When one uses a Nikon DX (APS-C) camera on an FX camera body, the camera automatically enters a "DX crop mode", using only the center portion of the sensor to match the DX format, as explained on Nikon's web site.

The FX format is roughly 36×24mm, while DX is 24×16mm, and it's this smaller size that DX crop mode uses. Notice that the short edge of the FX format is equal to the long edge of DX — and of course the image circle is perfectly round, not oblong. Do any Nikon full-frame cameras allow a "square DX" format, either 24×24mm (with some possible vignetting in the extreme corners) or slightly reduced?

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    Quick answer, No. But you can however compose your square format image in camera, then just crop out what you don't need in post. Turn on rule-of-thirds guide to help you with framing the image in the viewfinder. Also, check the manual for "Translucent Masking"! Side note: I'll never understand why would someone spend a lot of money for FX sensor then place DX lens over it. I guess if you are sports/wild life shooter, you might need that extra reach. But again there are tele-converters. If you place DX lens, on D700 for example, you get 5MP's image from effective 12.1MPs.
    – Alen
    Mar 8 '12 at 5:42
  • Also check out DX crop mode guide
    – Alen
    Mar 8 '12 at 5:46
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    For many years, Nikon didn't have any FX DSLRs, so someone who hoarded optics back then might now be stuck sitting on a pile of DX lenses. If he/she happens to like square format, this is a natural question to ask.
    – Imre
    Mar 8 '12 at 8:23
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    @Alen: if one is cropping to square anyway, the DX lenses basically cover the square center of the FX sensor, beyond the DX crop. So, in such a mode, one would actually not be missing out at all.
    – mattdm
    Mar 8 '12 at 14:40
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    One small point: it's the diagonal of the image that needs to fall within the image circle of the lens. A 16x24mm sensor has a diagonal equivalent to a square image of just a hair over 20.3mm on a side, so you'd gain vertical pixels at the expense of some horizontal ones. There would be more pixels in total than in a DX-format 2:3 image, but you wouldn't get a 24mm square image without severe (and perhaps complete, depending on the lens) corner vignetting.
    – user2719
    Mar 9 '12 at 18:24

Square format is not supported, but 5:4 aspect ratio offered by D3, D3s, D800 and D800E is only slightly wider, 30mm. The nice thing is that the viewfinder is electronically masked, so you can compose with good precision. This aspect ratio is not provided on a D700 or D3x.

Alternatively, you could always crop in post.

Of course, you'll also need to disable Auto DX Crop.

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    While Nikon (or anyone else with a similar lens mount compatibility) could theoretically offer, say, a 20.3-and-a-bit-mm square crop, it wouldn't be compatible with the tulip-shaped lens hoods usually included with the smaller-format lenses. Since vignetting is almost always an issue already with DX-format lenses, I can see avoiding that potential source of bad PR—no matter how many caveats you put in the manual, it's always going to be Nikon's fault in a lot of minds.
    – user2719
    Mar 8 '12 at 18:13

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