You can't go wrong with the Sigma 16mm f/1.4 prime for low light on the a5100. That f/1.4 aperture lets in tons of light and gives beautiful background blur. The wide 16mm focal length is very versatile too. It's sharp, well built and reasonably priced around $450 for a high quality prime.

For portraits on the a5100, I highly recommend the Sony 35mm f/1.8 prime lens. It's lightweight, compact, has great sharpness and the f/1.8 aperture allows nice background blur. It's an affordable lens around $250 but punches way above its price point in image quality.

Honestly, looking at some of these picks, I have had a bit of a rough run trying to save a buck on glass for my old a5100. I once picked up a really budget-friendly portrait prime thinking I was being smart... unfortunately, it was a total letdown in the field. It was not as good as expected and basically sat in my bag after the first week. The issues I ran into:

• The autofocus was so loud it actually ruined a few quiet moments
• It hunted constantly in anything but direct sunlight, which made low light shots impossible
• The build felt like a toy and the image corners were always mushy It taught me that sometimes cheap ends up costing more when you have to replace it six months later. Before I give you any real suggestions tho, what exactly is your actual budget range? Landscapes and portraits are totally different beasts... are you looking to carry just one lens or are you okay swapping them out in the field? Knowing if you are okay with used gear would help too.

Landscapes are where the Sony 10-18mm f/4 Wide Angle Zoom lens really shines on the a5100. It gives you an ultrawide field of view perfect for sweeping vistas and architecture. The f/4 aperture is bright enough for landscape work and the optics are tack sharp. A little pricey around $850 but worth it in my opinion.

For the Sony a5100, I highly recommend the Sigma 30mm f/1.4 DC DN Contemporary lens. Some key advantages of this lens:

• Fast f/1.4 aperture: Excellent for low light shooting and achieving shallow depth of field for isolating subjects. Creates beautiful background blur.

• Sharp optics: The Sigma 30mm produces tack sharp images with nice contrast. Impressive results for the price.

• Compact size: Balances well on the small a5100 body. Easy to carry around as an everyday lens.

• Versatile focal length: 30mm on APS-C equates to about a 45mm normal field of view - useful for many types of photography like street, portraits, food, etc.

• Affordable: Significantly cheaper than Sony's own 35mm f/1.8 while providing better light gathering capability with the f/1.4 aperture.

I've been using this Sigma 30mm on my a6000 and love the results. It rarely leaves my camera. A great addition to any Sony APS-C mirrorless shooter's kit.

If you are lookin for a zoom lens, get the Sigma 18-50mm f/2.8 DC DN Contemporary Lens.

For portrait photography, get the Sigma 30mm f/1.4 DC DN C lens.

Get some 7artisans lenses, they are manual focus, but very cheap, and image quality is not bad.