Pentax 100mm f2.8 A exposure issues

**stemked** · 04-13-2020, 12:20 PM

Hi All.

I'm just wondering about something. I just got a the 100mm f2.8 A macro, a beautiful lens (I love macros), but I've noticed an odd issue. If I actually use the lens on anything other than 'A' autoexposure and shoot Av it nails the exposure. When I put the lens on the 'A' setting it often way over exposes the image. What gives? I don't seem to have issues with other A lenses

Parallax · 04-13-2020, 12:31 PM

Sound like sluggish/sticky blades.

**Apet-Sure** · 04-13-2020, 12:38 PM

It sounds like the camera isn't able to actuate the aperture stop-down lever to the f-stop you've set in the camera. With the lens off the camera and on the 'A' setting, can you physically move the stop down lever and see the aperture closing down?

stevebrot · 04-13-2020, 01:52 PM - **1 Like**

Originally posted by stemked

If I actually use the lens on anything other than 'A' autoexposure and shoot Av it nails the exposure. When I put the lens on the 'A' setting it often way over exposes the image.

I agree that it sounds like the lens is not stopping down properly (e.g. "sticky/sluggish: aperture).

You can test by flicking the actuator lever on the back of the lens with your finger at various aperture ring settings. The action should be snappy with no hesitation at all settings.

Steve

**mikesbike** · 04-13-2020, 02:45 PM

Since the problem does not occur with your other "A" lenses, this lens is obviously defective. I would contact the seller regarding a return and refund.

I further suggest the much more recent Pentax DFA 100mm f/2.8. The current better-built WR version costs more, but has no aperture ring, if that is of importance to you, while the original DFA 100mm f/2.8 having the same optics can be found at a far lower price and does feature an aperture ring.

stevebrot · 04-13-2020, 04:18 PM

Originally posted by mikesbike

I further suggest the much more recent Pentax DFA 100mm f/2.8. The current better-built WR version costs more, but has no aperture ring, if that is of importance to you, while the original DFA 100mm f/2.8 having the same optics can be found at a far lower price and does feature an aperture ring.

The Pentax-FA 100/2.8 Macro (nice build) might also be considered as might the Pentax-F 100/2.8 since all four lenses share the same design and optical performance. That said, the OP's Pentax-A 100/2.8 is a bit of a classic with significant reputation and may be worth servicing, depending, of course, on how much they paid.

Steve

Wheatfield · 04-13-2020, 04:45 PM

Originally posted by stemked

Hi All.

I'm just wondering about something. I just got a the 100mm f2.8 A macro, a beautiful lens (I love macros), but I've noticed an odd issue. If I actually use the lens on anything other than 'A' autoexposure and shoot Av it nails the exposure. When I put the lens on the 'A' setting it often way over exposes the image. What gives? I don't seem to have issues with other A lenses

That happened to my A100/2.8 Macro. It was sticky aperture blades. I had it CLA's and there has been no recurrence.

pschlute · 04-13-2020, 07:12 PM

Originally posted by stemked

. If I actually use the lens on anything other than 'A' autoexposure and shoot Av it nails the exposure

Just for your info.......with the lens set to an actual aperture click stop, and in Av mode, the lens will always be wide open during exposure.

04-13-2020, 12:20 PM	#1
stemked Site Supporter Join Date: Feb 2009 Location: Indianapolis Photos: Gallery \| Albums Posts: 527	Pentax 100mm f2.8 A exposure issues Hi All. I'm just wondering about something. I just got a the 100mm f2.8 A macro, a beautiful lens (I love macros), but I've noticed an odd issue. If I actually use the lens on anything other than 'A' autoexposure and shoot Av it nails the exposure. When I put the lens on the 'A' setting it often way over exposes the image. What gives? I don't seem to have issues with other A lenses

04-13-2020, 04:18 PM	#6
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by mikesbike I further suggest the much more recent Pentax DFA 100mm f/2.8. The current better-built WR version costs more, but has no aperture ring, if that is of importance to you, while the original DFA 100mm f/2.8 having the same optics can be found at a far lower price and does feature an aperture ring. The Pentax-FA 100/2.8 Macro (nice build) might also be considered as might the Pentax-F 100/2.8 since all four lenses share the same design and optical performance. That said, the OP's Pentax-A 100/2.8 is a bit of a classic with significant reputation and may be worth servicing, depending, of course, on how much they paid. Steve Last edited by stevebrot; 04-13-2020 at 04:31 PM.

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04-13-2020, 12:31 PM	#2
Parallax Senior Moderator Loyal Site Supporter Join Date: Apr 2008 Location: South Dakota Photos: Gallery Posts: 19,332	Sound like sluggish/sticky blades.

04-13-2020, 12:38 PM	#3
Apet-Sure Site Supporter Join Date: Dec 2015 Location: Goldsboro North Carolina Photos: Gallery \| Albums Posts: 2,861	It sounds like the camera isn't able to actuate the aperture stop-down lever to the f-stop you've set in the camera. With the lens off the camera and on the 'A' setting, can you physically move the stop down lever and see the aperture closing down?

04-13-2020, 01:52 PM - 1 Like	#4
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by stemked If I actually use the lens on anything other than 'A' autoexposure and shoot Av it nails the exposure. When I put the lens on the 'A' setting it often way over exposes the image. I agree that it sounds like the lens is not stopping down properly (e.g. "sticky/sluggish: aperture). You can test by flicking the actuator lever on the back of the lens with your finger at various aperture ring settings. The action should be snappy with no hesitation at all settings. Steve

04-13-2020, 02:45 PM	#5
mikesbike Site Supporter Join Date: Dec 2016 Location: Southeastern Michigan Posts: 4,549	Since the problem does not occur with your other "A" lenses, this lens is obviously defective. I would contact the seller regarding a return and refund. I further suggest the much more recent Pentax DFA 100mm f/2.8. The current better-built WR version costs more, but has no aperture ring, if that is of importance to you, while the original DFA 100mm f/2.8 having the same optics can be found at a far lower price and does feature an aperture ring.

04-13-2020, 04:45 PM	#7
Wheatfield Moderator Join Date: Apr 2008 Location: The wheatfields of Canada Posts: 15,981	Originally posted by stemked Hi All. I'm just wondering about something. I just got a the 100mm f2.8 A macro, a beautiful lens (I love macros), but I've noticed an odd issue. If I actually use the lens on anything other than 'A' autoexposure and shoot Av it nails the exposure. When I put the lens on the 'A' setting it often way over exposes the image. What gives? I don't seem to have issues with other A lenses That happened to my A100/2.8 Macro. It was sticky aperture blades. I had it CLA's and there has been no recurrence.

04-13-2020, 07:12 PM	#8
pschlute Moderator Loyal Site Supporter Join Date: Mar 2007 Location: Surrey, UK Photos: Gallery Posts: 8,198	Originally posted by stemked . If I actually use the lens on anything other than 'A' autoexposure and shoot Av it nails the exposure Just for your info.......with the lens set to an actual aperture click stop, and in Av mode, the lens will always be wide open during exposure.