Your longest focal length lens

We always want something more powerful, but then we have so many constraints which limit what we can get. To name a few: your budget, your usage pattern, the portability, etc.

Last Sunday on an event, I brought along my Canon 200mm f2.8L with me. It's an indoor event so that the faster aperture is really useful. To increase the chance of success, I used ISO 1600 of my Canon 450D for all the shots. Don't get me wrong, I can use ISO 800 or even lower, but then the chance of a successful shot reduced. And in that event, I could not use a tripod, and I need to stay around 10-20m from my targets, and they are moving. So, a 200mm lens would be about right.

After using it for around an hour, I found that it's about my limit for continuous usage. My arms are so weak, I have to admit, but it's always a hidden constraint when you build up your gears.

If you plan to use your lens for astrophotography like me, you will be further limited by your mount. The mount is nearly always the heaviest component of your whole setup, and therefore, it plays a very important part in terms of portability. Given a mount, its loading capacity defines how heavy you can go, and it also limits your effective focal length given its tracking accuracy.

For me, 200mm is also the limit for my Takahashi TG-SP II, both in terms of loading capacity and tracking accuracy for longer exposure like a few minutes. Of course, if I were to use shorter exposure like under 5 seconds, I could use a heavier lens with longer focal length.

Therefore, you should know the whole thing before buying a particular piece of equipment, this is usually even more important than the limit of your bank account.