Originally posted by MadMathMind 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 ----------
Originally posted by stevebrot 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 ----------
Originally posted by Fogel70 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 ----------
Originally posted by K-Three 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 ----------
Originally posted by UncleVanya 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.