Why one should use DX mode on an FX camera

One shouldn't. Not always anyhow. But there are some moments.

As a reminder, a camera with a big sensor to capture the photo is called FX in the Nikon world. Most FX cameras can take photos using only part of the sensor in "DX mode". There are plenty of good reasons to use DX mode.

1. A photo taken in DX mode is smaller, so it takes up less space on your memory card, so you can shoot longer without changing card, and you can get away with smaller cheaper cards.
2. You can fit more DX photos on your harddisk, so when you travel you have more photos to show your grandma or prospective employer.
3. The smaller size of DX photos means they take less time to transfer to your harddisk or tablet.
4. The smaller size of DX photos means that synchronisation with any cloud site you use to store photos is quicker and takes less bandwidth from other people on the same network - like your grandmother if she uses your WiFi.
5. DX photos are quicker to edit and display in post-processing software. You will be able to edit more DX photos in one day than FX photos.
6. With most cameras, shooting DX allows more fps (frames per second) than FX.
7. Shooting DX does not fill up the buffer and allows you to shoot for a longer time with your camera, when you hold down the shutter button in continuous shooting mode.
8. If you "crop" a photo shooting in DX mode, you still get a full RAW file, but smaller. If you crop an FX photo in post processing, you either simply hide part of the photo, which means the file is as big as it was to start with, or you have to save it as jpg or some other format, and you lose the advantages of having shot RAW.

Then there are of course moments when FX mode is better. Why would you otherwise have paid for it?

1. You can get a more narrow depth of field.
2. You get more detail.
3. You can get a wider angle of view with the same lens.

Sometimes the FX advantages matter. Sometimes they do not matter at all. Sometimes they matter a lot.

If I one day take 99 photos in DX mode with my camera and just one in FX mode, and the subject in FX mode absolutely deserved it, I will be happy that I brought an FX camera instead of a DX camera.

Old Camera with Big Sensor - New Camera with Small Sensor

There is a widely accepted view that large camera sensors produce better pictures. In Nikon terminology, that means that a big FX sensor is better than a smaller DX sensor, which is better than the really small CX sensor. That is true in most cases. However, when one compares old cameras with new ones, occasionally the roles are reversed.

Some examples using DXOMark's criteria:

The 2012 DX camera D5200 vs the 2007 FX camera D3:

That's a five year difference. The DX camera has a better overall score, but it still lags behind the FX in some areas, like low-light ISO.

The 2013 CX camera Nikon 1 S1 vs the 2005 DX camera D70:

This is an eight year difference, and even though the CX Nikon 1 has an overall advantage in DXOMark's rating, it still lags behind the larger sensor when it comes to low light.

Like all comparisons, this is not the whole truth of course. There are many other aspects on cameras than what is measured by DXOMark. The point is simply that for certain things, new and small can be better than old and big, even when it comes to camera sensors.

A D800 is a Privilege - Not an Obligation

If one buys an expensive camera, it seems logical to make sure one uses it fully. “If it has 36 megapixels, use the 36 megapixels.” “If it has 14-bit files, use every last bit.” “If it has splendid autofocus and perfect auto-whitebalance, use them.” “If it has a Pet Portrait scene mode, use it!” No, wait! Not the last one. Your expensive camera is unlikely to have a Pet Portrait scene mode.

However, if one thinks about it, the camera should not tell you what to do. You should be able to tell the camera what you want. I tell my camera what I want.

• I use DX mode on my D800. Not all the time. When I need a really narrow depth of field or a lot of detail, I use FX. I'm glad it is there. However, when I do not need FX, I do not use it. With DX, my files are smaller; more of them fit on my harddisk, and they are quicker to edit.
• I use small basic jpeg files with my D800. Not all the time. Usually I shoot RAW. However, when I just want to take photos of labels or signs to remember what they said, there is no reason to use anything but the smallest bad quality files one can get. And, to be honest, even small basic jpeg files are pretty good with a D800.
• I use 12-bit lossy compression on my D800. I do that almost all the time. The average file size is less than 30 Mb. If I had used uncompressed 14-bit files, the average file sized had been around 75 Mb per picture. If there is a really delicate occasion, I can switch on lossless compression and 14-bit for a photo, but those delicate occasions are rare in my world.
• I use cheap lenses on my D800. I happily use a Nikkor 50mm f/1.8D. Not all the time. I have specialised high-quality lenses, for things like macro, zooms and wide angle purposes. I use those lenses when I want to do macro, zooms or wide angle photos. But when I want to carry light, I can use a 50mm f/1.8D, which in many cases does a decent job.
• I use manual focus on my D800. The D800 has a good auto-focus system, but sometimes I just feel I want to be in control, so I often twist the little focus ring on the lens instead of using the expensive auto-focus system built into the camera.
• I use manual ISO on my D800. People praise the auto-ISO feature, but I want to be in control, so I rarely use auto-ISO.

All those things do not mean that I paid money for things I do not use. I do use them. I know the camera is there for me when I want it, and that's why I bought it - to know that it has what I need when I need it.

And what about the Pet Portrait mode? If I use a camera with a Pet Portrait mode, do I take advantage of it? No. The scene modes of cheaper cameras frighten me. I do not know how they work, and I do not know how to take advantage of them. I much prefer to use the standard PSAM modes. If I buy a camera with a Pet Portrait mode, I pay for something I will not use.

It is all a matter of finding the right level.

FX - DX - CX - Why does the sensor size matter?

There is one not so obvious difference between Nikon cameras, and that is the sensor size. The sensor is the part of the camera that senses incoming light. Some high end cameras have big sensors and low end cameras tend to have small sensors.

There are three main sizes with Nikon cameras with exchangeable lense: FX, DX and CX. The big FX sensors are used in cameras like D3, D4, D700, D800. DX is used in cameras like D3200, D5100, D90, D300. The smallest CX sensors are used in J1 and V1 cameras.

A Nikon lens made for a big sensor works with any smaller Nikon sensor. This means that any Nikon lens fits a CX camera - in theory. (You need an FT-1 adaptor, and autofocus may not work and there may be some other limitations.) A DX camera can take both FX and DX lenses. An FX camera only works well with FX lenses.

The focal length of a lens does not change with the camera. This means that magnification, zoom and angle of view are the same for all lenses with the same focal length on that camera. If you take a 200mm CX lens and a 200mm DX lens and a 200mm FX lens and put them on the same CX camera one after the other, the magnification is identical for all of them.

However, the same 50mm lens magnifies more on a small sensor than on a big one. In the picture below there are three shots of the same object with the same lens and focal length, but three different cameras. 

The FX camera (a D800) covers the widest angle. The DX camera (a D5100) covers a smaller area. It has been given a green cast to make it stand out from the two other photos. The CX camera (a J1) covers just a small fraction of the FX area. (The photos have been resized to overlap, so the density of the pixels does not correspond to the size of each photo.)

If I had wanted to cover the same angle of view with each photo, I would have had to choose different focal lengths. If the FX camera uses 200mm, the DX camera would need to be 133mm and the CX camera 74mm to give the same angle of view (or zoom factor or magnification). A table with conversions of some common values can be found here.

Advantages with large sensor cameras:

• Less noise, better dynamic range and colour depth. This is basically because the sensor has a bigger area to gather information from. However, note that a new camera with the latest technology and a small sensor can perform better than an old camera with a big sensor in these respects.
• Wider angle of view. To get the same angle as a 14mm lens on an FX camera, you would need a 9mm lens on a DX camera.
• Narrow Depth of Field and more appealing bokeh. This is actually a tricky one. If you look at the three superimposed photos above, you will notice that they have more or less the same amount of bokeh on the background. That is because they were taken with the same focal length and the same aperture. In this case the bokeh is identical, but the image area is different. It is only when you try to cover the same image area with different sensors that the difference in depth of field is apparent. The FX camera has a focal length 200mm. To cover the same area, the DX needs 133mm. With a smaller focal length, the same aperture value (e.g. f/1.4) actually corresponds to a smaller physical aperture hole measured in millimeters. A smaller aperture hole means a sharper picture overall. Therefore, to get the same depth of field at a smaller focal length, the aperture hole needs to get bigger. If an FX camera has focal length 200mm with aperture f/1.4 with a target at five meter, a DX camera will need to be set to 133mm to get the same angle of view. To get the same depth of field, the aperture may need to be set to something as large as f/1.0. As hardly any Nikon lens has an aperture f/1.0, the same depth of field is impossible on the DX camera. Ergo, FX has a narrower depth of field.

Advantages with smaller sensors:

• Smaller, lighter and cheaper lenses possible.
• Smaller camera. (This is not always the case, but an FX sensor will not fit into a really small camera.)
• Cheaper than big sensor cameras.
• Wider choice of lenses.
• Less vignetting in some cases. (If you use an FX lens that has visible vignetting on an FX camera, the DX sensor will cut that part out.)
• Bigger magnification. (To get the same zoom as a 200mm lens on a DX camera, you need a 300mm lens on an FX camera.)