Discussions I have read on using a spotting scope on a DSLR have usually pointed out that the lenses are not as sharp as an OEM camera lens on the camera.The last one I read was on a scope which cost a lot more than the one you are showing. In the centre of the lens, the resolution is best, so for a smart phone it would probably be a good choice to get much closer to your subject. Since it comes with all of the adapters necessary, you could try it out and return it to Amazon if you are not satisfied with the results, so you have nothing to lose except some time, and you would have a usable scope for bird watching to boot!

Today I found this on Amazon, forgive me if you think I am advertising, I am not but just finger itching.
https://www.amazon.com/Gosky-Updated-Newest-Spotting-Carrying/dp/B07FMT95B5/ref=cm_cr_arp_d_product_top?ie=UTF8
I find this very nice and useful, because it can be used on smart phones and Nikons.
I have a D610 and my questions are:
1. Does anyone have experience with this kind of monoscopes on smartphones or Dslrs?
2. Can this thing do better job than my Nikkor(300mm, 4.5 manual) or similar? I also have a Tamron(200-400mm 5.6 auto)
3. Is this a good thing to own or you find other brands more versatile?

I've been using Zenfolio to sell prints, digital files and other printed products to customers for two or three years.

"300 ppi" is not necessary for most products they sell. It is recommended for a few. But some products can be printed with as low as 100 ppi, though 200 ppi or 250 ppi is recommended.

For more info, see this link: https://www.zenfolio.com/us/z/help/support-center#article/408060-having-sufficient-image-size-for-printing

(You will probably need to sign in to your Zenfolio account before you can access that link.)

For display in your galleries Zenfolio asks for a minimum file size of 1100 x 850 pixels, while their maximum is 1500 x 960 pixels. These files will be what's displayed in your galleries for customers to view. They are usually presented in a "grid" of smaller thumbnails, which the customer can click on to view in a larger size.

I don't provide Zenfolio with anywhere near that large "gallery" files to make the thumbnails and previews. I use Lightroom to batch process and have it set to 800 pixels no the long side. So an image with a 3:2 aspect ratio will end up about 534 x 800 pixels... and an image where I've cropped to a 5:4 aspect ratio will end up 800 x 640 pixels.

I DO NOT upload a finished, full size file to Zenfolio until an order has been placed. Until you know the size print or other product that the customer orders, any final finishing would likely just be a waste of time. I may upload a scaled down file of a "straight out of camera" shot that has a 3:2 aspect ratio, but the customer orders a 5x5" print (7:5 aspect ratio), an 8x10 print (5:4 aspect ratio) or an 11x14 print {14:11 aspect ratio}. Printable products such as coffee mugs, mouse pads, etc., etc. each have their own aspect ratios.

When I get an order, I go back to the image and "finish" it, per the customer's request. If they have chosen an 8x10" print, for example, I will crop the image (if needed) and size it to 2400 x 3000 pixels (note that both are the same 5:4 aspect ratio).

Yes, that's 300 ppi. I use that in nearly all cases just for sake of simplicity (the math is easy, I can do it quickly in my head) and because I know it's "enough or more than enough" resolution for any product offered through Zenfolio. I don't have to go check the recommended resolutions at the above link, each time a print or whatever is ordered.

Once an image has been finished for the customer's order, I upload it to a "hidden" gallery on Zenfolio. I then go into the order and tell it to use this finished version for printing purposes, rather than printing the smaller, lower resolution thumbnail.

You can upload larger files if you wish and leave the cropping, finishing to Zenfolio. In many cases that might not be to your preferences.

Your example image would need to be cropped and resized to print it. It's a weird aspect ratio (maybe because it was shot with a camera phone?) and I don't think it conforms with any of the standard print sizes on Zenfolio. While it would be possible to print it on oversize paper, that would also make it difficult for the customer to mat and frame it. They will need to go to custom matting and framing for odd size, which is VERY expensive (I can put a museum quality mat and frame on a standard size like 11x14 for under $50... to have the same print custom matted and framed would likely cost $200 or more).

It's best to instead deal with this up front and make the image fit the customer's requested size. Occasionally this isn't possible and I've had to consult with the customer and arrange with them to change the size. To minimize problems, I offer prints similarly or identically priced in two of the most common aspect ratios: 3:2 and 5:4. For example, I offer both 8x12" prints and 8x10" prints at the same price.

More often than not, though, I can simply adjust the image to meet the customer's requested size. For example, your image of the goose and gazebo could pretty easily be made to work for a 5:4 aspect ratio print size simply by adding a narrow strip of grass across the bottom (expanding the top... tree line and sky... is also possible, but would be more difficult). This would accommodate 8x10", 16x20" and some other fairly standard print sizes and is pretty easily done in most post-processing software). As a final step, you would downsize the image slightly to 2400 x 3000 pixels for that 8x10... Or resample and upsize it to 4800 x 6000 pixels for the 16x20 print.

It's possible also to make your image "fit" a 3:2 aspect ratio such as 8x12", 16x24". In this case you would only need to crop, unless you felt the gazebo on the right were a little to close to the edge and wanted to add some to the image there. The final size to make an 8x12" print would be 2400 x 3600 pixels... Or 4800 x 7200 pixels for a 16x24". (Note: These particular sizes are less commonly found in ready-made mats and frames, might be less to choose from or more expensive custom matting to a larger standard size frame or custom framing in it's entirety might be necessary.)

Some of the other common sizes that don't quite conform to those aspect ratios... such as 5x7" (similar to 3:2) and 11x14" (similar to 5:4)... are close enough that they can be pretty easily done using the same techniques.

Most inkjet printer manufacturers recommend 240 ppi resolution for highest quality prints. There's little to be gained by going higher than that. Although I use 300 ppi for inkjet prints I do myself, too... for sake of simplicity.

Don't confuse image resolution (ppi) with printer resolution (dpi). Most printers work at resolutions such as 720 dpi or 1440 dpi. 720 dpi essentially means the printer is using three "dots" to each pixel (240 x 3 = 720). 1440 dpi would mean the printer is laying down 6 dots per pixel in the image (240 x 3 = 1440). Most inkjets also work fine with 8-bit JPEGs, too.

If you use a slightly higher image resolution such as 300 ppi, it may slow things a little, but make little difference. But if you go to another file type at much higher resolution... such as a 16-bit TIFF at 720 ppi, it will likely take much longer to print and use more ink, although you're likely to see little to no difference in the final image.

There are exceptions. I recall a Fuji printing process that recommended something like 420 ppi images. Check with the printing service, such as the link above to Zenfolio's guidance.

Finally, notice that with digital downloads, if you decide to sell those via Zenfolio, there are two sizes... basically small, low resolution and high resolution. I usually just crop those either 3:2 or 5:4 whichever looks best. Note that there are minimum size requirements of those, in megapixels. So just do the math. I find something around 5x7.5" size (at 300 ppi, that's about 1500 x 2100 pixels) works for the smaller downloads, while for the larger download it's more like 10x15" (3000 x 4500 pixels at 300 ppi). Those are both 3:2 aspect ratio. Adjust accordingly for 5:4 or any other aspect ratio you might choose to use.