This wasn't even a topic that I thought I was confused about until I took a couple of photos today.
With my D800 set on Auto FX Crop, I took a picture. When I looked through the viewfinder, I could see the total area, and then I could see the smaller area inside that I assumed was the DX area. The cropped area was grayed out a bit. The RAW file size was about 19M.
I then turned off Auto DX Crop and took a photo with my 24-70 2.8 at 24mm. I switched lenses to the 12-24 DX and took another photo at 24mm. There was no crop area (as I expected) but the area covered in the photos was virtually identical. Additionally, both file sizes were about 40M.
I'm now confused. I guess I expected the DX lens to either produce a smaller size file or an image with a narrower field of view by 1.5X.
What makes a DX lens a DX lens and what effect does Auto DX crop have?If my file size and angle of view is identical from a dx to an fx lens it seems hard for me to justify selling the DX f4 to get the f2.8 FX, at least right now. I can continue to use my DX lenses on my FX and slowly upgrade.
Additionally, for example, if I were to place a 300mm lens (DX or FX) ON THE D800 and put it into DX mode, would I get the same photo as I would with a DX camera and a 300mm lens?
Sorry for the rambling question. If anyone can help me out of my confusion I would really appreciate it. Thanks.
#1. "RE: DX lens on a D800 - I am confused" In response to Reply # 0 Fri 12-Oct-12 08:52 PM by bkinthebay
In terms of focal length, FX and DX lenses are identical. If you only consider the central part of the image, the content is therefore 'zoomed' identically.
The difference lies in the ability of the FX lens to project on the sensor a wider field of view without vignetting on the sides. If you look at the back of your two lenses, you will notice that the FX one has a larger glass to achieve this.
Some DX lenses and zooms are however quite good at covering the FX sensor at certain focal lengths and focusing distance. At other focal lengths and/or focusing distances, severe vignetting will however occur.
Therefore, for the tests you did at 24mm, it shows that the 12-24 DX zoom nearly covers the FX sensor. If you do other tests at different focal lengths and focusing distances, you should observe severe vignetting in some cases when you switch off Auto DX.
#2. "RE: DX lens on a D800 - I am confused" In response to Reply # 0
Bernard hit it on the head. Just to add some tips with DX lenses on the FX bodies...
If you don't mind some extra steps to crop during post-processing, keeping the camera in FX mode with DX lenses will give you some extra field of view than what you'd get with a DX crop. You can potentially make your 12-24 act like a 10mm or 8mm lens depending on how it performs with vignetting on the FX sensor. I know my Sigma 10-20 gets me a good deal more image in FX mode than if I allow an auto DX crop. The downside of the DX lenses is that their IQ doesn't compare with true FX lenses. Also, the extra area you may get by keeping the camera in FX mode may be somewhat soft or have chromatic aberration. But it's a useful thing to know about if you need some extra field of view in a pinch.
#3. "RE: DX lens on a D800 - I am confused" In response to Reply # 0
I find it best to ignore the practice of quoting the "focal length" of a lens used in smaller format cameras as the focal length providing an equivalent field of view in the FX (full frame) format. While this may have been helpful as we were going from full frame film to smaller format digital cameras a few years ago, it is an over simplification best forgotten when trying to understand the coverage of different lenses on different cameras.
Focal length is a property of a lens. It has nothing to do with the camera on which the lens is used, whether FX, DX or any other format. The maximum available field of view provided by a lens of a given focal length is dependent on the size of the sensor for which it provides effective coverage. If used on smaller format cameras the field of view is reduced, on larger formats - vignetting.
As other commenters have pointed out, the difference between FX and DX lenses is the sensor area they are designed to cover.
I've also use the 12-24mm on the D800. There is no vignetting in the FX mode between 18 and 24 mm. As focal length is decreased below 18mm, the vignetting becomes progressively worse.
#5. "RE: DX lens on a D800 - I am confused" In response to Reply # 4
Kind of. Those statements originated back in 2007 when FX and DX were both 12mp. So a D3 put 12mp on the whole FX frame, while DX put the 12mp in the central 40% (ie the DX frame). So if you used 12mp, you could enlarge the DX to a significantly (50%) larger print with equivalent quality. Thus one effectively had 50% longer "reach" with the same lens.
However, at the moment, this isn't as true. For example, a D800 has more "reach" than a D300s, because the D800 can put 15mp into that central DX area, while the D300s only puts 12 there. Of course, all things being equal (and they often are not) a D3200 has even more, because its DX area is 24mp into that area whereas the D800 can only get 15mp...
Assuming that a D400 has at least 24mp, it too would offer somewhat better "reach than a D800.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!
#7. "RE: DX lens on a D800 - I am confused" In response to Reply # 4
We need to be careful when talking of "reach".
Focal length, sensor area, and field of view are all specified in hard numbers and are related to one another by the laws of physics. Their relationship does not change over time.
Pixel count is also specified by a hard number. However, it varies from camera to camera, being set by the designer.
"Reach" is the inverse of the field of view, i.e. the longer the "reach", the smaller the field of view. It is dependent on the lens focal length and the sensor area. To increase "reach", one must increase the lens focal length and/or reduce the sensor area.
In any given camera, sensor area can be reduced by cropping. As you note, in the D800 this can be done by mode selection in the camera or in post processing.
Basically, you are correct in saying that you can get the same "reach" with a FX camera as with a DX camera, either by putting it in the DX mode or by cropping. However, this is strictly true only when the number of pixels in the DX camera is the same as in the FX crop. If the DX camera image has more pixels, its resolution will be higher than that of the cropped FX image enabling you to crop the DX image to extend your "reach" beyond that of the cropped FX image. The amount you can crop, and consequently the amount of "reach" you can attain, using either the FX or DX cameras, will depend on your own criteria re sharpness. Different users may have different opinions, depending on required print size and sharpness criteria, on the level of "reach" achievable with any given camera/lens combination.
The D300 owner waiting for the D400 to come out for the greater "reach" of the DX mode does have a valid point. His existing lenses will continue to provide the "reach" he is currently enjoying with the D300. Also, the D400 will almost certainly give him a higher resolution than a cropped D800 image. Moving to a full (camera and lens) FX based system from a DX based system is expensive, particularly for a sports/wildlife photographer. If the advantage of the FX system is not required, he probably does not wish to incur the cost.
#9. "RE: DX lens on a D800 - I am confused" In response to Reply # 7
>We need to be careful when talking of "reach".
One of the problems is that (as you have explained) the term "reach" does not have a single universally-accepted meaning. Some folks associate it with magnification, others with a tighter crop, and yet others with more pixels on the subject.