What focal length is suitable for shooting flowers and plants with SLR? My shot is 50~ 135. Why is it that sometimes if you focus too close, it will still ring inside the lens?

Each lens has its own closest focal length and maximum magnification. Too close is out of focus, and the sound inside the lens is that the focus motor is "pulling the bellows"

50- 135 should be Pentax DA*50- 135/2.8. Nice lens, but not a macro head. The recent focusing distance is relatively large, and the maximum magnification is relatively small.

Usually, it is more suitable to shoot flowers, plants and insects with a macro lens with a magnification of 1: 1.

Pentax has its own 35mm bathroom, 50μ m and 100μ m ... In addition, we can choose the auxiliary factory Longteng 90μ m, Sigma 105μ m and 180μ m (this is especially suitable for taking close-ups of ants and other insects).

If you are not short of money, you can choose the original new Budweiser, also called Diewei (DFA 100), and the 90-micro SP90 that pursues cost performance is also good.