Digital camera sensors are of a certain size (the sensor 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 35 mm 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 attempts to show three things now:
Obviously, the top picture above is the view of the same scene, with the same lens, at same distance - if using the two sizes of sensors. The Subject size is obviously the same (again, same lens at same distance), but the frame size is not the same. The two drawn boxes simply show the size of the sensor that will capture the image, what the camera will see. This is simply how it works. When we enlarge the cropped 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. Nikon uses the terms FX for full frame sensor and DX for a 1.5x crop smaller sensor. 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 24 mm wide. The ratio of these two crop sizes is 36/24, which is 1.5 to 1, called Crop Factor (normally we compare sensor diagonals, but these two are the same 3:2 shape). The DX frame is cropped smaller (simply because its sensor is smaller). The FX view is obviously 1.5x wider than DX (more wide angle 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 (more telephoto than FX). So, the DX camera view "looks same as" if a 1.5x focal length were used on FX. The actual lens focal length is not changed, but the 1.5x number is called the DX "equivalent" or "effective" focal length (as compared to FX). The lens is NOT changed, but DX merely crops to see a different VIEW from it. DX would give same view as FX full frame would see if FX used the 1.5x longer lens at the same distance - or if with same lens, if DX stood back 1.5x times more distant than FX. Again, 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 wider view seen by a larger FX sensor.
The "effective" or "equivalent" focal length number (due to the crop factor) is simply the comparison to FX size, which is the same size as 35 mm film, which many of us were used to for many years. Back then, we knew what a 24 mm view did before, but it becomes something different on DX. And since the FX lenses are interchangeable (used on DX too), this comparison is important to some users. The only use or importance of this "Equivalent focal length on FX" is simply compare the DX VIEW to FX VIEW.
We know the 1.5x number, but sometimes we miss the significance until we actually look though the viewfinder once ourselves. (or the Field of View Calculator should make it be clear). 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 105 mm 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 a 105 mm lens on FX (D800). Because the FX sensor is larger, then compared to DX, FX is simply a wide angle view, 1.5x wider than the smaller DX sensor. The larger sensor extends farther out from center, so now we see pixels way out there too. Therefore, because the overall view is bigger, the subject is necessarily shown at a reduced size (scene is bigger, wider, but shown here in the same space, the objects in it are necessarily smaller than DX).
The first obvious reaction looking in a FX view finder is that (compared to DX with same lens), it shows the subject smaller - but the scene is larger, a larger area, visually appearing as if from a greater distance. These two pictures were at the same distance of course, just the view is different (DX is cropped from the FX view - see next one.)
This is the same 105 mm lens on DX (D300), on the same tripod at same distance. DX is just a smaller cropped version of the lens view, but which after enlarging 1.5x more to appear same size here, then it looks like a zoomed in view, "as if" it used a lens of 1.5x more focal length on FX. In this case, DX sees roughly an eight inch view instead of twelve inch view on FX (12/8 = 1.5). It is the same lens however. The focal length is not changed. It is only the "cropped view" that is different. After we enlarge the smaller sensor image more, then the view is zoomed "as if" it had been a longer focal length on FX.
A 10 mm lens is always 105 mm on FX or DX. The "Equivalent" term just means this is the "Equivalent" VIEW (as if seen on FX) of 105 mm x 1.5x = 160 mm focal length (if on FX). The lens is always 105 mm, but after the smaller DX image is enlarged more on the monitor, the VIEW we see on the cropped DX sensor looks magnified, "as if" FX used a longer lens, or "as if" FX were standing closer. Any cropping followed by more enlargement would simulate that. The DX sensor is smaller, so then we enlarge it more, the larger view appears as if magnified by the "crop factor".
The DX image above is actually smaller than the FX image (cropped by the smaller sensor), but it is enlarged half again more there, just to show the images as the same size.
But relatively (at same enlargement), the DX image is actually this size. The subject objects here are the same size as in the FX image.
The same lens of course projects the same image on the two sensors. The same image is of course the same size, but the DX sensor is cropped smaller. DX is a smaller image, but you can see that the area in this DX crop size matches the central area of the FX image above (assuming same lens at same distance).
FX is 1.5x larger, so in comparison, DX is only 2/3 of FX size. We would normally show them at same enlarged size (as above), but they are not same size. The FX sensor is half again larger than DX. DX has to be enlarged half again more, which is not an equal comparison of what we have.
Below is the Same FX image, but now it is marked to indicate a DX crop that matches the DX image area. And it is shown enlarged 1.5x more here, so that the cropped DX frame comes out the same size as the FX size above. DX is simply a cropped smaller view of the full size FX frame, enlarged more, simply because the DX sensor is cropped smaller.
Note that the cropped DX frame is smaller, and the FX frame is 1.5x larger. I am emphasizing the fact the smaller DX frame has to be enlarged 1.5x more to view or print it at the same size as the FX image. It is simply this crop (and resulting smaller angle of view) and the following greater enlargement, that zooms DX to give the apparent telephoto effect. No other magic illusions are involved. Any crop, done anywhere, anytime, will appear to show the same telephoto effect. The smaller image simply has to be enlarged more to view it at the same size. This is the entire cropped factor telephoto effect, due to the smaller sensor.
And you can also see this same "FX vs DX" view on any image in your editor, 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. Or simply just zoom in on any image in your editor (a smaller view, enlarged to same size, appears as a telephoto effect). 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 view at the 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, a typical compact camera with a tiny 7x5 mm sensor and shooting with maybe a 9 mm lens does even more of this, with a crop factor maybe 5 to 1 compared to 35 mm film FX size. Compacts use special very short focal length lenses to compensate for the tiny sensor. The lenses are not interchangeable with DSLR, so this difference is less discussed. The camera specs will mention the equivalent 35 mm film size equivalent focal lengths however (focal length multiplied by crop factor), since many of us were familiar with 35 mm, then we also know what this lens will do.
This is nothing new. In the film days, medium size roll film, and view camera sheet film, were very different sizes than 35 mm film, which again required lenses with different focal lengths to be used (to get the same normal viewing angle). Since these lenses were not interchanged among cameras of different film sizes, it is just something we knew, but did not worry about much.
FWIW, speaking of field of view... A "normal lens" is one with a focal length approximately same as the sensor diagonal, which is often considered to give what we remember as an eye's "normal" view of a scene. So different size cameras, each with their "normal lens", will take a picture with about the same "normal" view of the scene.
|iPhone 5||4.8 x 3.6 mm||6.0 mm||7.2||4.2 mm|
|APS C, or DX||24 x 16 mm||28.8 mm||1.5, 1.6||30-35 mm|
|35 mm, or FX||36 × 24 mm||43.3 mm||1||45-50 mm|
|120/220, 6×6||56 × 56 mm||79.2 mm||0.55||75-80 mm|
|120/220, 6×9||84 × 56 mm||101.0 mm||0.43||105 mm|
|4×5 sheet film||118 × 93 mm||150.2 mm||0.28||150 mm|
|8×10 sheet film||194 × 245 mm||312.5 mm||0.13||300+ mm|
Pure and simple, in all cases of same lens at same distance, FX shows a view 1.5x wider than DX, simply because DX is cropped to a smaller view, and FX of course is not. This has perceived effects.
A 24 mm lens on FX is serious wide angle, twice wider than a normal lens. In comparison on DX, the same 24 mm lens effectively looks like a 36 mm lens (would look on FX), which is actually mild telephoto on DX, certainly not wide angle at all. We can put a 16 mm lens on DX which would look like 24 mm on FX, which extreme wide angle then involves more lens distortions.
A 200 mm lens on FX acts like 200 mm. In comparison on DX, the cropped view from the same lens looks like 300 mm (would look on FX). in that way, DX shows 1.5x effectively longer telephoto view (as compared to FX). The lens and its image does not change, but DX simply sees a cropped view on the smaller sensor (which when enlarged later, appears telephoto). This is perceived as an advantage at longer range, and DX is popular for sports or wildlife, for this reason.
Of course, it does not matter when we crop it (to get same view). We could crop with the smaller camera sensor area, 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. You can see this same telephoto effect in your photo editor by simply zooming in. 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 would only have about 40% of the pixels left. Starting from the D700 12 megapixels, cropping FX to DX leaves 5 megapixels. The D600 24 leaves 10 megapixels, and the D800 36 leaves 15 megapixels. But this is the same result, regardless if cropped in the camera, or done later. So if you are shooting the distant wildlife with FX, and wishing for the DX telephoto effect, either select the DX menu, or just simply plan to crop it later. However, there is a bigger difference. If instead using the real DX camera, it returns its normal 16 or 24 megapixels for that DX view, which is a plus.
The purpose of the effective focal length comparison (compares lens view to that using 35 mm film frame size) is simply 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 30 mm lens means the same thing on FX (same view) - that a 30 mm lens always meant for 35 mm film. So this is another relative advantage of FX - FX is like "Old Home Week" again - the way we learned to think of it in the past. "Effective focal length" (actual focal length x sensor crop factor) is used to compare a lens view to 35 mm film format, which helps old timers "know" or predict what that other lenses will do. Which is surely pointless to newcomers who never used 35 mm film, but nevertheless, it is quite important to those who grew up thinking that way.
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:
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.
However FWIW, be aware that the big downside of using one walk-around lens like a 24-120 mm on a FX body in DX mode is that 24 mm offers no wide angle when switched to DX mode. DX requires 16 mm to do what 24 mm does on FX, so plan on needing the 16-85 mm lens for DX mode.