Upgrading lenses. Which to get?

I'm a natural light photographer who specializes in children, senior, engagement, and family photography. I'm planning on using my tax return money to upgrade my camera to a full frame sensor and upgrade my lenses. I currently own a 50mm 1.8 and an 85mm 1.8 (all nikon products). I thought that the 50 1.4 would be great for my natural light studio sessions (as my studio is a tight sunroom in my home). But the 85 1.4 would be great for my outdoor sessions. I can only choose one or the other… Which would you suggest?

Well, only you can answer that - it depends on the proportion of time you spend on location vs in the studio.

Tbh you're not gaining much in terms of stops of light between f1.8 and f1.4 - the 'bokeh' is creamier in the f1.4 lens but that's not the reason you really want it for is it?

I also have to add that you may be restricting yourself as a 'natural light' photographer. That often just means someone who doesn't know how to use flash. Now that might not be the case with you, but what do you do when you have clients booked and the 'natural' light isn't enough, of the right quality or the right direction? Do you rebook? Or shoot in poor light?

If you added a couple of off camera flashes to your repertoire, along with some decent modifiers, and learned how to light, you'd be in a far better position to offer your clients a more creative service. And penny for penny, lighting gives you far more 'bang for your buck' than fast glass or upgrading your camera body ever will.

Just look at the lovely wrap of light Joe McNally gets on his subject in this video;

You may be thinking that flash is too harsh - well it needn't be, not if you learn to use it properly. It's all about crafting your light source (whether that be ambient or flash light). Light is light after all - it obeys the same rules whether it's 'natural' or flash.

Lenses are fitted based on need. Additionally the digital sensor (format size / film frame size) plays a governing role as to what focal length will be suitable. Your decision process should start by finding out what is "normal" for your camera.

A lens is considered normal if it delivers an angle of view that is not wide-angle and not telephoto. This will be an angle of view of about 45⁰ with the camera held in the horizontal (landscape) position. This happens when the camera is fitted with a lens with a focal length that is approximately equal to the diagonal measure of the format size. The full frame format measures 24mm height by 36mm length. This rectangle has a diagonal measure of about 45mm. The industry has chosen to round this value up to 50mm. Thus 50mm is regarded as the "normal" focal for the full frame.

Now wide-angle is generally labeled as 70% of "normal" or shorter. That's 35mm or shorter. Telephoto is characterized as 200% of "normal" or longer. The ideal portrait focal length is regarded by many to be 2x thru 2.5x of normal. That works out to a range of 100mm thru 125mm. Most authorities set this value at 105mm. Such a lash-up delivers portrait images that are considered to have the best perspective (distortion free).

Let me add that the now popular compact digital format measures about 16mm height by 24mm length. The diagonal measure is 30mm. Thus 30mm is "normal", 20mm or shorter is wide-angle, 60mm or longer is telephoto and portrait is 60mm thru 75mm.

The above is just a rule-of-thumb, art has no rules, and you are free to follow your heart.