Changing frame size from FF to DX acts as a teleconverter ?

Hi

I have read that a popular combo with the Nikon 8-15 fisheye is to add to the setup a 1.4 teleconverter (for shooting sharks etc.) 

However in a  Z7 2 camera (Like more advanced mirrorless cameras) - I can change the size of the frame from FF to DX via the menu. If I do so - It is the same as using a 1.4 teleconverter ? Am I wrong ?

I am guessing that will just use a DX size bit of the FF sensor. A digital crop / zoom. So maybe yes for the field of view, but not with same resolution.

44 minutes ago, Cerianthus said:

I am guessing that will just use a DX size bit of the FF sensor. A digital crop / zoom. So maybe yes for the field of view, but not with same resolution.

Yes. But there is enough resolution... 45mp will be about 20mp: The same size as the Canon R6

The TC also reduces the FOV of the lens...and helps to reduce the fisheye effect...

2 hours ago, adamhanlon said:

The TC also reduces the FOV of the lens...and helps to reduce the fisheye effect...

The DX mode does not solve the FOV issue ? This is the only reason for changing from FF to DX from time to time.

I guess this is eventually not a TC substitute. 

14 hours ago, ido said:

The DX mode does not solve the FOV issue ? This is the only reason for changing from FF to DX from time to time.

I guess this is eventually not a TC substitute. 

What FOV issue are you trying to solve.  The crop mode is exactly what it says a crop, you could take the same image full frame and crop in photoshop and the result is exactly the same.   The main reason for the crop factor is many cameras allow an increased frame rate.  The Raw file is also smaller which could be useful in some circumstances.

In the crop mode you have 20 MP, the same number of pixels as R6, however it is a DX crop and there are subtle differences. first the Depth of field is about 1.5x  as much as if you framed the same way with fullframe 20MP sensor.  Not always apparent if you used crop mode instead of getting closer.  You also gather less light because the sensor area you use is only 42% of the fill frame sensor, So you have only 42% of the photons. 

For most purposes my view is you are better off just taking the image rather changing to DX mode for UW, you don't have to fiddle around to switch it back and forth and because of the the view you have it might just encourage you to get closer which is usually never a bad thing.

Thanks Chris. You are  100% right.  I Know that the DX will give me "tele version" of my setup. I thought It will replace the need for TC in order to "fix"the barrel distortion in some cases.  

However I played with LR with some relevant photos and I saw that it works quite well. 

Barrel distortion decreases progressively from edge to centre - anything that restricts the view to the centre of frame, be it TC or cropping will reduce distortion.

IDO, I have the APS-C crop assigned to a button on my Sony A7R IV housing and use it most with macro lenses like the Sony 90mm and 50mm. When I move to the APS-C settings with the 1.5X crop the lenses act like 135mm and 75mm lenses and I get a 26MM APS-C sized image.

With a 1.5X tele converter I would get the same 135 and 75 reletave focal lengths the difference is that they would cover the entire 61MP full frame of the A7R IV sensor. 

Thanks. I believe my main photo shoots will be wide angle UW photography 

There are more differences, I have just moved to a FF camera and I what I have find out is that I need tighter diaphragms to have definition on  the corners and depth of field. And due to that you need more powerfull strobes. So it's a compromise. In my opinion if I coul crop the sensor in my camera I wouldn't do that. I prefer to use the TC for wide angle. It's really more challenging in terms of illumination and composition.