[Lowell Goudge]

Thank you Lowell,
of course your method is the most correct and prevents from unwanted under/overexposures due to different colors or reflections of the subject(s).

About my "unorthodox" attempt of using non pentax lenses in continuous stop down mode, I guess that my problem is caused by either all-shorted pins (the shape of metal mount pentax lenses seems to me recessed and not allowing pins to make contact) or/and by some mechanical interference between zeiss lens and pentax camera levers.

Similar to what found and declared by Leitax the issue has different effects on different lenses, although I couldn't devise hard interferences. I'll keep trying to understand, anyway

Ciao,

Stefano

one thing to consider, and this is related to the not all pins(except the A pin which is recessed) shorted is that if the data pin 7th from lens locking pin is not shorted lat cameras do not move the aperture lever. This has been encountered on extension tubes most notably

[seppstefano]

After several tests of yesterday evening, perhaps I found the existence of a possible solution to my issue on non pentax lenses (of course on pentaxforums )

It's here: the lenses I tried have black painted lens mount...

https://www.pentaxforums.com/forums/pentax-beginners-corner-q/137718-manual-a...etering-2.html

I'll report after this evening's test

Ciao,

Stefano

[seppstefano]

As promised, for who cares, here's my followup.

I'm finally starting getting more consistent exposure results with my K-x and "strange" lenses

Quick and dirty, I placed a "C" shaped aluminum foil covering (shortening) all pins on camera lens mount, so any foreign lens was "sensed" like a M42 (of course with metal mount). The pattern is 111*11, the camera therefore should consider the mounted lens as with [1.2 - 22] f/stop range.

I fixed ISO (6400), set M mode, SR on, MF, and metered (the mirror did flip this time ) with Green Button (or EV button) and took some shots. Some data:

- P50/1.4 - Good exposures from 1.4 up to f/8, value from which I started getting underexposure and the metered shutter value was blinking.
- P100/2 - Good exposures from 2 (perhaps a bit bright) up to - again - f/8 where I got underexposure and the metered shutter value was again blinking.

Then I did the same with my SMC-M 50/1.7 in stop down mode (not fully locked in) - Good exposures (perhaps 1.7 and 2.8 a tad brighter) but - again - at f/8 I started getting underexposure and shutter time showed in the viewfinder was blinking (we're talking of 1/5 sec, far from the limit of the lightmeter)

Anybody caring to explain me what can be the reason of this strang behaviour?

Kind regards,

Stefano

[JimJohnson]

On genuine A-series lenses (A through DA) the aperture lever changes the aperture in a linear fashion. On M-series and older lenses, this is not true other than when the lens is wide-open. The exposure, while typically consistent, will have an effective aperture somewhat different than shown on the lens.

You have two choices-
#1- Run a few test shots for a lens and note how much exposure compensation is needed for proper exposure for each f/stop on the lens. Create a 'cheat sheet' for each lens.
#2- Set the body to the widest aperture, make sure the lens is stopped down to the desired f/stop and THEN meter.

[seppstefano]

Jim thanks for answering.

But what you're writing is valid in case di lenses in continuous stop down?

The linearity variation from M to A series of the aperture lever doesn't seem to me influent in this case, does it?

I mean that all the lenses I tested and reported had their diaphragm already stopped down when I metered, exactly as you write at #2, yet I'm getting that strange behavior...

Stefano

[JimJohnson]

Are you shooting

1. in either manual or aperture priority (Av) mode,
2. with the body set to the widest aperture (f/1.2),
3. the lens aperture ring set as desired,
4. the lens stopped down to this aperture when metering,
5. and if using Av mode, making sure the exposure is locked until after the shutter is released?

[seppstefano]

Sorry for late reply, due to intercontinental time-shift

1. all tests were made in M mode, as written, and lens effectively stopped down (I mean the image in the viewfinder is darker than at full aperture ). With Av I keep getting underexposed, "inaccurate" metering, as declared by Pentax herself. With M mode, shutter time evaluations get clipped when f/stop is smaller than f/5.6 and can't understand why.
2. no, the lens is already stopped down to the desired aperture, before metering. I seem I remember body does not show any f/ info.
3. yes
4. yes
5. N/A. See point 1, I did shot in M mode.

I'd love to know if other Pentax users do always get good exposures even e.g. at f/11 as per Pentax officially method (custom menus properly set, M mode, green button) with stopped down lenses (e.g. old M42).

In test the clipped evaluated shutter time was blinking even if the value was quite far from metering limits. For example it blinked at 1/10.... while the effective light was demanding longer shutter speeds. It seems that the shutter speed which is set (and showed) doesn't match with the low light blinking warning. It seems there is "something" externally clipping the shutter value.

Thanks for your time,

Stefano

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Last edited by seppstefano; 11-16-2012 at 04:35 AM.

[Lowell Goudge]

If you now have the system "working" as you think it should, then take one of your lenses and shoot a uniformly lit block wall, at all apertures. Then look at the histogram and grey scal value and plot this as a function of aperture. You will see two things.

You will see a shift, as a function of true open aperture, because the viewfinder is only accurate for non A lenses with open aperture at F4-5.6 and also you will see a slight "Hump" with apertures below F 5.6 tending to unr expose, and amputees above F5.6 over exposing from F8-11 then falling back towards correct at F22-32

The total variance will be in the range of +/- 2 stops, wherre a stop is about 45 greyscale.

If you consider this good then that's fine go and shoot. But my Sigma 70-200/2.8 is +/- 2 greyscale from F2.8-F32 or as close to perfect as the conditions would allow, so my definition of the cheat "working" is a little different. If it worked across all apertures even to +/-1 stop it might be workable, but it isn't.

[seppstefano]

Thank you Lowell,
as soon as possible I'll take the shoots for the test you're suggesting.

Honestly I'm actually finding no less than unacceptable these wildly wrong evaluations given by Pentax meter.
I remember when LX exposure system was one of the best of its time, and I am missing its accuracy and simplicity.

I'll be back with some more data...

Stefano

[seppstefano]

Ok,
here am I. I did take some shots of a grey pavement and I'm just trying to evaluate the shifts among several lenses. BTW how do you "quantify" the histogram shifts on latest Pentax Digital Camera Utility?

Contrary to what I found the night I made my early test, this time I found reasonable differences among metered EVs through following lenses: C/Y P50/1,4, C/Y P100/2, C/Y D28/2,8, M50/1,7, both with M+GB (Green Button) and/or Av mode when lens is not fully locked in (continuous stop down). However this happens IF AND ONLY IF I hear the "swishing" sound after pressing the Green Button.

It seems infact that sometimes my green button doesn't seems reacting to first pressure (even if showed in-finder values do change), and needs a second action to wake up and meter. This happens e.g. after closing apertures from 8 to 11 and not when mounting the lens first time.

Did anyone ever experience anything similar? Does this happen only to my K-x? This doesn't seem a faulty pressure: I got it several times.

Stefano

[fb_penpho]

I am interested with this thread because, a manual lens without an A pin, the wireless option is turned off. I could not think of any good reason why this should be so!

[aurele]

i tried to do this on a cheap lens, but as far as i remember, it was a failure : the calculation of exposure is differently achieve with A lens compared to K/M lens .

In the original K-mount its displacement is proportional to the diameter of the diaphragm opening. In the KA mount, it is proportional to the area of the diaphragm opening, and thus to the selected f-stop. This leads to an identical stop-displacement between any two consecutive f-stops, and thus greatly simplifies the operation of the body in the Tv and P operating modes. All KA lenses have the same stop-displacement.

Features and Operation of the Ka Mount (see Linear operation of the diaphrgam actuator)

in clear, the smallest aperture you set, the more "off" the exposure will be.

So to be clear, if you want it to work well, you need to change the aperture lever mecanism too.

[JimJohnson]

Shown in pictures:
https://www.pentaxforums.com/forums/pentax-flashes-lighting-technique/190819-...ml#post2021062