What is the specific use of the lens for. Most Full Frame lenses aren't cheap. The best option is used at this price point.
What lenses do you already own or have used in the past?
One exercise I did years ago when first moving to a full frame camera (the EOS 6D in fact), was to look at all my captured photos and see what was the most-used focal lengths. Close to 90% of the time the field-of-view equivalent was 50mm. Thus, I picked up the EF 50mm f/1.2 L and used that as my only lens for a few years. I went for the f/1.2 since a common theme amongst my personal favorite photos was shallow depth of field.
If you find you also prefer a 50mm, then the EF 50mm f/1.8 is very inexpensive. Though I'd recommend the better build quality of the EF 50mm f/1.4.
Also, what specifically do capture photos of? A single lens will most likely be very limiting. But if you plan to later add additional lenses, then the above excercise can help you understand the best initial lens to get.
This is easy. The lens of choice is the ef 24-70mm f/2.8l ii usm. The hard part is the $800 dollar limit. This means you will have to go to the used market where $800 is doable. Best and cheap don't generally go together!
If someone told me tomorrow I can have only one lens it would be the ef 24-70mm f/2.8l ii usm without question or doubt. There is no better lens!
As the title says, I’m looking for the best lens I can possibly get for under $800 to go on my 6d. It would be my only lens. Needs to be fast but don’t care too much about portability.
What is your definition of “best”? Yeah, I know. You said that you’re looking for an all around lens. Even that is wide open and is subject to interpretation, as evidenced by the variety of responses that you have received so far. In my mind, the best lenses for Canon DSLRs are Canon L Series of lenses, which pretty much beyond your budget.
A “standard zoom”, like the suggested EF 24-70mm, is considered to be an “all-purpose lens.” But, it is not one lens that can do everything, which is what I suspect you are looking for. Such a lens does not exist. If it did, then there would not be such a wide variety of lenses to choose from.
I find the most useful lenses I own are primes, not the zooms. They tend to have wider apertures and better image quality than most zoom lenses. I typically carry one prime in my camera bag.
My favorite prime is the Sigma 35mm Art lens. My favorite zoom is the EF 70-200mm f/2.8L IS II USM. I usually carry just two lenses. When I carry two zooms it is the EF 24-70mm f/2.8L II USM and the EF 100-400mm f/4.5-5.6L IS II USM.
All the lenses that I might consider to be “the best” cost a lot more than $800. My first L series lens was the EF 24-105mm f/4L IS USM. It is much more affordable than the EF 24-70mm, and it does not perform quite as well. However, the 24-105mm is still outside your budget, not unless you buy a used one…..from a reputable used camera dealer, I might add.
To sum it up, the others have offered good suggestions and good rationale to support those choices. Again, there is not one lens that can do everything, if that is what you mean by “all purpose” lens.
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