Canon lens "24- 105", what is the difference between with and without red circle?

There must be a difference. You can tell from the price.

In fact, the "red circle" (L prefix) is just a slang term in the photography field where Canon uses fluorite lenses. The quality of L-head is generally high, except for the lens with "small ternary" F4 aperture constant, which is generally expensive, but the imaging quality and durability are quite reassuring.

The new 24- 105, the maximum aperture is not constant F4, but F3.5-F5.6, indicating that STM means that the lens adopts Canon's STM motor. STM can probably be understood as the speed between mechanical motor and ultrasonic motor, which is close to or equivalent to the mute of ultrasonic motor.

In fact, a shot like this can be regarded as an economic version of the L-head, which gives users one more choice. I think this is a good thing. If you need a lens with this focal length, then I think you will know how to choose it by looking at your budget.