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11-14-2018, 07:09 PM - 1 Like   #1
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Why use Green button instead of Av?

Whenever I use an M lens and I don't want to do full manual, I just put the dial to Av mode and it decides what shutter speed to use without me having to push the green button.
Everyone says "press the green button" which is more effort.

I've tried taking photos in Av, and in Manual and pressing green button and it picked the same shutter speed.

I must be missing something, is using the green button more accurate or something?

11-14-2018, 07:16 PM   #2
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QuoteOriginally posted by pentax360 Quote
Whenever I use an M lens and I don't want to do full manual, I just put the dial to Av mode and it decides what shutter speed to use without me having to push the green button.
Everyone says "press the green button" which is more effort.

I've tried taking photos in Av, and in Manual and pressing green button and it picked the same shutter speed.

I must be missing something, is using the green button more accurate or something?
This will only work when your lens is wide-open, since that's what the meter goes by.

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11-14-2018, 07:23 PM   #3
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I wonder the same thing. I've been a green button pusher for a while, and just recently switched to Av mode with my K lenses.

It's such a habit, I'm still hitting the green button in Av mode, but catch myself doing it. So I think I can cure that.

---------- Post added 11-14-18 at 09:30 PM ----------

QuoteOriginally posted by Adam Quote
This will only work when your lens is wide-open, since that's what the meter goes by.
Ok. I think I understand now. Lens is always wide open, even though I'm turning the aperture ring.

Last edited by Riggomatic; 11-14-2018 at 07:31 PM.
11-14-2018, 07:58 PM   #4
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QuoteOriginally posted by Riggomatic Quote
I wonder the same thing. I've been a green button pusher for a while, and just recently switched to Av mode with my K lenses.

It's such a habit, I'm still hitting the green button in Av mode, but catch myself doing it. So I think I can cure that.

---------- Post added 11-14-18 at 09:30 PM ----------



Ok. I think I understand now. Lens is always wide open, even though I'm turning the aperture ring.
QuoteOriginally posted by Riggomatic Quote
Ok. I think I understand now. Lens is always wide open, even though I'm turning the aperture ring.
Correct. It will still stop down at the time of capture, which means the effective aperture will be smaller than what the camera thought, which means the final image will be underexposed.


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11-14-2018, 08:10 PM - 3 Likes   #5
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QuoteOriginally posted by Adam Quote
Correct. It will still stop down at the time of capture, which means the effective aperture will be smaller than what the camera thought, which means the final image will be underexposed
Actually, when using a K or M lens in Av mode the camera will not stop down the lens at all. You will get correctly exposed pictures, but they will always be at the lens' widest aperture, irrespective of where you have set the aperture ring.

The only way to get correct exposure is in M mode using stop-down metering.
11-14-2018, 10:19 PM   #6
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QuoteOriginally posted by pentax360 Quote
Whenever I use an M lens
Define "M" lens. I have seen that term used to mean Manual, M42, and Pentax-M. If you are shooting with K-mount lens without the "A" contacts on the mount, M mode is your only option if you want to shoot other than wide-open. If M42 screw mount, you have a choice of M mode or Av mode.


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11-14-2018, 10:26 PM   #7
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If there is no aperture control and you set it on the ring, shouldn't that get a correct meter reading in Av?
It seems like it should and it also seems like it does. But I have never really objectively tested.
11-14-2018, 11:10 PM - 1 Like   #8
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QuoteOriginally posted by mattb123 Quote
If there is no aperture control and you set it on the ring, shouldn't that get a correct meter reading in Av?
It seems like it should and it also seems like it does. But I have never really objectively tested.
It will if the lens has manual aperture (M42 with A/M switch in M position). K-mount lenses, OTOH, are wide-open until stopped down by the body and unless an "A" contact is detected, the lens remains wide open at all times except in M, X, and B modes.


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11-14-2018, 11:45 PM   #9
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QuoteOriginally posted by pschlute Quote
Actually, when using a K or M lens in Av mode the camera will not stop down the lens at all. You will get correctly exposed pictures, but they will always be at the lens' widest aperture, irrespective of where you have set the aperture ring.

The only way to get correct exposure is in M mode using stop-down metering.

I just checked and you're totally right, I don't know how I didn't notice that.
But in my defense I shoot in manual like 90% of the time, and don't use my K mount manual lenses much.
11-15-2018, 05:07 AM   #10
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Sometimes, just sometimes, but fairly repeatable, especially with my Tamron 500mm mirror (fixed aperture), the Av option just "gets it wrong" and I haven't figured out why. The "M-Mode + Green Button" option just gets it right every time so, for me, it becomes a no-brainer, definitely get the picture or risk being a couple of stops out and miss the opportunity.
With the Tamron I admit there might be a random factor depending on which mount I've fitted to it (auto or manual) or whether or not I'm using a 2x, but the instance does also occur occasionally with other fixed-mount lenses that don't have an "A" setting on the aperture ring.
I suspect the issue may be caused by bad connections between the back of the metal mount on the lens and the electronic contacts on the camera body ... maybe one day when I've got absolutely nothing else better to do I'll sit down with a couple of bodies and a box-full of lenses and try for some replication and elimination tests ... in the meantime, I'll rely on the Green Button
11-15-2018, 05:21 AM   #11
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perhaps this explanation found below the linked chart might be of assistance to some readers

Pentax K-Mount Lens Series Explained: The differences between various Pentax lens series

In a nutshell

There are several series of Pentax K-mount lenses; the ones currently being produced include the DA (digital), DA L (cheap digital kit), D FA (full-frame, digital optimized), and FA (full-frame) series. While all Pentax K-mount lenses ever produced are compatible with all current Pentax DSLRs (in some cases with restrictions in that not all exposure modes of the camera are supported by old lenses), there are significant differences between each lens series and each has different features. Furthermore some of the newer lenses do not work with older cameras. We'll be exploring the differences between the various K-mount series lenses in this article: . . .

smc PENTAX-FA

The second generation of autofocus lenses
Sub-groups: FA* (professional) and FA Limited (compact high-end) series
Image format covered: 24 x 36 mm
Suitable for APS-C digital sensor format and 24 x 36 mm "full frame" format
Autofocus with screw drive system
Aperture ring
Automatic aperture setting is available*
Certain zoom lenses feature the power zoom function**

In production: Only five of the original 42 lenses are still in production (as of May 2010: FA 35, FA 50, FA 30/43/77 limited)
The FA series lenses are autofocus and allow for certain modern cameras (film and digital) to automatically set aperture (Tv mode), shutter speed (Av mode), or both (Program mode). You can also set exposure manually if the camera body allows for this. On recent camera bodies (e.g. *ist and all digital SLR cameras) you use dials on the camera body to set the aperture value (the aperture ring on the lens must be in the "A" position at all times). On older models (such as ZX-5N, MZ-5N, LX, ME, MX, K1000) the aperture is set with the aperture ring on the lens.

Three FA "Limited" lenses were made available in 2001. Except for these and the 35mm f/2 and 50mm f/1.4, the FA series, which consisted of 42 lenses total, has been discontinued.

While FA lenses are identical in operation to F lenses, they feature improved electronics that support the MTF program line.
The FA series also includes 11 "star" lenses (FA*) that employ the finest of Pentax optical and mechanical engineering.

*Exceptions: The smc PENTAX-FA Soft 85mm F2.8 and smc PENTAX-FA Soft 28mm F2.8 lenses have no "A" setting on the aperture ring. Hence they work like smc PENTAX-M lenses with respect to exposure; they are still autofocus. The "A" setting has been removed because the degree of image softness is controlled by the aperture ring and it would be meaningless to let the camera decide the setting.

**Power zoom is only supported by certain camera bodies. The following lenses feature power zoom:
-SMC Pentax-FA 28-80mm F3.5-4.7
-smc PENTAX-FA 28-105mm F4-5.6
-smc PENTAX-FA 70-200mm F4-5.6
-smc PENTAX-FA 100-300mm F4.5-5.6
-smc PENTAX-FA* 28-70mm F2.8 AL
-smc PENTAX-FA* 80-200mm F2.8 ED [IF]
-smc PENTAX-FA* 250-600mm F5.6 ED [IF]


. . .

smc PENTAX-A

The third generation of manual focus lenses
Sub-categehory: A*
Image format covered: 24 x 36 mm
Suitable for APS-C digital sensor format and 24 x 36 mm "full frame" format
Automatic aperture setting is available
In production: No

Remarks: smc PENTAX-A lenses are manual focus. Regarding exposure control they work like FA lenses.
The A series also includes 9 "star" lenses (A*) that exemplify the finest in Pentax optical and engineering.




smc PENTAX-M

The second generation of manual focus lenses (generally smaller than K lenses)
Sub-categhory: M*
Image format covered: 24 x 36 mm
Suitable for APS-C digital sensor format and 24 x 36 mm "full frame" format
Manual aperture
In production: No

Remarks: smc PENTAX-M and smc PENTAX (commonly know as "K") lenses are manual focus and have no "A" setting on the aperture ring. These lenses allow for the camera to automatically set shutter speed (if it has the Av auto-exposure mode). You can also set exposure manually if the camera body allows for this. The ZX-5N is an example of a body which provides Av mode as well as manual exposure mode with M and K lenses. Almost all K and M lenses have an automatic diaphragm.

Some recent bodies cannot use M and K lenses or only with some limitations. An example are the *ist D and K (digital) series. In M mode you set the aperture using the aperture ring on the lens; the camera will set the shutter speed when you press the green button (or AE-L button if the camera has no green button). Or you can set the shutter speed manually (there is no exposure read out in the finder to assist you).


The M series includes a single "star" lens (M*) that exemplies the finest in Pentax optical engineering. That is the smc PENTAX-M* 300mm F4.


Pentax K-Mount Lenses Explained: The differences between various Pentax lens series

Last edited by aslyfox; 11-15-2018 at 05:27 AM.
11-15-2018, 08:44 AM - 3 Likes   #12
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The main reason people are having trouble figuring out why their K-mount manual lenses don't work without using the Green button, is that Pentax "crippled" the K mount with the current digital bodies.

While I've seen explanations offered as to why the new mount couldn't have the little internal lug that transmitted aperture ring position to the body, I still don't entirely buy it.

Obviously there's a cost-cutting issue. Deleting a feature most of your new customers would never miss (weren't we all going to rush out and buy brand new DA type lenses to go with our shiny new K100D in 2006?) is helpful. But it's interesting to note Nikon deleted their similar Ai lug on the entry-level DSLRs, but made sure it was present on the higher-end bodies - they still do this. Is it really impossible for the K-1 to have the metering lug?


But without that lug, the camera body has no way of knowing where you've set the aperture ring. In Av mode, the body doesn't even actuate the aperture stop down lever, so you're always shooting wide open. But in manual mode, stop down does occur during exposure, so you have to take a reading with the lens at that aperture before shooting. The Green button is what does that.

In a lot of ways, it's like using an old Spotmatic. Since those cameras used stop-down metering (no little lug to tell the body what aperture you might be using) you had to flip on the meter switch, which also stopped down the lens. You took your reading to balance shutter speed and aperture. You could then turn off the meter, and open up the lens for fine focusing, knowing your exposure was ready to go when you decided to click the shutter. The Green button on the digital bodies is a tiny bit quicker, because it actually sets and shows you the "correct" shutter speed for your chosen aperture. Since you're in manual mode, you're free to adjust it afterwards.
11-15-2018, 08:52 AM - 3 Likes   #13
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I have some time off work, with no plans other than cleaning my workshop, office area...the to do list that I keep putting off. So to help with my procrastination, I did a little test between M mode and Av mode, using an SMC-K 50 f1.4.

Setup: Objects placed on flat surface with the ME Super at the back, and the watch face at the front. Focus was on the Pop-It box which is about the midpoint of the objects. Remote and 3sec delay used on all pictures. I had to touch the camera to change the aperture and hit the green button. Approximately, 36" from the pop-it box to the camera.
Lighting: crappy light from the fixture in the center of the room, and a lamp off to the side.
Equipment: K-5, SMC-K 50mm f1.4, Tripod, remote, jpg images developed by the camera.

M mode:

Picture1 f1.4, green button used to meter (calculated DOF: 0.72")


Picture2 f8, green button used to meter (calculated DOF: 3.96")


Picture3 f16, green button used to meter (calculated DOF: 8.04")



Av mode:

Picture1 f1.4, (calculated DOF: 0.72")


Picture2 f8, (calculated DOF: 3.96" no visible change from f1.4)


Picture3 f16, (calculated DOF: 8.04" no visible change from f1.4)




Conclusion: Shoots wide open in Av mode, no matter where you set the aperture ring.

Back to M mode and clicking the green button with my K glass.
11-15-2018, 09:24 AM - 1 Like   #14
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QuoteOriginally posted by Riggomatic Quote
I have some time off work, with no plans other than cleaning my workshop, office area...the to do list that I keep putting off. So to help with my procrastination, I did a little test between M mode and Av mode, using an SMC-K 50 f1.4.


Conclusion: Shoots wide open in Av mode, no matter where you set the aperture ring.

Back to M mode and clicking the green button with my K glass.

The interesting thing here is that at f/1.4 in manual mode the camera appears to have under-exposed compared to the subsequent two manual exposures, both of which appear slightly over exposed in comparison to the Av exposures, so even with the best will in the world it's not always "perfect"!
11-15-2018, 09:29 AM   #15
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QuoteOriginally posted by kypfer Quote
The interesting thing here is that at f/1.4 in manual mode the camera appears to have under-exposed compared to the subsequent two manual exposures, both of which appear slightly over exposed in comparison to the Av exposures, so even with the best will in the world it's not always "perfect"!
Which is why I "tweak" my SOOC JPEGs - but Green Button gets it sufficiently close that only minor adjustments are necessary.
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