Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
08-26-2015, 06:31 PM   #1
Veteran Member
maltfalc's Avatar

Join Date: May 2010
Location: Winnipeg
Photos: Albums
Posts: 394
Could Av, Tv, etc. potentially be enabled with Pentax-M lenses through firmware?

Just a random thought I had. Currently, M lenses only work in manual mode because the camera has no way to know the difference in light levels wide open vs stopped down. Why not set the aperture ring to the smallest f-stop and use one of the buttons or a menu item to take a series of meter readings while closing down the iris to get the info the camera needs to meter wide open? one extra button press is a small price to pay for enabling all modes with M lenses.

08-26-2015, 06:37 PM   #2
Pentaxian
MadMathMind's Avatar

Join Date: Dec 2013
Location: Houston, TX
Photos: Gallery
Posts: 1,646
The problem is that the camera has no idea what the aperture is like it does for A series and later lenses. The camera measures wide open. Let's say that's f/4. Then it says "Oh, I'm going to use f/8, so I need to double my exposure time to make the light equivalent." With an M lens, the camera has no way to know what the aperture is. It doesn't know it measured at f/4 and doesn't know it will be set to f/8.

The camera does know the difference in light levels wide open vs. shooting aperture but it needs to stop the lens down and then take the metering. It just doesn't know the difference in aperture between those two settings. If the light were perfectly constant, it could deduce the relative difference in f-stop by computing the ratio of light level, but if the user changes the f-stop with the ring, that calculation will be incorrect. And the camera has no way to know that the user made this change.
08-26-2015, 06:40 PM   #3
Administrator
Site Webmaster
Adam's Avatar

Join Date: Sep 2006
Location: Arizona
Photos: Gallery | Albums
Posts: 43,710
QuoteOriginally posted by maltfalc Quote
Just a random thought I had. Currently, M lenses only work in manual mode because the camera has no way to know the difference in light levels wide open vs stopped down. Why not set the aperture ring to the smallest f-stop and use one of the buttons or a menu item to take a series of meter readings while closing down the iris to get the info the camera needs to meter wide open? one extra button press is a small price to pay for enabling all modes with M lenses.
Av will work with lenses that have manual diaphragms, but certainly not Tv as that would require the camera to be able to set the aperture. Supporting Av natively would require the "non-crippled mount" / aperture linkage.

Adam
PentaxForums.com Webmaster (Site Usage Guide | Site Help | My Photography)



PentaxForums.com server and development costs are user-supported. You can help cover these costs by donating. Or, buy your photo gear from our affiliates, Adorama, B&H Photo, or Topaz Labs, and get FREE Marketplace access - click here to see how! Trusted Pentax retailers:

08-26-2015, 07:03 PM   #4
Veteran Member
maltfalc's Avatar

Join Date: May 2010
Location: Winnipeg
Photos: Albums
Posts: 394
Original Poster
ok, you're both just restating the problem with using m lenses and not getting my proposed solution. the idea is to have the camera take a series of meter readings not while the lens is closed down, but rather while it's transitioning from wide open to closed down, so that it can calculate the specific positions at which there is a 1 stop difference in light levels. the aperture ring would be kept at the smallest aperture, just like a pentax-a lens in a-mode. it'd be t-stops rather than f-stops technically, but close enough, and the max aperture could be set in a prompt at startup just like focal length is for sr.

08-26-2015, 07:27 PM   #5
Pentaxian
MadMathMind's Avatar

Join Date: Dec 2013
Location: Houston, TX
Photos: Gallery
Posts: 1,646
QuoteOriginally posted by maltfalc Quote
ok, you're both just restating the problem with using m lenses and not getting my proposed solution. the idea is to have the camera take a series of meter readings not while the lens is closed down, but rather while it's transitioning from wide open to closed down, so that it can calculate the specific positions at which there is a 1 stop difference in light levels.
This is not possible because the meter is located in the pentaprism. Once the mirror goes up, no light can reach the meter.

This is why when you use "Mirror Up" mode it takes two shutter presses--one to put the mirror up, one to fire. The meter can't operate while the mirror is up so it would not work to have the mirror up at all times. Note that when you put the mirror up, the exposure is locked.
08-26-2015, 07:34 PM   #6
Site Supporter
Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 27,691
QuoteOriginally posted by maltfalc Quote
the idea is to have the camera take a series of meter readings not while the lens is closed down, but rather while it's transitioning from wide open to closed down, so that it can calculate the specific positions at which there is a 1 stop difference in light levels
Oh, you mean when the green button is pressed? That is the only way it could work is with a manual stop-down. As noted above, the shutter sequence begins with mirror up. There is also the problem that many aperture mechanisms are poorly modulated, meaning that they stop down, even full down, with very little movement of the actuator lever.


Steve
08-26-2015, 08:09 PM - 1 Like   #7
Pentaxian
Fogel70's Avatar

Join Date: Sep 2009
Location: Stockholm, Sweden
Photos: Albums
Posts: 1,748
QuoteOriginally posted by maltfalc Quote
ok, you're both just restating the problem with using m lenses and not getting my proposed solution. the idea is to have the camera take a series of meter readings not while the lens is closed down, but rather while it's transitioning from wide open to closed down, so that it can calculate the specific positions at which there is a 1 stop difference in light levels. the aperture ring would be kept at the smallest aperture, just like a pentax-a lens in a-mode. it'd be t-stops rather than f-stops technically, but close enough, and the max aperture could be set in a prompt at startup just like focal length is for sr.
But what should the camera do with this information? As the stopping down of the aperture goes very quickly it will be difficult to get a accurate readings from the meter during stop down.

The camera will still not know in which position you set the aperture before the lens is stopped down. The aperture will always be fully stopped down before a images is captured, so you always have to set the aperture on then lens to the f-stop you want to use for the image.

The only extra you would get by entering largest aperture is that exif could be updated with an approximation of the aperture used.
08-26-2015, 08:13 PM   #8
Pentaxian
chickentender's Avatar

Join Date: Jan 2008
Location: Seattle, WA
Photos: Gallery | Albums
Posts: 1,993
I'm not sure this is even a problem. You're able to use a near 50 year old series of lenses on brand new camera technology with shake reduction and you can even meter properly using the Green Button. I've never thought a solution was in order for this.

08-26-2015, 08:47 PM - 1 Like   #9
Senior Member




Join Date: May 2014
Location: Pugetopolis, WA
Photos: Gallery
Posts: 283
I think that Maltfalc's idea is to set something up to pre-calibrate the lens to the body when you put it on, possibly have a "library" of lenses in the camera's memory so when you put one on you pick the lens (similar to putting a non-AF lens on and telling the SR how long it is).
The challenge I see is that the lens is closed down during exposure by the camera releasing the aperture arm on the lens, and letting the lens mechanism close down to the preset aperture. Think of this as closing down an old pre-set lens just before exposure, you rotate the ring until it stops, and don't have any indication of where that point is, you are just turning until it stops. The camera does not change its arm motion, it just gets it out of the way so the lens can stop down to where its set. There is nothing to calibrate in the motion. It is an on/off motion, no subtle degrees of movement involved.

The only indication to a camera body of where the aperture is set on the old M and K lenses is the tab outside of the bayonet, that coupled to a similar tab in the old film bodies. This is the "Crippled" part of the modern K-mount. The modern bodies don't have this coupling to transfer the lens f-stop data to the metering system.
It is a shame in some ways, likely a cost cutting move, possibly they needed that room in the mirror box for something else, but really for those of us that remember the change from screw to K-mount, not much different. You always had to do stop-down metering with screw mount lenses. Going to the "Modern", open-aperture K-mounts was an "enlightening" experience.
Be grateful you can painlessly mount all those old lenses and use them at all.
08-26-2015, 09:02 PM   #10
Loyal Site Supporter
Loyal Site Supporter
UncleVanya's Avatar

Join Date: Jul 2014
Photos: Gallery | Albums
Posts: 10,118
QuoteOriginally posted by MadMathMind Quote
This is not possible because the meter is located in the pentaprism. Once the mirror goes up, no light can reach the meter.

This is why when you use "Mirror Up" mode it takes two shutter presses--one to put the mirror up, one to fire. The meter can't operate while the mirror is up so it would not work to have the mirror up at all times. Note that when you put the mirror up, the exposure is locked.
How does LiveView meter then?
08-26-2015, 09:09 PM   #11
Pentaxian
MadMathMind's Avatar

Join Date: Dec 2013
Location: Houston, TX
Photos: Gallery
Posts: 1,646
QuoteOriginally posted by UncleVanya Quote
How does LiveView meter then?
The sensor measures the amount of light electronically and adjusts its drive accordingly. One advantage to live view is that the entire scene is used, as opposed to a discrete number of metering segments like in optical view.
08-26-2015, 09:14 PM   #12
Loyal Site Supporter
Loyal Site Supporter
UncleVanya's Avatar

Join Date: Jul 2014
Photos: Gallery | Albums
Posts: 10,118
QuoteOriginally posted by MadMathMind Quote
The sensor measures the amount of light electronically and adjusts its drive accordingly. One advantage to live view is that the entire scene is used, as opposed to a discrete number of metering segments like in optical view.
My point is that the adjustments could be done using the same mechanism... there are other practical difficulties but metering does not seem like the main issue.
08-26-2015, 09:42 PM   #13
Veteran Member
maltfalc's Avatar

Join Date: May 2010
Location: Winnipeg
Photos: Albums
Posts: 394
Original Poster
QuoteOriginally posted by MadMathMind Quote
This is not possible because the meter is located in the pentaprism. Once the mirror goes up, no light can reach the meter.

This is why when you use "Mirror Up" mode it takes two shutter presses--one to put the mirror up, one to fire. The meter can't operate while the mirror is up so it would not work to have the mirror up at all times. Note that when you put the mirror up, the exposure is locked.
who said anything about the mirror being up? that's not in any way required for this.

---------- Post added 08-26-15 at 11:48 PM ----------

QuoteOriginally posted by stevebrot Quote
Oh, you mean when the green button is pressed? That is the only way it could work is with a manual stop-down. As noted above, the shutter sequence begins with mirror up. There is also the problem that many aperture mechanisms are poorly modulated, meaning that they stop down, even full down, with very little movement of the actuator lever.


Steve
neither the shutter nor the mirror would be involved in calibrating the camera to a lens. the max aperture would be entered manually when the lens is first mounted, the aperture lever would be run through it's full range of movement and the changes in light level recorded in order to map a specific location of the lever to a specific t-stop.

---------- Post added 08-26-15 at 11:57 PM ----------

QuoteOriginally posted by Fogel70 Quote
But what should the camera do with this information? As the stopping down of the aperture goes very quickly it will be difficult to get a accurate readings from the meter during stop down.

The camera will still not know in which position you set the aperture before the lens is stopped down. The aperture will always be fully stopped down before a images is captured, so you always have to set the aperture on then lens to the f-stop you want to use for the image.

The only extra you would get by entering largest aperture is that exif could be updated with an approximation of the aperture used.
you leave the ring at the smallest aperture. the aperture doesn't need to be fully stopped down. it isn't when using pentax-a or newer lenses. the lens will be stopped down to multiple positions with a meter reading at each then the camera will map out the non-linear movement of the aperture and use that map to determine the correct position of the aperture lever while shooting.

---------- Post added 08-27-15 at 12:07 AM ----------

QuoteOriginally posted by K-Three Quote
I think that Maltfalc's idea is to set something up to pre-calibrate the lens to the body when you put it on, possibly have a "library" of lenses in the camera's memory so when you put one on you pick the lens (similar to putting a non-AF lens on and telling the SR how long it is).
The challenge I see is that the lens is closed down during exposure by the camera releasing the aperture arm on the lens, and letting the lens mechanism close down to the preset aperture. Think of this as closing down an old pre-set lens just before exposure, you rotate the ring until it stops, and don't have any indication of where that point is, you are just turning until it stops. The camera does not change its arm motion, it just gets it out of the way so the lens can stop down to where its set. There is nothing to calibrate in the motion. It is an on/off motion, no subtle degrees of movement involved.

The only indication to a camera body of where the aperture is set on the old M and K lenses is the tab outside of the bayonet, that coupled to a similar tab in the old film bodies. This is the "Crippled" part of the modern K-mount. The modern bodies don't have this coupling to transfer the lens f-stop data to the metering system.
It is a shame in some ways, likely a cost cutting move, possibly they needed that room in the mirror box for something else, but really for those of us that remember the change from screw to K-mount, not much different. You always had to do stop-down metering with screw mount lenses. Going to the "Modern", open-aperture K-mounts was an "enlightening" experience.
Be grateful you can painlessly mount all those old lenses and use them at all.
the aperture adjusting mechanism in the camera is not a simple on/off motion. with newer lenses it moves the lever to a specific location for any given f-stop setting. the whole point of my idea is to give the camera a way to teach itself what lever position corresponds to which f(or t)-stop setting.

---------- Post added 08-27-15 at 12:14 AM ----------

QuoteOriginally posted by UncleVanya Quote
My point is that the adjustments could be done using the same mechanism... there are other practical difficulties but metering does not seem like the main issue.
no need even for that. a modified version of green button stopped down metering is all that's needed, with meter readings taken at 10 percent stopped down, 20 percent, etc. instead of just at 100%, then just a bit of connect the dots to calculate the full curve.

Last edited by maltfalc; 08-26-2015 at 10:15 PM.
08-27-2015, 06:41 AM   #14
A Proud PENTAXER !!!
Loyal Site Supporter
fwcetus's Avatar

Join Date: Sep 2006
Location: New England
Photos: Gallery | Albums
Posts: 1,249
QuoteOriginally posted by chickentender Quote
I'm not sure this is even a problem. You're able to use a near 50 year old series of lenses on brand new camera technology with shake reduction and you can even meter properly using the Green Button. I've never thought a solution was in order for this.
I'd agree with all of that ~except~ "and you can even meter properly using the Green Button" -- I think the metering is actually quite poor with K and M lenses (and, I assume M42 lenses, too).

Having to use the Green Button is not a huge inconvenience, all things considered, but the non-linearity of metering is very frustrating to me (YMMV).
08-27-2015, 07:07 AM   #15
Site Supporter
Site Supporter




Join Date: Jan 2010
Location: Michigan
Photos: Gallery
Posts: 2,200
QuoteOriginally posted by maltfalc Quote
the aperture adjusting mechanism in the camera is not a simple on/off motion. with newer lenses it moves the lever to a specific location for any given f-stop setting. the whole point of my idea is to give the camera a way to teach itself what lever position corresponds to which f(or t)-stop setting.
As I understand it:
For the M lenses, the stop down lever is a simple on/off motion that only releases the M lens to spring down to the setting of the ap ring.

For A lenses in A mode, the f/- is sent in binary via 2 ( or 3) gold contacts.
For DA lenses, f/- is sent as data

As mentioned, I think Pentax users benefit from the new cameras still having all the above functionality, duplicated so the old K lenses still work.
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!
aperture, camera, ds, dslr, f-stop, lens, lenses, lever, light, map, meter, mirror, motion, move, movement, on/off, pentax, photography, position, post, shutter, stop
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
av or tv retired2007 Troubleshooting and Beginner Help 11 02-20-2015 05:57 AM
HD-DA Rear converter 1.4x - could it be used with FA lenses? zzeitg Pentax SLR Lens Discussion 3 12-31-2014 04:46 PM
could ISO 80 be possible on the k3 with a future firmware update? Rice Pentax K-3 2 05-19-2014 09:21 PM
Hyper TAv, M, Av, Tv, Sv, Bulb and X? Mareket Photographic Technique 5 05-01-2012 04:22 PM
DOF preview with M lens and Av/Tv mode atekant Pentax K-5 14 02-23-2012 07:58 PM



All times are GMT -7. The time now is 04:49 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