Here we are over a decade into the D1 revolution and we're still going through focal length agony all over again. If you haven't been paying attention, when the D1 came out with its smaller-than-film sensor, we interchangeable lens folk got introduced to the "equivalent focal length" jargon that compact digital camera users had been safely ignoring for years. Ever since then there have been yelps and confusion about focal lengths and which lenses should be in your kit.

Let's recap a bit, just so we're all on the same starting page. Here are a few things you need to know:

. The Nikon D3 was the first FX camera they produced (Kodak produced two, the Pro 14n and the followup SLR/n, though they didn't refer to them as FX; for the purposes of this article, you can consider them FX cameras). Since then we've gotten more. FX cameras: D3, D3s, D3x, D4, D600, D700, D800, D800E. The Kodak SLR/n was also full frame, and thus "FX".**Nikon refers to digital cameras that have the same size sensor as film as FX**. Until the D3 appeared, all Nikon DSLRs were DX. The FX frame is 1.5x the size of the DX frame (e.g. the horizontal axis of the capture area for DX cameras is 24mm while it is 36mm for the FX cameras). DX cameras: D1, D1h, D1x, D2h, D2hs, D2x, D2xs, D40, D40x, D60, D70, D70s, D80, D90, D100, D200, D300, D300s, D3000, D3100, D3200, D5000, D5100, D5200, D7000, D7100, plus all the Fujifilm DSLRs.**Nikon refers to digital cameras that have a smaller-than-film sensor as DX**. All Nikkor lenses prior to the introduction of the D1 (other than the ones for the short-lived APS film bodies) have an imaging circle that covers the FX sensor size. Lenses introduced after the introduction of the D1 that are marked DX have an imaging circle that covers the DX sensor size, though some of those may cover the FX sensor size at certain focal lengths.**Lenses are designed with an imaging circle that covers a particular capture area, either FX or DX**. Both DX and FX lenses are marked with their actual focal length. That's because serious photographers may need to use the actual focal length in calculations, such as for depth of field.**Focal length is focal length**. If you have a lens that covers both the DX and FX frame—for example the 14-24mm set to 18mm—it is an 18mm lens on both the DX and FX cameras. But the angle of view covered for the scene being photographed is smaller on the DX camera, as the sensor is smaller. Sometimes for convenience we cite equivalent focal lengths. For example, 18mm on a DX body has the equivalent angle of view as a 28mm lens on an FX body. Indeed, just divide the actual focal length by 1.5 to get the equivalent focal length for DX (28/1.5 = 18.6, which we tend to round to 18, since 19mm isn't a common focal length). Thus, if you use a 28mm lens on an FX camera and want to shoot the same angle of view on a DX body, you need an 18mm lens. But if you use the same 28mm lens on both an FX and DX body, on the DX body you get the equivalent angle of view of a 42mm lens (28 * 1.5). We multiply in this case because the DX sensor is cropping the angle of view compared to the larger FX sensor. The 1.5 is a rounded number, and the 35mm film (FX) and digital DX frames aren't always exactly the same proportion, thus this 1.5 only gets a very close approximation. If you want the actual numbers and the exact horizontal, vertical, and diagonal angle of views for all the lens sizes for DX, you'll find them in my books.**Angle of view is different for the same focal length on DX and FX**

Basically, FX lenses can always be used on DX bodies. (Note: Nikon doesn't always refer to lenses as "FX." Essentially all Nikkor lenses that aren't marked "DX" (or 1 Nikkor for the Nikon 1 bodies) should be considered "FX.") Some DX lenses can be used at some focal lengths on FX bodies (the most notable and interesting is the 12-24mm, which makes a fine 18-24mm FX lens).

What follows are some simple (near) equivalent lenses you can use to figure out what you've got and how it fits if you decide to shoot both DX and FX.

First, let's map equivalent *focal lengths for lenses*. By equivalent lenses I mean that the FX lenses I list first will have about the same angle of view equivalence as the DX lenses I list second. Put another way, if I'm shooting with a 28mm lens on my D3 (FX body) and want the same angle of view on my D300 (DX body), I'd put an 18mm lens on my D300:

14mm FX = 9mm DX

18mm FX = 12mm DX

20mm FX = 13mm DX (14mm is close)

24mm FX = 16mm DX

28mm FX = 18mm DX

35mm FX = 23mm DX (24mm is close)

50mm FX = 33mm DX (35mm is close)

85mm FX = 57mm DX (55mm is close)

105mm FX = 70mm DX

135mm FX = 90mm DX

200mm FX = 133mm DX (135mm is close)

300mm FX = 200mm DX

400mm FX = 267mm DX

500mm FX = 333mm DX

600mm FX = 400mm DX

Indeed, those of us who shot on DX bodies during the past years built up new lens sets that were *equivalent* in angle of view to our old. For example, I have a 200mm f/2 for my DX kit where I use a 300mm f/2.8 for my film and FX kits. Likewise, I have a 12-24mm f/4 DX lens for my DX body when I might use a 17-35mm f/2.8 lens for my film and FX bodies. At the wide angle end it was difficult to get exact equivalents, especially of the fast aperture lenses.

Now let's look the other direction. Let's say that I only have one lens and need to use it on both my D4 (FX body) and on my D7100 (DX body). On the D4 I'll have a full angle of view but on the D7100 I'll have a cropped angle of view. Here's the figures that express that:

14mm on FX becomes 21mm equivalent on DX

18mm on FX becomes 27mm equivalent on DX

20mm on FX becomes 30mm equivalent on DX

24mm on FX becomes 36mm equivalent on DX

28mm on FX becomes 42mm equivalent on DX

35mm on FX becomes 53mm equivalent on DX

50mm on FX becomes 75mm equivalent on DX

85mm on FX becomes 128mm equivalent on DX

105mm on FX becomes 158mm equivalent on DX

135mm on FX becomes 203mm equivalent on DX

200mm on FX becomes 300mm equivalent on DX

300mm on FX becomes 450mm equivalent on DX

400mm on FX becomes 600mm equivalent on DX

500mm on FX becomes 750mm equivalent on DX

600mm on FX becomes 900mm equivalent on DX

This second table illustrates the basic dilemma we had back at the film-to-digital transition, and now have again in the DX-plus-FX era. If I carry only one set of lenses that work correctly for my FX body, my DX backup body is going to perform differently with those lenses, essentially giving me angle of views that are quite a bit cropped from what I see on my FX body. But I'm going to propose a solution (and one you might not have guessed).

Let's say that I want to be able to shoot from a 100 degree angle of view down to a 12 degree angle of view. On an FX body, that's 18mm (100 degrees) to 200mm (12 degrees). On a DX body, that's 12mm (100 degrees) to 135mm (12 degrees). Believe it or not, I can do that in for both bodies with only three lenses:

. This provides 12-24mm on the DX body (100 degrees down to 78 degrees). But that lens also manages to cover the full FX frame from 18-24mm, so I have my 100 degrees down to 84 degrees for FX, too.**12-24mm f/4 DX**. This new lens gives me 84 degrees down to 34 degrees on the FX body, and 78 degrees down to 23 degrees on the DX body.**24-70mm f/2.8**. This older lens takes me down to 12 degrees on the FX body and a bit over 8 degrees on the DX body.**70-200mm f/2.8**

Plus you get a side benefit: all 77mm filter threads on those three lenses. And here you've been agonizing over lenses! Of course, if you're into primes or specialized lenses, things get more difficult, as there are gaps all over Nikon's line.

Unfortunately, all is not quite that simple. "Equivalence" is a convoluted subject. Read my article on equivalence over at sansmirror.com for more.