The AF-P NIKKOR 70-300mm f/4.5-5.6E ED VR is an advanced lens that draws peak performance from Nikon's latest generation of FX and DX cameras. Its versatile 70–300mm focal length range (105–450mm on DX cameras) is great for everything from portraits to close-ups of nature and wildlife.
The Nikon D3100 has got external dimensions of 124mm x 96mm x 75mm (4.9" x 3.8" x 3.0") along with a weight of 505 grams (1.11 lbs) while the Pentax K-50 has sizing of 130mm x 97mm x 71mm (5.1" x 3.8" x 2.8") along with a weight of 650 grams (1.43 lbs). Take a look at the Nikon D3100 versus Pentax K-50 in the new Camera and Lens Size Comparison
Lens mount: Nikon F: Focal length mult. 1.5 ×: Articulated LCD: Fixed: Screen size: 3 ″ Screen dots: 230,000: Max shutter speed: 1/4000 sec: Format: MPEG-4: Storage types: SD/SDHC/SDXC: USB: USB 2.0 (480 Mbit/sec) Weight (inc. batteries) 505 g (1.11 lb / 17.81 oz) Dimensions: 124 x 96 x 75 mm (4.88 x 3.78 x 2.95 ″) GPS: Optional The Nikon D3100 is 30 grams lighter than its predecessor, the Nikon D3000, with a weight of only 455 grams without the lens, making it the lightest Nikon DSLR camera. The Nikon D3100 is largely made of plastic, with the exception of the metal lens mount. But the 70-300 f/4-f/5.6 is listed as an AF lens, meaning that it needs a camera with a motor for autofocus. So that lens won't autofocus with a D3100. But all the exposure modes will work. The newer 70-300 f/4.5-f/5.6 AF-S lens will work with all Nikon DSLR's and is a much better lens besides. -- Leonard Migliore. Features Nikon's 14.2- megapixel Nikon DX format CMOS sensor with 12 Bit Resolution. Nikon EXPEED 2 image processor. Active D-Lighting. Automatic chromatic aberration correction. Sensor cleaning and airflow control system. 3.0-inch 230,000-dot resolution fixed TFT LCD Continuous Drive up to 3 frames per second. Live view mode. jdLnXP.