Aperture Mechanism

[iskoos]

Say you are on Av (or A for Nikon) mode and set the f stop to f/4.0. When does the camera stop down?

a. At the moment you set the f stop
b. At the moment where you half press the shutter button (to do the metering)
c. Right before the shot is taken...

I believe the answer is "c" but don't know the reasoning behind.

 

[Derrel]

C is the correct answer.

 

[iskoos]

C is the correct answer.

I see it. But do you know why Derrel?

If the camera keeps the aperture at max till the last moment and only stops down for the exposure to be taken and then gets back to fully open, there must be a reason for it.

Would this be the same pre-autoexposure/auto aperture cameras?
I would think not... And also this shouldn't matter for the old-day cameras because those cameras wouldn't need to meter.

Anyways, yeah the answer is "c" but can someone explain why?

 

[Overread]

On the camera (somewhere) there is a depth of field preview button. If you press and hold that button it will close the aperture blades on the lens to the set aperture - set the aperture to something small (eg f8) and press the button and see what happens.

What you will get is a much darker viewfinder image - the result of closing the aperture blade and reducing the amount of light getting into the camera, thus making focusing by either manual or automatic methods much harder to perform. This the aperture blades are kept open the whole time till the shot is taken - to preserve the brightest possible viewfinder image.

 

[Phranquey]

C is the correct answer.

I see it. But do you know why Derrel?

If the camera keeps the aperture at max till the last moment and only stops down for the exposure to be taken and then gets back to fully open, there must be a reason for it.

Would this be the same pre-autoexposure/auto aperture cameras?
I would think not... And also this shouldn't matter for the old-day cameras because those cameras wouldn't need to meter.

Anyways, yeah the answer is "c" but can someone explain why?

If you weren't aware, among other things, aperture controls depth of field of your focus, hence the reason most slr's have a "DOF preview" button. When you push this button while looking through the viewfinder, it will close down to the selected aperture without taking the photo to give you an idea of what will be in focus, and what will not, but you will also note that when this happens, everything in the viewfinder gets darker, especially if you have it stopped way down. It is left open until right before the photo in order to give you the brightest view possible while composing.

EDIT: DAMMIT, Overread!!!!!

 

[Overread]

hehehe !

 

[Dwig]

The reason modern SLR cameras are designed to stop the lens down only when the picture is being taken (generally just as the mirror is lifting) is that stopping the lens down darkens the image from the lens. If it occurred earlier, while you were still viewing, the VF would darken.

Most, but not all, SLRs have had "fully automatic diaphrams" for the last half century or so. The earliest SLRs, though didn't. Even when some degree of "auto diaphram" (automatic closing of the diaphram when the picture is taken, no relation to any other "auto" function such as exposure or focus) what implemented in the camera, not all lenses for that camera would have the complimentary functionality.

The classes or diaphram action are:

Manual: aperture closes when set on the lens. User must manually open the aperture to view and focus and then manually set the desired aperture just before taking the picture and manually reopen the aperture just after the picture to view again.

Preset: This is the same as manual except there are two aperture rings on the lens. One, generally marked "O - C" for "Open - Close", is the actual aperture control. The other, marked in f/stops, serves as a limit settings. You "preset" the desired aperture on the f/stop ring and, when you take the picture, you manually turn the O-C ring from its open position as far to the close end as it will go. The f/stop ring limits the O-C ring's travel so that it stops at the preset aperture. The is faster than true manual. You don't have to look at the ring or count clicks when closing the aperture to take the picture.

Semi-auto: These lenses stop down automatically when the picture is taken, but don't reopen after the picture. You have to manually recock the lens to reopen the aperture.

Full auto: These lenses automatically close for the picture and reopen afterwards. This is what we are all used to today. This is the "auto" that is referred to on the early Nikkor lenses for the Nikon F; they were marked as "Auto-Nikkor" when they had auto diaphram.

SLRs also have an issue with their mirror system. Early SLRs would automatically lift the mirror just before firing the shutter but wouldn't automatically return the mirror to viewing position after the exposure. You had to manually recock the mirror system after the exposure in order to view again. Auto return mirrors were a significant advancement and didn't become common until the end of the '50s.

There have been many hybrid mixes of these functions. Canon's first SLRs had a system where the mirror was auto return and the lens was a semi-auto diaphram lens. What as somewhat unique was that cocking the camera body recocked the lens. Other semi-automatic diaphram systems required that the lens be recocked separately.

Many medium format SLRs have lacked and auto return mirror and have no need for full auto diaphram lenses. Most such modern cameras (Hasselblad, Mamyia RB67, ...) integrate the mirror return and recock, shutter recock, and diaphram recock into a single action, often including film wind in the process.

 

[iskoos]

On the camera (somewhere) there is a depth of field preview button. If you press and hold that button it will close the aperture blades on the lens to the set aperture - set the aperture to something small (eg f8) and press the button and see what happens.

What you will get is a much darker viewfinder image - the result of closing the aperture blade and reducing the amount of light getting into the camera, thus making focusing by either manual or automatic methods much harder to perform. This the aperture blades are kept open the whole time till the shot is taken - to preserve the brightest possible viewfinder image.

Mucho thanks... That's what I was looking for...
I tried the DOF button and I can hear and see the aperture blades closing down...

Okay was I right on my comment about the oldy all-manual cameras?
If you had a camera from 1960-70s and when you set the aperture manually, would camera close the blades right away? Or would it still wait till right before the exposure taken?

 

[KmH]

Somewhere in your camera's users manual is a description of what your DOF preview button does.

Wonder how many other camera features you're not aware your camera has.

If by chance you don't have a uses manual for your camera, most of them can be downloaded from the camera makers web sites.

 

[Dwig]

...
Okay was I right on my comment about the oldy all-manual cameras?
If you had a camera from 1960-70s and when you set the aperture manually, would camera close the blades right away? Or would it still wait till right before the exposure taken?

Old is relative. Cameras from the 1960-70's are not "old" by the standards of this discussion. In 1960, almost all SLRs had either full auto diaphram or semi-auto diaphram. I can't think of a model introduced after 1962 that didn't have full auto diaphram. True manual diaphram only SLRs generally date from before WWII. The Graflex SLRs had a semi-auto diaphram from somewhere around 1900-1910 onward. Very, very few 35mm SLRs were manual diaphram only. There were always some lenses that were manual diaphram only, though there were usually special purpose lenses.

 

[Derrel]

The only cameras that would close the aperture when the aperture was set would be rangefinder or viewfinder cameras or press and view cameras. To this day, lenses for rangefinder/viefinder cameras and press/view cameras have simple diaphragms without any type of auto-diaphragm function. Perhaps you've seen older Leica or Contax, or Canon or Nikon rangefinder lenses, where the diaphragm opens and closes as the user adjusts the aperture. Even the most modern Leica lenses adjust in this venerable way.

 

[Josh66]

can someone explain why?

Because it's better that way?

If it stopped down before the picture was taken, it would darken the viewfinder...

 

[iskoos]

...
Okay was I right on my comment about the oldy all-manual cameras?
If you had a camera from 1960-70s and when you set the aperture manually, would camera close the blades right away? Or would it still wait till right before the exposure taken?

Old is relative. Cameras from the 1960-70's are not "old" by the standards of this discussion. In 1960, almost all SLRs had either full auto diaphram or semi-auto diaphram. I can't think of a model introduced after 1962 that didn't have full auto diaphram. True manual diaphram only SLRs generally date from before WWII. The Graflex SLRs had a semi-auto diaphram from somewhere around 1900-1910 onward. Very, very few 35mm SLRs were manual diaphram only. There were always some lenses that were manual diaphram only, though there were usually special purpose lenses.

Hey sorry Dwig!.. You gave me a complete answer in your previous post but I didn't get to see it. I started typing the above question before your answer but then got a phone call and got away from my computer for an hour; then came back to complete my post. But meanwhile though you answered everything I just didn't get to see it on time.
So thanks for that detailed answer. I found everything in it...

 

[iskoos]

Somewhere in your camera's users manual is a description of what your DOF preview button does.

Wonder how many other camera features you're not aware your camera has.

If by chance you don't have a uses manual for your camera, most of them can be downloaded from the camera makers web sites.

Thank you. Yes, I wasn't entirely familiar with the DOF button on my camera. And now I am; but the DOF button/function wasn't the answer to my question. I asked why the camera doesn't stop down immediately and a few guys answered this in detail above. Thanks to them again.
I have my camera manual and I do read it very frequently and I do use internet to look for answers but sometimes I can't find what I am looking for and come here to ask. This does NOT show that I got a camera in my hands and started asking others to explain me what every button on it does. Camera manuals are made to show the user how that specific function works. They do not go deep and explain why. That's why I was asking here.
I absolutely have many features on my camera that I don't know(this is my first DSLR) though I will learn all of them.