DX lens on FX camera

I fully understand that the DX lens will not utilize the full senor of a full frame camera, but I do have some basic questions about using DX lens. I have several DX lens and even have several old manual lens from my earlier photography days. So, I have been experimenting with some of the DX lens and my biggest question regards a DX lens but with the setting called "Auto DX" set OFF. The photo seems to be full size, not cropped??? Why is that?? Also, the manual says that the image might have some darkening around the edges. Why? And a relate question... if I shoot with the "Auto DX" off, and then crop the image later, would I end up with the same exact image that I would have if I had "Auto DX" set to ON?

If my old lens are originally made for a 35mm film camera, why would there be any cropping when used in a full-frame camera (D750)? Isn't the sensor exactly the area of 35mm film? I must be missing something....

Hope this is clear, but I can elaborate if needed. Thanks.

Jim_Y said:

I fully understand that the DX lens will not utilize the full senor of a full frame camera, but I do have some basic questions about using DX lens. I have several DX lens and even have several old manual lens from my earlier photography days. So, I have been experimenting with some of the DX lens and my biggest question regards a DX lens but with the setting called "Auto DX" set OFF. The photo seems to be full size, not cropped??? Why is that?? Also, the manual says that the image might have some darkening around the edges. Why? And a relate question... if I shoot with the "Auto DX" off, and then crop the image later, would I end up with the same exact image that I would have if I had "Auto DX" set to ON?

If my old lens are originally made for a 35mm film camera, why would there be any cropping when used in a full-frame camera (D750)? Isn't the sensor exactly the area of 35mm film? I must be missing something....

Hope this is clear, but I can elaborate if needed. Thanks.

Click to expand...

There wouldn't be. Lenses made for 35mm film cameras are not DX lenses.

The camera may still be manually set to DX mode.

"Auto DX" means the camera automatically detects a DX lens and sets itself to DX mode. But you also have the option of not only turning that feature off, but choosing FX or DX modes for yourself.

Another possibility is you have a DX lens that, by chance, covers the FX sensor. There's a few that will.

When I switched from the D3300 to the D750 I used the 10-24mm DX in the beginning.

You can use them in FX mode if they're zooms but they only fill the FX frame from a certain focal length. For me that was around 15mm on that lens if I remember well. You can shoot the lower end too in FX mode but you'll get funny shots and have to crop the good part out of them. You can use crop modes to get around that problem.

Whether you can shoot them on the D750 is not the issue but whether you like what they deliver. That's what pushed me towards FX wides. A DX lens isn't really made to be shot on an FX and especially when shooting them in FX mode, you'll notice how weak they perform in the corners. The center is pretty ok but the further you go from that, the lower the IQ becomes. Not to mention the distortion depending what lens you use.

So yes it is possible but most likely you'll get tired of the results pretty fast.

As the others said, some dx lenses could work on a fx without much of noticeable problem. The image may not be up to lens spec at the corners and edges. It may depend on zoom or aperture to demonstrate a problem. The problem could be distortion at the edges or vignetting.

For example a 35mm lens is 35mm whether it is a dx or fx lens. In the dx lens, corners are cut. I'm saying that figuratively and literally. If it is a dx lens, a simpler design may get the job done. The dx sensor is smaller, so the lens can be designed concentrating on the more center portion of the image that the smaller sensor will see. Put this dx lens on a fx camera or 35mm film, and we may see vignetting or just lesser quality at the edges. These lesser quality outer areas are where the dx lens was not expected to perform since a dx sensor would never capture this area.

And yes, if you crop later, it should work out the same as auto dx. With auto dx, you will just get smaller files from the start since the image is pre-cropped to dx size.

nickt said:

For example a 35mm lens is 35mm whether it is a dx or fx lens. In the dx lens, corners are cut. I'm saying that figuratively and literally. If it is a dx lens, a simpler design may get the job done. The dx sensor is smaller, so the lens can be designed concentrating on the more center portion of the image that the smaller sensor will see. Put this dx lens on a fx camera or 35mm film, and we may see vignetting or just lesser quality at the edges. These lesser quality outer areas are where the dx lens was not expected to perform since a dx sensor would never capture this area..

Click to expand...

So, how would my old fully manual lens that worked great on a 35mm film camera work on an FX camera? Those lens were certainly designed to yield good results over the full 35mm image area. Also, does that also mean that a DX lens is not exactly (glass-wise) the same as the old 35mm film lens??

Jim_Y said:

So, how would my old fully manual lens that worked great on a 35mm film camera work on an FX camera? Those lens were certainly designed to yield good results over the full 35mm image area. Also, does that also mean that a DX lens is not exactly (glass-wise) the same as the old 35mm film lens??

Click to expand...

A DX lens is meant to only cover the area of a DX sized sensor. Your old Film camera lenses will cover the entire area of a full frame sensor.

Jim_Y said:

So, how would my old fully manual lens that worked great on a 35mm film camera work on an FX camera? Those lens were certainly designed to yield good results over the full 35mm image area. Also, does that also mean that a DX lens is not exactly (glass-wise) the same as the old 35mm film lens??

Click to expand...

Technically, there is no such thing as an 'FX' lens. There are Nikon lenses, and there are Nikon DX lenses designed for crop (APS-C) sensors. If it's not specifically marked DX, it's a full-frame lens. 'FX lenses' are not marked FX.

Jim_Y said:

So, how would my old fully manual lens that worked great on a 35mm film camera work on an FX camera? Those lens were certainly designed to yield good results over the full 35mm image area. Also, does that also mean that a DX lens is not exactly (glass-wise) the same as the old 35mm film lens??

Click to expand...

Your old lens is an FX lens. We just didn't have the term back then. FX is digital terminology for full (35mm) frame. ...and yes, less or simpler glass needed to fulfill the needs of the smaller sensor. You could say an fx lens is simply over designed for a dx sensor because it casts a quality image larger than the sensor, so the extra image is 'wasted'.

To show this, here's my D600 ("Full Frame/FX") and my D7000 ("Crop Sensor/DX"), side-by-side, with both in Mirror Up for Cleaning mode.



The blue-green rectangles are the actual sensors. Notice how the FX sensor on the left is larger (Nikon specs are 24m x 35.9mm) than the DX sensor on the right (Nikon: 23.6mm x 15.6mm)

I put a 50mm FX lens on the D600, it sees this:



When I put a 50mm DX on the D600, it sees this:



This is exactly what you would see in the viewfinder as well. These images are merely reduced in scale for posting here. No other editing was done. Notice how everything in the scene is rendered the same size?


The only difference between the two is the DX lens is not designed to cover the entire FX sensor.... it only needs to cover the smaller DX sensor. That's why the DX lens shows the black areas... both in the VF and on the sensor.

If I put either the FX OR the DX lens on the D7000 (crop sensor) , or on the D600 and shoot in DX mode, they will both record this:








When I enlarge the DX image to match the same dimension of the FX image, it appears I'm using a longer lens. I'm not. They're both 50mm. What changes is the field of view caused by the 'crop factor' of the smaller sensor.


An analogy would be like putting a film negative into an enlarger and running it up to make an 11x14. Then replace the 11x14 with a sheet of 8x10...... without changing anything else. The subjects in the 8x10 will measure the same as they are in the 11x14. There would also be no more detail in one compared to the other. Or, you could take the 11x14 and cut it down with scissors to an 8x10.... there would be no more detail when you're done doing that.

Blacktop said:

A DX lens is meant to only cover the area of a DX sized sensor. Your old Film camera lenses will cover the entire area of a full frame sensor.

Click to expand...

So, if that's true, then I could actually use my old lens on the FX with no cropping necessary, full frame? I've never see any reference to using those older manual lens on the FX cameras. My old manual lens might be old, but they are extremely good lenses. I know they would have to be used in manual mode, but that's what I grew up on, and I'm still very comfortable with full manual operation. This gives me some experimenting to do....

Jim_Y said:

So, if that's true, then I could actually use my old lens on the FX with no cropping necessary, full frame? ........

Click to expand...

Absolutely! All pre-digital lenses are full-frame. They're not marked as 'FX' because Nikon didn't make 'FX' bodies back before digital cameras, they made 35mm film bodies.

Lenses NOT marked "DX" are full-frame. This is true for ALL F-mount lenses, no matter how old. Lenses suitable for FX digital bodies are not marked as FX. Even a G-series lens today is not marked 'FX' if it is full-frame.

I'm not sure if pre AI lenses can be used. Certainly 99% of AI onward old glass can be used on your D750. Read the manual under non CPU lens and it will explain how to enter the focal length and maximum aperture into the camera

...you need to be aware of auto-focusing as well... check what your camera supports and what the lens presents. Most older lenses on a modern digital body are going to become manual focus.

Oh, I'm fully aware that those old lens would make my D750 into a fully manual camera. That doesn't bother me, though. I am very comfortable with a manual camera. In fact, that's how I use the camera much of the time anyway. I don't always trust the camera to make the decisions that I want. And it really isn't all that difficult to control the camera manually.

Jim_Y said:

And it really isn't all that difficult to control the camera manually.

Click to expand...

Try to shoot a bird in flight at 600mm.

J-see said:

Try to shoot a bird in flight at 600mm.

Click to expand...

I don't think that would be very easy under any conditions!