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Illustration of Limitations of Nikon Compressed Raw Format?

Chris Fraser , Feb 18, 2006; 01:08 p.m.

Over the past few months, a number of people have asked if there is any difference in quality between NEF and compressed NEF formats, and several have suggested that they've yet to see any practical examples of the difference. I think I've come across several examples recently, which show that, as Thom Hogan and others suggested, compressed NEF results in a limited ability to post-process highlight detail.

Attached is a 100% crop of a D2X compressed raw file shot at ISO 800. The crop shows the edge of a person's arm lying against an ivory-colored pillow. Notice that the bright part contains both ivory colored regions and what appear to be blown-out highlights. In Nikon Capture 4.4, if I reduce the "Exp Comp" by 0.67 EV, the blown-out highlights vanish -- that is, in the "highlights" view, the image goes totally black. According to the "Information" palette, the RGB value of the washed-out areas seen in this crop is R228, G231, B228 -- nowhere near blown out.

However, whether in Capture or in PS-CS2, those areas continue to *look* blown out, and no processing technique I'm aware of can bring out any color or texture there. (I am not a PS expert, so if you know of such a technique, I'd be grateful to hear about it.) As a color photo, this capture is effectively ruined (maybe it might still work as a B&W, I'm not sure).

I've recently processed at least a half-dozen images with similar problems.

My hypothesis is that there was nothing wrong with my original exposure, and that if I'd used uncompressed NEF, the image would have recoverable highlight detail. But because I shot this in compressed NEF, those highlights really are blown, even though the average RGB value is only 229.

If that's correct, then the lesson is: Use compressed NEF only for low-contrast images without bright highlights.

Do you agree that NEF compression explains what's going on here, or is there another explanation?


Highlights with average RGB of 229 appear blown out

Responses


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Arthur Yeo , Feb 18, 2006; 01:25 p.m.

I think I experienced loss of partial pixels when I look a the crop at 100% of some shots I have taken with compressed RAW.

I have learned my lesson well.

Chris Fraser , Feb 18, 2006; 01:30 p.m.

Arthur, in principle the loss should be confined to the highlights -- is that what you've found? Compressed NEF is a big convenience, but I'm turning it off and investing in more storage space and memory for my computer.

Andy Radin , Feb 18, 2006; 02:27 p.m.

"My hypothesis is that there was nothing wrong with my original exposure, and that if I'd used uncompressed NEF, the image would have recoverable highlight detail.

Do you agree that NEF compression explains what's going on here, or is there another explanation?"

What? Shoot it again uncompressed, and find out! Likely to yield a better answer than asking us.

Lex Jenkins , Feb 18, 2006; 06:49 p.m.

Have you compared highlights in compressed vs. uncompressed NEFs at ISO 200? I would think that the limited dynamic range at 800 would make it even more important to shoot uncompressed NEFs when maximum quality is needed.

I've tried various highlight recovery utilities but after a certain point all they do is make the highlights gray. They can only recover just so much detail - if none was captured it can't be created.

Since I don't have a high powered PC my main reason for avoiding compressed NEFs, after a couple of experiments, is because they take for-freakin'-ever to open. I tried to time it but my watch died of old age.

It might not matter with a P4 and 2 GB RAM, but on a P3 with 128 MB RAM it's painful. (And forget about adding RAM - this is only a backup PC and adding RAM would cost more than buying a new low end P4.)

Jon Curtis , Feb 18, 2006; 09:37 p.m.

With out an example of an uncompressed photo of the same image and same conditions, your just guessing.

Set up an example and a couple test shots, then it won't be a bunch of people guessing whats going on.

David Moore , Feb 19, 2006; 12:56 a.m.

Would you mind posting the original raw file somewhere? We can examine it to see if the detail in the pillow was actually blown out or not. If it was truly blown out, the only solution would have been to lower your exposure compensation at the time the shot was taken.

Chris Fraser , Feb 19, 2006; 02:13 a.m.

Thanks for the comments. The friend in the photo would prefer that I not post the original file. I could post one with a similar problem, though. But in any case I can confirm that when I move the "Information" pointer over the area shown in the crop above, the RGB values are all around 228-231, not 255. So the RGB tells me the highlights are not blown, but they look blown and reducing the EV doesn't help.

Lex may be right about the impact of high/low ISO. I just did a few test shots at home on a white towel (detailed surface texture) placed next to a bright window to compare highlight recovery from NEF and compressed NEF at ISO 100. In a shot intentionally overexposed by 1.7 EV, highlight recovery from both was impressive. Areas for which RGB were initially all 255 could be brought down to 220-230 and looked good. I'll repeat the test shots at ISO 400 and 800 another day (don't have time at the moment). But it could be that compressed nef is fine at low ISO and a difference emerges only at high ISO (if that's what's really going on here).

Chris Fraser , Feb 19, 2006; 04:40 a.m.

Sorry, my previous post was confused. Let me correct. The highlights in the *original capture* shot at ISO 800 really are blown: they're at 255. Adjustment in post-processing indicates that they have been recovered: The RGB value given for those pixels is no longer 255, it's about 230. Visually, however, there's no data there. So the oddity here is that Nikon Capture indicates those pixels are not at 255, but in fact they do seem blown out.

The question I'll look into further is whether, in a similar shot at ISO 800, using uncompressed NEF instead of compressed will make a difference. I'm guessing it probably will. I'll do some test shots later to see.

Ilkka Nissila , Feb 19, 2006; 05:44 a.m.

If you have pixels at 255,255,255 and then you adjust the brightness of the image in post-processing, you will of course get darker values as you make stronger adjustments. However, there is no detail as the detail was clipped in the original exposure.


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