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AE lock does not compensate for varying aperture while zooming

tyurek

Member
Hi all,

I just purchased a used N1 with the 24-85 and 70-300 zooms. What I noticed today is that if I
lock the exposure say at 1/125 @ f/4 at 70mm while using the AV mode, the shutter speed
does not get compensated when I rack it out to 300mm, at which time the aperture readout
automatically changes to f/5.6; however, the shutter speed stays at 1/125! The camera
effectively underexposes by one stop! Logic dictates that the shutter speed should be
readjusted to 1/60, especially since I'm using the aperture priority mode. If I unlock the AE at
300, it immediately gives 1/60 @ f/5.6. The reverse happens when I go from 300 to 70. The
situation is the same with the 24-85 VS.

Is this just my camera or is it a known bug with the N1? Are there any known firmware upgrades for N1?

Thanks and best regards...
 
J

jgban

No, it's not your camera. It happens also with the NX. If you lock the exposure in AV, the shutter speed remains locked even if you change the focal length of the zoom.

Darn! Now I have to measure the exposure with the focal length I am going to use to actually take the picture!

Juan
 

tyurek

Member
But this is a horrible engineering mistake. It probably is a bug the Contax engineers overlooked.
Imagine you are at a lake side. The sun is shining on the surface of the water. There are
constantly moving birds of varying tonalities. Many things to fool the in camera meter. So you
read the exposure off of a gray rock at the shore, which is in the same light and lock it.
You start framing the birds, constantly zooming to catch new formations. Some of your exposures
will be plain "wrong" as one stop is a lot of error with slide film. I can think of a lot of
other scenarios where one might want to zoom after locking the exposure. This is one serious problem
I cannot believe the N1 team has overlooked while inventing tools such as AF bracketing. I do like
the camera very much in all other respects, but I'm sorely disappointed at this problem.

Does anyone know or guess if there is any rationale behind this behaviour? Now that Juan
says the NX also behaves this way, I'm wondering if the Contax engineers have something in mind
which I cannot see.
 

bobbl46

Well-Known Member
Out of interest, if you leave the 70-300 at 70 and lock it at f4, what happens if you deliberately change the aperture to f5.6? Does it correctly go to 1/60th?

Also, is your problem "cured" if you start at 70mm/f5.6 and then zoom?

I will experiment with my AX and report back.

Cheers, Kyocera Kid.
 

fotografz

Well-Known Member
Tolga, yep you're right. I checked it on the N1 and then the ND.
I actually took two ND shots with the AE Locked after metering a neutral grey. One full stop difference because the shutter speed stayed the same. It's okay if you're shooting neg. film or even digital. But with slides it is a problem.

Two possible solutions:

Meter the grey area manually at 70mm, and when zooming to 300mm adjust the aperture ring a stop.

Or set AV and meter the difference between the scene and the grey neutral area... then set the compensation wheel.

The latter is what I would do as the difference is the constant.
The AV setting will then adjust normally as the lighting may vary, but the difference would still stay the same relative to each other.
 

afranklin

Well-Known Member
Can you check if when you manually change the aperture after engaging AE locking, does it change the shutter speed?
 
J

jgban

Yes, if you change the aperture after locking, the shutter speed varies accordingly. This is described in the manual, and it happens as described.
You can see the same thing happens with the AX when you zoom. The "locked" value does not adjust for the light loss (of course, when you zoom with the AX, the aperture value on the display does not change).
 

afranklin

Well-Known Member
Juan,

This is bizzare then...if you change the aperture after locking (assuming the N1/NX, as that is the camera of discussion), and it displays this change in the display, and the shutter speed DOES change as well, then why, if the aperture changes when you zoom, wouldn't the shutter speed also change with the zoom changes? It *should be* the exact same mechanism. That surely seems like a design flaw.

As far as the manual focus zooms, if the display doesn't change, then the aperture hasn't changed...as it's a mechanical linkage, and the camera doesn't know if you have a zoom on the camera or not. Are the aperture values listed on the zoom barrel? Isn't a 5.6 on the barrel, a 5.6 no matter what the zoom? The blade opening (or the ring on the barrel) doesn't change with the zoom, right? I don't know much about zooms, but isn't it only the minimum aperture that changes with the zoom range, or does the entire range shift?

Regards,

Austin
 
D

djg

I believe that would be because the change in aperture through the ring is an explicit command, whereas the change in aperture when extreme zooming is inherent in the optical mechanism.

Now we know it can detect that, but it looks like it's only wired to adjust the display but not the data path to the processor that calculates exposure? Don't know how their internal architecture is set up.
 

afranklin

Well-Known Member
Hi DJ,

Are you saying that if I select f4 on the marked aperture scale, it may NOT be 4, but f5.6, depending on the zoom selected? And, that though the barrel has f4 selected, the display shows f5.6? This is significant at determining what is going on here.

Has anyone actually *verified* that the images are exposed incorrectly? Why I ask, is perhaps it is making this compensation without informing you...

Regards,

Austin
 
J

jgban

Austin,
I don't know anything about the technical questions here, so I can just report observations.

I think it is a design flaw. However when I said that the same thing happens with the AX I was just trying to point out that every single Contax camera has had that limitation with AE lock and "variable aperture" zooms. In my test with the AX I used the 100-300mm 4.5-5.6 zoom. My understanding (I may be wrong) is that, in that lens, when the aperture ring is set to 4.5 but the focal length is 300, the effective aperture is, in fact, 5.6. This seems equivalent to the 70-300 f4-5.6 for the N1-NX.
The difference is that the N1/NX tells you the change in effective aperture in the viewfinder display. The aperture ring on the lens may point to "4", but the viewfinder says 5.6.

This information is passed to the processor that calculates exposure. If you zoom on a grey wall (WITHOUT locking the exposure), the speed does change appropriately, at the same time you see the change in the displayed aperture. The issue seems to be what the camera actually "locks" and how this is unaware of the changes caused by the zooming.

Juan
 

bobbl46

Well-Known Member
Forgive me, if anyone has answered this ..... I get confused when people just say "Yes....etc" when a few members have asked different questions!

If you set the lens to 70mm f5.6, then AE Lock, then zoom to 300mm, does the aperture reading in the viewfinder change, and does the shutter speed change?

Indulge me, someone!

Cheers, Kyocera Kid.
 

fotografz

Well-Known Member
Austin. yes I checked it (read my post above) I did it with a ND and the 70-300 f/4/5.6. Trained on the same subject.

I first read a neutral grey at 70mm and AE locked it in then shot the subject at 70mm. Then zoomed to 300 on the same subject... the result was clearly a stop darker at the 300 end.
 
D

djg

Austin,

Indeed, as I zoom from wide to tele the f-stop display in the finder changes. Not the speed, though. Maybe as you say it may be compensating but not indicating it, although in Av mode the speed display should be coming form the actual speed setting the camera has determined to use.

I should point out I used the 24-85 already on my ND, as I was too darned lazy to switch zooms. Hey, it's Sunday, OK?
 
J

jgban

Kyocera Kid,

By now you have seen the answers, but I just want to make sure you don't feel your posts get overlooked:

With the NX and the 70-300mm: With the zoom at 70mm and the aperture ring at 5.6, if you zoom to 300mm the display shows the aperture is 8. The shutter speed varies accordingly.

With the AX and the 100-300mm: The AX will also change the speed appropriately, but it does not change the aperture value on the display.

If you lock the exposure (on either camera), the speed does not change. In the NX/N1, the aperture value changes on the display.

I may be completely off here, but the way I think about it is: until you lock the exposure, the metering system is measuring the light. When you zoom (regardless of what the display says) less light gets through the lens, and the resposnse is appropriate. Once you lock the exposure, no more metering takes place.

I think the bottom line is you have to lock the exposure with the focal length you are going to use. I also think this is exactly what most of us do most of the time, and this may explain why this has not created any uproar before amonth the Contax faithful. You can certainly come up with situations where it will be a problem, but in general (consciously or not)) one probably locks the exposure with the focal length [close to the one] one is going to use.

Just to be complete: in the G series, when you lock you lock the shutter speed only. If you switch the aperture after locking "the exposure" (obviously a misnomer), there will be no automatic adjustment. This is explained in the instruction manual, ("Be careful", it says on page 136).

Juan
 

afranklin

Well-Known Member
Juan,

"With the AX and the 100-300mm: The AX will also change the speed appropriately, but it does not change the aperture value on the display."

How does the AX know the zoom has zoomed and to what zoom position? There are no electrical contacts...only mechanical linkages. Can you see if any of the mechanical linkages vary with zoom position? There *is* a way I can think of, and that is when you zoom, the min aperture indicator changes, but that should change the display as well...

Regards,

Austin
 

afranklin

Well-Known Member
Juan,

"I may be completely off here, but the way I think about it is: until you lock the exposure, the metering system is measuring the light. When you zoom (regardless of what the display says) less light gets through the lens, and the resposnse is appropriate. Once you lock the exposure, no more metering takes place."

It *does* change the exposure based on changing the f-stop and/or shutter speed, after AE locking...so why can't it do it with the zoom, as it does change the display (so it has the appropriate info). This doesn't require re-metering, just adjusting the exposure accordingly.

Regards,

Austin
 

bobbl46

Well-Known Member
Juan - your kind thoughts are much appreciated, as is your reply.

I guess I am astounded by this movement from 5.6 to 8 on the N series! I thought, like Austin implied/queried, that the aperture reading would remain static once moved away from its maximum(variable 4~5.6) setting!

To put it another way, I assumed the problem would exist only at maximum aperture, and no other, for a lens with a variable max aperture.

Marc seems to have confirmed that this is a real effect, and that no "internal" compensation is applied.

I will play with my AX with the Tamrons (must borrow a CZ to play with too) and make a mental note of the above findings in case any of the Contax Postal Club members make a similar enquiry about the N series.

It's discussions like this that are so useful to help other groups.

Oh Oh! Incoming from Austin x2, even as I write .... I'll watch the replies on these too.


Thanks All, Kyocera Kid.
 

pim

Member
Austin,

"How does the AX know the zoom has zoomed and to what zoom position?"

The AX doesn't need to know the zoom has zoomed for this functionality: less light reaches the light meter, so shutter speed is changed as long as you have not locked in the exposure.
No magic involved.....

Regards,
Pim
 
J

jgban

Austin,
"How does the AX know the zoom has zoomed and to what zoom position?"
I don't think it does, that is the reason the aperture displayed in the viewfinder does not change. But the fact is that less light gets to the meter, so the speed changes.

"Can you see if any of the mechanical linkages vary with zoom position?"
When I look at the mechanichal linkages on the bayonet, I cannot see anything changing when I zoom.

Regarding your next comment: you are, of course, right. Regardless of if there is light metering involved or not, the camera could have been set to react appropriately to the zooming effect. That is the reason I think it is a flaw.

I precisely said "the way I think about it" to mean that, whether this is the explanation or not, it is helpful enough for my own grasping of the situation.

Juan
 
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