Possible reasons to "need to update & upgrade" from D5100 to D7500...
- Wanting to use an AF-P lens. These affordable lenses... such as the $300 AF-P 10-20mm VR... aren't supported by the D5100. A newer camera model is needed.
- Planning to shoot more challenging subjects such as sports or wildlife, where a faster frame rate (8 fps vs 4 fps) and a more advanced AF system (51-point vs 11-point) would be helpful. D5100 has a single higher performance "cross type" AF point at the center... D7500 has 15 cross type points scattered around the array.
- WiFi and higher quality video wanted.
- Touch screen controls on the newer camera's slightly larger, higher resolution LCD screen.
- Higher image sensor resolution (21MP vs 16MP)
- Slightly more dynamic range and color bit depth.
- Higher usable ISO for low light shooting.
- Larger battery and approx. 50% more shots per charge.
- A true pentaprism viewfinder that's larger and brighter than the D5100's penta-mirror viewfinder.
- Higher specification shutter with 1/8000 top shutter speed and 1/250 flash sync (versus 1/4000 and 1/200).
- Rated for 50% longer life span (150,000 "clicks" vs 100K) and has add'l sealing for dust/weather resistance.
For what you shoot, the D7200 might be an option too. Compared to D7500 it has higher resolution (24MP), a little more dynamic range and color bit depth, slightly higher usable ISO. It also has dual SD memory card slots, as well as an available Nikon (OEM) battery grip (n/a for BOTH D5100 and D7500!). D7200 DOES NOT have the articulated LCD screen you prefer. Also, although it's an older, now discontinued model, refurbished D7200 are less common and are selling for about $100 more than refurbished D7500.
I don't agree with the recommendation to "go full frame". That's actually bad advice for a lot of people. It can often lead to greater expense from having to replace smaller, lighter less expensive DX lenses with bigger, heavier, more expensive FX lenses. Plus an FX camera simply isn't needed for a lot of peoples' "real world" uses.
I do agree that "better glass" typically brings greater rewards than camera upgrades. A higher quality lens or a lens that expands your capabilities will make for more noticeable differences in your images. Hard to say in your case, since we don't know what lenses you're using now.
Yes, it's GAS.Not always, but sometimes GAS happens for good reason! (Every time I have a midnight snack from Taco Bell!
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