David Hobby (THE Strobist) posted about the Nikon D600's tragic flaw: its 1/200 sync speed. If you're new to flash photography, you may be wondering why there was a groan from lighting enthusiasts when they found out about this.
What is sync speed?
Sync speed is the highest shutter speed that will allow your camera to use a flash without HSS (discussed below). Imagine your shutter is like a canvas in front of a window. When you press the shutter, the window (which represents the shutter curtain) moves across the canvas for the exposure. Above a certain speed, that window can't get any faster. Instead, to get a higher effective shutter speed, the window becomes narrower -- narrower than the canvas -- while sliding across at the same maximum speed. The narrowing of that window poses a problem for flash photography because the entire flash burst happens in an instant. Since the canvas is never fully exposed (because the window becomes narrower than the canvas), the flash burst is partially blocked. The blocked portion of the canvas receives none of the flash.
The maximum speed the shutter can travel without narrowing that window is the sync speed. If you somehow force the camera to exceed the sync speed while using flash, the shutter curtain will block a portion of the image and that portion of the image won't be lit by the flash. Oftentimes, this makes the image unusable. For this reason, when you're using a flash, the camera either will stop you from using a higher shutter speed, or will allow you to exceed the shutter speed but with HSS activated, which has disadvantages of its own (see below).
The reason sync speed is critical for flash photography is it can control the ratio of ambient light to the flash. Remember that the flash burst is nearly instantaneous. If you reduce the shutter speed, the amount of ambient exposure will decrease. However, the exposure from the flash is not affected as long as the shutter speed is slower than the duration of the flash, and you don't exceed the sync speed.
So it goes like this: let's say you're shooting in day time (let's say your exposure is ISO 100, 1/125, f/8). The light isn't flattering so you decide to use a flash with an umbrella. You take a shot. If you didn't change your exposure then the flash's additional light would cause the shot to be overexposed. But you knew that so you underexposed the ambient light by 1 stop, let's say. ISO 100, 1/125, f/11. At ISO 100 and f/11 your tiny speedlight in an umbrella doesn't have enough power to illuminate the subject so she's underexposed. You could double your flashpower by adding another speedlight. Or instead, you could use ISO 100, 1/250, f/8. Your ambient is the same but at ISO 100 and f/8, your speedlight has twice the exposure than ISO 100 and f/11. In other words, the sync speed allowed you to decrease the amount of ambient light without affecting the flash, thus increasing the flash power relative to the ambient light.
The other reason sync speed is important is because a higher sync speed allows you to shoot at high shutter speeds, which is useful when you're freezing action.
The D600's sync speed is only 1/200, compared to high-end Nikons with a sync speed of 1/250. Everyone kind of knew that the D600 was a lower-end model compared to the D4 and D800 so there had to be some sort of compromised spec. So, I expected a shorter shutter life like 150,000 instead of 200,000. Maybe it wouldn't have a magnesium alloy body (fortunately, it does). I did not expect Nikon to screw us with the sync speed. That is a serious handicap, like not having an ISO button or WB button. (Nor did I expect a shutter speed max of 1/4000.) I guess Nikon wants to make absolutely sure no professional will opt for the D600 instead of the pricier D800 or D4.
What about ISO?
One cool feature of the D600 is the low 100 base ISO. DSLRs of the previous generation usually had a base ISO of 200. One question is whether the D600's 100 ISO can compensate for the sync speed for overpowering the ambient. The answer is no. When you lower the ISO you indeed decrease the ambient. Awesome! But you're also decreasing the flash exposure. Argh. So in terms of ratio of ambient to flash, you haven't changed anything. Indeed, this is also why having a high base ISO of 200 is not an issue with respect to overpowering ambient. (Yes your ambient is twice as strong, but so is your flash.) Don't get me wrong - I'm glad the D600 has a low ISO because it means I can shoot at an aperture one stop wider (for shallow depth of field) than I could with ISO 200.
What about HSS?
Middle and high-end Nikons have a high speed sync (HSS or Auto FP in Nikonese). Remember that narrow sliding window? What HSS does is to rapidly pulse the flash while the window is moving across the canvas like a continuous source of light, thus illuminating the entire canvas. No more shutter curtain showing up in the shot. The problem with HSS is that it decreases your flash power by more than two stops. You read that right. Your flash has less than 1/4 the power it has when it is not in HSS. So HSS doesn't help at all when you're trying to overpower the ambient. (It does help for freezing the action or for using a wider aperture for shallow DOF.)
Although the 1/200 sync of the D600 is disappointing, there are a couple of mitigating factors.
First, the D600 CAN sync at 1/250 without using HSS. The catch is the flash range will be reduced (i.e. the flash exposure will be decreased). The decrease is not as big as when you switch to 1/250 with HSS. However, it is a decrease nonetheless. I've had a D300 which had a similar feature (1/250 sync with non-HSS 1/320) and the decrease was noticeable but didn't seem like much. I will try to run tests to quantify the difference. David H. guesses the difference is 1/3 of a stop, which if true means there is effectively no power advantage to the 1/250 sync (again, there's still a benefit from freezing the action or using a wider aperture).
I expect the D600 to have a very good highlight dynamic range of newer cameras like the D7000 (5 stops of highlight range), D800 (5 stops of highlight range) or D4 (over 5 stops of highlight range), significantly better than the highlight range of the D700 (3.4 stops) or D3 (3.9 stops). If I can block the ambient light on the subject then I believe I can achieve a similar effect as having a higher sync speed by bringing down the exposure of the background during postprocessing. It's more work but hey, it's better than nothing.
Darn you Nikon but I'm sticking with my D600 preorder. If you're on the fence, I suggest you preorder it now and ask questions later. You can always cancel your preorder or scalp it on ebay.