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12-01-2014, 09:59 AM   #1
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Changing aperture doesn't darken viewfinder image?

When I change the aperture, why doesn't the brightness of what I see through the viewfinder change?

For example, I'm at f/3.5 and I look through the viewfinder. Then I change it to f/11 and look through viewfinder again and I see the same image. It's not darker. Why is that?

12-01-2014, 10:05 AM   #2
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Assuming you are using a k-mount lens, not m-42, the aperture is set but does not change until you press the shutter button. When you do the camera stops down the aperture and takes the picture.

On m-42 it would be different and if it is not changing that is a problem.
12-01-2014, 10:13 AM   #3
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Thanks. That answers my question. I'm using the 18-135mm WR lens.

So the camera changes the aperture only when you take a picture. What is the default aperture that the lens is normally at? Meaning, after the picture is taken, to what aperture does the camera set it back again? I assume wide open to let more light through for AF...?
12-01-2014, 10:18 AM   #4
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Yes, it's wide open as it has benefits for AF and viewing through the viewfinder.

12-01-2014, 10:25 AM   #5
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There is a lever mechanism in the camera that keeps the aperture wide-open until you dress the shutter release (which will stop down to the aperture setting you desire via the dial). This is the main reason why you need to set the lens aperture to 'A' setting if you are using the older lenses. For DA lenses, there is no aperture ring, therefore, you always have to change aperture of the lens using the camera dial. If you use the older type of m42 lenses (with A/M switch), there is no aperture lever, therefore, when you stop down the lens, the aperture closes and the viewfinder gets darker.
12-01-2014, 10:25 AM - 1 Like   #6
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The other thing to remember is that, because you're viewing thru a fully open aperture, you're not seeing the true depth of field. If you're stopping down a bit, more of your image will be in focus than you're seeing thru the viewfinder.
12-01-2014, 10:27 AM   #7
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As a side note, on the modern camera bodies, the "on" switch has a "diaphragm" icon when you toggle it over (past the "on" setting),the camera will stop down the lens for you (to aid in previewing the depth-of-field).

Maybe this is what you're looking for?
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12-01-2014, 10:30 AM   #8
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QuoteOriginally posted by Moropo Quote
Thanks. That answers my question. I'm using the 18-135mm WR lens.

So the camera changes the aperture only when you take a picture. What is the default aperture that the lens is normally at? Meaning, after the picture is taken, to what aperture does the camera set it back again? I assume wide open to let more light through for AF...?
Aperture will always be wide open when you don't take a picture.
What is that number!? Depends on the lens you have...

12-01-2014, 10:39 AM   #9
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Focusing is easier with the lens held wide open. Camera designers worked on this for years with the M42 mount, inventing many ways to hold the aperture open for focusing and framing, yet closed to a point you select when shooting. The K-mount incorporated all those ideas.

Look at a lens off the camera. The blades are normally fully closed. You can move a spring-loaded lever on the back of the lens to open them. Put the lens on the camera and watch the blades. The camera has an arm to control that lever. As the lens rotates into place and clicks, the arm opens the blades. When you press the shutter button, the arm closes the blades to your selected aperture, the front shutter curtain opens, the flash may fire, the rear curtain closes, and everything resets for the next shot.

The original K-mount lenses had an aperture ring to set the aperture on the lens. With the lens off camera, you can move the ring to a particular aperture. The lever still opens the blades fully, but when they close, they only close to the aperture you choose on the ring. The original K-mount cameras had an arm to move the lever, but it only had two positions, open or closed. A few years later, Pentax wanted to add Program modes to the camera. That meant the camera body needed to control the aperture, not just the aperture ring. They added a special A position to the aperture ring on the latest lenses, plus some contacts so the camera knew some basic information about the lenses. They standardized the lenses so moving the lever a certain distance opened the aperture blades one stop. Then they designed the arm in the cameras to move in precise small distances. With the new lenses, the camera body could control the aperture. New cameras could add many features. Old lenses were still compatible by making the arm still move on or off like it used to.

For a while, lenses retained aperture rings so they were compatible with old cameras, and bodies retained a small tab so they had some idea of where the aperture ring position was. Pentax started introducing some film cameras without the tab and lenses without the ring in the 90s. No Pentax DSLR has the tab, which is why some references call it the crippled mount - it is not 100% compatible with the original lenses. The *ist D was introduced without a way to use these M or K lenses conveniently. A firmware update added the work-around to use a lens with an aperture ring but no A position.
12-01-2014, 11:05 AM   #10
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The only time viewfinder will get darker is when you are using a completely manual lens, one that has no relation to the camera and the aperture is moved by hand. These are usually m42 mount lenses. The other time viewfinder can get dark is if you customize the buttons, so Green button is "DoF preview."
You can test by simply looking at the lens from the front. With lenses where the camera can physically stop down the aperture, it will keep the aperture wide open for composition and focus, and then stop down in the split second during which the photo is taken. So just look into the lens and press the shutter, you will see.
Lenses that have A setting on aperture ring (or no aperture ring at all) can have the aperture set directly on the camera. Lenses that have an aperture ring, but no A choice, will only stop down to the selected aperture if you Enable aperture ring in the camera's Menu, and use M mode (without auto ISO). M42 preset lenses move the aperture without the camera's knowledge, so you need to keep it open to focus and then stop down by hand to meter and take the photo.

tl;dr: DA 18-135mm you never have to worry about aperture anywhere except on the camera screen - the camera does everything for you. The aperture is wide open for composing and focusing, but gets closed to the chosen fraction when you take the photo or use DoF preview (customize button)
12-01-2014, 12:24 PM   #11
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QuoteOriginally posted by jatrax Quote
Assuming you are using a k-mount lens, not m-42, the aperture is set but does not change until you press the shutter button. When you do the camera stops down the aperture and takes the picture.

On m-42 it would be different and if it is not changing that is a problem.

Or the A/M switch is in the wrong place. (M42 lens).
12-01-2014, 02:38 PM   #12
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QuoteOriginally posted by kjg48359 Quote
As a side note, on the modern camera bodies, the "on" switch has a "diaphragm" icon when you toggle it over (past the "on" setting),the camera will stop down the lens for you (to aid in previewing the depth-of-field).
Some of the Pentax bodies DOF preview has to be mapped to one of the button. Also note there are two DOF preview modes: Optical and Digital. Optical will stop down the lens (provided it has a lever) and you can see the results in the viewfinder. Digital will take an exposure into the image buffer (not saved to the card) and you can view it in the LCD screen.

If you are using manual focus lenses this should be set to "Optical". Provided the body supports this function (flagship models) operating the optical DOF preview in manual mode will stop down the lens and turn on the meter and exposure bars. You can then set the exposure with the eDials and/or the aperture ring (if any) on the lens.
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