It’s hard to fault the excellent Sony 100-400 but as a longtime pro I’m at times reluctant to take a 3 lb. lens on the road or hike long back country treks with it. I’ve been seeking a lighter long tele zoom option and have compared four alternatives in depth. Let’s discuss in detail starting with the best in class Sony 100-400 f 4.5-5.6 that costs $2,500, out of the price range of most. Can others compete? (See photo comparing lens sizes) Read on:SONY 100-400mm f4.5-5.6 GMI specialize in tech reviews and am well known in the photo world. I have regularly used the finest glass from Canon, Nikon and Sony including 300mm f2.8, 400mm f2.8 and 500mm f4 lenses. I’ve had this Sony for a year and think it’s a great lens. So why look elsewhere? Simply said, weight! At 3+ pounds, it can be fatiguing although optically it is the best lens in this group. I’m not selling the Sony but went on the hunt for a lighter long zoom to carry in the outback, or to pack for shooting trips.SIGMA 100-400mm f5-6.3 DG DN OSAt 38% the cost of the Sony, this is a marvelous lens! After testing four alternatives to the heavy Sony, I am enthusiastically keeping this one, too. Let’s talk its strengths and a few weaknesses you may have read about: Optically, it gives s 95% of the quality, again at 38% of the cost. I use Sony’s most demanding body to compare optical quality, the 61 MP A7rIV. Although I don’t bother with pixel peeping when getting down to the joy of seeking great shooting situations, when buying a lens it’s wise. The Sigma goes head to head with the Sony in focusing speed and tracking. You may have read some reviews of pre-release lenses that slightly trailed the Sony. The firmware has been updated. This lens tracks well. As a pro whose reputation rests on getting THE shot, I’m confident in it.In terms of “speed” the Sony is 1/3 of an f-stop faster. That’s only the difference between choosing an ISO of 1000 vs 800. Today’s mirrorless cameras are superb with higher ISO shooting and for the first time you should feel confident shooting at 3200 ISO and even higher. I recommend the superb DeepPRIME software from DxO for noise reduction. It’s the best but Topaz Denoise is also quite good.Both Sony and Sigma has excellent image stabilization. I have learned that in body stabilization (IBIS) is much better for wider lenses. For long telephotos like these, you’ll need it for a higher hit rate at shutter speeds under 1/250.Now for the biggest Sigma selling point. It weights 2.5 pounds versus 3.4 for the Sony. Yes, you will realize this after only five minutes carrying it. For a lens with 400mm reach this is excellent. It's also thinner and easier to stow in a camera bag without a separate sling bag, depending on your shooting style. Thus, I’m happily keeping this razor sharp and comfortable to carry Sigma. Let’s explore other good options, too:TAMRON 70-300mm f5-6.3 RXDIn short an absolutely great lens at a fantastic price, if you can live with the shortcomings. I’ll include a link below to my review: The Tamron weighs mere 19 ounces at 20% the cost of the Sony. It’s reach is only 300mm but that’s enough for many. On the downside, it’s a lens only for bright light. With no image stabilization built into the lens, it does not compete for highest image quality when you must use a lower shutter speed. In good light, it exhibits terrific and sharp optics, indeed. Here’s a link to my review of the not quite as well-built Tamron but still recommended Tamron:https://www.amazon.com/review/R2SD7APT0NVX1G/ref=cm_cr_srp_d_rdp_perm?ie=UTF8&ASIN=B08K3MXZCZSONY 70-350mm f4.5-6.3 G OSSThe Sony 70-350 is 22 ounces and with the higher weight comes has longer reach although it is slightly shorter than the Tamron. In APS-C mode this Sony gives truly impressive 105-525mm reach. Yet, I don’t recommend it for non APS-C cameras. I’ve tested two and sharpness does not compete with any of the lenses above on full frame cameras. However, for APS-C cameras, it delivers reliable images with the advantages that come with image stabilization, too, However, my keeper rate with this lens incorporating Sony OSS was not a great deal higher than the Tamron without it. Here’s my review:https://www.amazon.com/review/R15P98AHU5QXKF/ref=cm_cr_srp_d_rdp_perm?ie=UTF8&ASIN=B07X72DMLCSONY 70-300mm f4.5-5.6 G OSSSimply said, this lens has an older tech focusing motor and is not as sharp as any of the lenses above. There is nothing wrong with this lens but it is overmatched and way overpriced in today’s market. Not worth considering although I am in general a huge fan of Sony’s awesome optics. Look for a future redesign of this lens to again make it competitive.Hope this too long review has been of some small help. Keep shootin’ strong!