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AE lock does not compensate for varying aperture while zooming

J

jgban

Kyocera Kid,
To me, what has always been a mystery is how come there are zooms with fixed aperture. In my ignorance, I thought the f value was the ratio between the diameter of the diaphragm and the focal length. From this standpoint (which, I acknowledge, must be wrong) it seems difficult to keep a constant f value when your zoom changes the focal length by a factor or 3 or more.
Anyone who can shed light on this specific lens design problem will be most welcome.

Juan
 

afranklin

Well-Known Member
Pim,

"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."

You are right, the amount of light *should* change, given the same diameter of the aperture...but...the metering system DOES need to know the minimum aperture of the lense for that amount of light, and if this changes via the zoom, it needs to know this, or it can't make an accurate exposure. The lense apparently does not indicate this information to the camera body, so if the AX does in fact change it's exposure given the zoom value (when locked) it IS magic ;-)

Also, I was talking about AE locked...and given the function of AE lock, it won't change the light value it has stored...though it *can* change the exposure based on changing aperture/compensation/ASA/shutter speed.

So, I believe my question/comment still is unanswered.

Regards,

Austin
 

bobbl46

Well-Known Member
Hi Juan et al.

I wish I could add something scientific to all this, but I am limited to belief, variable memory accuracy and a tiny smattering of physics only!

I think constant aperture in a zoom lens has something to do with the "apparent" aperture of the blade openings as "seen" or "magnified", by the moving lens elements, at the film plane. I'm sure I have read that (up to a point!) the diameter of the front element is a secondary consideration, and that the aperture (f) is a function of the entrance pupil size (apparent or effective aperture). Seems contrary to popular believe, doesn't it!

This, coupled with focal length, is the clever bit about keeping a constant aperture.

Lets hope someone with in-depth knowledge puts us out of our misery!

Cheers, Kyocera Kid
 
J

jgban

Austin,

AX. Zoom 100-300.
1. Unlocked. Spot metering on a grey wall.
Focal length 100. Display: 4.5, 1'' (kind of dark in here, isn't it?)
Focal length 300. Display: 4.5, 1.4"-2" (it goes from 1.4 to 2 and even 2.8").

2. Locked. Spot metering on a grey wall.
Focal length 100. I measure 4.5" and 1". I lock the exposure.
I zoom to 300. The display shows 4.5 and 1". If I change the aperture ring, the display changes: 5.6-1.4", 8-2.8", etc.

This is what actually happens. (What actually just happened). Explanations (other than magic) welcome.

Here is my explanation:
When the exposure is not locked and you zoom, truly less light gets to the light meter. The meter does not know or care if there is less light because you zoomed or because you are pointing the camera at something else. The measurement is always done at full aperture anyway. Less light is less light regardless of the cause. So the behavior seems quite straightforward.

When I lock the exposure, the camera is "set" to give a fixed EV (around 4 or so (ISO 100)). This is f:4.5 (that the mechanical conection of the lens to the camera tells it) 1", or f5.6 2" or f8 4" etc). So when I change the aperture ring to 5.6, the response is to appropriately lengthen the speed. As the camera is not measuring light anymore, and there is no way in which it can know that one zooms, it cannot adjust for that. The N1/NX "could", because the CPU (or whatever it is these cameras have) is being informed of the effective aperture (at least to show it on the display) , but they don't. So here is another reason for a new model! (hopefully ND II).

Please, no more tests today. The cameras work beautifully, even if one can fool them.

Juan
 
J

jgban

Kyocera Kid,
Your explanation sounds good and believable to me.
Regards,

Juan
 

tyurek

Member
Juan's comment on the difference between how AX and N1/NX behave makes sense. I don't have any
manual focus Contax zooms but if the camera cannot tell the lens is zoomed, the effect will
be the same as the light intensity changing (cloud comes over the sun or you point at a
different tonality subject) and the camera is supposed to keep the EV value locked. But as
Austin and Juan both point out, N1/NX already knows the lens is being zoomed since the aperture
readout changes just by zooming. Hence a fix should be possible by a firmware upgrade. What
drives me crazy is that this is not a simple bug with little consequences. This is a major issue
which renders AE lock useless when zooming in A LOT of situations. I think this is a big blow
to the N cameras as "photographic tools", and I'm sorry if I'm angry but I am angry! Just to make
sure I asked a friend with a Canon EOS 10D to check how his camera behaves with his 28-135
Canon consumer zoom and he verified the EOS10D *does* compensate the shutter speed after he
locks the AE at the 28mm end, wide open, and zooms to the 135mm end. Just like it did when he
manually changed the aperture after AE lock, without zooming.

N1/NX should also show the same consistency in terms of behaviour between these two operations.
But they don't and this is very sad because you don't expect it from Contax engineering...
 
J

jgban

Tolga,
Thank you very much for pointing out this problem. As you have seen, several forum members were not aware of it, and I am happy to know about it.

I understand you are upset, as the machine you just bought has shown to be below your expectations. I still think you bought a great camera, even with this limitation (hey, the AE lock is not any worse than in the Aria...)

I wish there was an easy way of notifying Kyocera of these problems, so they can be addressed in future models. Does anyone know what the best venue for that is?

Kyocera Kid?

Best regards,


Juan
 

tyurek

Member
I do love almost everyhing else about the camera. I had been longing for a Contax camera that had an exposure compensation bar graph in the viewfinder and the N1 has surpassed my expectations with 1/2 or 1/3 EV selectability. I am indeed upgrading from an Aria and prime manual focus lenses. I consider the lens part also an upgrade because the zoom lenses increased the photographic opportunities many fold for me. I simply find much better compositions in such a quick amount of time (I find the good old advice "just walk closer or farther with a prime lens" very impractical and many times not possible). So overall, I'm thrilled with my upgrade. I just happen to use spot metering and AE lock a great deal in my photography.
Thanks everyone for your valuable inputs.
 

bobbl46

Well-Known Member
Juan .... I'll discuss this issue with the Manager of KY UK .... He will know who to pass it on to.

Cheers, Kyocera Kid.
 
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