Manual lenses can be a lower cost alternative to those that support automatic operation.
The disadvantages are pretty obvious.
But I still prefer to manually set the camera before releasing the shutter.
Although my m4/3rds Olympus E-M10 has various automatic and semi automatic modes, including auto-focus, I much prefer to manually set exposure and adjust focusing.
A manual lens generally has no mechanical or electrical control linkage with the camera body. So both the aperture and focus must be set manually by you, the photographer.
Now that I'm able to use my old Pentax K mount lenses on my Olympus, I've rediscovered the joy of a focus control with 'distance' markings and end-stops. Compare this with my auto-focus lenses, which have no scale and just go round and around. So I tend to blindly turn the focus control hoping I am going in the right direction!
I also seem to fight with the focus control acceleration on a typical auto-focus lens, where it adjusts either quickly or slowly based upon some notion of whether you turn it fast or not.
|Olympus E-M10 + Pentax K mount 80-200mm lens via adapter|
I stopped using the Olympus view finder within a few weeks of getting the E-M10 and now pretty much exclusively use the rear display. The brightness of the image in this display varies as you adjust exposure (i.e. shutter speed, aperture and ISO).
The "focus peaking" feature (where the 'in focus' areas of the display turn red when correctly focused) does not work with manual lenses. However, I don't like focus peaking. I much prefer to press the zoom button and focus on a small area (usually x3 size).
|Olympus E-M10 + 28mm Pentax K mount lens via adapter|
Apart from my 40 year old Pentax K mount lenses, there are lots of new, low cost manual m4/3rds lenses on the market from companies/brands such as: 7artisans, Meike & Neewer. These often have good optical performance, and so may represent good value for money for those happy to forgo automatic operation.
|Olympus E-M10 + Olympus 9mm manual lens|
However, as I currently have 5 manual lenses and 2 auto lenses (plus I have access to another 3 auto lenses) I can't build much of a case to spend money on any more at the moment (...although I'm tempted by the 7artisans macro lens).
And finally, I should point out that there are plenty of photographic situations where automatic operation is a god-send (e.g. sport, children, Elvis riding by on a unicorn). However most of my photography seems to involve static scenes or slow moving objects.