FX - DX Lens Crop Factor

Digital camera sensors are of a certain size (the area capturing the image), much like film frames were a certain size. Nikon calls their DSLR sensor sizes FX (full frame) and DX (APS frame). Canon calls them Full Frame and APSC. Full frame compares to the same size of a 35mm film frame (36x24 mm), and APS is the smaller APS film frame size (about 24x16 mm).

The lens projects a circular image onto the sensor, the diameter designed to cover the corners of the sensor or film. Then the camera sensor captures a center rectangular portion. The picture above (in Adirondacks in North Carolina) attempts to show three things:

1. The circular image projected by the FX lens.
2. The red box marks the scene seen in a FX sensor.
3. The blue box marks the scene seen in a DX sensor with same lens (1.5x crop factor, FX frame is 1.5x larger).

Obviously, it is the view of the same scene, with the same lens, at same distance - using the two sizes of sensors. This is simply how it works. When we enlarge the cropped DX image to be displayed at same size, it appears to be telescopic, as if with a longer lens, or as if standing closer. Any cropped image shows the same telescopic effect when enlarged to same size. DX is simply a smaller sensor, and this "crop" changes the viewed area, which causes the differences between FX vs. DX.

The digital DX camera simply uses a smaller sensor to capture the center of the lens image, which is said to "crop" the image (edges are cut off, as shown above). A full frame lens is shown above, but DX lenses are designed smaller, to project a smaller circle, which only covers the corners of the smaller DX frame (which causes vignetting if on a FX body). Nikon shows a clear description of FX vs DX. Compact cameras use even a much smaller sensor, around 7x5 mm is a common size, but they include several sizes, all tiny (crop factor of 5 or 6 is common).

The difference in these sensor sizes causes different visual effects. For example, the FX sensor is 36 mm wide, and the Nikon DX sensor is 24mm wide. The ratio of these two crop sizes is 36/24, which is 1.5 to 1, called Crop Factor. The DX frame is cropped smaller (simply because its sensor is smaller). The FX view is obviously 1.5x wider than DX. But the smaller DX view, when enlarged to show it same size, is magnified 1.5x more than the FX, which is the same effect seen when FX uses a lens that is 1.5x longer focal length. So, the DX camera view "looks same as" if a focal length x 1.5x were used on FX, which is called the DX "equivalent" focal length (compared to FX, DX would give same view as FX full frame would see if FX used the 1.5x longer lens at the same distance). The lens itself is unchanged of course, it still does whatever it always does in both cases, but the view seen by a smaller DX sensor is simply a cropped and enlarged view, different than the view seen by a larger FX sensor.

We know the 1.5x number, but sometimes we miss the significance until we actually look though the viewfinder once ourselves. Using the same lens, and relative to each other, FX makes a wide angle view (wider view of more area, but with necessarily smaller contents within that view), whereas DX makes an apparent zoomed in telephoto view (technically, a cropped view), which shows less scene area of course, and the subject is magnified when it is viewed enlarged to be the same size again.

Below are D300 DX and D800 FX images, using the same 105mm lens, on the same stationary tripod, at same distance. The only change was that the bodies were swapped out. The camera viewfinder shows these same views.

This is the 105mm lens on DX (D300). DX is a cropped view, which then looks like a zoomed in view, as if it were with a lens of 1.5x more focal length on FX.

"Equivalent" of 105mm x 1.5x = 160 mm focal length

(technically this comparison of these sensor sizes is 35.9 mm / 23.6 mm = 1.52x).

This is same 105mm lens on FX (D800), at same distance. Compared to DX, FX is a wide angle view, 1.5x wider. Therefore, the subject is necessarily shown at a reduced size.

The first obvious reaction looking in a FX view finder is that (compared to DX with same lens), it shows the subject smaller, but in a larger area, visually as if from a greater distance. These two pictures were at the same distance of course, just the view is different (cropped).

This is the Same FX image, but now it is marked attempting to indicate a DX crop that matches the DX image area. Not exactly centered here because the two cameras were not pointing at exactly the same angle, but the idea is still the same.... DX is a cropped smaller view of the full size FX frame, simply because the DX sensor is smaller. Shown enlarged here, so that DX comes out the same original size.

Note that it is simply this crop (and resulting smaller angle of view) that gives DX the telephoto effect. No other magic illusions are involved.

And the point is, you can see this same "FX vs DX" view in your editor on any image, by simply marking a crop box that is 2/3 the dimensions of the original 3:2 image you use (1/1.5 is 0.667, or 2/3). Or use any crop factor, but this 2/3 dimension will show exactly the view a 1.5x DX camera would see, as compared to the FX view, assuming if both are at the same distance with the same lens. This crop (shown just above) is the only difference of DX and FX. DX is simply a smaller sensor, which cannot capture a view as wide. When you can believe that, then you've got it.

So DX is simply a smaller sensor, which crops out a smaller central area, which necessarily has to be enlarged more, to be same effective size. This does change the view it sees - DX sees the same view FX would see if FX used a telephoto lens 1.5x longer (at same distance). Or, if with same lens at same distance, then DX sees a smaller view width, which is necessarily enlarged more (to be the same size print).

FWIW, regarding equivalency, we could get the same equivalent views, in these other ways:

• We could put a 1.5x longer lens (157 mm instead of 105mm) on FX, to see the same DX view with FX.
• We could put a 1/1.5 or 0.667x focal length lens on DX (70 mm instead of 105mm), to see the same wider FX view with DX.
• We could step back to 1.5x greater distance with the same lens on DX, which would see the same wider FX area, 1.5x wider, but smaller, less enlargement.
• We could step up closer to 1/1.5 or 0.667x distance with the same lens on FX, and see a 1.5x enlarged DX view of larger but cropped subject.

We can analyze this in detail and make it seem complicated, but every effect seen is only a result due to that smaller crop, shown above. The lens itself of course does not change in any way at all, not just by putting it on a different camera body. This effect is named "crop factor".

Enlargement and magnification also increase effect of camera shake. The common rule of thumb about our ability to hand-hold slow shutter speeds has been to use at least a shutter speed of 1/focal length seconds. For example, use at least 1/200 second shutter for a 200mm lens. That comes from the 35mm film era however, and while it still applies to FX, the crop factor must be included, to use 1/(focal length x crop factor) seconds, i.e., use a 1/300 second shutter for a DX 200 mm lens, to compensate for the necessary enlargement increase. This is not exactly a disadvantage of DX, since it is only about the magnification of the effective focal length you choose to use (it is same situation if you put the 300mm lens on the FX body, for the same view). And people vary individually, some cannot hold this speed, while others can hold slower. Faster is better, and image stabilization options can improve this too.

K.I.S.S. tells me not to complicate things, but who listens? Just a quick note: Changing where we stand with the camera to get the same view may give the same angular view, but it does still change a few things:

• Distance affects depth of field - all other things equal, closer distances have less, and greater distances have more. DX has more depth of field, when we stand back farther to get the same view with same lens. But also, greater enlargement of smaller views reduces apparent depth of field.
• Photographic perspective depends only on the distance of where the camera stands. Simply changing lens focal length while standing in same location does not change perspective, but moving the camera does. Perspective is only about where we stand, and not about which lens we use. But we can use shorter lenses for portraits with DX, because FX 105mm and DX 70mm equivalents force us to stand at the same distance for head and shoulders shots, to prevent making a close nose look larger. The rule is, for proper perspective, we should always stand back (about six feet or more), for portraits with any lens (zoom in as desired, but stay back).

Advantages of FX or DX

The significant advantage of DX is that DX increases the apparent range of our lenses, a telephoto effect relative to FX. A 200 mm lens on DX "acts like" (shows same view as) a 200 x 1.5x = 300mm lens on FX. At least it is perceived as an advantage at longer range, and DX is popular for sports or wildlife, for this reason. DX generally has a little more depth of field (because it must stand farther back).

Of course, it does not matter when we crop it (to get same view). We could crop with the small camera sensor, or we could do exactly the same thing later at home, by simply cropping the FX image to be smaller DX size, and then enlarging more (assuming of course the edges contain nothing that the view will miss when cropped off). Other than the initial file size and the cropped telescopic view, the only difference and concern might be about the pixel density - the final cropped dimensions should still have sufficient pixels left. Starting from the D700 12 megapixels, cropping to DX leaves 5 megapixels. The D600 leaves 10 megapixels, and the D800 leaves 15 megapixels (only about 40% remaining).

The significant advantage of FX is FX increases the apparent width of our wide angle lenses, increases field width relative to DX. A 24mm lens on FX shows a 24mm wide view ("equivalent", so to speak), which is quite wide angle. Put the same 24mm lens on DX, and it shows a 1.5x or 36mm focal length view ("equivalent", relative to FX), which for DX, is only a mild wide angle view. It requires 16mm on DX to match the wide view of 24mm on FX. Extreme wide angle is tough to obtain on DX. FX generally has less depth of field (because we must stand closer), which could be an advantage when trying to blur the background.

The purpose of the "effective focal length" comparison (compares lens view to 35 mm film size), and another relative advantage of FX, is that for anyone who was long accustomed to 35 mm film (same size frame as the FX sensor), now our lenses act that same familiar way in FX digital - a 30mm lens means the same thing on FX (same view) - that a 30mm lens always meant for 35 mm film. So FX is like "Old Home Week" again, the way we learned to think of it in the past. 30mm FX is moderately wide angle, where 30mm DX is very close to a "normal" lens. "Effective focal length" (actual focal length x sensor crop factor) is used to compare a lens view to lenses on 35mm film, which helps old timers "know" or predict what that other lenses will do. Which is surely pointless to newcomers who never used 35mm film, but nevertheless, it is quite important to those who grew up thinking that way.

Early on, a major advantage of FX was that the same megapixels in a larger sensor creates larger pixels, with lower noise, resulting in outstanding high ISO performance. But today, the number of megapixels is increasing, and sensor performance is increasing too, and perception of this obvious advantage is becoming less clear.

The Nikon FX models (D700, D600 and D800) give you both choices - you can shoot DX or FX mode, for both Raw and JPG. That makes DX show different in the viewfinder (not like actual DX cameras), in that the DX frame is seen as the smaller cropped area bordered with a red box inside the full FX frame (not enlarged in viewfinder - the DX viewfinder view is very much like my last picture example above - a smaller box marked inside a larger frame). The viewfinder is optical, but Live View is digital, so DX mode in Live View can show the enlarged DX frame. The final DX image result will be necessarily enlarged. Or of course, we can always crop FX to the DX view and size anytime later.

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