首先HLG是一条用于交付的Gamma曲线，向下兼容SDR内容，这也就注定了它的上限比较低，这也是为什么大部分人对它不感兴趣的原因(另一个原因是它是HDR）。HLG的动态范围(尤其高光)比不过N-LOG，更别说Slog3了。暗部表现则比其他LOG曲线要好一些。它最大的优点就是所见即所得，不需要LUT还原，也(基本)不需要向右曝光，特别适合HDR直出。不过如果你只做SDR的话，还是选N-LOG吧。只是2025年了，拍N-LOG/NRAW获得12+档动态范围，然后再压缩到8档，我只能说历史的包袱太沉重了。可以参见：再谈苹果XDR显示器与HDR之殇

响应曲线

我在Z8上实测响应曲线，测试方法：调整曝光，记录不同曝光值下中灰的IRE，并和理论值做对比。为了统一起见，我将中灰(Stop 0)都曝光到36%。Note: HLG中灰的理论值应该是38%，不过差别不大，大概0.2档左右。结果实测N-LOG的时候，同样用了95 (37.2%)的斑马线，最后得到的曝光是38%，不管了，差不多就行了。

Z8 HLG实测曲线和理论曲线在[-7, +3]的范围内贴合的非常不错。实测的HLG底噪大约是3%,。两者在高光部分差别比较大，HLG中灰以上一档就开始变成对数曲线，和Stops应该是线性关系，但实测下来，Nikon对+3档以上的高光做了一些保护，曲线非常平滑，因此也比理论多了1档的高光，中灰以上5.3档才会完全过曝。 但比起N-LOG还是差了一档多。另外HLG模式下，IRE最大值不是100，而是97左右，斑马线需要设置在245才会有效果。

N-LOG的曲线大致上贴合的还是不错的。noise floor，13%左右。-7档开始就贴着了。只有在+1档到+2档的附近有点波动。高光部分的斜率还是太大了，竟然比理论值还大一些。中灰以上6档就过曝了。只比HLG好了不到1档。

名义ISO

众所周知，N-LOG的基础ISO是800，第二档则是ISO 4000。HLG的基础ISO是400，第二档是ISO 2000。无论是N-LOG还是HLG，它们的ISO都是名义ISO，或者说等效ISO。在相同照度下，我使用相同的参数(1/400s F/4 ISO800)拍摄灰卡，N-LOG和HLG的IRE是36%左右，SDR则为52%。三者还原之后中灰的亮度大体上相同（SDR稍微亮了一点），证实了”名义”ISO。但实际上传感器使用的ISO是多少呢？其实都是ISO 100左右，如何证明呢？拍摄同样的白卡，记录刚刚过曝时的曝光参数。
N-LOG: 1/30s F/4 ISO 800
HLG: 1/30s F/4 ISO 400
SDR: 1/30s F/4 ISO 80 (NL profile, Active D-lighting off)
进光量相同，传感器同时过曝了，表明实际ISO是相同的。
等效ISO也可以这么理解：
N-LOG拍摄时欠曝3档，后期还原时把中灰提亮3档，以起到保护高光的目的。
HLG拍摄时欠曝2档，后期还原时把中灰提亮2档，以起到保护高光的目的。
这也就是为什么按照标准曝光（将中灰曝到36% IRE)，N-LOG暗部噪点爆炸，需要使用向右曝光的原因之一。
HLG则相当于找了一个平衡点，欠曝2档，等效于使用ISO 400拍视频，对于全画幅来说勉强可以接受吧。

HLG quality 设置

由于是面向直出的交付曲线，只有H.265编码支持HLG。虽然Nikon贴心的提供了HLG质量选项，但这里有个坑，在默认的设置下，相机是会对HLG的画面进行锐化的，导致使用超级锐的Z卡扣镜头时会产生过度锐化的情况。需要将Quick Sharp设置成-1才能获得未锐化的画面。有需要的话，也可以将对比度和饱和度也降低一些，推荐-1。

不同Quick Sharp模式下的波形图

调整对比度

调整饱和度

I did some test about IRE vs stops in HLG mode and compared that to the theoretical N-Log curve.

Setup: I expouse the 18% midgray to 36% IRE in HLG mode to match that with N-Log. Adjust the exposure time first then aperture, ISO to over and under expose the midgray for IRE readings.

Note: Techincally, you should expose midgray to 38% IRE in HLG mode (ref), diffuse white will be around 75% IRE, 75%+ is for specular highlights (0~3 stops over diffuse white).

Stops vs IRE

First of all, we can see that HLG curve is more contrastive than N-Log. Highlights will be clipped after 5 stops over midgray which I think is OK for a lot of scences. N-Log retains more infornation in the highlights (if not over-exposed) up to 6.3 stops at the cost of noiser shadows. HLG has more detials in the shadow area, noise floor is about 2% at ISO 400 (12.5% for N-Log).

Another thing intresting is that the HLG curve is very steep from -1 to +3 stops but quickly saturated afterwards (non-linear anymore), highlights are compressed and will clipped around 96% IRE, making sure to set the high zebra to 245 when shooting HLG.

Base ISO for HLG and N-Log

Nikon Z8’s HLG has a “nominal” base ISO 400. Under the hood, the sensor is working at ISO 100, shadows will be boosted by 2 stops while highlights will be intact, bascially you are shooting at ISO 100 with 2 stops under to protect highlights.

N-Log has a “nominal” base ISO 800. Same as HLG, the sensor is working at ISO 100, shadows will be boosted by 3 stops while highlights will be intact, bascially you are shooting at ISO 100 with 3 stops under to protect highlights. However, you will see a lot of noise in shadow area due to insufficient light and 1.3 ~ 1.7 stops “over-exposure” is usually needed to compensate that to get clean image.

Consolution

HLG has a pretty good balance between highlights and shadows, very user friendly for HDR workflow. Unlike N-Log, there is no need to over-expose (even 1 stop under looks good to me), no camera LUT needed for restoration, zero or minimum color grading needed. Another advantage of HLG over N-Log is monitoring, most of the time you can rely on the screen/viewfinder (with zebra on) to adjust the exposure and only open the waveform if needed.

ProRes Raw 12Bit

ProRes Raw has a very low compression ratio of 2, which suggests that it might be lossless compressed, which takes more spaces than Nikon’s lossy N-RAW, 2.5x of the high quality and 4.5x of the normal quality!

ProRes HQ 422 10Bit

With compression ratio of 5.2, ProRes is still 2x the size of the normal quality n-raw! Though the former one has 1.5x number of pixels to deal with (YUV422 vs Bayer).

I hope Nikon could provide more options for ProRes via firmware updates. A 10x~12x compression ratio version (ProRes LT 422) will be very useful to balance between quality and disk space.

H.265 420 10 bit

For H.265, there is no option for all-intra, and only in 420. Compression ratio is much higher given long GOP. One thing to note, that bitrate is variable depend on the content, i.e. very dark scene will have a much lower bitrate (<100Mbps for 4K60).

NRAW High Quality 12 bit

First of all, given 4.5 compression ratio, NRAW is definitely a lossy codec. Interesting thing is that 8K’s compression ratio is higher than 4K’s. 4.5 vs 3.7.

NRAW Normal Quality 12 bit

For normal quality, the compression ratio is even higher, reaching 7.5:1 which is not bad for all-intra.