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Question: Hi, i have the G5 with the 14-42mm kit lens but i would like to know how big is the difference in light gathering between the 14-42mm and 14mm 2.5 ? Does really worth to invest $200.00 ?  Thanks i appreciate any response or advice in order to take better low light pictures.
Document: I would compare it to the quick focusing and silence of the 14-42mm non-pancake lens at 14mm. It is noticeably faster and quieter than the Panasonic 20mm. The 14mm also supports continuous AF (AFC) and flexible AF (AFF) which the 20mm does not when shooting pictures.With an f/stop of 2.5, the 14mm lets in twice as much light as the 14-42mm kit lens at 14mm. The kit lens has stabilization whereas the 14mm pancake does not.
There is very noticeable chromatic aberration on Olympus bodies in high contrast areas like tree leaves and branches. It's a very solid lens you can get for a low price. If you already have a good kit lens in 14mm range, it may not be worth it unless you really like it's super small size and f2.5 aperture.
The lens is perfect for environment photos or group shots and features a sharp image quality. While the max aperture of 2.5 is decent, bokeh (or blurring of the background) only occurs with close up shots. Shooting in low light environments can be challenging and obviously a faster lens like the 12mm f2.0 and 20mm f1.7 are better in this regard.The best part is it's affordable price.
I was never pleased with the image quality (IQ) of the 14-42mm kit lens, good center sharpness but very soft edges and corners, and was hesitant on purchasing the 14mm pancake since some reviewers said its IQ was only on par with the kit zoom lens. Doing an actual IQ comparison test between the 2 lenses on a G5, the 14mm pancake produced noticeably sharper images than the 14-42mm kit lens.
There's currently a lens shortage, so I decided to get this 14mm/2.5 while I'm waiting for a 20mm/1.7 to be available from Amazon. I can only compare this lens with the Olympus 14-42 kit lens that came with my E-P1, and I have to say it's definitely worth the purchase. The sharpness and color are noticeably superior compared to the kit lens, and bokeh is pretty nice. I love how compact this makes the E-P1.
The 28mm equivalent view is great. I love being able to take pictures indoors of large rooms and capture what's going on. As some of the reviewers have mentioned, this lens is very soft at f2.5 but performs well at f4. I think shooting at f4 and increasing the ISO makes shots turn out better, especially in lower light conditions.
Quick Summary:* Unbelievably compact and small* 14mm wide focal length (equivalent to 28mm)* Minimal bokeh and low light capability with f/2.5* Affordable new and SUPER affordable if you buy new from Amazon/Ebay that was part of a kit package (~$180)Detailed Review:The Panasonic 14mm wide angle prime has all the characteristics that fans of Micro Four Thirds love.
When paying quite a lot of money for a lens that shoots at only one focal length, and has the rather un-aggressive maximum aperture of only f/2.5 (not wide enough to get much of a blurred background when taking photos of people), one at least expects the lens to have good optical qualities, right?However, this lens is noticeably less sharp than the Lumix G Vario 14-42mm I kit lens.
I recently bought the 14mm pancake used from Amazon for around $250. I've had a couple weeks to play with it, though it didn't take that long to fall in love with.I'm just starting my first system with m43 and the Olympus E-PM1. After getting used to the kit lens,  it's nice to use this prime with very quick AF. There is also quite a difference in IQ from the Olympus kit lens, even to a novice photographer like myself.
Stopping the 14mm pancake down to f/4 and f/5.6 the edges and corners become crisper producing much better sharpness in the edges and corners than the 14-42mm. In the IQ comparison test I also noticed a slight improvement in the center sharpness with the 14mm pancake across the respective f/stops. While the 14mm pancake did produce noticeably sharper images than the 14-42mm I wouldn't say at any point the sharpness along the edges or corners was outstanding.
Answer: The 14mm f/2.5 will give you a little less than a stop of difference, which is quite a bit, it will mean on it's widest setting, you can gather light almost twice as fast as with the 14-42mm.  But where the real difference comes in is the sharpness of the lens.  A prime lens is always going to be sharper than a zoom lens.  It is also a tiny lens, making the camera ultra portable.