Here's another good video by Peter Hurley (Mr. Shabang) explaining the Inverse Square Law (light falloff over distance).

http://www.youtube.com/watch?v=xO-J42VM448&ab_channel=Fstoppers

This is an oldie but goodie from Joe Edelman describing how to see light by taking pictures of an egg while moving a permanent light.
It sounds silly but it actually has some very good fundamental information to see the light (pun intended).
Joe Edelman has dozens of excellent youtube videos that are informative and very entertaining.

http://www.youtube.com/watch?v=qM7CcUrUD2g&ab_channel=JoeEdelman

The lens appears to be the Yongnuo 14mm f/2.8 with the Canon lens mount (red band).

Here also something else way more interesting on more or less the same subject. Apparently Yongnuo is in the process of releasing a smart device camera with a (yet to be specified) DSLR-type lens mount. I would love to have the ability to have my current lens collection mount to a smart device.

Bill, I am terribly sorry for forgetting to put smiley faces with my comment and I sincerely hope you can find it in your heart to forgive me! After all I grew up in a time when the only smiley faces were in the fotos my family took when we had birthdays or when we were on vacation.

I might add though that even my wife understood the humor of my much maligned post. I really have to credit her since she is considerably younger than me and part of the emoji generation. I might also add that she has absolutely no interest in photography and always rolls her eyes when I try to make her understand the (considerable!) difference between heavy and or focus breathing.

I also thought that it was really funny (insert smiley face here!) that some people described me as some sort of ignoramous or even a technophobe. 35 years as a professional (analyst) in the computer industry have certainly taught me a thing or two and I am pretty sure I have at least a grasp on where technology (even something as wildly sophisticated as a mirrorless camera!) might be going.

Oh, and in closing I would like to add that I am pretty sure that the vast majorty of the younger generation would never trade their iPhone for any type of digital camera unless it is absolutely needed for professional work.

Camera manufacturers still have not grasped that social media drives the majority of photography (and video) today and unless their attitude on that drastically changes the more or less single task digital camera will soon be going the way of the dinosaurs.

What I get from all these responses is that right now the typical enthusiast convert moving from a DSLR to a mirrorless system is an older lady/gentleman that is out of shape and frail but optimistically looking into his/her life expectancy and the future.

The latest incarnations of full frame mirrorless cameras prove me right since they are specifically targeted at the above mentioned group of users. "Look at me!" "Aren't I young and modern and pretty and sexy?" "Yes (old fart), you can still take all those technically perfect full frame pictures at half the weight (and twice the price)!" "And you won't have to worry breaking a hip lugging all your old, outdated and heavy DSLR gear!" "So come right now (you might be dead later!) and spend your hard earned retirement money on me and you too can be young and sexy again!"

The 18-200 might have been a good lens on a 12MP camera but today's 24MP DX cameras without the low pass filter (like the OP's D3400) will brutally show the flaws of any lens.
I have a D3300 and D7200 and switched to short range zooms (i.e. Tokina 11-16 f/2.8 and Tamron 17-50 f/2.8) for exactly that reason. My much beloved 18-105 was great even on my old 16MP D7000 but it was soft on the D7200 at all apertures even after calibrating the lens to the body. I now shoot mostly with the the above mentioned zooms and Nikon 35mm, 50mm and Tamron 90mm Macro primes. The Tamron 90mm f/2.8 Macro also doubles as a very capable portrait lens.

First of all, everything Tony Northrup says you have to take with a grain of salt!!!

You can safely mount (with a few rare exceptions) all Nikon (and third party) F mount FX and DX lenses on your camera .

All Nikon F mount lenses (FX and DX) with the AF-S or AF-P designator are fully compatible and will work great with your D-3400.

Older AF and AF-D lenses will meter and set aperture but will only manual focus since your camera does not have a built in focus motor like the D7xxx series and D500 DX cameras.
The 50mm AF-D lens you have coming will therefore NOT autofocus on your camera.

You have the idea of FX lenses being inferior on a DX camera backwards.
Since only the center portion of the lens is used on DX it will actually perform better on a DX camera.
The sharpest part of any lens IS THE CENTER SECTION!

You pretty much duplicated lenses with the 35mm and 40mm primes you have coming. The 40mm is not very popular since it is too short a focal lens for macro. You have to get very close to your objects which can create unwanted shadows. Also shooting insects will be difficult since they are easily disturbed. A good Macro would be for example a 90mm Tamron or 100mm Tokina even on a DX camera and can serve double duty as portrait lenses.

If your 70-300mm (you didn't specify) is the older AF-D design it will also not autofocus on your camera.


OK, I now will try to answer your questions in order

Is [ Nikon AF-P DX NIKKOR 10-20mm f/4.5-5.6G VR ] my best option for getting a wide angle?
There is a wide array of decent wide angle lenses available from Nikon and third party manufacturers like Tamron, Sigma and the Tokina line of lenses (11-16, 11-20, 12-24, 12-28, etc.).
They all work great on your camera but make sure you get a model with the built-in focus motor.

Is there a prime 10mm for DX?
There are some third party manual focus options from Korea.
Not something I would put on my camera.

Is there a 50mm prime for DX?
Yes any Nikon 50mm AF-S (FX) lens will work absolutely great on your DX camera.
The Nikon AF-S 50mm f/1.8 is one of my favorite portrait lenses on my D3300 and D7200. Very sharp and nice BOKEH.

Is the 50mm prime, above, about 75mm on a DX?
It will always be a 50mm lens but on DX it has the field of view of a 75mm lens.
BTW., your 35mm DX lens will behave like a 52mm lens and your 40mm DX like a 60mm lens because of the crop factor. It does NOT matter that they are DX lenses!
They will actually work on an FX camera but because they are physically smaller (designed for the smaller DX sensor) they will show severe vignetting on FX on larger apertures.

What would be some interesting discontinued lenses to consider?
WIth your D3400 you are somewhat limited with older lenses since it does not have the built-in focus motor.
I would stay away from older lenses anyway since they might have had a rough life and could be mechanically unreliable or have fungus.

Should I avoid CX lenses, unless I don't mind a recalculated focal length?
CX lenses are much much smaller than F mount lenses and therefore INCOMPATIBLE with F mount cameras.
There is an adapter to mount F mount lenses on a Nikon 1 camera but NOT the other way around.

When a lens is advertised as FX/DX, are they assuming one will recalculate the focal length for DX?
As explained above the focal length on the lens DOES NOT CHANGE!
Only the viewing angle is tighter making it look like a longer lens on DX (by the 1.5x calculated crop factor) .

This one looks promising: [ Tokina AT-X PRO 11-16mm f/2.8 SD MF DX IF AF Lens ]
I own it and it is a beast! Very well made, very sharp (rivaling primes!) and very heavy.
You do have to be careful with the positioning of lights (or the sun) because you can easily get unwanted flare.

I hope this answers all your questions (and raises some new ones)!

He has a full-frame D750. The 18-400 is a DX lens.

If I remember correctly you won’t be able to disable VR on the AF-P lenses with a D7200. Other than that it should be fully compatible.

I showed this to my wife with clear instructions that she would be allowed to use the shotgun in case this happens to me...LOL

I also have a D7200 and alternate a Nikon 50mm f/1.8 and a Tamron 90mm f/2.8 Macro for portraits. Both are inexpensive but very sharp and have very decent Bokeh. I mostly use 2.8 on the 50mm and 5.6 on the 90mm for decent sharpness and Bokeh and don't need image stabilisation with studio flashes.

I have had a D7200 for about 2 years now (stepping up from a D7000 and a D5100) and the one thing I've noticed very quickly is that this body (and sensor) needs excellent glass. I can almost guarantee that you would not be very happy with the performance of either your 18-55 or the 18-400 from Tamron. I shoot a 17-50 f/2.8 from Tamron as a walkaround which is nice and sharp. The 55-300 (which I still have) is ok but not spectacular. I am not familiar with your Sigma 105mm (I shoot a 90mm Tamron Macro) but at least your 35mm (which I also have) should do just fine. Should you decide to get the D7200 make sure to calibrate the focus with all your lenses which is absolutely critical for best performance. I now shoot mostly prime lenses with the D7200 (Portraits and Macro) and use a D3300 (with the 17-50 or 35mm) for walkaround.

Yes, I completely agree!
And before you mess around more with your camera I'd suggest you go to Google and search for "exposure triangle"

You won't get much for the D3300 so I would suggest keeping it as a second body! I have a D7200 for more serious stuff but my D3300 with a Nikon 35mm f/1.8 or a Tamron 18-50 f/2.8 goes everywhere with me because of the small size and light weight. Picture quality (except in low light) is pretty much identical. Also, I would recommend going to a (maybe factory refurbished) D7100 or D7200) instead of a D5xxx body.