My way is/was quite simple. I started out with a relatively cheap used "super zoom" - A Sigma 18-250mm Macro. I knew it would be too dark of a lens for my tastes, but that wasn't the point. I shot for 6 months... several thousand pictures. That timeframe also included a vacation (a cruise with several offshore excursions - both the cruise and the excursions used telephoto lengths that I don't often use in day-to-day shooting).
After 6 months, I looked at the focal length metadata and image counts in LR. I threw out the 18mm (because presumably I was looking to go even wider) and the 250mm (because presumably I was looking to go more telephoto).
I then made a spreadsheet listing from that data listing the image counts and calculating the % of the total shots taken. That gave me a good starting point to figure out what I need.
Note that the ranges will differ for every user/photographer, but here's my breakdown:
- Nearly 70% of my images fell into the 46-90mm range. An important note here is that the distribution fell almost evenly on each side of 70mm.
- I don't think it's an accident that 70mm is where Canon chose to split the focal lengths of their most popular zooms (forcing the average enthusiast to buy 2 expensive lenses), but constantly carrying two heavy lenses and missing shots due to non-stop lens switching doesn't work for me.
- An additional 9% fall into the 91mm-150mm range, with 5% of them under 110mm.
- About 8% are in the 19mm-45mm range.
- About 5% are macro.
- The remainder (18mm, 250mm, and the 151mm-249mm) are less than 5%.
This gave me a pretty good purchasing strategy:
- Looking through the images, I realized I needed a flash first. Helloooo 600EX-RT and 8x12 softbox! Purchased.
- Get the Sigma (better than the Canon) 24-105mm. It covers 83% (70% + 5%(under 110mm) + 8%) of my shooting. I wish it was f/2.8 instead of f/4, but it's a darn good lens and has IS. Purchased.
- Get a Sigma (non-art) or Canon 50mm f/1.4. Great bang for the buck, and I LOVE the 50mm. It never comes off my other camera and 25% of my shots were between 46mm-62mm. It makes sense to have a good prime here. Purchased.
Note: I still want/need to purchase the below, but truthfully, the only thing I really wish I had at this point is the full macro setup. That's right, 3 lenses and I'd be a VERY happy camper. 5 (add the 20mm and 85mm) and I could not find a *valid* excuse to buy anything more.
- Most of my occasional (vacation) telephoto needs can be handled by a 1.4x teleconverter. They are cheap and light weight. Ideally, from a photographic perspective, I'd probably have a 70-200mm f/2.8 with a 2x teleconverter. Realistically, this is mostly needed for vacations, and there is no way I'm carrying that beast on all day walks, especially if those walks are hikes up a mountain!
- Most occasional (ooh, look, pretty flower!) macro work can be handled with extension tubes. Again, cheap and lightweight.
- I'd like to get something along the lines of a 20mm prime for certain types of shooting. It makes up such a small percentage of what I do that I can be patient about this one and buy used/cheap when the right deal comes along and there's money burning a hole in my pocket.
- I didn't do much of it, but I LOVE extremely close macro. I really want a Canon MP-E 65mm f/2.8 1-5x lens. Saving for the ($1k) lens isn't bad, but there's a whole slew of stuff that goes with it (Manfrotto 190 CF tripod, Manfrotto 401 geared head, RRS focusing rails, Canon twin and/or <brand?) ring lights, ad nauseum.. This isn't just a $1k lens, it's a $4k endeavor, so save it for last.
- There are always going to be times that we wish for a little less or little more focal length. Deal with it. Something to keep in mind: Missing wide-angle is harder to deal with than missing telephoto. You can crop and zoom to make up for missing telephoto length, but if you don't have the wide angle, you just don't have it (though photo-stitching can help).
- If I was to buy anything else after this, it would likely be an 85mm prime as about 32% (IIRC) of my shooting was in the 80-90mm range).
So, that's my way of figuring out what makes sense and the conclusions I came to *for my type of shooting*.
Now, if you REALLY want to drive yourself crazy, go back and rate all of those photos, determine which of your best shots were taken at each focal length and cross reference to the above. I tried, and I strongly advise against it.