Left, No flash, f/8, 1/250 second. This bright window gets the attention of Matrix metering, and makes it harder. This is camera Manual mode for ambient, but Matrix does change the ambient metered target goal, which affects TTL BL balance.
The flash does its job well, but much of what we see happens in TTL BL 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. So a little "How To" info is added.
A FX D800 is used in all of this section below, to show the zoom coverage seems usually acceptable on FX 24 mm (if with enough exposure, but FX is NOT OK at 18 mm).
VK750: Manual flash mode, direct flash, full power, 24 mm, ISO 200, f/16.
The distance was measured to be precisely eight feet (with tape on the floor). The VK750 guide number for 24 mm is 91.9 (feet). For ISO 200, multiply by 1.414 which is GN 130. Therefore, this exposure is GN 130 / 8 feet = f/16, ISO 200, determined from the Guide Number chart.
The guide number is a little high in this case. Still is a good ballpark first starting point though.
SB-800: Guide Number is 98 (0.18 stop more), but this is same 24 mm f/16 on same tripod (full manual power). Very little power/exposure difference.
Unless otherwise specified, this group of test samples are: FX D800, camera manual mode M, 1/250 second, ISO 200, Matrix metering. Nikon 24-70 mm lens at 24 mm. Direct flashes on hot shoe.
VK750: Full manual power, 24 mm, ISO 200, f/8. Backed off 2 stops. Slightly too much exposure, we could debate if 1/3 or 2/3 stop excessive. I do like bright, but if people were in it, I'd say 2/3 stop too much.
The FX frame is not much trouble at 24 mm, SB-800 is not perfect either. WB correction 7000K tint -16.
SB-800: Full power, 24 mm, ISO 200, f/8, to compare. The SB-800 has slightly more power, but not much. These are easily in the same power class. WB correction 5900K Tint +6.
Slight clipping is not always bad, it depends on what detail is clipping, and how much (if it is important or not). Adobe stuff shows "what" and "how much" clipping by holding the PC Alt key while moving Exposure or White Point.
VK750: TTL menu mode (actual TTL BL), f/5.6. Any TTL needs a little power margin, for scene variations, for compensation, for faster recycle, etc. Routinely in Nikon systems, TTL BL intensity is down a bit. It is the camera system that meters it, and sets the flash power level accordingly. The Matrix metering is seeing the bright sky outside, and is turning the TTL BL mode down slightly (TTL BL is fill flash mode). For comparison, see the actual TTL mode below, where the actual metered value is used instead. The TTL flash is still automatic exposure even in camera M mode.
Photoshop ACR white balance on the white card (on floor) was 7050K tint -14. Which is a bit high, but no big deal, flashes vary color with power level, situations will vary, and all images always need attention. It's rather easy, once you accept that it needs to be done. FWIW, I do WB in Raw.
SB-800: TTL BL menu mode, f/5.6. SB-800 exposure is a little less with TTL. Photoshop ACR white balance on the white card was 6000K tint +9.
SB-800 was the first iTTL flash, ten years old now ($320 back then), but it is a classic, still the standard, in my notions. 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.
This is a Porta Brace White Balance Card (on floor), 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. In this case, the white porcelain pot would have worked as well, but we do need something known to be neutral white.
VK750: How much correction is that 7050K -14?
Above is the same image just above, alternating with standard Flash White Balance (5500K in Adobe).
At standard Flash WB, 5500K +0, the white card color is RGB(204,210,229). At 7050K -14, it is neutral white, RGB(211,211,211)... no color cast.
Flash color varies, everything varies, just how it is. This is NOT abnormal for speedlights. Also see the last frame in this series, bounce flash. Only beginners imagine their flash color and camera exposure ought to always be right. If you have never learned to correct white balance, it may be a bit blue, but no big deal if you're going to fix it anyway. Learning to deal with White Balance is the only solution. It is actually quite easy, and extremely satisfying.
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.
VK750: This is TTL menu (but actual TTL BL), f/5.6 with +1 EV flash compensation.
Obviously if f/8 is near maximum manual power, we cannot expect +1 EV at f/8 to work. But TTL BL is not quite doing maximum power at f/8 (so we can, and often should). TTL BL exposure is affected by ambient level, and is routinely down a little, much of the time.
SB-800: TTL BL menu mode, f/5.6, +1 EV. We should pay attention to results, and simply do what we see we need to do, to make it come out like we want it.
Only beginners see no need for flash compensation or white balance correction. But it's very easy to do it right.
VK750: TTL mode (actual TTL now), f/5.6, zero compensation, 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). The indoor ambient is too low to matter, 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. The Neewer seems unaffected by D-lens distance, but ambient does affect TTL BL (except not in this Spot TTL case - See More). And the manual ambient exposure of the leaves is -3.3 EV below what metering their ambient would want. So here, Spot simply forces TTL mode, which is often desirable indoors. Normally, TTL is a little brighter flash than TTL BL mode (not held back by balancing with ambient or D-lens).
Here is the big point: These last two (+1 EV TTL BL and +0 EV TTL) are similar, because the matrix metering is being affected by the window. It doesn't know the situation, it just sees some very bright ambient light and suspects a possible problem. The TTL BL Balanced system is reducing the flash a bit to be fill level for the bright ambient (mostly to NOT overexpose the ambient). But TTL mode (switched by Spot metering) does not back off, it comes ahead on, as metered, regardless of ambient. Note that the flash system does NOT do Spot metering, flash has its own system, but Spot does switch flash mode to be TTL (no BL balancing). I prefer Center Weighted metering, but this was Matrix.
Bottom line: We just need to watch results, and simply do what we see we need to do, to make results be as we want them. That is our job to do. Some users cannot be bothered, but a little flash compensation works wonders.
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 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 in camera A or P modes, the 1/60 second Minimum Shutter Speed With Flash normally prevents properly exposing the weaker ambient (sunlight will be not be negligible). But Spot is only about ambient (and this is camera M mode which cannot respond), and flash system does Not use the Spot metering (flash has its own system). But Spot does convert TTL BL metering to be TTL mode.
Two factors, Matrix metering "balancing" TTL BL, and for the Nikon (Neewer seems exempt), the D-lens distance. The Exif reports Nikon 24-70 mm D-lens distance was 2.0 meters at 24 mm. However, the actual 8 foot measured distance is 2.4 meters. Zoom lenses vary, but the camera is being thought closer than it is, the D-lens system thinks 2 meters worth of flash should be enough. We can put it back with +EV compensation, or indoors (insignificant ambient) we can shut both Matrix and the D-lens off with Spot and TTL mode (the previous picture above without either is darker). Here, Matrix metering sees the sunny ambient outdoors, and the white roof and red fence are clipping. So there are a few things happening, and in general, near +1 EV flash compensation routinely does help TTL BL indoors. The Neewer is subject to Matrix metering, but not to the D-lens distance.
This need is not due to any flash unit, this just happens in the Nikon TTL BL system. There are multiple reasons.
VK750: Bounce flash, TTL BL, f/4, 90 degrees up at ten foot ceiling, Not using pull out bounce card. Zero flash compensation (a bit would be good).
In this bounce case, Photoshop ACR white balance on the white card (on floor) was 4950K -4 tint.
SB-800: Bounce flash, f/4, ten foot ceiling. Not using pullout bounce card. TTL BL menu mode. Zero flash compensation.
Photoshop ACR white balance on the white card was 4750K +11 tint.
VK750: Bounce flash, f/4, 90 degrees up at ten foot ceiling, using pull out bounce card.
In this bounce case, Photoshop ACR white balance on the white card was 5200K -3 tint.
Not much to see this time, but notice the direct card shadows of leaves on wall behind.
SB-800: Bounce flash, f/4, ten foot ceiling. pullout bounce card. TTL BL menu mode.
Photoshop ACR white balance on the white card was 5100K +7 tint.
SB-800 has menu to select TTL mode, which I would normally use indoors. But a bit more +1 EV flash compensation instead does the same thing. Spot metering does it too (good indoors). We have to watch either mode, so it is merely a matter of degree. Just use Flash Compensation, and simply do what you see you need to do, to get what you want
VK750: Bounce flash, f/4, 90 degrees up at ten foot ceiling, using pull out bounce card, and +1 EV flash compensation.
In this bounce case, Photoshop ACR white balance on the white card was 5150K -7 tint.
SB-800: Bounce flash, f/4, ten foot ceiling. pullout bounce card. TTL BL menu mode, and +1 EV flash compensation.
Photoshop ACR white balance on the white card was 5000K +8 tint.
When using bounce flash, the bounce card is really important to add catchlights in human eyes (adding sparkle for a sense of liveliness or vitality). Pay attention to the catchlights in the TV news desk anchors eyes - that is intentional.
The card adds a bit of frontal fill too, but it also adds a set of direct shadows, so don't overdo it. It is slightly larger than the Nikon card, and we don't have to pull it all the way out. But we still have gradient shadows to show shape detail, not purely flat frontal light.
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.
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.