Why do lens caps do that?

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Blonkster

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When I put the bottom cap on my Sony/ Minolta lens it opens the aperture wide.

Why does it do this? Won't it weaken the springs if stored this way?

Thanks

Blonkster
 
I'm not familiar with the Sony/Minolta mount. But I do have an idea.

Are you familiar with Occam's Razor? It's a principle for deciding between competing hypotheses, and it recommends that the simpler one - or the one that makes the fewest assumptions - is generally the better.

Here we seem to have two hypotheses:

(A) Hypothesis: The spring is under tension when the lens is stored.
Necessary assumption: The team of Minolta engineers who designed the mount made a basic error.

(B) Hypothesis: The spring is not under tension when the lens is stored.
Necessary assumption: You have misunderstood how it works.

Hmmm, what would Occam's Razor say? ;)
 
surely the apperture should be wide open by default, for focusing? Could it be an optical illusion?
 
StewartR said:
I'm not familiar with the Sony/Minolta mount. But I do have an idea.

Are you familiar with Occam's Razor? It's a principle for deciding between competing hypotheses, and it recommends that the simpler one - or the one that makes the fewest assumptions - is generally the better.

Here we seem to have two hypotheses:

(A) Hypothesis: The spring is under tension when the lens is stored.
Necessary assumption: The team of Minolta engineers who designed the mount made a basic error.

(B) Hypothesis: The spring is not under tension when the lens is stored.
Necessary assumption: You have misunderstood how it works.

Hmmm, what would Occam's Razor say? ;)
Click to expand...

You really though about this didn't you ;-)

A) they failed

b) it is under tension, as when the lens is off the body the lever that the cap moves spring s back closing the aperture.
 
Trev Rich said:
How can you see the aperture if the lens cap is on.... and if the camera is turned on it would open wide as its getting no light in? :popcorn:
Click to expand...

Its the bottom cap (whatever the correct name is). When its off the camera you can look through the front lens as see the blades are open.
 
Blonkster said:
You really though about this didn't you ;-)

A) they failed

b) it is under tension, as when the lens is off the body the lever that the cap moves spring s back closing the aperture.
Click to expand...
Hmmm. That is interesting.

All SLR lenses have to have the aperture wide open when they're on the camera, so that the metering and focus systems can work. They stop down, if required, when a picture is taken. So I guess there are two ways of designing it:

(1) The aperture is open by default, and the spring (or equivalent) needs to be tensioned in order to stop down.
(2) The aperture is stopped down by default, and the spring needs to be tensioned in order to open it up.

Canon use design (1) as far as I can tell, though it's difficult to be 100% sure since there are no moving parts in the mount and no way to affect the aperture manually. But Canon lenses are definitely wide open when off the camera.

Nikon, on the other hand, definitely use design (2). The aperture is stopped down when the lens is off the camera, and there is definitely tension in the spring when the aperture is opened up. But the design of the Nikon rear lens cap is such that it does not touch the aperture control lever, so the spring stays untensioned and the aperture stays stopped down when the lens cap is fitted.

It sounds like Minolta opted for design (2), the same as Nikon. But then that would imply that the Sony/Minolta lens cap engages the aperture control mechanism in a way that the Nikon lens cap doesn't.

This suggests that the design fault, if there is a design fault, is in the lens cap rather than the lens itself. Is it a feature of all lens caps for the Sony/Minolta mount (ie older Minolta lens caps, newer Sony ones, and lens caps made by third parties such as Sigma)? Or just some of them?
 
The shutter release springs of the Rollei 35 cameras always stay tensioned when the lens is collapsed in . This is a peculiarity of the design. After 35 odd years of remaining in constant tension; the springs still function fine.

I suspect if what you say is by design, it'll be a design peculiarity rather than a fault; and the engineers knew that the spring can easily take the tension for a few decades and survive.
 
I think you OP is talking about lens which are stopped down mechanically; this is how lens all use to work.
There is a pin on the mount that is activated by the plate inside the camera body mount.
Most modern lens are stepped down electronically now with a stepping motor. The like of lens OP is referring to I think also have a manual ring to adjust the F-stop if I can remember that far back lol
 
Chaz Photos said:
Most modern lens are stepped down electronically now with a stepping motor.
Click to expand...

Some moderns lens are - not most. Canon lens are stopped down like this. Nikon Lens are (with the exception of tilt shift lenses) stepped down mechanically.

Hugh
 
I thought OP was concerned that the spring inside the lens might get weakened if its kept under constant tension. I was drawing an analogy with a similar ( albeit slightly different ) situation, where the Shutter release spring has been designed to be under constant tension; with no detriment to its functioning after 35 odd years....
 
is it just me, or does the nikon system of various mechanical links between body and lens seem a bit odd?
 
I can't remember the rationale but this is by deliberate design by Minolta .
there are plenty of 50 year old Minolta lenses still working fine so it doesn't seem to be an issue.

there were actually 2 types - the black only opened the aperture 1/2 way whereas the translucent white opened it fully (making it easier to inspect for dust/fungus etc.)
 
pearce_jj said:
is it just me, or does the nikon system of various mechanical links between body and lens seem a bit odd?
Click to expand...

the reason is Nikon's F mount is largely unchanged from when it was first introduced - you can mount, just about, and nikon lens on any nikon body although you may lose some functionality, AF and metering are the two most common.

Canon made the decision to change to the EOS mount in the late 80s and in doing so made all lenses made for earlier Canon bodies not mount at all.

I don't know who made the right decision, you could argue it either way, and it is possible for Nikon to lose all the mechanical connections now with the f mount. Unfortunatley only d300 and up bodies have the functionality to work an electronic apperture

Hugh
 
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