Should You Buy a Top End Camera Lens?
There is rarely a single reason to buy a high end lens. It's usually a bunch of reasons, that when added up, justify dropping some big bucks on an expensive piece of glass. On average, a top end lens is going to be north of $1,500, while an entry level lens can be had for around $500. Perhaps even cheaper if you buy it used.
But make no mistake about it, a more expensive lens does not make you a better photographer. That comes down to composition, subject and choosing the right focal length which have nothing to do with the quality of the glass. Yes, it can make your images better, but you might be surprised to find out they won't be shockingly better under most circumstances.
As an example, one of the best nighttime astrophotography lenses is the manual focus Rokinon 14mm F2.8 that can be found for under $500. It performs better than most high end lenses that cost 4x as much.
Anyhow, below are some important questions you should ask yourself before dropping serious money on a lens.
How much time do you spend doing Photography?
If you're just kinda into photography (be honest with yourself), this should be a sign that you may not be ready to buy the higher end lenses. Especially if you only go out and shoot once every month or two. This is a clear sign your interest is probably below average in the hobby and you might want to reconsider. A lens that lives in your camera bag has great bragging rights, but little practicality.
Are you going to publish your photos on the web, or make large prints?
If you're not selling your photos online, nor printing them on paper greater than 8x10, there is almost no chance anyone is going to notice a difference between a top end lens or an entry level lens. There simply isn't enough resolution to bring out the imperfections. Definitely a reason not to upgrade. However, if you are publishing online, making large prints, selling to clients, and you notice shortcomings with your current lens (flare, soft edges, distortion that can't be corrected post-processing), then an upgrade may be warranted.
Are you currently frustrated with your current lens image quality?
If you don't notice any issue with your current lens(es), then why upgrade? For example, if you don't notice your lens is soft in the corners, has horrible coma, produces distortion that cannot be corrected in post-processing, the colors are off, or has an auto-focus that misses regularly causing you to miss shots, then why upgrade? Don't just look at online reviews and think your lens isn't good enough because the pixel-peepers say so in a controlled lab. Better gear doesn't necessarily make your photos look better. Again, virtually no one will be able to tell the difference, except maybe you or a potential client.
What Camera Body are you using now?
The higher 30+ megapixel DSLR bodies may benefit from a better lens. Some of the entry level lenses just don't have the optics to match the higher resolution photo sensors of the newer camera bodies. If you have a high megapixel camera, you could run into image quality problems. However, if you're using DSLR that's around 20 megapixels, it's unlikely you'd see a difference due to resolution issues.
Do you mostly shoot in JPEG? Or RAW?
Most people shoot JPEG, because they don't want to do the post-processing themselves and would rather have the camera do it. That's not to say the JPEG format is somehow inferior to RAW. Some professionals shoot JPEG. But when post-processing a RAW photo in Lightroom or Photoshop, there is a greater chance you will notice the imperfections of a lower quality lens because you'll inevitably see "something" you don't like while editing, such as out of focus corners, inconsistent focusing across the image, chromatic aberrations, etc. This isn't a reason to upgrade, but rather just something else to think about.
Here's my story ...
My first EF lens was the Canon 28-85mm f/3.5-4.5 USM lens paired with the Rebel XTi, and I thought for years it was great. I only printed at most 8x10 for family and rarely put anything on the web. When I upgraded to the Canon 6D (full-frame), got more serious, started shooting in RAW and post-processing myself in Lightroom, I noticed the lens was producing very, very, very soft corners. The overall sharpness was also very poor. I also started printing larger landscape photos at 11x17 for around the house, and the imperfections were quite obvious.
I started devoting a lot more time to the hobby and was now shooting a couple times a month, and sometimes during the week. Every time I can home and started post-processing I would get frustrated at how poor they looked. I was actually losing interest because I knew if I went out and got a great shot, it was probably going to look bad when I got home. I was also starting to sell photos on Shutterstock and Dreamstime and I could see this old entry level lens was going to be a problem.
It was time to upgrade ...
I ended up buying the Canon 16-35mm f/2.8L USM II and also upgraded my camera body. I was mostly shooting landscapes, but also wanted to do some night time photography. This lens paired with my new Canon 6D was leaps and bounds better than the Rebel XTi and the Canon 28-85mm f/3.5-4.5 USM.
A few years later, I found myself in a similar situation where I just wasn't happy with the results I was getting with the newer expensive lens. Monitor resolutions increased to 4K, I was now printing at 20x30, and even larger. The 16-35mm f/2.8L USM II lens was very soft in the corners and produced a lot of coma when shooting the stars. The Canon 6D produced a ton of "red" noise when trying to lift the shadows, and the auto-focusing system was quite out-dated. So after a few years, I again upgraded to the Canon 5D Mark IV and traded in for the Canon 16-35mm f/2.8L USM III (third iteration of the lens). Despite the pronoucned vignetting of the Mark III lens, I'm happy with what I have now, but I know some day I again will want to upgrade. And as long as I have a reason to, I will.
What's the point to all this? There is no single reason to upgrade and a more expensive lens won't necessarily make your photos any better, unless you've reached the limits of your existing lens. A better wide-angle lens isn't going to help your composition. It's not going to help you frame the shot. It's not going to look any better at 8x10 or even at 11x17.
For example, I'm using a $700 manual focus Rokinon SP 14mm f/2.4 lens that costs 1/4 the price of my L-series Canon Mark III. It is most definitely and entry level lens, but it works fantastic for astrophotography. Upgrading to a better quality lens won't make the photos any better, in fact, it could make them worse due to shortcomings of the more expensive lens.
I also have the Sigma 50mm ART lens. This is perhaps one of my favorite lenses for people and landscapes. It's incredibly sharp, which makes it great for 20x30 prints (or larger).
The other lens I have is the Canon 100-400mm f/4.5-5.6L USM IS II. This is another great lens for portraits, but also wildlife. The image stabilization is a huge benefit and the lens is sharp.
Whatever you do, don't upgrade because you think it'll make you a better photographer. Upgrade when you have a very compelling reason to improve the image quality for that focal length.