Take any advice, including mine, with a grain of salt, different photographers have different views. Though in some forums, a standard view is accepted and deviances from the mainstream are not tolerated.
Also, I do not give this advice from the lofty perspective of a successful portraitist; I am in a similar situation than you. I am now preparing to go full time pro as a second career in a couple of month. I believe that I have the experience, ability, and business savvy to be successful in this very competitive market.
- Shoot, shoot and shoot
The best way to progress is to shoot a lot. That is the best hands on experience you can get. You can start with your kids, playing around, sleeping... you will develop your abilities and also your tastes in what you want to do, like for instance on location kids at play pictures. Then you can start expending on your experience and building your portfolio with models willing to pose for time.
Else you can start finding paying customers. Some may disagree, but I see no problem using paying customers to grow in your craft. Afterall even when you are a master portraitist, you should continue to grow and learn with every client. Understand that pro photography is a lot more about running a small business than photography per say. Do it if you really want to or need to...if it is just to justify the price of gear investment, it is not worth it.
- Generic or unique
There are many ways to approach portraiture, the most obvious formulaic approach works, but so do others.
The main problem, IMO, I see in photography today is that most pictures look like each other. This is particularly concerning amongst professionals. If your professional pictures look like the other pros or amateurs, your product becomes generic, hence a differentiation by lowering prices, hence struggling to make ends meet or going bankrupt.
There are formulas, or general advice on what constitute portraits, you should start by learning that, but be careful that you don’t end up with photographs that are boring, lifeless and look like any one of 50,000 generic photographers took it.
Also, sometimes genre photography gets pigeonholed, and adopts narrow-minded views. This is why you often hear that portrait photography is different than landscapes. Sure if you follow formulas, portrait recipes are very specific and different than other types.
You will also find that good photographers, and famous ones, almost always do break every single "rules" of what constitute good portraits. Often the best inspiration and technical approach is outside the field, like using street, art, low light, or commercial photography methods for portraits. Expose yourself to other arts like moviemaking, painting or design; it will make your pictures stronger.
As you come from landscape, play to your strengths and try to adapt your methods to portraiture.
Your mastery of light in landscapes will come in handy for portraits too.
In digital approach, you will often hear expose to the right. This can be valid but not always. Some photographers do the reverse, exposing to the left like it was done with slides. This gives richer shadows with more color.
For me, I find that when I can manually meter, I produce better exposure than the grey smudge of matrix. It is an oversimplified approach of the zone system exposure; I spot meter the highlights and do down 2-3 stops. This gives me the best results, and usually you do it only once and continue shooting manually like that. I also find that that gives much more consistent results instead of the camera deciding at every step what the light is. Then again, I am also used to shoot on old film cameras with no meter, and never found that to be a hindrance.
As an experienced nature photographer, you know everything you need to know. The human figure is a landscape waiting to be explored.
Most lighting gimmicks give flat boring images. Sometimes you want that, but not always. This type of lighting, particularly if further compressed by a telephoto lens gives a flat low contrast image, grayish blah.
Learn to light for wrap around, but also learn not to be afraid of negative space, dark shadow and contrasts. Adapt the type of light to the subject personality and mood you try to portray.
- Artificial lighting
The only thing you may be lacking is flash photography. Yet, you don’t even need that at the beginning, do natural light portraits and maybe use reflectors as fill lights. Then maybe later a fill in flash with a soft box.
However, a lot of successful photographers use only available light.
But if you want to start shaping light, use reflectors, then fill in flash, then accessories, then full flash with modifiers, then a 3 lights systems….it depends mostly on the type and style of portraits you take. If for instance you want to a glamour commercial portrait, then artificial light mastery is essential, refer to the excellent post by WayneF
If you don’t know anything about portraits, start by reading books, or watching videos about posing. Then forget about them, if you can. There is nothing worse than posed, resentful kid’s photographs. Learn to capture the moment.
With posing it is very easy to end up totally formulaic, head at that angle, light at that angle….
However, these formulas do work, so you need to know them. Sometimes, you also need to forget your artistic integrity and compromise yourself to eat. Most of the classical poses are the most flattering to the customers and you may have to apply them. There are even techniques to make fat people seem slimmer; some customers are ready to cover you in gold if you make them consistently younger, more beautiful, thinner…..
In this way, a portrait is a lie, and that is even before touching it up with Photoshop or alien skin….
So, I find that I can’t really give you advice here, just be aware of what you are doing and why. I have seen professionals using formulas so much that they approach photography as detergent soap selling.
Doing environment portraits showing the background is relevant if it is part of the story being conveyed.
Mostly the best portraits show who the person really is.
If you want the best esthetic look, go wide open and blur the background totally for that bokehlicious backlighted effect, particularly for woman and kids.
However this is a taste preference and it is stereotypical, but here I believe that being stereotypical is warranted. I understand if coming from nature photography you may not like razor thin shallow dof, though I advise you to try it with your 50/1.8 and with faster lenses, particularly with round iris blades, if you can borrow or rent one.
Also coming for landscape if you have larger formats keep them and offer a film service, you can always scan the negatives, there is nothing quite like a scanned 4x5 negative, well an 8x10…. Also the larger you the shallower dof you get, which is why professional wedding photographers in the film era carried a medium format camera. Consider buying an older digital back like the Phase One P25, which is also an easy way to differentiate your pictures.
Typical “ideal” tele for portrait is in the 70-90 range. Wider than 70 you start to have distortions of proportions if you get to close, longer and you flatten the environment. Most photographers take portraits, and most photos at the 24-200 range, however you can get great and unique portraits by going above and beyond the extremes.
There are no ideal lenses. Every lens has limitations, but you learn to work around them. However if you are careful you can use different focal lengths for great effects.
They often use something like 24 or under for groups and indoors, 35 for environmental and more creative close up portraits, 50 for a regular look, 85 for a nice portrait, 135 for close up, 180, 200 or longer for a flatter close up.
You bought a 50/1.8 that’s the first good step on your journey. Some people will try to tell you that their $2000 24-70/2.8L is so much better than your cheap plasticy $100 50/1.8. It may be more convenient and versatile, but borrow one or look for pics produced by these zooms and you will see that your lens outperforms them. In sharpness L or pro zooms can match primes, but their design and number of elements tend to eat light and produce very low contrast, flat, grayish images.
Get an adaptor for your nikkor glass, there is no need for AF, you can manual focus much faster and more accurately than an AF lens in 99% of the situations with some, not even much, practice.
This is one of the subjects I get down voted for, that and no zooms, no AF and staying at base ISO.
Some photographers, often the ones that quote XO mark, love sharpness. IMO, sharpness is one of the worst enemies of nice portraits, and is why some special “soft” lenses have cult status amongst discerning photographers.
When you take portraits with modern lenses, particularly zooms you are confronted with that issue. This is also one of the reasons to avoid the 100-105 mm range where a lot of macro lenses are. For example at 105 you have the Nikkor 105 micro, or the end of the canon 24-105/4.
I can’t think of a worse lens than that the 105 macro for portraits. Macros are uber-sharp slow lenses, about the reverse of what you would want; you want a flattering look, not to count the number of pores on their nostrils
Even if you use zooms, it is better to avoid their extremes, so a 24-105 zoom, at 105 is at its worse, and is a slow f4, that probably need to be at 5.6 to be good, and a spectrum gobbling zoom at that…brrrr consider a pinhole shoebox camera instead
Consider old manual lenses, they are full of aberrations, bubbles, rare earth glass formulas, or even radioactivity….all considerably better than the modern laser cut perfect aseptically sharp lenses; there are whole forums on these lenses where people haven’t forgotten what good photographs are.