My Idea of Choosing a perfect lens for the shoot.
There are different types of lenses and finding a perfect one for your requirement is tricky. It is true any lens can give better quality than a smartphone. So, it is impossible you will get a bad picture if you using at least a DSLR especially a full frame DSLR. Whether you are choosing a mirrorless or DSLR with mirror, you can find plenty of lens choices from camera manufacturer brands like Canon, Sony and Nikon and from third party manufacturers like Sigma and Tamaron. It is better to stick with flagship lenses of each camera manufacturer rather than buying and trying various fancy lenses unless you have a particular need which can be fulfilled by those lenses only. Because, believe it or not, you can make your career in fashion photography with just two lenses.
If you know the basics of DSLR, you will be knowing that there are various types of lenses like Prime, Wide Angle, Telephoto, Fisheye etc…I assume you know the basics of it. So, I am skipping to the practical aspects of shooting. While shooting a model or any portrait, you need a picture without distortion. So, obviously your choices will narrow down to telephoto lenses and popular ones are 85mm Prime and 70-200 Telephoto. You can find the same focal length lens from every manufacture, only difference will be the aperture. Usually prime lenses have wider aperture around f1.2 or f1.8 and flagship 70-200 lenses have aperture of f2.8 but in my opinion 70-200 lens with f4 is also gives the same optical quality as of 70-200 f2.8. Only difference is their massive price difference and the wider aperture, everything remains same. So, unless you take lot of headshots (for which you need wider aperture), you can easily get away with cheaper f4 70-200 lens.
When choosing between prime and zoom lens, you need decide two things. First, how much mobile you can be because, with prime you cannot change the composition or frame with your fingers, you need to use your feet and actually need to move around a lot. With zoom lens, it’s very much easier that all you have to do is just turn the zoom ring in your lens until you get a desired composition. Benefit of using Prime is its larger aperture and its price – Prime lenses have greater optical quality than any zoom lens and its price is also less. Personally I prefer zoom lens and my favourite is 70-200. It helps me to do everything from close-up to full length without moving around much and its quality is also great.
There are many other things go into consideration before buying a lens and choosing one for the shoot. But, for me above things are most important not because other things are not important. The idea is that if you buy a flagship lens of a brand, you can be rest assured everything will be there. In fact, most of the time, I go by this logic because if you decide to learn all the features in a product, it can really be overwhelming, just stick to what you need and focus on a good brand and popular product.