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09-23-2021, 10:57 AM   #1
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Problem with Pentax 300 mm, M* F4 lens

I need help to determine if I have a problem with this lens. When used with my K70, I am able to change the shutter speed using the front knurled wheel when the top dial is set to M. Any other setting such as T, TAV, or AV does not allow the speed to be changed. However, on my K50 I cannot change the speed when using any of the M, T, TAV, or AV settings. Why the M setting only works on the K70 puzzles me. To check to see if the problem is specific to the 300 mm lens I ran tests on my Pentax DA 18-135 lens and found that both cameras behaved similarly and the speed could be changed as long as the top dial was set to M. All other settings did not work. I might add that the cameras were always set to manual focus, but I don't think this causes the problem, if indeed it is a problem.

What say ye, learned Pentaxians?

09-23-2021, 11:08 AM   #2
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Your K50 has a problem?

Last edited by AfterPentax Mark II; 09-23-2021 at 11:09 AM. Reason: unquoting the quote
09-23-2021, 11:36 AM   #3
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Make sure you enable aperture ring in the custom function menu, C 24.

Also, in the owners manual of the K-70, it says:
If the aperture ring is set to a position other than A, the
camera operates in Av mode regardless of the mode dial
setting except when set to M or B mode.

This describes what you are experiencing. Make sure it's in Manual mode because it's a manual lens without an A setting on the aperture ring.
09-23-2021, 11:37 AM   #4
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QuoteOriginally posted by psoo Quote
When used with my K70, I am able to change the shutter speed using the front knurled wheel when the top dial is set to M. Any other setting such as T, TAV, or AV does not allow the speed to be changed
With the M*300 that is correct behaviour.


QuoteOriginally posted by psoo Quote
To check to see if the problem is specific to the 300 mm lens I ran tests on my Pentax DA 18-135 lens and found that both cameras behaved similarly and the speed could be changed as long as the top dial was set to M
This is correct for Manual mode.

QuoteOriginally posted by psoo Quote
All other settings did not work
This is not correct. You should be able to change the shutter speed (with a modern lens) with the front dial when the camera is in (Hyper) P mode; TAv mode ; Tv mode ; as well as Manual. You cannot change it in Av mode obviously.

With your M* 300, any of the Auto modes default to Av wide open, and the front dial will do nothing. In M mode you should be able to change the shutter speed.


Last edited by pschlute; 09-23-2021 at 11:49 AM.
09-23-2021, 11:46 AM   #5
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QuoteOriginally posted by psoo Quote
What say ye, learned Pentaxians?
I'd suggest checking the E-Dial Programming for each camera: e.g., K-70 Operating Manual, p. 109
09-24-2021, 06:23 AM   #6
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QuoteOriginally posted by builttospill Quote
Make sure you enable aperture ring in the custom function menu, C 24.

Also, in the owners manual of the K-70, it says:
If the aperture ring is set to a position other than A, the
camera operates in Av mode regardless of the mode dial
setting except when set to M or B mode.

This describes what you are experiencing. Make sure it's in Manual mode because it's a manual lens without an A setting on the aperture ring.
I did use the M setting on the K70 and I can alter the shutter speed on my K70. I was wondering why the M setting on my K50 did not allow me to alter the speed. I would expect that both cameras should behave similarly

---------- Post added 09-24-21 at 09:38 AM ----------

QuoteOriginally posted by builttospill Quote
Make sure you enable aperture ring in the custom function menu, C 24.

Also, in the owners manual of the K-70, it says:
If the aperture ring is set to a position other than A, the
camera operates in Av mode regardless of the mode dial
setting except when set to M or B mode.

This describes what you are experiencing. Make sure it's in Manual mode because it's a manual lens without an A setting on the aperture ring.
I did have the aperture ring enabled. I always leave the camera at this setting. However, I still can't get the K50 to allow speed adjustement when the camera is set to M.
09-24-2021, 06:55 AM   #7
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What mode does the K50 display in upper left corner on the rear screen when you have dial set on M-mode?

It may not be in manual mode even if the dial is set to M-mode.
FI if you enable auto ISO i manual mode you enter TAv mode, which then will switch to Av-mode as the lens do not have A-setting on the aperture ring.


Last edited by Fogel70; 09-24-2021 at 07:02 AM.
09-24-2021, 07:45 AM   #8
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QuoteOriginally posted by Fogel70 Quote
What mode does the K50 display in upper left corner on the rear screen when you have dial set on M-mode?

It may not be in manual mode even if the dial is set to M-mode.
FI if you enable auto ISO i manual mode you enter TAv mode, which then will switch to Av-mode as the lens do not have A-setting on the aperture ring.
Hi. When I set my K50 to M, the top left screen shows Av. When I set my K70 to M the screen shows M. Does this give you a clue as to what the problem is?

Also, I have the camera set to auto ISO but I still can't manually change the shutter speed. I'm confused.
09-24-2021, 07:51 AM   #9
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On the K50 in M mode you must use a fixed ISO, auto ISO doesn't work in M Mode
09-24-2021, 08:08 AM   #10
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QuoteOriginally posted by psoo Quote
Hi. When I set my K50 to M, the top left screen shows Av. When I set my K70 to M the screen shows M. Does this give you a clue as to what the problem is?

Also, I have the camera set to auto ISO but I still can't manually change the shutter speed. I'm confused.
As you have set ISO to auto, you are not in M-mode, you are in TAv-mode. TAv-mode is only supported if the aperture ring is set to A-position. But your lens do not have that so the camera switch to Av-mode.

To switch from TAv-mode to M-mode you need to set ISO manually.
09-24-2021, 08:23 AM   #11
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QuoteOriginally posted by pvanni Quote
On the K50 in M mode you must use a fixed ISO, auto ISO doesn't work in M Mode
AHHA!! You have solved my problem. I just set my K50 to a fixed ISO and lo and behold I can now adjust the shutter speed. I would never have guessed that this was the solution. I was beginning to think that the problem was not fixable and that I would just have to use the lens on my K70. However, it's nice to have two camera bodies in use, instead of switching lenses all the time. I sometimes use the 300 mm lens to photograph the moon and I find that the speed adjustment feature gets me quickly to the best exposure for a very bright small image like the moon.

Thanks to all respondents to my post, you've been a great help.
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