Posted: Sat Oct 11, 2008 7:20 am Post subject: Histograms: tone or saturation?
I've been learning Photoshop for quite a while, I've done lots of tutorials and read a few books, but I can't find the answer to this simple thing: In the histogram palette menu there are red, green and blue histograms to give the saturation of those colors, and a luminosity histogram to give the overall ( greyscale) brightness. There is also an RGB option, presumably to give a composite version of the red, green and blue saturation. In Levels, however we have the same thing, except there is no luminosity option, only RGB. But RGB in this case cannot be a composite of the three color channels, as we use it to adjust brightness and contrast; in other words it is presumably the same as Luminosity in the histogram palette. Despite this, when I do adjustments to an image the RGB histogram in Levels looks identical to the RGB histogram in the histogram palette. I'm obviously missing something, but I don't know what. Can anyone help?...I'm using PS 3 Extended.
Joined: 24 Sep 2007
Posts: 3515
Location: Haverhill, UK PS Version: Lightroom 5, CS4 & Elements 11 OS: Windows 8.1
Posted: Sat Oct 11, 2008 5:46 pm Post subject:
Hi vampirelover,
Yes it can get quite confusing!
An RGB and Luminance histogram both measure luminance info but do so in different ways. A luminance histogram calculates luminance as our eyes see it, so green is assigned a higher value than blue, and then each pixels total luminance is calculated and added to the appropriate line in the histogram. An RGB histogram doesn't allow for the human eye, it just adds up the total luminance in each channel and then adds up the channels to the composite histogram. Not suprisingly the RGB composite doesn't really give accurate shadow and highlight information, which is why a lot of photographers prefer to use the luminance and/or color histograms.
As you can probably tell, this is a huge topic, but a neccessary one to understand if you work with photographs and the like. Luckily, I created a few tutorials based on your questions, they can be found here-
An RGB and Luminance histogram both measure luminance info but do so in different ways. A luminance histogram calculates luminance as our eyes see it, so green is assigned a higher value than blue, and then each pixels total luminance is calculated and added to the appropriate line in the histogram. An RGB histogram doesn't allow for the human eye, it just adds up the total luminance in each channel and then adds up the channels to the composite histogram. Not suprisingly the RGB composite doesn't really give accurate shadow and highlight information, which is why a lot of photographers prefer to use the luminance and/or color histograms.
As you can probably tell, this is a huge topic, but a neccessary one to understand if you work with photographs and the like. Luckily, I created a few tutorials based on your questions, they can be found here-
Hi Mattyboy, Many thanks for that. I've just done the first tutorial and it's really clear...now why couldn't Deke explain it as simply as that?
I don't know if this is too far off topic, and maybe I should post again, but I have Vista with four gigs of ram. I'm wondering what I should set the ram to in PS Preferences. Deke says it's best to set it at 50%; other gurus say 70%, or even 100%, since PS only handles less than 2 gigs of ram so I've got 2 gigs left over for the OS, right? Or maybe it's not that simple....?
Joined: 24 Sep 2007
Posts: 3515
Location: Haverhill, UK PS Version: Lightroom 5, CS4 & Elements 11 OS: Windows 8.1
Posted: Sun Oct 12, 2008 3:45 pm Post subject:
No probs, always a pleasure helping a fellow Deke fan!
Yes you're pretty much correct in saying that Photoshop cannot see the 4GB of ram available on your system. When you set the usage amount in the preferences you should see not only a percentage, but an actual amount of RAM too. I wouldn't advise setting it to 100% under normal circumstances, although with a 4GB system you may get away with it! I wouldn't like to guarantee that though. Personally I'd say you're safe uping yours to 80% at least.
Another thing to note is that the OS has already been deducted from the total before Photoshop sees the figure. So if 100% equals 2GB then you have 2GB available after the OS has used its chunk. Keep in mind that you'll need to leave RAM available for all other software you plan to run whilst using Photoshop, this includes other Adobe products (such as the Bridge) and Photoshop plugins. Even a web browser can use around 20-30MB of RAM, so it's better to be safe than sorry.
Interesting subject that, loads to talk about (and learn). If you have any other questions on RAM, probably best starting a new thread and I'll do my best to help!
HI Matt, I've been working my way through all the levels tutorials on your site...very clear and easy to understand. I wonder if you could explain for me the difference between Brightness, Luminosity and Luminance; apparently they are not the same thing.
On the subject of ram, I reduced the cache level in the histogram palette to 1 so it would refresh immediately. I was working on a large file and I ran out of memory so PS just closed down. I gather that this is not a good idea? Cheers, vampirelover.
Joined: 24 Sep 2007
Posts: 3515
Location: Haverhill, UK PS Version: Lightroom 5, CS4 & Elements 11 OS: Windows 8.1
Posted: Thu Oct 16, 2008 3:04 pm Post subject:
Brightness, Luminosity and Luminance mean exactly the same thing, they are used interchangeably by most folks (especially me lol). Sorry, very confusing I know!
Choosing a cache level of 1 will definitely effect performance especially for larger documents. The default setting is fine for most purposes, personally though, I wouldn't go further than 4? Everything seems to work great for me at 6 so it's not something I've really had chance to play around with. If you're experimenting I'd be interested to hear the results?
Good luck _________________ Matt
3photoshop.com
http://www.3photoshop.com
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