Advice on upgrading lens - please may I request (2 Viewers)

[Earnest lad]

It is now almost five years since I obtained my first DSLR (Nikon D7100) as well as my first long lens (Sigma 150mm to 600mm). At the time I had been doing my birding with a bridge camera and wanted to go so another level. The second hand market proved useful to me for the gear as I had and have a limited budget
Being more of a birder with a camera than a photographer per se, have found my experience with the set up mentioned really good. Thousands of nice pics have been accrued.

As any birder knows, quite often the bird is often a good distance away. Even with the lens largely extended one has to crop heavily.
What I am seeking is an upgrade that can afford better quality of a bird that is a long way off. I read somewhere that the sigma lens in question has a deteriorated performance when it is fully zoomed. I cannot comment on this because it is for all intents and purposes the only lens I have had: Still some of the images I have obtained where the birds were distant have been somewhat poor. (see two recent cropped images attached) Having said all that I have been wondering whether a different lens would be conducive to better photos of that nature.
Specifically I've been thinking of the Nikkor 500mm pf .
Please does anyone have any experience to share of upgrading from a zoom lens to the Nikkor 500mm pf?
ps I do appreciate that knowledge and skill with a camera set up is important, so I continue to work on that. But I am seeking advice on the benefit of an aforementioned potential "upgrade" on the basis of "all other things being equal"
Many thanks

 

[MelospizaMelodiaFan]

That 500mm prime looks like a nice lens. Are you planning to use it with a teleconverter? Otherwise you will have to get closer to your subjects, which will help your current lens as well.

 

[Earnest lad]

That 500mm prime looks like a nice lens. Are you planning to use it with a teleconverter? Otherwise you will have to get closer to your subjects, which will help your current lens as well.

Thank you. I've tried a teleconverter with the Sigma 150-600 lens. However it seems to me the quality degrades to the extent you are just as well off it not better off cropping.
Its a good point about the insufficient reach of a 500mm. I hear a teleconverter works better with a prime lens. So maybe it would do well with that lens and a teleconverter..

 

[njlarsen]

Several years ago, a popular topic of discussion was that zoom lenses just about never reach the stated reach when used in real life, while this is less common for primes. You may therefore find that there is less difference than one would expect.
Niels

 

[Earnest lad]

Several years ago, a popular topic of discussion was that zoom lenses just about never reach the stated reach when used in real life, while this is less common for primes. You may therefore find that there is less difference than one would expect.
Niels

Thank you so much. I wasn't aware of this. It is definitely a factor to consider. Another thing is, I think: - I have recently noticed that my zoom lens, after nearly five years, has definitely gotten some dust particles inside the lens. You can see it when you look down the barrel. Perhaps the dust can get in when one is zooming in and out. There must be at least a bit of a gap. I imagine, in contrast, that a prime lens would be more hermetically sealed.

 

[njlarsen]

Thank you so much. I wasn't aware of this. It is definitely a factor to consider. Another thing is, I think: - I have recently noticed that my zoom lens, after nearly five years, has definitely gotten some dust particles inside the lens. You can see it when you look down the barrel. Perhaps the dust can get in when one is zooming in and out. There must be at least a bit of a gap. I imagine, in contrast, that a prime lens would be more hermetically sealed.

Some lenses have internal zoom and some have internal focusing movements, and some have neither. But a zoom lens that gets longer when zooming definitely must have some level of opening, though one would hope that it was tiny.
Niels

 

[Earnest lad]

Some lenses have internal zoom and some have internal focusing movements, and some have neither. But a zoom lens that gets longer when zooming definitely must have some level of opening, though one would hope that it was tiny.
Niels

I am guessing that is true because after five years there is a considerable amount of dust within the lens.