The following are copy from several different user's post.
Hopefully this can help more newcomers (or not) to know more about the great Green Button.
Thanks to Peter Zack, RuiC, sezme.

In manual mode (A,FA, DA etc series lens) you can push the button and it sets the exposure for the program mode and the program line. Program line is the setting you have selected in the camera menus, like MTF (where the camera sets the lens at the best aperture for that lens, which is programmed into the lens by the manufacturer). Then you can fine tune the exposure to your liking from this setting. Very old school but an absolute requirement for a serious shooter in tough lighting and many other situations. (Peter Zack)

Step Down Metering
With an M or M42 lens, you can use the button to stop the lens down and take a meter reading. For those that own these lenses, it's an approximate reading and you have to understand (with practice) the way your particular lens will meter on the attached body. In most cases, these lenses meter well wide open but tend to over expose in middle apertures. So you use the green setting recommendations as a starting place to adjust to the correct exposure. (Peter Zack)

Return to HyperProgram(P) after changes
In HyperProgram (P) mode you can turn the front e-dial to adjust exposure time other than P suggests and it changes to Tv mode. If you turn the rear e-dial it becomes Av mode. Then press Green-button it returns to P mode again. (RuiC)
image from Digiichi - Next generation hyperprogram.

From Full Manual mode to P, Av or Tv
The green button in M mode get you to P, Av or Tv dependant on what mode is chosen in the green button menu set up. (excellent if you want to use it like an on-demand spot-meter)

The green button doesn't change from M (manual) mode to any other mode - it leaves you in M mode. It just momentarily gives you the effect of being in one of those other modes (which one depends on how you've set it up in your custom options) - setting the same aperture and/or shutter speed that you'd get it you were in one of those other modes, but then leaving that set. As opposed to constantly readjusting as the light changes.

(thanks lol101, Marc Sabatella, adwb)

Act as Reset button: Reset Exposure Compensation, Flash Compensation
If you have set exposure compensation, hold the exposure compensation button plus the green button will reset the meter to zero E.C.
* You can also reset the Flash Compensation with green button. (thanks rockmaster1964)

Fast access to set Auto ISO
if you press the ISO button (OK button on K10/20) plus the green button, it sets ISO to auto. A very useful button!
Also in Av/Tv if ISO is made accessible via one of the control dial, green button switches to auto-ISO. (thanks lol101)

Zoom in in Playback mode
In playback, after you zoom, Green button zooms IN and EV+- zooms OUT

[Waldo]

I have a k20d and have been evaulating purchasing either the k7 or k-x. Since the k-x does not need the "green" button for manual lens to establish the shutter speed does the k7 perform likewise? Or is the k7 just like the k20d in the use of the "green" button to establish the shutter speed for the correct exposure using manual lens? I have 5 manual lens, so the answer to my inquiry will significantly impact my final decision. THANKS.

[Marc Sabatella]

You might be confused. While it's true the K-x can assign that stop-down fucntion to a button other than Green button, that's only because it doesn't *have* as many buttons as the K-7, so it's more important to be able to customize which button does what. The K-x still needs you to press a button like all other Pentax DSLR's; it just happens to be the case that you can choose which button does that function. This is *not* necessarily a good thing, because assigning that function to a different button means that button now cannot perform it's regular function.

The K-7 hs no need to assign the stop-down function to a different button.

[Waldo]

Marc,
Thank you for the clarification...appreciated.
JimC

[jpzk]

Just wondering if anyone can help with this:

Cosina 55/1.2 mounted on a K7 ... usage of the green button:

Camera on Av, (lens is fully manual of course), aperture set at f1.2, I push the green button and nothing happens except to show me the infamous F_ _ _ . I take the picture and it seems "OK";
I did make sure I had the settings to allow for "using aperture ring" ON.
Will the camera ever display the shutter speed, and am I wrong to believe that when I press the green button, nothing happens?
If I stop down to F2.8, for instance, will the pic be taken at F2.8?
Sorry, I am always confused about this no matter how much I read the OP post.
JP

[adwb]

You need, if it is like my K10 ,to use the M [manual] setting not the AV. In av the lens will be held wide open no matter what you set it to, In M if you press the green button you will hear, and if you look through the viewfinder at the same time see, the shutter close to the aperture that you have set on the lens, at the same time you will see the speed reset to suit this selected aperture.

[jpzk]

You need, if it is like my K10 ,to use the M [manual] setting not the AV. In av the lens will be held wide open no matter what you set it to, In M if you press the green button you will hear, and if you look through the viewfinder at the same time see, the shutter close to the aperture that you have set on the lens, at the same time you will see the speed reset to suit this selected aperture.

Great, and thanks for the reply.
So, that is why I couldn't see the f-stop in the EXIF's? It just showed F*_* (not being bad mouthed here), that is what it showed.
Cheers.
JP

[Marc Sabatella]

The simple fact is that camera cannot read the aperture from the aperture ring. That's the reason the camera displays F--, that's the reason no aperture appears in the EXIF, that's the reason Av mode doesn't work, and that's the reason that when using M mode, you need to press a button to get the camer to stop down in order to take a meter reading.

[jpzk]

The simple fact is that camera cannot read the aperture from the aperture ring. That's the reason the camera displays F--, that's the reason no aperture appears in the EXIF, that's the reason Av mode doesn't work, and that's the reason that when using M mode, you need to press a button to get the camer to stop down in order to take a meter reading.

Makes sense now.
Thanks for the reminder and the explanation, Marc.
JP

[Aussieflicker]

Hi - I've a Green Button related question on my Kx

In P mode - when I set the green button to P shift - the dial only seems to adjust shutter speed (puts it into Tv mode. Can I set it to adjust aperture?

Thanks

J

[monching]

Wow!!! what can I say...the site is really informative to the extent that you don't need to have an instruction manual to learn about your camera, everything is in here not to mention the helpful, unselfish traits of the guys (members) in one way or the other extending and sharing their knowledge to beginners/newbie like myself. I was wondering about this green button myself when I happened to see this and boy was I happy to ever came across this thread. My hat's off to you guys...keep it coming....I also read somewhere that a year is not enough for me to fully learn and understand my K-7 so I will be doing a lot of reading.
keep on posting guys...I'm learning a lot just by reading your posts..I have a K-x and a K-7 both been bought in a matter of weeks of each other, one for the daughter (k-x) and one for me (the K-7 of course )


God Bless you all
Monching

[Brangdon]

You might be confused. While it's true the K-x can assign that stop-down fucntion to a button other than Green button, that's only because it doesn't *have* as many buttons as the K-7, so it's more important to be able to customize which button does what. The K-x still needs you to press a button like all other Pentax DSLR's; it just happens to be the case that you can choose which button does that function. This is *not* necessarily a good thing, because assigning that function to a different button means that button now cannot perform it's regular function.

This makes it sound as though the K-x user has a choice about what the AV button does. We don't: it always stops-down with a manual lens in M-mode. The Green button is the only button that can be assigned by the user.

[gadeco]

这真是一个非常好的信息！

[johnb-123]

I looked for a magic latern book for K-X it's not available, but I found a K200D how close is the K200D to the K-X in functionality

[adwb]

It's close the K200d and the Km where the same thing I think different names for different markets, but the K-x has more functions and features in the menu. So I would not get a magic lantern book unless it was specific to your camera. IMHO the Pentax book supplied tells you all you need to know and is clear enough to understand. use the detailed index at the back to find what you are looking for.

[johnb-123]

The book I found on amazon was for K20D, again I don't know if it would be any help unerstanding my K-X

John