Legacy lens and the green button

Neville Holmes · 03-12-2017, 01:34 PM

I am considering buying a Pentax DSLR to use with my old SMC M lenses. What I can not find out is what happens to the aperture after the 'green' button is pressed? Does the aperture stay stopped down or does it remain fully open to permit clear focusing?

ZombieArmy · 03-12-2017, 01:52 PM

The green button just stops down to get exposure then goes back to wide open.

pschlute · 03-12-2017, 04:43 PM

You will also get focus confirmation (green hexagon in the viewfinder) if you leave the camera set to autofocus with a non-autofocus lens.

And if you shoot your M-series lenses at widest aperture, you can also use Av mode (aperture priority) in addition to manual/green button.

Paul Pheiffer · 03-13-2017, 07:24 AM

If you have not done so yet, please look/read these two articles for more information on the use of manual lenses on a Pentax DSLR and the Pentax Green Button Guide:
How to use/meter Manual & M42 Lenses on all Pentax DSLRs (K-1, K-3, K-5, K-30, etc) - PentaxForums.com
Pentax Green Button Guide - Articles and Tips | PentaxForums.com

I love my M 50mm f1.7 and it was only after reading the first mentioned article that I was able to use it correctly.
I prefer to use my K5 in M mode when I use manual lenses because I want to be in control of the DOF.
A word of caution: not all manual lenses meter correctly on the smaller lens openings (using the green button to obtain a reading) - mine tends to underexpose.

Wheatfield · 03-13-2017, 08:00 AM

Originally posted by Paul Pheiffer

A word of caution: not all manual lenses meter correctly on the smaller lens openings (using the green button to obtain a reading) - mine tends to underexpose.

This is probably because you are outside of the meter's range when trying to meter at small apertures.

Neville Holmes · 03-13-2017, 09:35 AM

Many thanks for all the replies. I am a retired Professional Engineering so if the specification is not clear then I assume a problem. All seems well so am I the first to go film-mirrorless- DSLR?

**boriscleto** · 03-13-2017, 09:51 AM

Originally posted by Neville Holmes

All seems well so am I the first to go film-mirrorless- DSLR?

I don't know about that, but many here have gone film-DSLR-mirrorless. I'm sure some have even gone film-APS-C DSLR-mirrorless-FF mirrorless-FF DSLR...

**monochrome** · 03-13-2017, 10:53 AM

Originally posted by boriscleto

I don't know about that, but many here have gone film-DSLR-mirrorless. I'm sure some have even gone film-APS-C DSLR-mirrorless-FF mirrorless-FF DSLR...

Some have gone Compact film -> APSc dSLR -> film SLR -> FF dSLR.

stevebrot · 03-13-2017, 11:40 AM - **1 Like**

Originally posted by pschlute

You will also get focus confirmation (green hexagon in the viewfinder) if you leave the camera set to autofocus with a non-autofocus lens.

You get focus confirm even when the AF switch is on the manual setting.

Originally posted by Paul Pheiffer

A word of caution: not all manual lenses meter correctly on the smaller lens openings (using the green button to obtain a reading) - mine tends to underexpose.

The truth is that metering is fairly inconsistent for almost all lenses when using stop-down metering on modern dSLRs and can happen at both wide and narrow apertures. The causes are complex and related to the focus screens optimized for use on auto-focus cameras. The effect varies by both lens and aperture and is not easily countered by simply applying exposure compensation. Fortunately, current generation Pentax dSLRs behave much better than my K10D from 2007. That camera could vary as much as three stops over/under exposed between maximum and minimum aperture with some lenses.

Originally posted by Wheatfield

This is probably because you are outside of the meter's range when trying to meter at small apertures.

This is a very important observation. The lower limit of meter sensitivity is very quickly reached when doing stop-down metering at even moderate apertures in dim light. As the meter loses sensitivity at lower light levels, underexposure is the result. The user manuals for legacy stop-down meter cameras such as the Pentax Spotmatic often came with a chart indicating the combination of shutter speeds and ASA/ISO that were outside the "coupling range". The Spotmatic goes so far as to have a red warning flag that displays when the combination of shutter speed and ASA/ISO is outside the meter's range. On that camera at ASA/ISO 100, the full range of apertures (f/1.4 - f/16) and shutter speeds (1s - 1/1000s) are available for metered exposure. At ASA/ISO 400, the lowest valid metered shutter speed increases to 1/4s. The meter needle will respond at lower speeds, but the reading will not be accurate.

The GPD and RGB meters on modern cameras are more sensitive, but the same concept applies. For example:

Stop-Down Meter Limit: Pentax K-70
(sensitivity 0 EV100)

ISO	Max Exposure
100	2s
200	1s
400	1/2s
800	1/4s
1600	1/8s
3200	1/16s
6400	1/32s

(Calculated for f/1.4 from published meter sensitivity. Actual values may be different.)

Steve

**pres589** · 03-13-2017, 11:43 AM

Agreed; my M 50 1.4 bounces all over the place with green button use. Press it five times in a row a few seconds apart and I usually get at least two, usually three different exposures in M mode without me changing ISO or f-stop.

I use it as a rough guess and then go about doing it myself, usually with the newbie-style "point-click-check back screen-repeat" dance.

Wheatfield · 03-14-2017, 08:45 AM - **1 Like**

Originally posted by Neville Holmes

Many thanks for all the replies. I am a retired Professional Engineering so if the specification is not clear then I assume a problem. All seems well so am I the first to go film-mirrorless- DSLR?

Hubris is a beautiful thing.

**psoo** · 03-15-2017, 01:29 PM

Originally posted by ZombieArmy

The green button just stops down to get exposure then goes back to wide open.

My Kx has a green button and I will use this to get the correct exposure for a legacy lens as Zombie says. However, I have a K100D which doesn't have a green button and I think that to get the correct exposure I need to press the AE-L button when using a legacy lens. Now the Kx has both a green button and an AE-L button. Can I also use the AE-L button on my Kx to get the right exposure? I will check it out myself if nobody can give me a quick answer.

stevebrot · 03-15-2017, 01:47 PM

Originally posted by psoo

Can I also use the AE-L button on my Kx to get the right exposure?

No

Steve

**psoo** · 03-15-2017, 02:40 PM

Originally posted by stevebrot

No

Steve

Short and sweet. Thanks Steve

**clackers** · 03-15-2017, 03:56 PM

Originally posted by Neville Holmes

Many thanks for all the replies. I am a retired Professional Engineering so if the specification is not clear then I assume a problem. All seems well so am I the first to go film-mirrorless- DSLR?

(Laughs)

There are people here who have done an *enormous* amount of photography, Neville.

Enjoy dusting off the old lenses!

03-12-2017, 01:34 PM	#1
Neville Holmes New Member Join Date: Mar 2017 Photos: Gallery Posts: 9	Legacy lens and the green button I am considering buying a Pentax DSLR to use with my old SMC M lenses. What I can not find out is what happens to the aperture after the 'green' button is pressed? Does the aperture stay stopped down or does it remain fully open to permit clear focusing?

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03-12-2017, 01:52 PM	#2
ZombieArmy Pentaxian Join Date: Jun 2014 Location: Florida Posts: 3,210	The green button just stops down to get exposure then goes back to wide open.

03-12-2017, 04:43 PM	#3
pschlute Moderator Loyal Site Supporter Join Date: Mar 2007 Location: Surrey, UK Photos: Gallery Posts: 8,222	You will also get focus confirmation (green hexagon in the viewfinder) if you leave the camera set to autofocus with a non-autofocus lens. And if you shoot your M-series lenses at widest aperture, you can also use Av mode (aperture priority) in addition to manual/green button.

03-13-2017, 07:24 AM	#4
Paul Pheiffer Senior Member Join Date: Nov 2011 Location: Bushveld, Ellisras - South Africa Photos: Gallery \| Albums Posts: 202	If you have not done so yet, please look/read these two articles for more information on the use of manual lenses on a Pentax DSLR and the Pentax Green Button Guide: How to use/meter Manual & M42 Lenses on all Pentax DSLRs (K-1, K-3, K-5, K-30, etc) - PentaxForums.com Pentax Green Button Guide - Articles and Tips \| PentaxForums.com I love my M 50mm f1.7 and it was only after reading the first mentioned article that I was able to use it correctly. I prefer to use my K5 in M mode when I use manual lenses because I want to be in control of the DOF. A word of caution: not all manual lenses meter correctly on the smaller lens openings (using the green button to obtain a reading) - mine tends to underexpose.

03-13-2017, 08:00 AM	#5
Wheatfield Moderator Join Date: Apr 2008 Location: The wheatfields of Canada Posts: 15,991	Originally posted by Paul Pheiffer A word of caution: not all manual lenses meter correctly on the smaller lens openings (using the green button to obtain a reading) - mine tends to underexpose. This is probably because you are outside of the meter's range when trying to meter at small apertures.

03-13-2017, 09:35 AM	#6
Neville Holmes New Member Join Date: Mar 2017 Photos: Gallery Posts: 9 Original Poster	Many thanks for all the replies. I am a retired Professional Engineering so if the specification is not clear then I assume a problem. All seems well so am I the first to go film-mirrorless- DSLR?

03-13-2017, 09:51 AM	#7
boriscleto Loyal Site Supporter Join Date: Dec 2009 Location: North Syracuse, NY Photos: Gallery Posts: 16,477	Originally posted by Neville Holmes All seems well so am I the first to go film-mirrorless- DSLR? I don't know about that, but many here have gone film-DSLR-mirrorless. I'm sure some have even gone film-APS-C DSLR-mirrorless-FF mirrorless-FF DSLR...

03-13-2017, 10:53 AM	#8
monochrome Loyal Site Supporter Join Date: Mar 2008 Location: Working From Home Photos: Gallery \| Albums Posts: 26,276	Originally posted by boriscleto I don't know about that, but many here have gone film-DSLR-mirrorless. I'm sure some have even gone film-APS-C DSLR-mirrorless-FF mirrorless-FF DSLR... Some have gone Compact film -> APSc dSLR -> film SLR -> FF dSLR.

03-13-2017, 11:43 AM	#10
pres589 Site Supporter Join Date: Jan 2014 Location: Wichita, KS Photos: Albums Posts: 4,533	Agreed; my M 50 1.4 bounces all over the place with green button use. Press it five times in a row a few seconds apart and I usually get at least two, usually three different exposures in M mode without me changing ISO or f-stop. I use it as a rough guess and then go about doing it myself, usually with the newbie-style "point-click-check back screen-repeat" dance.

03-14-2017, 08:45 AM - 1 Like	#11
Wheatfield Moderator Join Date: Apr 2008 Location: The wheatfields of Canada Posts: 15,991	Originally posted by Neville Holmes Many thanks for all the replies. I am a retired Professional Engineering so if the specification is not clear then I assume a problem. All seems well so am I the first to go film-mirrorless- DSLR? Hubris is a beautiful thing.

03-15-2017, 01:29 PM	#12
psoo Site Supporter Join Date: Jan 2012 Location: Long Island, NY Posts: 485	Originally posted by ZombieArmy The green button just stops down to get exposure then goes back to wide open. My Kx has a green button and I will use this to get the correct exposure for a legacy lens as Zombie says. However, I have a K100D which doesn't have a green button and I think that to get the correct exposure I need to press the AE-L button when using a legacy lens. Now the Kx has both a green button and an AE-L button. Can I also use the AE-L button on my Kx to get the right exposure? I will check it out myself if nobody can give me a quick answer.

03-15-2017, 01:47 PM	#13
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by psoo Can I also use the AE-L button on my Kx to get the right exposure? No Steve

03-15-2017, 02:40 PM	#14
psoo Site Supporter Join Date: Jan 2012 Location: Long Island, NY Posts: 485	Originally posted by stevebrot No Steve Short and sweet. Thanks Steve

03-15-2017, 03:56 PM	#15
clackers Otis Memorial Pentaxian Join Date: Jul 2013 Location: Melbourne Photos: Albums Posts: 16,397	Originally posted by Neville Holmes Many thanks for all the replies. I am a retired Professional Engineering so if the specification is not clear then I assume a problem. All seems well so am I the first to go film-mirrorless- DSLR? (Laughs) There are people here who have done an enormous amount of photography, Neville. Enjoy dusting off the old lenses!