light and shadow to show 'Out of Focus' blurr and produce Bokah effects.. the SCENE is much more conducive to that effect before you even start;.. and look at the cobble stones beneath the girl..... they are in focus from the edge of the frame.... a little cheeky peek, but I counted five or six rows of bricks infront of her foot that are in focus.....I only copunted four rows of bricks behind, before they started going OoF.... Hmmmm.... May be a flook.... BUT, that suggests to me that there's actually more DoF infront of the subject than behind, which is contrary to the way that the electrickery would tend to work with AF, that would put 1/3 infront 2/3 behind or maybe 50/50.... but with more infront than behind? SUGGESTS that that photographer was NOT relying on equipment to get Shallow Focus, but turning off the AF, and exploiting Selective Focus to put the DoF zone where they wanted it, not where the camera's electrickery best guessed it aught to be.....
So THAT is how you would get photo's 'like' that one...... start with your composition and picking a back ground more conducive to the Bokah OoF effect you want; then dont just rely on the gear 'giving' you shallow focus, but make it, exploiting Selective Focus, using Manual Focus or if you must use AF, putting red dot on something infront of your subject or at shorter range than your subject and using Focus-Lock before recomposing and firing shutter...
Has little or nothing to do with the kit in your mitt, but the craft in your cranium.....