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terryw
27th August 2008, 07:36 PM
I have and E-1, normally shoot in RAW, use Adobe RGB for my shooting colour space and use Oly Viewer 1.4 for conversion, mainly to 16 bit TIFF.

When I convert to TIFF the histogram looks like a comb to a greater or lesser extent but this evening I converted to high quality .jpg by mistake and the resulting histogram was as smooth as anything.

My question is why is this happening since TIFF is a supposed to be a lossless compression system and the comb artifacts should not surely be occuring?

Any thoughts and advice on this matter would be very much appreciated.

Many thanks


Terry

Graham_of_Rainham
27th August 2008, 10:29 PM
Terry,

I could speculate on this but that would not be of any help what so ever.

Better just to say welcome and hope you get the answer from one of the many knowledgeable folk we have on this forum

*chr

Jim Ford
27th August 2008, 10:37 PM
I think it's to do with 16 bit TIFFs having 65025 levels, but jpegs (8 bit) only 255. When you convert the 16 bit to 8 bit, you lose the gaps. In other words, each bit of data in the 8 bit jpeg histogram is represented by 255 bits in the 16 bit TIFF one. Taking arbitary numbers, if you have a level of 19890 in a 16 bit tiff and the next level is 20145, you have a gap of 255 in the histogram. But when both are converted to 8 bit, they come out respectively 78 and 79 - consecutive levels without a gap in the histogram.

I think the above is the answer, but would welcome a better (or clearer) explanation!

Jim

terryw
28th August 2008, 08:19 AM
Jim

Thanks for the thoughts. I tried coverting to 8-bit TFF but this too produces exactly the same type of spiky histogram.

Terry

art frames
28th August 2008, 09:34 AM
Terry

The comb appearance usually happens when a photograph that has a peaky, perhaps unbalanced histogram is spread around as part of the processing. Perhaps when a shadow/highlight rebalance is done to open up dense shadows. I don't often use Master but can speak from conversions in photoshop CS3. Once you rebalance the peaks then the levels that Jim explains have a bigger impact - but you probably would have said.

Are you able to post an example of a photograph that exhibits the worst signs, and is it a problem that leads to issues with the quality of the output?

Welcome to the site.

Peter

terryw
31st August 2008, 01:49 PM
Peter

Sorry it has taken so long to respond but I have been trawling through all my images that I have converted from RAW to TIFF (both 8-bit & 16-bit) and they all exhibit the same comb like histogram. With regard to effect on the ouput image, they look O.K. to me but I don't have a good example to compare them with so I could be losing out without knowing it. Could it be that the RAW converter in Viewer 1.4 is at fault and that an alternative converter might be better. If there is one. I understand that Adobe have a .dng RAW converter but will it work with PS7?

Thank you again for yours and Jim's supportive input.

Maybe I should just convert from RAW to high quality .jpg format.

Terry

Graham_of_Rainham
31st August 2008, 01:58 PM
There is a New Version of Olympus Master 2 (2.06)

Why not try that and see if it helps.

*chr

snaarman
31st August 2008, 01:59 PM
This is a mystery:
You usually get spiky histograms if you have adjusted the image brightness or colour mix in some way - making it a litttle lighter or darker etc. I see the effect after I have used the PS levels controls for example.

Generally a spiky histogram is not visible in the image.. (Cue the old joke. "Doctor doctor, when I look at my histograms they are all spiky: Well, stop looking at your histograms...")

I just wonder if the conversion from RAW to TIFF involves a change of colour space that you are not aware of? That might do it.

Pete

terryw
31st August 2008, 02:34 PM
Just in case it was a colour space glitch (I shoot in AdobeRGB and convert using the same colour space) I've just tried converting using sRGB. The only change is that the spikes are different. No information thrown away (dips) just computer generated guestimates (spikes).

Since nobody else seems to have this problem, and I'm sure there must be other E-1 users that shoot in RAW and use Viewer 1.4, maybe it's a computer rather than software problem. There again maybe it's not worth bothering about a relatively small amount of information being thrown away and spurious information being added to the histogram.

I think I will try Graham's suggestion, since at least it will provide a comparison with another software programme.


terry

:confused:

crimbo
31st August 2008, 03:05 PM
you take the image as a RAW...it has no colour space regardless of what the camera is set at.
You convert the image and in doing that apply various bits of processing including assigning a colour space.

so my question to you...in which software are you viewing the histograms?

Paulpp
31st August 2008, 03:46 PM
Have followed this with interest, but am unclear about RAW and colour space. With my camera set to Adobe RAW, and shooting in RAW, the program (Aperture) metadata shows Adobe RGB as the colour space. Same if I change the colour space to SRGB in the camera then the program metadata shows sRGB. Which I have presumed meant that RAW included a colour space.

crimbo
31st August 2008, 04:16 PM
the metadata of the RAW file says that this is the colour space I would like the RAW data to be seen in...if the viewing program reads it...unfortunately most monitors approximate sRGB colour space so you may not get exactly what you would see if you printed the image.
If you make a jpeg capture the colours will be processed to fit into the Adobe RGB colourspace.
It does get very confusiing and I do not pretend to understand it...
Most of my images are seen on screen or I send them to a printer that uses sRGB so I set the camera to sRGB so that if I do a jpeg capture it is in that colour space.
When converting a RAW I also select the sRGB colour space rather than any other

terryw
1st September 2008, 08:31 AM
Yippee! And thank you.

This information explains a lot, and almost certainly provides me with an answer for the spurious "comb" effect that occurs with .tiff but not with .jpg files. It certainly explains why I cannot see a problem when I produce prints, even up to A3 size.

I am currently converting using Oly Viewer 1.4 (this came with my E-1) and manipulating images in PS7. Just to clarify, the "comb" effect is present in the levels histogram before I even attempt any changes to the image file whatsoever.

Many thanks for everyone's input since at last my personal fog is beginning to clear.


Terry

HughofBardfield
1st September 2008, 09:26 AM
If you are shooting RAW and using Viewer you are almost certainly missing out on some of the advantages of shooting RAW IMHO.

Could I suggest giving RAW Therapee a try ( http://www.rawtherapee.com/ )? It's a free download (donation ware) and gives you far more control over the final image. It also has some useful capabilities for recovering almost-blown highlights that Adobe Camera Raw doesn't have. UFRAW is also free and worth a look, in conjunction with The GIMP - which has most of the functions of Photoshop but is open source and therefore free.

Personally I am a fan of Lightroom for most RAW conversions, but I know not everyone wants to give Adobe yet more money for a variety of reasons!

theMusicMan
1st September 2008, 12:00 PM
Hugh - have you upgraded to LR2...? if so, how are you finding it...? I think it has become slower for me and I have raised the question with Adobe.

/back on topic - sorry!

theMusicMan
1st September 2008, 02:12 PM
Sorry OP - will start a new thread... :)

OlyPaul
3rd September 2008, 10:50 AM
The thing to also bear in mind is that the histogram (at least in PS and proberly others ) only displays 256 levels even if the image is in 16 bit.:)