I had a Nikon 7100 and a 70-300mm lens. Upgraded to a Nikon d750, and want to upgrade my lens to one that will allow me to take photos at comparable distances. (Going full frame decreases my effective "reach".) I bought the 24-120mm 1:4G ED, but that won't work for wildlife. And although technically my old 70-300mm "works" on the d750, it seems so much less effective. Suggestions?
To get the equivalent FoV ("reach") on full frame that a 70-300 has on 1.5 crop would require a (70-300)*1.5 = 105-450mm lens. This will probably be at least twice as expensive as a 70-300, and will definitely be larger and more difficult to handle. Make sure you're ok with the possibility of relearning technique or adding stabilization--and possibly rent--before going there. Your likeliest bet is going to be the AF-S Nikkor 80-400 VR, but most likely you'll end up landing on one of the Sigma supertele zoom lenses, instead: e.g., the 120-400 OS HSM or 50-500 OS HSM, on cost.
To get the same reach as the long end of your 70-300mm lens on an APS-C body, you need a lens with a focal length of 450mm on the D750. To get any Nikon lens with that kind of focal length at f/5.6 or wider requires a substantial expenditure compared to what you paid for the 70-300. The Nikon 500mm f/4 sells for about $7,900 new. The 400mm f/2.8 runs a little over $10K!
Closer to the concept and image quality of your 70-300, the Nikon 80-400mm f/4.5-5.6G ED VR sells for around $2,700.
If you're willing to go third party the latest offerings from Sigma and Tamron are much better than their older counterparts. The Tamron 150-600mm f/5-6.3 is well-liked by several of my friends who shoot birds in bright daylight.
When you took the photo with a 300mm length, it got cropped on a crop sensor. You didn't actually get a 450mm photo, but still a 300mm cropped photo.
With full frame, it will still take the same photo but without any cropping. If you crop it in photoshop then you will get the exact same photo as your crop camera... you are basically not missing out on any details with full frame.
Or buy the 28-300, it's $1,000, but will get you that reach.
Somewhat ironic, but the FX 750 will get you less resolution over the same area. Even with my 16MP DX D7000 that I owned previous, I got more pixels per square inch at the same focal length than with my D750.
Or buy a new D7200 body to re-use your 70-300.
You still get the full 24-megapixels for a longer reach, and the 2nd body is still cheaper than a longer full-frame telephoto lens.
A D7200 + 70-300 maybe lighter and more compact than a stand-alone full frame telephoto lens longer than 300mm.
I have a D610 and I'm beginning to realize this.
The answer here is simple - if you liked the 70-300 on the D7100, just keep using it on the D610.
The 70-300 is not a sharp lens. You physically get a 24MP image out of the D7100, but it is completely limited by the softness of the lens, especially at 300mm.
If you shoot in crop mode on the D610, or just crop in post, you won't have any less detail in the final image than you would have had on the D7100. You'll have slightly fewer megapixels, but the lens simply isn't sharp enough for those megapixels to matter.
See below the perceptual mexapixel map for the 70-300 mounted on the D7100 vs the D610. It's immediately clear that, especially at the long end, the sharpness just falls right off.
You get more apparent reach with the D7100, but because of the smaller sensor you are blowing up the imperfections in the lens and sampling them at a locally higher resolution. The weaknesses of FX lenses on DX bodies are "magnified" (ie: sampled at higher resolution), in a sense, along with the rest of the image.
For midrange glass like the 70-300, you really gain nothing by putting it on a crop body. The illusion of more reach is just that - an illusion. You don't get any more detail further away, you just get a smaller frame. Just crop your D610 shots and be done with it - trust me, your results won't be any worse than they were on the D7100. (I own this 70-300 as well as a D7100 and D610 - I've done the test : this isn't just theory, I assure you it's the truth).
In the meantime, save up for a Tamron 150-600 (as Michael Clark suggested) - if you like to shoot wildlife and don't have a huge budget, this is probably the lens you'll want next.