Image Area and Crop

A little egret stands on a mudbank about 50 m out, and when it lifts off its wings span roughly a metre. With the Nikkor Z 180-600mm f/5.6-6.3 VR at 600 mm, that metre of bird lands on the Z6III's sensor as a strip about 12 mm long, a third of the frame's width. Change one menu setting and the same egret spans half the frame's width. The lens has not changed, the bird has not moved, and not one extra pixel lies across its wings. What changed is how much of the sensor the camera kept.

That choice is the image area. It looks like a zoom and behaves like a pair of scissors, and once the arithmetic is on the table the menu items explain themselves.

01.What the Image Area Is

A sensor is a grid of light-sensitive sites, and each site becomes one pixel. The Z6III's grid covers a rectangle 35.9 mm wide and 23.9 mm tall and carries 24.5 million effective pixels (p. 997). The image area is the part of that rectangle whose readings are written to the card. Everything outside it is discarded at capture, and no later software can bring it back.

Nikon names its formats by the angle of view they match. "FX" means an angle of view matching a 35 mm film camera, the format called full frame, and "DX" one matching the smaller APS-C format (p. 36). The camera offers four photo areas (p. 116), with pixel counts in the image-size tables (pp. 119-120) and megapixels on p. 998. The last column is my arithmetic.

16:9 keeps the full width and trims bands from top and bottom. 1:1 keeps the full height and trims the sides. DX trims all four edges in proportion, so it keeps the 3:2 shape and simply shrinks it. The RG illustrates the four as nested regions of one sensor (p. 116), and the figure draws them centred, to one scale.

The rule to carry away is that every option other than FX records a subset of the FX frame. None reaches a part of the sensor that FX leaves out, and that one fact settles most arguments about cropping in camera.

02.Crop Factor From First Principles

How Big the Bird Is on the Sensor

A lens throws a small inverted picture of the scene onto the sensor. For anything far away compared with the focal length, its size follows from similar triangles. With focal length $f$, subject size $S$ and distance $D$, the image on the sensor measures

$$s \approx \frac{f \, S}{D}$$

For the egret, in millimetres, $f = 600$, $S = 1000$ and $D = 50\,000$:

$$s \approx \frac{600 \times 1000}{50\,000} = 12 \text{ mm}$$

Now count pixels. The sensor is 35.9 mm wide with 6,048 pixels across, so the pixel pitch, the spacing between neighbouring pixels, is $35.9 / 6048 \approx 0.00594$ mm, about 168 pixels to the millimetre. The wingspan covers

$$12 \times \frac{6048}{35.9} \approx 2020 \text{ pixels}$$

That number is set by the lens, the distance and the pixel pitch. No image-area setting can raise it. Pixel shift shooting, which merges several raw frames into one larger picture, is meant for static subjects with the camera on a tripod (pp. 552-554), so it is no help with a bird.

The Same Bird in a Smaller Frame

Switch to DX and the camera keeps a rectangle 3,984 pixels wide, about 23.7 mm of sensor. The egret's image is still 12 mm long and still covers the same 2,020 pixels. Only the frame around it has shrunk. In FX the wings span $12 / 35.9$ of the width, about a third, and in DX $12 / 23.7$, about a half. The bird looks bigger because there is less picture.

A lens that really is 900 mm, on the full FX frame, behaves differently. The same formula gives $s = 900 \times 1000 / 50\,000 = 18$ mm, about 3,030 pixels across the wings. That is magnification. The image is half as large again in each direction, so the bird carries $1.5^2 = 2.25$ times as many pixels in all. The DX crop gives the framing of that lens with none of its extra detail.

Crop Factor and Equivalent Focal Length

The crop factor is the ratio of the two widths:

$$c = \frac{6048}{3984} \approx 1.518$$

Nikon rounds this to 1.5 and says that multiplying the focal length by it gives the approximate 35 mm focal length for the same view (p. 116). That product is the equivalent focal length:

$$f_{\text{eq}} = c \, f \approx 1.5 \times 600 \text{ mm} = 900 \text{ mm}$$

So 600 mm on DX frames like 900 mm on FX, but it does not resolve like 900 mm. The wingspan is 2,020 pixels long in both pictures, and the DX picture is the middle of the FX one. On the rest of my kit, DX turns the Tamron 70-180mm f/2.8 Di III VC VXD G2 into a 105 to 270 mm view and the Nikkor Z 85mm f/1.8 S into about 128 mm.

What the Crop Costs

Pixel count follows area, and area scales with the square of the width ratio:

$$\frac{1}{c^2} = \frac{1}{1.518^2} \approx 0.434$$

The RG's pixel counts agree:

$$\frac{3984 \times 2656}{6048 \times 4032} = \frac{10\,581\,504}{24\,385\,536} \approx 0.434$$

That is the 10.6 megapixels Nikon prints on p. 998. Dividing 24.4 by $1.5^2 = 2.25$ gives about 10.8 instead, which is why it pays to work from pixel counts rather than the rounded factor.

The loss shows when the picture is enlarged. At 300 dots per inch the RG gives a Large FX file as about 51.2 by 34.1 cm and a Large DX file as about 33.7 by 22.5 cm (p. 119). Print the DX picture 51 cm wide and every pixel covers about 1.5 times as much paper in each direction, so a feather is drawn with fewer, larger dots.

Raw is no escape. NEF (RAW) files are always recorded at Large, but the RG says the pixel count follows the image area (p. 119). A DX raw file holds the DX pixels and nothing else.

03.The Photo Image Area Item

Two settings live here. Choose image area picks one of the four options, with FX (36×24) as the default, and DX crop alert ships OFF (p. 446). With the alert ON, the image-area symbol on the shooting display blinks whenever DX (24×16) is active (p. 116). That symbol is one of many on the display (p. 54), and a blink is much harder to miss, so I turn the alert ON on day one. It does nothing for 1:1 or 16:9.

A separate item, Image size settings, holds Enable DX image sizes and Image size (DX), which give JPEG and HEIF pictures shot in DX their own size setting (pp. 119-120, 446). Those are file settings and live in RAW, JPEG, HEIF and Tone Mode. Mount a DX lens, one built to cover only the smaller format, and the camera locks the photo area at DX (24×16) (p. 116), which explains a menu that refuses to change.

The menu is slow, and the RG offers faster routes:

• Custom Setting f1 can add Choose image area to the i menu, the quick-settings screen (p. 699). Neither default i menu includes it (pp. 83-84).
• A button given Choose image area through f2 lets you hold it and turn a command dial through the options (p. 712).
• A button given Switch FX/DX through f2 jumps from FX to DX, and from any other area back to FX (p. 706).
• The U1, U2 and U3 mode-dial positions store photo shooting menu options, and Image area is not among the exclusions (pp. 142-143).

The i menu and button assignments get their own chapters later. The point here is that Switch FX/DX cannot strand you in 1:1 or 16:9, since a press from either lands on FX.

d8 Limit Selectable Image Area

This setting chooses which options appear when you turn a command dial to change the area, either in the i menu or while holding a button assigned to image area (p. 670). Highlight an option, press OK or right on the multi selector to tick or untick it, and press MENU to finish. FX (36×24) is always ticked and cannot be removed, and the other three start ticked (p. 624).

I untick 1:1 and 16:9, so the dial becomes a two-way switch between FX and DX. By the subset rule, a square or widescreen frame can be cut from an FX file later at the same pixel count and in any position, so those options buy only composing in the final shape. If you shoot squares for a feed and want to see them as you compose, keep 1:1. Otherwise each extra stop is one more place to land by accident with your eye at the viewfinder.

04.The Video Image Area Item

The video recording menu has its own Image area item, with its own Choose image area and DX crop alert. The choice is FX or DX, the defaults are FX and OFF (p. 578), and with the alert ON the symbol blinks while the DX crop is in use (p. 211). Video frames are always 16:9 (p. 210). The FX-based crop is about 35.9 by 20.2 mm and the DX-based crop about 23.7 by 13.3 mm (p. 211), and $35.9 / 23.7 \approx 1.515$ is the same factor of 1.5 as for stills.

Video differs from stills in one decisive way. A still crop shrinks the file, but a 4K video frame is 3,840 by 2,160 pixels whichever area feeds it. The RG lists 4K sizes drawn from the DX area, such as 3840 by 2160 at 120p (p. 210), so DX video is a full 4K file. Shoot FX 4K and crop it to the DX view in the editor, and you keep

$$\frac{3840}{1.515} \approx 2535 \quad \text{by} \quad \frac{2160}{1.515} \approx 1426 \text{ pixels}$$

which must be stretched back to 4K for delivery. Choosing DX in camera puts 1.5 times as many pixels across each dimension of the same view. This is the strongest argument for DX anywhere on the camera.

The video choice is also overridden more often. Raw video, meaning the N-RAW 12-bit (NEV) file type and ProRes RAW HQ 12-bit (MOV), the same idea in Apple's raw video format, takes its area from the frame size and ignores the Image area item (p. 210). Frame sizes marked FX record from the FX area and those marked DX from the DX area (p. 201), and the DX raw frame is 3984 by 2240 (p. 200). The 5376 by 3024 frame is locked to FX, and 3840 by 2160 at 120p or 100p to DX (p. 210). A DX lens selects the DX-based format by itself, and Electronic VR, the electronic stabilisation option, shrinks the crop further by an amount the RG does not state (p. 211). Frame sizes in full, and Hi-Res Zoom, belong to the video part of this guide.

Custom Setting g1 can add Choose image area to the video i menu (p. 745), and g2 can give a button Choose image area or Switch FX/DX (pp. 751, 755). The area cannot change while recording (p. 755), so the switch works between takes. For gimbal dance work that is handy, because one button gives a tighter shot from the same spot, still in 4K, with no lens change and no rebalancing. Time-lapse video, an item in the photo shooting menu, has its own FX or DX choice (p. 532) and records with the video crop (p. 534).

05.When Cropping in Camera Beats Cropping Later

By the subset rule, a DX still holds the same 3,984 by 2,656 sensor sites as a centred crop of an FX still. So the case for cropping in camera can never be more detail, and the candidates have to be weighed on other grounds.

Video wins clearly, for the reason above. A DX file keeps its full 4K frame, and a later crop of FX footage keeps about 2,540 by 1,430 pixels.

The buffer is the weakest reason on this camera. The buffer is the fast memory that holds frames until the card absorbs them. Nikon quotes over 1,000 frames for every raw and JPEG setting it lists, in FX with the electronic shutter at Continuous H (extended), a release mode that shoots about 20 frames per second (pp. 1000, 1012), and the online capacity table agrees. A burst ends sooner for another reason. Custom Setting d2 caps every burst at 200 shots by default (p. 624), about 10 seconds at that rate, and setting it to no limit works only with the electronic shutter (p. 663). With no limit set, the buffer lasts at least 50 seconds. Nikon publishes no DX buffer figures, so I make no claim that DX extends it.

File size is a modest win. A DX picture stores 10.6 million pixel readings instead of 24.4 million, so files are smaller, cards fill more slowly and a long burst is quicker to sort. The RG gives no DX file sizes, so I will not put a number on the saving.

Framing is the real argument for stills. When the bird is small, distant and staying put, and the keeper will be cropped to about the DX view anyway, composing in DX lets you place it and check the edges while it is still there. That is a working method, not a gain in quality, and it is the only reason I press the DX button with the 180-600.

Sometimes the camera decides. High-Speed Frame Capture +, a set of very fast JPEG-only release modes, offers FX or DX at C15, C30 and C60, but only DX (24×16) at C120 (p. 157). If you want 120 frames per second, DX is the price.

06.When It Does Not

Birds in flight argue firmly for FX. The DX crop is centred and fixed at capture, so a bird that drifts off-centre in a burst loses a wingtip that FX would have kept. A later crop can sit anywhere, take any shape and be tightened differently for each frame. Since the DX file holds no pixel the FX file lacks, shooting FX and cropping later loses nothing. With the 180-600 tracking a moving bird, I stay in FX.

The 85 mm is the cleaner counter-example, because with a portrait lens you can usually move. Rearranging the formula above, an FX frame at distance $D$ takes in a scene $35.9 \, D / f$ wide. At 3 m the 85 mm covers $35.9 \times 3000 / 85 \approx 1267$ mm, and DX narrows that to $23.7 \times 3000 / 85 \approx 836$ mm.

Now step forward to 2 m and stay in FX. The frame covers $35.9 \times 2000 / 85 \approx 845$ mm, almost exactly the DX view from 3 m, but with 6,048 pixels across it instead of 3,984, so 24.4 megapixels instead of 10.6. At the same f/1.8 the closer shot also has a shallower band of sharp focus for that framing, and a softer background is why most people buy an 85. Perspective depends only on where the camera stands, so the face gains slightly more depth from nose to ears. That is a choice made with your feet, and it beats cropping.

Stage dance on the Tamron sits between the two. From a fixed seat you cannot step forward, and 180 mm in DX frames like 270 mm. Dancers move unpredictably, though, and a centred crop can clip a raised arm. A crop is also enlarged more for the same screen, so low-light grain is enlarged with it. I stay in FX and crop later. Street and landscape follow the same logic, since street rewards margin and a tripod gives time to crop with care.

07.Where the Camera Chooses for You

Several features force an area or refuse one. The RG spreads them across many pages, so the table gathers them in one place.

Three rows punish a forgotten 1:1 or 16:9. High-Speed Frame Capture + offers only FX or DX (p. 157). Auto capture for photos, which takes pictures by itself when a subject meets conditions you set, will not run with either selected (p. 562). The interval timer, combined with its Time-lapse video option, will not release the shutter while 1:1 is the photo area (p. 523).

08.Recommended Settings

For birds on the 180-600, shoot FX and press Switch FX/DX only for a small, static subject you already know you will crop to the DX view. Leave it in FX for anything that flies, and accept DX when you want C120. For stage dance, portraits, street and landscape, stay in FX. For gimbal dance video, record FX for wide shots and switch to DX between takes for a tighter one.

09.Beginner Mistakes

Believing DX adds reach. DX gives the framing of a 1.5 times longer lens and none of its detail. The egret has 2,020 pixels across its wings in both, so only getting closer or a longer lens puts more pixels on the bird.

Assuming raw keeps the whole sensor. A DX raw file holds 10.6 megapixels (p. 119), and the discarded border is gone for good.

Forgetting that DX is on. With the alert OFF by default, a camera left in DX after one distant bird discards 57 per cent of every frame for the rest of the day. Turn the alert ON in both menus.

Landing on 1:1 or 16:9 by accident. Photo auto capture and High-Speed Frame Capture + then refuse them, and an interval time-lapse with 1:1 will not fire. Untick both in d8 unless you shoot those shapes on purpose.

Cropping the 85 mm instead of stepping closer. Moving from 3 m to 2 m gives the DX framing with all 24.4 megapixels and a softer background.

Cropping 4K video later instead of choosing DX. The later crop leaves about 2,540 by 1,430 pixels to stretch, while DX records the same view as a full 3,840 by 2,160 frame.

10.Related Chapters

• RAW, JPEG, HEIF and Tone Mode: image quality, NEF (RAW) and the JPEG and HEIF sizes each area produces, including Enable DX image sizes.
• Card Slots and File Handling: how the CFexpress and SD cards share the files.
• Menus and Controls: the menu tree, U1 and U2, and how buttons take on roles.
• First-Day Setup: where the image-area choice sits among the first-day settings.
• Glossary: FX, DX, image area and the other terms used here.