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08-18-2021, 05:01 AM   #1
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Pentax LX with modern lenses

Hi all, can you help ? I want to use my new 15mm F4 Limited lens and my 100mm f2.8 WR Macro on my Pentax LX. Neither lens has an aperture dial, so does each lens default to the widest aperture when it's attached to the LX ? Thanks in advance, All the best Alex.

08-18-2021, 05:11 AM   #2
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the DA 15 Limited may vignette on the LX when shot wide open, while the 100/2.8 Macro should not....

both will only work wide open, however...
08-18-2021, 05:12 AM   #3
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this is the chart you need to review:

QuoteQuote:
Pentax K-mount Lens Compatibility Chart
Copyright 2006-2020 Mosphotos.com. All rights reserved.
The Pentax K-mount Camera Lens Compatibility Chart

QuoteQuote:
Lens or
lens group:

D FA 50mm,
D FA 100mm non-WR,
F, FA
A
AF
M, K
will work properly with an LX

another good chart

QuoteQuote:
Pentax K-Mount Lens Series Explained: The differences between various Pentax lens series
Pentax K-Mount Lenses Explained: The differences between various Pentax lens series
08-18-2021, 06:02 AM   #4
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You need an aperture ring with the LX if you want to control aperture. Later cameras like the PZ-1 can control aperture in a way that is similar to a dslr. Many DA lenses will vignette, some won’t; consult the test thread for more specific info on the lens performance on full frame.

08-18-2021, 06:21 AM - 1 Like   #5
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QuoteOriginally posted by pepperberry farm Quote
both will only work wide open, however...
Just the opposite. Lenses without an aperture ring will only work full stopped down.
08-18-2021, 06:33 AM   #6
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QuoteOriginally posted by boriscleto Quote
Just the opposite. Lenses without an aperture ring will only work full stopped down.
thank you - more coffee, please...
08-18-2021, 06:59 AM   #7
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QuoteOriginally posted by pepperberry farm Quote
thank you - more coffee, please...
Usually a good idea

Also good idea, for me at least, - rereading the chart before posting

08-18-2021, 08:32 AM   #8
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I think KAF4 lenses would be shot wide-open on cameras like the LX. KAF3's would be shot full stopped down. D-FA*85 on Portra 400 anyone?
08-18-2021, 08:38 AM   #9
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QuoteOriginally posted by pres589 Quote
I think KAF4 lenses would be shot wide-open on cameras like the LX. KAF3's would be shot full stopped down. D-FA*85 on Portra 400 anyone?
for the LX

DA 55-300mm ED
PLM WR RE
DA 18-50mm,
DA L 18-50mm

Although the lens fits the body, it cannot be used: The shutter will not fire (manual focus lens) or the lens cannot be focused (DA focus-by-wire lens).

D FA* 50mm F1.4,
D FA* 85mm F1.4,
D FA 70-210mm F4

Not Recommended. Aperture will remain wide-open and cannot be controlled

KAF3

Not Recommended. While the lens does fit the body, the lens will always stop down to its smallest aperture during exposure (for example F22), which limits practical shooting. DA lenses will furthermore produce vignetting as explained above.

according to the chart

The Pentax K-mount Camera Lens Compatibility Chart
08-18-2021, 08:58 AM   #10
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It should meter and shoot as if fully stopped down the aperture is wide open during metering. I'm not sure if all the DA/DFA lenses have a dummy aperture setting linkage. The dummy (in the slot opposite the stop-down actuating lever) is set to the fully closed position.

I tested a DFA lens on my K1000 and that seems to be what it is doing. The LX is K mount, no contacts.
08-18-2021, 12:06 PM   #11
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QuoteOriginally posted by ARA Quote
Neither lens has an aperture dial, so does each lens default to the widest aperture when it's attached to the LX ?
Both lenses will stop down to minimum aperture for exposure. As noted above, chances are good the "dummy" aperture simulator present on many (all?) DA/D FA lenses will reflect that behaviorindicate it is set to minimum aperture, so metering will be accurate accept for vignette.


Steve

Last edited by stevebrot; 08-20-2021 at 02:54 PM. Reason: clarity and accuracy
08-18-2021, 07:18 PM   #12
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Just checking the back of the DFA85, there's no aperture simulator. So it's basically like a mirror lens on a camera which cannot control the aperture nor the focus motor, but which can be focused manually.

However, this won't apply to the 55-300 RE because it focuses by wire; so it cannot be manually focused either.

You can also use the DOF preview to assess what the shooting aperture will be, but in the case of most lenses with no aperture ring, it will stop down all the way.
08-20-2021, 12:47 PM   #13
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QuoteOriginally posted by jslifoaw Quote
Just checking the back of the DFA85, there's no aperture simulator.
Look again. While there is not aperture actuator, all photos I have seen of the lens mount shows the slot for the aperture simulator and by width, it appear to proper for wide-open @ f/1.4. As long as one is happy with shooting wide-open, it should work fine on the LX.




Steve
08-20-2021, 01:26 PM   #14
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When the D-FA*85 and other KAF4 lenses are sitting out on a table, are they wide open or are they full stopped down, in that sans-body condition? I thought they were wide open and only stop down when commanded to stop down by a compatible body.
08-20-2021, 01:58 PM   #15
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QuoteOriginally posted by stevebrot Quote
Look again. While there is not aperture actuator, all photos I have seen of the lens mount shows the slot for the aperture simulator and by width, it appear to proper for wide-open @ f/1.4. As long as one is happy with shooting wide-open, it should work fine on the LX.




Steve
Right. When I referred to simulator, the lever is what I meant. Presumably it's nitpicking since a conventional K mount lens set to the maximum aperture wouldn't move the lever on the body's mount to indicate to the camera meter that the lens is set to stop down, noting that the actual f-number is irrelevant; the wide open position is the same for all lenses to enable the simulator to measure the exposure difference between the set aperture and maximum aperture.

So at the end of the day, the lens will work like any other fixed aperture lens like a mirror lens since nothing is telling the camera's meter that the lens aperture will be closed by any amount (and to compensate when metering), and the release of the aperture control lever when the shutter fires won't affect the lens.

Unlike mechanically linked DA lenses, the KAF4 lenses are wide open by default when not mounted.

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