Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
11-16-2010, 08:32 AM   #91
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,400
QuoteOriginally posted by blende8 Quote
Lowell, what I want to find out is this:
Dave said that he can put his M42 lens on his A-modified body and it meters correctly. I am questioning this based on the above reasoning.
M42 lenses do not necessairly meter correctly because with this mod, the camera thinks the lens is an F1.2 and will applyt what ever corrections for an F1.2 lens that camera needs based upon the linearity of metering vs native F stop. See the multitude of threads on this subject
QuoteQuote:
Dave said that that the difference is that M42 lenses do not have a "diaphragm actuator" (as Bojidar calls it here) or "activation lever" as you call it.
this is true, there is no activation lever, hence the lens stops down when you turn the ring,.
QuoteQuote:
But is this detected by the camera?
no, what the camera sees is the change in exposure when you stop down the lens, but it has no reference aperture to compare to, so there may be all sorts of errors and offsets.
QuoteQuote:





Really? Strange.
You mean r2?
r2 appears to be just a pin for the aperture encryption.
you are correct,
QuoteQuote:
And what do you mean by "insulated base". If it is already insulated, why again insulate it?
I think the reference to insulated bases is the fact that M42 lenses (at least many of them) have insulated paint or anodyzing that stops them from contacting the other pins

11-16-2010, 08:46 AM   #92
Veteran Member
blende8's Avatar

Join Date: Feb 2007
Location: Bremen, Germany
Photos: Albums
Posts: 1,484
Ok, Lowell, I think we then agree against Dave.
He said that his A-modified camera meters correctly with M42 lenses irrespective of the aperture set on the body.
11-16-2010, 09:03 AM   #93
Veteran Member
lguckert79's Avatar

Join Date: Jun 2010
Location: Pittsburgh, PA
Photos: Gallery | Albums
Posts: 588
so in lamens terms when you short out the pin and use av mode and it you set the body to say f:4 then set the lans to f:4 it will work but here is the ? when in p mode and when you meter the light and the body says to set the apature to say f: 4 then you set the lens and it will corectlly expose?
11-16-2010, 09:09 AM   #94
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,400
QuoteOriginally posted by lguckert79 Quote
so in lamens terms when you short out the pin and use av mode and it you set the body to say f:4 then set the lans to f:4 it will work but here is the ? when in p mode and when you meter the light and the body says to set the apature to say f: 4 then you set the lens and it will corectlly expose?
that's the idea, but note the following.

With this modification, you also need to drill holes to match the pattern of the lens coding into the base of the lens, ot otherwise insulate those sections of the lens mount so that the camera knows the correct aperture.

Also note, that the activation lever will move to what it thinks is the correct location, and not always allow the lens to open correctly for the aperture programmed, so errors will still exist.

If you do not duplicate the lens coding pattern the camera will meter the wrong way because it thinks you have an F1.2 lens.

to me, this entire thing is a waste of effort and damages otherise good lenses because no one will buy them from you afterwards

11-16-2010, 09:22 AM   #95
Veteran Member
jolepp's Avatar

Join Date: Oct 2010
Location: Finland
Photos: Albums
Posts: 3,196
QuoteOriginally posted by blende8 Quote
Really? Strange.
You mean r2?
r2 appears to be just a pin for the aperture encryption.
And what do you mean by "insulated base". If it is already insulated, why again insulate it?
Actually no :-) It seems my experiment with the k-x and the edge of a sheet of a paper between the lens and the mount was probably only about the lens not going all the way to the lock position and hence the aperture lever not being in contact with its body counterpart (this would apparently work if someone really insists on using an M/K this way :-). I retried this while making sure that the lens locks in position and could not reproduce my earlier result.

M42 lenses tend to have a coating on the base (= non-conductive), whereas the base of K is chromed (conductive), I put the remark in my previous post as there could be lenses with the M42 mount, where the base surface is conductive.

r2 gets a special treatment though, shorting this (or not) with no lens mounted causes the k-x to operate (or not operate) the aperture mechanishm when the green button is pressed in M-mode. Also, I've read here that shorting this enables catch-in focus for M42 lenses. As to why this should make a difference is a mystery to me :-)
11-16-2010, 09:59 AM   #96
Inactive Account




Join Date: Dec 2008
Location: Ames, Iowa, USA
Photos: Albums
Posts: 2,965
QuoteQuote:
The camera thinks that it is an f1.2/22 lens, because all contacts are shortened.
yes
QuoteQuote:
It assumes that the lens is in the A-position.
yes
QuoteQuote:
It thus assumes that the lens is fully open (F1.2).
yes - but for a reason I don't understand, to get correct metering, I must set the e-wheel to F2.8 for my Macro-Takumar 50:4, F2.0 for my MIR 65:3.5, etc...
QuoteQuote:
It then will take the f-reading from the camera (which you have set with the back wheel) and will calculate the speed.
Yes, speed is calculated when you press the green button. Afterward, you can change the e-wheel f-stop if you want - that'll change what's written to exif, but doesn't affect actual exposure.
QuoteQuote:
This will happen independent of there being an aperture lever or not.
yes
QuoteQuote:
Or does the camera recognize that there is no aperture lever?
no; the camera does not know there's no lever. It thinks the lens is an A type which has a lever.

Dave

PS I think the above is consistent with Lowell's observations.
11-16-2010, 10:21 AM   #97
Senior Member




Join Date: Mar 2010
Location: Berlin
Posts: 112
Since i'm home with the flu and i've nothing better to do, i made a little test using my KX and an M50 f1.4 lens.
The test has a total of 24 picture, the first 8 are with the M50 half mounted (the lens lever isn't mechanically affected by the body this way) and shotted moving the lens aperture ring to f1.4-2-2.4-2.8-3.5-4-4.5-5.6.
The second 8 pics are done after shorting the "*" pin and moving the lens aperture as above.
I've set the lens locking notch so the pin needed to say to the body the lens has an aperture range between f1.4 and f22 doesn't make contact.
I don't know why (dunno cause i remember i did use the M50 in thuis way before and metering was spot on) but i had to push the exposure copensation by + 2 and 1/2 stop to have a correct exposure wide open (maybe i shorted other pin also because using LV the shutter was twice as fast 0_0...).
The last 8 was done has the A set but using flash (to show P-TTL usage).

I didn't match the f-stop between the body and the lens, all i've done is closing the aperture on the lens itself while in AV mode and thake the picture.

Here's the full set of 24 images
11-16-2010, 10:24 AM   #98
Inactive Account




Join Date: Dec 2008
Location: Ames, Iowa, USA
Photos: Albums
Posts: 2,965
QuoteOriginally posted by matam Quote
If you're interested only in AV mode you may remove the aperture lever (if reversible) or don't completly mount the lens and the camera will give you an accurate metering with full manual aperture control. (note that if you stop down to f16 the viewfinder will darker)
yes. the "don't mount all the way" trick works well; it can be made more secure by putting an indent into the lens base so it locks on the camera before the standard position.

QuoteQuote:
If you need matrix metering and P-TTL short the pin on the body also. (blende8 is the * on the picture)
Easier said than done in my experience - the reason I did the foil mod under the mount ring was I could not find a way to easily & reliably short the * (A) pin on an ad-hoc basis.

After the foil mod under the mount ring, I've found no significant downside to the camera's thinking all my lenses are A type.

QuoteQuote:
The drawback is that your picture exif will show always f1.2 and you can't select the f-stop on the body.
After speed is set in M mode with green button or whatever, you can change the e-wheel setting of f-stop to what you want recorded in exif.

QuoteQuote:
Keeping the situation above in mind, if you use a K50 1.2 and put f4 on body and stop down the lens at f4 your picture will'be severly over exposed because the body will read the light that is passing trough the lens at f4 thinking it is fully open.
Yes that's true if you change the body f-stop before you determine the shutter speed.

In practice, you press the green button while the camera thinks the lens is open to a particular f-stop, constant for that lens*; that sets the correct shutter speed for the light intensity coming thru the lens. Later you can change the e-wheel f-stop to whatever you want recorded in exif.

QuoteQuote:
All of the above can be done easily without arming or modifing the mount on the lens.
"Arming"?

You are correct that it is easy IF you can find an easy way to short the A pin; I could not find an easy way after many attempts.

Dave

*logically f:1.2 should be set on the e-wheel but in practice I find it varies with the particular lens


Last edited by newarts; 11-16-2010 at 10:36 AM.
11-16-2010, 10:30 AM   #99
Inactive Account




Join Date: Dec 2008
Location: Ames, Iowa, USA
Photos: Albums
Posts: 2,965
QuoteOriginally posted by blende8 Quote
Ok, Lowell, I think we then agree against Dave.
He said that his A-modified camera meters correctly with M42 lenses irrespective of the aperture set on the body.
Dave never said that.

The metering is done at a particular body f-stop setting that has nothing to do with the actual aperture setting on the lens.

Dave
11-16-2010, 10:34 AM   #100
Inactive Account




Join Date: Dec 2008
Location: Ames, Iowa, USA
Photos: Albums
Posts: 2,965
QuoteQuote:
Ok, Lowell, I think we then agree against Dave.
What is this? A vote for reality contest?

I am not making this up, am reporting experimental results..

Dave
11-16-2010, 10:40 AM   #101
Senior Member




Join Date: Mar 2010
Location: Berlin
Posts: 112
You're right in M mode you can meter stopped down, and then change the value on body.
As far as i know there's no easier way than shorting the pin on the body itself.
In any other mode you could lock the AE and change the value on body and whould still work...but isn't so fast and user friendly to do it .

I wrote this in another thread, i think the only thing that could be done (other than using a not crippled mount...) would be to have a menu like the one that ask for the FL when you mount a K/M lens so you can input the aperture range, then you chose the aperture on the lens and on body and when you shoot the camera completley stop down the lens.
This could be easily done with a firmware upgrade for all the body.

Sorry for my bad english, i mean without damaging the lens mount with "arming"
11-16-2010, 10:44 AM   #102
Inactive Account




Join Date: Dec 2008
Location: Ames, Iowa, USA
Photos: Albums
Posts: 2,965
QuoteOriginally posted by lguckert79 Quote
so in lamens terms when you short out the pin and use av mode and it you set the body to say f:4 then set the lans to f:4 it will work but here is the ? when in p mode and when you meter the light and the body says to set the apature to say f: 4 then you set the lens and it will corectlly expose?
No, I've been referring to M mode. The body is not set to match the lens aperture when the green button is pressed to set the shutter speed, rather it is set to a particular aperture for that lens like maybe f2.0.

P mode was never mentioned.

Dave
11-16-2010, 11:00 AM   #103
Inactive Account




Join Date: Dec 2008
Location: Ames, Iowa, USA
Photos: Albums
Posts: 2,965
QuoteOriginally posted by matam Quote
Since i'm home with the flu and i've nothing better to do, i made a little test using my KX and an M50 f1.4 lens.
The test has a total of 24 picture, the first 8 are with the M50 half mounted (the lens lever isn't mechanically affected by the body this way) and shotted moving the lens aperture ring to f1.4-2-2.4-2.8-3.5-4-4.5-5.6.
The second 8 pics are done after shorting the "*" pin and moving the lens aperture as above.
I've set the lens locking notch so the pin needed to say to the body the lens has an aperture range between f1.4 and f22 doesn't make contact.
I don't know why (dunno cause i remember i did use the M50 in thuis way before and metering was spot on) but i had to push the exposure copensation by + 2 and 1/2 stop to have a correct exposure wide open (maybe i shorted other pin also because using LV the shutter was twice as fast 0_0...).
The last 8 was done has the A set but using flash (to show P-TTL usage).

I didn't match the f-stop between the body and the lens, all i've done is closing the aperture on the lens itself while in AV mode and thake the picture.

Here's the full set of 24 images
These results are entirely consistent with what I've posted.

Try setting the e-wheel f-stop to a higher value before you meter; that will give the offset you want.

You have temporarily shorted the A pin - I've found no compelling reason to not leave it that way.

Dave
11-16-2010, 11:04 AM   #104
Inactive Account




Join Date: Dec 2008
Location: Ames, Iowa, USA
Photos: Albums
Posts: 2,965
QuoteOriginally posted by matam Quote
....As far as i know there's no easier way than shorting the pin on the body itself......
Sorry for my bad english, i mean without damaging the lens mount with "arming"

Your English is great. I thought "Berlin" in your profile was perhaps "Berlin, New York"!

Dave

PS there's very little risk of damaging the lens mount - it is neither delicate nor difficult work. Use the locking pin to realign the mounting ring when you replace it and don't lose the little screws.
11-16-2010, 11:13 AM   #105
Veteran Member
jolepp's Avatar

Join Date: Oct 2010
Location: Finland
Photos: Albums
Posts: 3,196
QuoteOriginally posted by Lowell Goudge Quote
Note your last point is incorrect. With an M lens (I.e. K mount with no A or an A lens out of the A position) the camera will meter the scene for wide open aperture, and take the shot with the correct exposure for open aperture regardless of where the ring is set. The camera does not allow the aperture to close when shooting a manual aperture lens in any mode other than M (and bulb)
Looking through the lens when I press the shutter fully I can see the k-x closing the aperture to the ring setting in Av mode with a Pentax M 50mm 1:1.7. It would make more sense if it did not as the resulting picture does not reflect the ring position, that is, all positions result in a exposure with a fully open lens and a shutter time to match that (as long as the amount of light does not change).
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
k-mount, pentax lens, slr lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
How to make a non-A lens work like an A-lens Egg Salad Do-It-Yourself 6 06-06-2010 04:18 AM
Wanted - Acquired: Cheap "donor" KA-mount 28mm f/2.8 lens - any make panoguy Sold Items 2 03-04-2010 08:02 PM
Is there something that can make my lens into a zoom lens? CrossStealth Troubleshooting and Beginner Help 15 02-14-2010 01:26 PM
Is DXO lens correction really able to make a lens THAT much better? shaolin95 Digital Processing, Software, and Printing 10 09-22-2009 07:53 PM
Why doesn't Pentax make a camera that uses another manufactures lens mount? steffi Pentax DSLR Discussion 44 04-18-2007 08:14 AM



All times are GMT -7. The time now is 08:45 PM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top