Exposure control for adapted manual focus lens

[thehippo]

The problem:

I use Zeiss Contax lens (e.g., contax 50mm 1.4) adapted to Canon 5DIII.
Since there is no electric aperture control.  a normal process for using lens
is as follows for taking a photo with f8.

1. adjust camera aperture setting to f 8.0
2. set Av mode.
3. adjust lens aperture to f 1.4 and then focus and then AE lock.
4. while in the AE *lock mode, adjust the aperture to 8.0.
5. click the shutter button.

It's inconvenient to use AE lock and then adjust the aperture to the
desired aperture.

What I want is pre-adjust the lens aperture to the desired aperture
say f8.0. And focus use the LCD live screen. So I don't need
the step of AE lock and re-adjustment of the lens aperture.

The problem of this is that it will have over exposure.

The reason is  that at Av mode,   the camera assume that my lens
aperture during focusing is  max aperture (f1.4) and at time of taking
the photo,  the camera will be able to automatically adjust the camera
setting value of f8.0.  So the calculation of exposure is based on the above
assumption, and metering is done at (focus time f1.4)
and auto compensate the final aperture(f8.0).  With my above procedure,
the metering is actually done at f8.0, give the camera a wrong
impression that the light it obtained at f1.4 (not f8.0), so in order to have enough
exposure for f8.0, the exposure time needs to be increased. Therefore the result
of the calculation will be over exposed.


The work around for solve this problem is :
set the camera aperture value as lens max aperture(f 1.4 in this case),
but adjust real lens aperture to a desired value, say f8.0.
So the final exposure will be correct (as it will not compensate for
the difference of the 1.4 vs 8.0. Because, camera thought the light
it get is already at max aperture(and final aperture is also max 1.4).


But the issue of the work around is that the EXIF data recorded for the photo will always
be f1.4.  But the photo is actually taken at f8.0.

The solution:
Magic lantern add a function so that the exposure will be calculated
as if the aperture setting on the camera is lens' max aperture. But
the recording of the EXIF,  will still use the camera aperture setting.


Thanks

[a1ex]

I'm not sure I understood the request. Can you rephrase it, add some screen captures, or write it in your native language?

[thehippo]

Thanks for reading my post. I updated as below,
will add photos if needed.


The problem:

I use Zeiss Contax lens (e.g., contax 50mm 1.4) adapted to Canon 5DIII.
Since there is no electronic aperture control.  a normal process for using lens
to take a photo with following parameter (ISO: 100, Av Mode, Aperture: f8.0)
is as follows:

1. Set camera ISO as fixed 100, set exposure as Av mode.
2. Adjust lens aperture setting on the * camera* to f 8.0
[Note:  The lens setting is only on the *camera" itself, it will never be
able to actually control the lens to this aperture value since this is
not a native EF lens]
3. Adjust lens aperture to f 1.4
[Note: This the actual lens aperture setting on the lens itself, not the lens aperture
Setting on the camera.]

4. Press the camera rear AE *lock button.
[Note: This will lock the camera exposure, typically will be like:
ISO 100
Aperture: f8.0
Speed:  1/100

5. While continue press the AE lock button, adjust
the lens aperture on the *lens* to f8.0 [Now it equals the lens aperture setting
On the camera]

6. While continue press the AE lock button, click the shutter button.
[Note: step 4+step 5+step6, basically simulate the native electronic controlled
lens, which the camera will control the lens diaphragm to the aperture
value set in the camera (in this case f8.0) and then take the photo]


The above  process is inconvenient.   What I want is pre-adjust the  *lens*
aperture to the desired aperture say f8.0 directly, same value as the
setting on the camera (of course, with small aperture, it  will be difficult to
do focusing, but put aside this problem). So the procedure will be simplified as:

1. Set camera ISO as fixed 100, set exposure as Av mode.
2. Adjust lens aperture setting on the * camera* to f 8.0
3. Adjust lens aperture on the *lens* to f8.0
4. Press the shutter button to take the photo.


Unfortunately, the above process will have an over exposed photo, it's
Like(value not accurate, just for illustrating purposes):
ISO 100
Aperture 8.0
Speed: 1/10s

The reason is at time of press the shutter button, the camera
Metering thinks that the light it detected is at the lens max aperture
(f1.4) like a normal EF lens would.  But since the lens setting is now at
the final aperture of f8.0, so the light it detected is much darker,
it figures out it should have a longer exposure time.

The solution:
Magic lantern add a function so that the exposure will be calculated
based on the fact the light it detected is the actual aperture setting,
In this case at f8.0, not like a normal EF lens, which is the maximum
Aperture of f1.4.

[Note:  This is based on the understanding that the camera detects the
light at  the lens max aperture,  then based on the light, the ISO and the
final lens aperture to calculate the speed needed]

[Audionut]

You're lens adapter must be doing something funky, because cameras meter through the lens.

http://en.wikipedia.org/wiki/Through-the-lens_metering

[dmilligan]

Whenever I attach a manual lens to my camera, the aperture reading just says: "f0.0", and I can't adjust it, and everything works fine. It sounds like you have some kind of "active" lens adapter that makes the camera think that it can adjust the aperture, when in reality it can't. That is what is causing your problem.

It is true that the camera meters and focuses with the aperture wide open, then right when you press the shutter, the camera commands the aperture to close down to the requested size. So the camera calculates the exposure based on this. It expects that when it sends the command to the lens to close down the aperture, that this command will actually work. Your lens adapter is confusing the camera and making it think that it can adjust the aperture when in reality it can't. Perhaps you should tell some about the adapter itself and the settings for it.

I would suggest you either give up on having correct metadata and leave the camera aperture setting on wide open, or simply use EC (exposure compensation), to compensate for the difference in aperture. Of course you'll be limited to 5 stops of difference with this method (which would be f/8.0 on a f/1.4 lens).

[chris_overseas]

I've encountered this same problem with my Samyang 14mm f/2.8 when used in conjunction with an AF confirm chip attached. The one I use is the Optix V5+, but presumably the same thing applies to any AF confirm chip that reports aperture to the camera.

My current "solution" is what dmilligan recommends - just give up on having the lenses real aperture in the EXIF and set the camera to the max aperture instead (in my case, f/2.. Having said that, it would be quite nice if ML could told to let the camera do everything it normally does, EXCEPT stop down the aperture when actually taking the photo. That would make it relatively easy to end up with the correct EXIF info being written out. Even better would be if it was possible to register the lens with ML so it switched to this mode automatically for as long as the lens was attached.

[thehippo]

Yes, the work around of setting max aperture works,  but that sacrifice the good EXIF feature that the  *chipped adapter" brings in.

I think adding this feature will help a lot of people using the adapted lens with near native lens experience. Hope this feature
can be added .

[dubzeebass]

I use Leitax adapters on all my Contax Zeiss: 28/2,35/1.4,50/1.4,85/1.4,100/2.8makro,135/2.8 and they all report max aperture. I also shoot in M with the Autoexposure or ETTR modules, though it works great shooting in Av too. Try the Leitaxs... Expensive but way more solid than a non screwed-in adapter.

[dreamingof8a]

I would like to revive this old thread as there has not been a final answer whether or not this would be possible with Magic lantern at all:

1) We're using a chipped manual lens which has maximum aperture of the lens stored (e.g. f/2. and simulates an adjustable aperture
2) set the lens to desired aperture (eg f/4)
3) in Av, also set the "camera aperture" to f/4
4) When taking the photo, ML will let the camera meter but then after metering (for example 1/100s), ML calculates the actual exposure based on the Av setting as compared to the maximum aperture and uses this value for the actual exposure (in this case 1/200) to avoid overexposure

I understand that it would be easier to just forget about the true aperture feedback, however that's the only reason I installed those chips on my Samyang lenses ....

Cheers
Felix