To first show what the flash below is doing.
Left, No flash, same f/13, 1/60 second.
The flash does its job well, but much of what we see that happens in TTL flash mode is not actually about the flash unit. Even for a third party flash unit, the camera metering system runs the TTL show. All any TTL flash has to do is to respond correctly (it flashes), with the programmed power level. It helps us if we realize what the flash is being asked to do. This review tries to describe that.
This test is sort of "by formula", to be the same procedure as the Yongnuo YN565EX review, allowing direct comparison with it too.
NW985N: Manual flash mode, direct flash, full manual power, 24 mm, ISO 200, f/13, 1/60 second. Subject distance 10 feet (measured on the floor). White Balance is adjusted here to 6600K -19.
It is nearly one stop underexposed (due to my f/13 setting, the SB-800 too), but it shows that the maximum flash power compares with the SB-800.
Unless otherwise specified, this group of test samples are: D300, camera manual mode M, 1/250 second, ISO 200, Matrix metering. Nikon 14-24 mm lens at 24 mm. Direct flashes on hot shoe.
SB-800: Guide Number is 98 (computes near f/13 at ISO 200), and this is same 24 mm f/13 1/60 second on same tripod (full manual power). The Neewer NW985N is the same power class as the SB-800 (which is a lot). White Balance is adjusted here to 5850K -8, but they are closer in the bounce shot below. We always need to check it anyway.
SB-800 was the first iTTL flash, ten years old now ($320 back then), but it is a classic, still the standard, IMO. It has recently been serviced with a new flash tube (routine service check, $75 each), which restored its color. But flash color still just varies, with power level, and with age.
NW985N: TTL mode (actual TTL BL), f/5.6 (1/250 second for the daylight). TTL flash is still automatic metering even in camera M mode. I am really talking of my SB-800 experience, but routinely in Nikon systems, TTL BL intensity is down a bit. This seems less true of the NW985N. It is the camera system that meters it, and sets the flash power level accordingly.
Photoshop ACR white balance on the white card was 6800K -18 tint (a little more blue and green, corrected now). Which is no big deal, flashes vary color with power level, other situations will vary, and all images always need attention.
SB-800: TTL BL menu mode, f/5.6
Photoshop ACR white balance on the white card was 6100K -3 tint.
Hanging is a Porta Brace White Balance Card, 5x7 inches, plastic, durable, washable, accurate, $5 at B&H. It is all that is needed, plus some software to click it. The WhiBal card is also very good.
FWIW, I do WB in Raw, but use camera Auto WB for the embedded JPG shown on the rear LCD.
Above is the same image just above, alternating with standard Flash White Balance (5500K in Adobe).
White Balance is NEVER one single constant value, not for flash, not for anything. Color varies with power level, so it is questionable for a variable power flash to even specify a color temperature (Nikon and Canon flashes do not, only the camera says what it will assume). There are many colors of flash, of incandescent, of daylight, etc. My point is that a white card makes handling WB be trivial, especially if Raw. All images here are White Balance corrected. Correct White Balance can matter more some times than others, but it always matters.
NW985N: TTL mode (actual TTL), f/5.6, Spot metering (on green leaves) to force TTL. The flash system does NOT use Spot metering. Only ambient watches the spot (but ambient level can affect TTL BL level). And this is camera M mode anyway, so ambient settings did not change (and the green leaves were no factor here). Since TTL BL balancing is not possible with ambient Spot metering, Spot metering switches metering of the flash to be actual TTL mode. This switches Off both D-lens distance monitoring, and also balancing with ambient.
This NW985N seems unaffected by D-lens distance, but ambient does affect TTL BL (except not in this Spot case - See More).
The manual ambient exposure of the leaves (1/250 f/5.6) is -3.4 EV below what metering indoor ambient would want. So here, Spot simply forces TTL mode. Normally, TTL is a little brighter flash than TTL BL mode (not held back by balancing with ambient or D-lens).
SB-800: TTL mode, f/5.6, Spot metering, which switches TTL BL menu to be actual TTL mode.
Spot metering is often misunderstood by novices. Spot is only about the metered ambient, but it does NOT make the spot be correct exposure. It only makes the ambient spot be equivalent of a middle gray tone, which may or may not be what you want (probably is not correct for faces, not without knowledgeable +1 EV compensation). Typically indoors, the 1/60 second Minimum Shutter Speed With Flash prevents properly exposing the weaker ambient. But Spot is only about ambient (and this is camera M mode which cannot respond), and flash does not use the Spot metering (flash has its own system). But Spot does convert TTL BL to be TTL mode.
NW985N: TTL mode (actual TTL BL), f/5.6, Matrix again, with +1/3 EV flash compensation, which is routinely often better flash exposure for TTL BL flash indoors.
My +1/3 EV compensation was not needed, I was just thinking I probably needed some (and my normal +1 was excessive here). There is still slight clipping, which is not always bad, it depends on what is clipping, and how much (if it is important or not). Adobe stuff shows "what" and "how much" by holding the PC Alt key while moving Exposure or White Point.
SB-800: TTL BL menu mode, f/5.6, Matrix again, with +1 EV flash compensation.
There are multiple reasons possible for variance in TTL systems. A little compensation works wonders. Simply do what you see you need to do, to get what you want.
Note that +1 EV compensation requires double flash power. If we are at maximum manual power, we cannot expect +1 EV to have any effect. So we have to keep TTL down under a safety margin, so we can adjust it. TTL BL exposure is affected by ambient level, and is routinely down a little, much of the time.
NW985N: Bounce flash, f/4, 90 degrees up at ten foot ceiling, using pull out bounce card. TTL, +1 EV Flash Compensation (because it is TTL BL). 1/250 sync shutter speed is limited for f/4 outside, it probably would have done f/5.6. This +1 EV compensation is in no way unusual for TTL BL, for any flash. TTL BL reduces the flash level according to the ambient it sees (which is quite bright here). Just use Flash Compensation, and simply do what you see you need to do, to get what you want.
Photoshop ACR white balance on the white card is 5050K -14 tint.
SB-800: Bounce flash, f/4, ten foot ceiling. pullout bounce card. TTL BL menu mode, +1 EV Flash Compensation.
SB-800 has menu to select TTL mode, which I would normally use indoors. But a bit of +EV flash compensation does the same thing. We have to watch both modes, so it is merely a matter of degree. Do what you see you need to do.
Photoshop ACR white balance on the white card is 5100K -5 tint.
The card can add a bit of fill too, but it also adds a set of direct shadows, so don't overdo it. We see bounce shadows under the window frame, but the other shadows are direct shadows from the bounce card. We still have a little gradient tones on the pot, but we don't have to pull the bounce card quite all the way out (It is slightly larger than the Nikon card).
This is about enough fill. The pullout bounce card is plenty large enough. If you are bouncing, don't play he-man and imagine you need some heroic size super-card - which simply obliterates the bounce, and all you get then is direct flash. We can learn to simply look at results and then fix it.
I'd say the Neewer is certainly holding its own, and maybe outdoing the Nikon a bit on exposure. Wondering if that might be the newer firmware version? The White Balance may not be quite as close, but WB always has to be looked at and adjusted anyway, and it will fall with use.
Frankly, the NW985N 18 mm zoom is very creatively rated, they have to be assuming DX only. Not sure that is fair, without saying so. It might be 24 mm for FX.
Bare wall at 4 feet. That's a small WhtBal card taped to it, helps focus. D800 FX and D300 DX, TTL mode. The flash heads will go seven degrees lower than level, and I should have been more conscientious about doing that at only four feet.
Neewer NW985N TTL
Nikon SB-800 TTL
See a report on the ability of third party flashes (non-Nikon brand flashes) to ignore bad effects of D-lens distance on flash on hot shoe cords, and/or from the many zoom lenses reporting inaccurate D-lens distance.