What I look for in a lens.
If I’m to target my images for 4K displays, I should be able to use the Tamron 28-300mm or almost any other lens on my 6D body and have just enough sharpness across most of its focal length and aperture ranges to achieve the desired results by using the lens at, or closer to, its sweet spot to attain more sharpness. That reduces the usability of the lens for low light conditions, narrow depth of field or fast moving objects to some extent as stopping down to F8 not only has a huge depth of field, but also loses five stops of light compared to the Sigma 50mm wide open. Five stops means a longer shutter speed (1/100 becomes 1/3) causing blur or higher ISO setting (100 becomes 3200) or a mix of both (eg ISO 1600 at 1/50). If I already needed a high ISO setting and slow shutter speed before stopping down, I’d end up with an image I cannot use because it is under exposed, too noisy or too blurry. Clearly not a good choice for low light work. The Sigma would be a lot better under those conditions but would not have the versatility of the Tamrons’ focal length range.
If I’m to target my images for 8K displays, I’ll need to use the highest resolution sensor I can find for my full frame lenses. Today that is Canon’s 50MP sensor. That still only allows me to use some of my acceptable quality lenses, and only at their sweet spots assuming perfect technique. Alternatively, I’ll need to use some of the better or ideal quality lenses to achieve the desired results. Neither the sensor nor desired lenses fall within my budget today. So I guess 8K will have to wait a bit.
Putting such demands on your images does mean that the usability of various lenses is reduced to the point where you’d rather not use some of those lenses at all, or you need to follow the new trend of soft, blurry images and call it artistic fine art.
Therefore, I tend to buy only lenses that are affordable, that can achieve the results I’m looking for, and that I know I’ll use a lot. By not buying the poorer quality lenses, I save money and learn to work with what I have. If I need a particular lens for only one shoot in a year, then it is better to rent it.