I have some tiny seeds on a plate that I want to take pictures of, and I want them to appear big in pictures so I can identify them at the same time, covering the whole or at least half of the plate. I can put them under the microscope and get pictures of them, but my field of view gets very small, and I can only take pictures of hardly one square cm. Is there any way I could do it with the phone camera? I have tried Keywing's 20x macro lens and other lenses, but you need to be at a distance of 1 inch, and it covers a very small portion of my plate that contains seeds. I have attached a picture of a plate containing seeds (this is a full zoomed-in picture from an iPhone 8, even though bigger seeds are recognizable, but the smaller darker seeds are different types that you can not tell from this picture) and a picture of how I see them under a microscope (I was not able to add the picture). I don't need microscope-level magnification, but enough that I can tell small seeds apart.
You cannot have high magnification and large field of view simultaneously... so what you want is a compromise. And that's not something add-on phone lenses really offer.
You can have a higher resolution with a wider field of view, which allows zooming in more to see more detail. But that requires lenses better than generally available for a phone, and more importantly sensors that are not overly diffraction limited. Most cell phone cameras (like your iPhone 8) are operating at/very near their diffraction limit, even at max aperture.
So a phone camera isn't really a great choice for this. You might be able to do better with a more current phone with a higher resolution camera, more zoom, and/or macro function (i.e. iPhone 13 or 15). But a compact camera with macro function may still be the better/cheaper choice (i.e. olympus TG series)
On Amazon, order "close focusing monocular, about 25 dollars. This monocular will provide magnified views of the seeds. Adjust the view you see through the monocular that satisfy your needs. We are talking zoom amount and focus. Now place the eyepiece of the monocular over the lens of phone camera. Take a few shots, within a few minutes of practice you will be taking good magnified images of these seeds. This technique is called "afocal photography".
As an alternative, purchase a 10X Hasting Triplet jewelers loupe. This is a pocket magnifier thirty dollars. Place the loupe over the phone camera lens and compose and shoot.
The Hasting Triplet Loupe functions as a super close-up lens. This works flawless.
Try a scanner:
If it's really just about having an image of these seeds (for documenting them etc.) and getting the focal length right for all of them, then the easiest way to get really nice high-res images is with a scanner.
Leave the lid open and use in a dim room to get a nice pitch black background; close the lid if you want a white background OR put some paper on the seeds so as not to crush them.
I accidentally discovered this technique in high school (more than twenty years ago) -- you can do some really creative stuff with it. But I recently heard somebody using a scanner on the Frames photography podcast. Can't remember the artist's name of episode though.
Edit: Found the artist. The name is Robert Schultz.