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09-03-2015, 08:33 AM   #1
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basic question on Pentax-M lenses

I'm a new member. I have a K-50 which I use exclusively with 'old glass', mainly Tamrons. I want to be sure that I understand what websites mean when they say that Pentax-M lenses are 'manual aperture'. I just want to confirm that this means I set the aperture using the ring, but still can focus fully open - when I shoot (and when I use the green button to determine exposure) the camera stops down. I ask because to me, 'manual aperture' sounds like the camera has no control of the iris at all.


thanks
Andrew

09-03-2015, 08:45 AM   #2
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M-series lenses have no camera control of the aperture. That came in the A series lenses. When you use the M series it works as so:

Set lens aperture via ring.
Set ISO desired.
Focus and compose your image.
Press green button to meter and camera decides shutter speed.

In A series, it's exactly as using Av mode with manual focus. In this, the lens remains wide open for focusing and the camera stops down to your desired aperture as part of the process.
09-03-2015, 08:48 AM   #3
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QuoteOriginally posted by mistertransistor Quote
... I set the aperture using the ring, but still can focus fully open - when I shoot (and when I use the green button to determine exposure) the camera stops down. I ask because to me, 'manual aperture' sounds like the camera has no control of the iris at all.

thanks
Andrew
You are correct on exposure, and yes the lens is fully open for composition. It only stops down at green-button press and taking the shot.
09-03-2015, 12:04 PM   #4
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QuoteOriginally posted by mistertransistor Quote
I'm a new member. I have a K-50 which I use exclusively with 'old glass', mainly Tamrons. I want to be sure that I understand what websites mean when they say that Pentax-M lenses are 'manual aperture'. I just want to confirm that this means I set the aperture using the ring, but still can focus fully open - when I shoot (and when I use the green button to determine exposure) the camera stops down. I ask because to me, 'manual aperture' sounds like the camera has no control of the iris at all.


thanks
Andrew
The terminology "manual aperture lens" has been used erroneously by those who learned photography after the shutter-priority mode was added to auto-exposure systems. Manual aperture lens really means that you focus with the aperture wide open but you have to manually stop down the aperture (turn the aperture ring on the lens) to achieve an appropriate exposure as indicated by the light meter ("stop-down metering"). Therefore, late screw-mount Takumars and all of the K-mount lenses are auto-aperture lenses (including the M series). That means that, even when you are using manual exposure mode, you focus and meter with the lens wide open, but when you press the shutter release, the lens automatically stops down to the aperture chosen to achieve an appropriate exposure as indicated by the light meter. These lenses are termed "auto-aperture" lenses. They cannot be described as manual-aperture lenses but rather as: not compatible with the shutter-priority mode of auto-exposure systems (in which the user chooses the shutter speed and, when the shutter is tripped, the auto-exposure system chooses an aperture size for an appropriate exposure and then stops down the aperture to that size). If there are no electrical contacts on the lens mount for the camera to transmit the command to the lens to stop the aperture down to the aperture size chosen by the auto-exposure system, then the auto-exposure system cannot choose an aperture size.

Therefore, the M-series was the last Pentax lens series to be shutter-priority incompatible.
The A-series was the first to be shutter priority capable.
Both are auto-aperture lenses

09-03-2015, 02:34 PM   #5
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QuoteOriginally posted by mistertransistor Quote
I understand what websites mean when they say that Pentax-M lenses are 'manual aperture'.
What they mean is that the lens has "manual aperture control". All K-mount lenses feature "auto aperture actuation".


Steve
09-04-2015, 05:19 AM   #6
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The only thing manual about the K and M mount lenses is the requirement to set the aperture yourself on the ring, as the camera can't do it for you.

The true manual aperture - the one you have to stop down yourself - went out with the M42 mount Takumars, and even the last of those could meter wide-open on the late-model Spotmatics. AFAIK no lens with "Pentax" on the front ring has ever had a manual aperture of that kind.
09-04-2015, 08:08 AM   #7
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thanks

Thanks for all your helpful replies.
Andrew
09-04-2015, 09:34 AM   #8
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QuoteOriginally posted by mistertransistor Quote
I'm a new member. I have a K-50 which I use exclusively with 'old glass', mainly Tamrons. I want to be sure that I understand what websites mean when they say that Pentax-M lenses are 'manual aperture'. I just want to confirm that this means I set the aperture using the ring, but still can focus fully open - when I shoot (and when I use the green button to determine exposure) the camera stops down. I ask because to me, 'manual aperture' sounds like the camera has no control of the iris at all.


thanks
Andrew
Manual aperture has changed over time.

Originally manual aperture required you to manually set the aperture.

This was soon replaced by automatic aperture where the camera let you focus wide open and then stopped the lens down to expose the frame, but his still required to stop down manually or metering

The next evolution was to have linkage to tell the camera what the aperture was set to (specificAlly how many stops down from wide open you were. This way you could both focus and meter wide open.

Next were lenses with the A position. This permitted the camera to set the aperture though the control of the aperture lever and based upon knowledge of the lens maximum and minimum aperture from the contacts.

Today this is what people refer to as automatic aperture

Sadly for you. The M seri s l need lack the contacts to l t the camera set he aperture. So you n the d to use the green button

09-04-2015, 10:12 AM   #9
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QuoteOriginally posted by pathdoc Quote
The true manual aperture - the one you have to stop down yourself - went out with the M42 mount Takumars
Almost all the Super/S-M-C/SMC Takumars have automatic aperture actuation and I believe even the earliest have at least semi-auto or pre-set apertures. There are a few specialty Asahi M42 lenses with preset or manual aperture. Good examples would be the Bellows Takumar 100/4 which has a preset aperture and Takumar ultra-telephotos, all of which have fully manual apertures.

As for the K-mount Pentax lenses, the Pentax-K 28/3.5 Shift has a preset aperture and the K/M-series ultra-telephotos all have fully manual apertures.


Steve
09-04-2015, 10:15 AM   #10
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QuoteOriginally posted by mistertransistor Quote
Thanks for all your helpful replies.
Andrew
BTW...Welcome to the Pentax Forums!


Steve
09-04-2015, 10:36 AM   #11
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QuoteOriginally posted by stevebrot Quote
Almost all the Super/S-M-C/SMC Takumars have automatic aperture actuation
The capability is there, yes,but the point I was trying to make is that the manual actuation option (the clutch system) had been removed with the move to K mount and it was ONLY auto-actuation from then on.

I didn't know about the preset on the shift lens, but I guess it makes sense that it would have it. I stand corrected there.
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