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03-08-2010, 05:28 PM   #1
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Aperture stop down behaviour on Kx

I've just got a Pentax A 50mm f1.2, and I'm a bit puzzled by the camera's (K-x) behaviour with the lens.

First, I noticed that the depth of field preview makes surprisingly little difference to the view in the optical view finder at low f stops (barely noticeable -if at all- difference between f2 and f1.2 either to the brightness, or depth of field) and the depth of field in the view-finder is always much wider than the depth of field in the actual shot. This makes it even harder to find the critical focus at very wide apertures.

But when I preview depth of field on the screen in live view, I get an accurate impression of what the depth of field will be in the shot-

so how come? why can't I accurately see the depth of field through the optical viewfinder, even though I can verify by looking down the lens that the aperture is wide open?

The other irritating thing is that when I release the DOF preview button, the aperture momentarily closes right down to f22 before opening up again- is this normal behaviour? I'm sure my film camera never did that.....



Dan

03-08-2010, 06:07 PM   #2
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Perfectly normal, and due to many reasons.

First, the focusing screen on modern auto-focus cameras isn't ideally suited to manual focusing in the first place. Second, the depth of field on an APS-C sensor is different than on a 35mm film camera.

You won't get the same depth of field in the viewfinder as you would in the resulting picture. It's annoying, but it's inevitable.

edit: this isn't your K-x's fault, my K-7 is the same way. Any digital camera will behave the same, except maybe full frame cameras. A four-thirds camera would be worse.
03-08-2010, 06:27 PM   #3
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QuoteOriginally posted by GoremanX Quote
You won't get the same depth of field in the viewfinder as you would in the resulting picture. It's annoying, but it's inevitable.
I understand the lens will behave differently than if it was on a 35mm camera, but putting that aside, on my K-x, shouldn't the path to the focusing screen be the same distance (as the path to the sensor), and the focusing screen be the same size (more or less) as the sensor? I'm not sure I understand why the depth of field should differ between the viewfinder and the captured image, unless the path to the focussing screen is restricted in such a way as to effectively act as a second, narrower aperture- (which, thinking about it now, might make sense and also explain why I don't see much brightness difference between f1.2 and f2)
03-08-2010, 06:32 PM   #4
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No, the focusing screen isn't the same size as the sensor, so its focusing plane is different. Camera manufacturers make an effort to align the focusing screen so that when the auto-focus sensor deems a picture is in focus, it will appear in focus in the viewfinder. That's about as far as it goes. This isn't like the old manual-focus days, where critical focus was so important and depth of field could be determined through the viewfinder. A lot of these intricacies aren't really catered to anymore in this digital age.

03-08-2010, 07:00 PM   #5
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thanks Goreman- errrr...... I think I need to go and re-read my old high-school optics notes....
03-09-2010, 05:09 PM   #6
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QuoteOriginally posted by GoremanX Quote
No, the focusing screen isn't the same size as the sensor, so its focusing plane is different.
Sorry if I'm going to appear pedantic, but..... I've re-read what you've written, thought about it, and I still don't understand how this can be the case- otherwise wouldn't you need one focus setting to get a sharp image on the focusing screen, and a different setting to get a focused image on the sensor?

For the focusing screen to have a sharp image on it and for it to be of any use as a focusing screen (even if, as you say, the primary focus is normally done by other means these days), it must, by definition, be in the focal plane of the lens as defined by the position of the sensor (ie the same distance from the lens as the sensor is) and therefore the screen and the sensor must have similar dimensions in order to have the same (or similar) field of view- the only thing I can think of that could influence/alter this would be if there were any other optics between the mirror and the focusing screen- which I don't believe there are?

.....maybe I have a fundamental misunderstanding of the way an SLR works?
03-09-2010, 05:16 PM   #7
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I dunno, take a ruler and measure it for yourself. It's right there in the mirror box when the lens is removed.

Focusing Screen--How to adjust focusing screen--

All I know is, you won't get the same depth of field in the viewfinder as you will from the camera's sensor. You can wish for it all you want, but it won't happen.
03-09-2010, 05:40 PM   #8
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QuoteOriginally posted by GoremanX Quote
All I know is, you won't get the same depth of field in the viewfinder as you will from the camera's sensor..
.... which was my question in the first place- why isn't the depth of field the same?

The link you provided pretty much says exactly the same as my previous post- L1 must = L2 for the camera to function properly.

I take your point and will now stop wishing for the impossible. But I still want to understand why it behaves the way it does.

03-09-2010, 05:43 PM   #9
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Circle of confusion - Wikipedia, the free encyclopedia
03-09-2010, 05:52 PM   #10
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haha- I thought "circle of confusion" was a comment on this discussion....

that's good reading Goreman- I'll go away and digest.
03-09-2010, 05:58 PM   #11
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QuoteOriginally posted by smokeydan Quote
haha- I thought "circle of confusion" was a comment on this discussion....

that's good reading Goreman- I'll go away and digest.
I find it very technical, myself. And there's more to it than just the size of the circle of confusion. But it's a good start, and the links at the bottom of the article point to more information that's a bit easier to read.

The fact that we can't preview depth of field accurately anymore is kind of a loss, but it's mitigated a fair bit by the fact that we can take countless pictures at different settings now and not worry about wasting film. Also, I've gotten pretty good at guessing what my depth of field will be like through a lot of experimenting. If you need to know accurate depth of field before taking a shot, the Online Depth of Field Calculator comes in handy.
03-09-2010, 10:07 PM   #12
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It's the design of the focus screen itself, and the little microlenses that form the image. These are what cause the DOF to appear to great at large apertures. At leats that's the explanation I've read in articles such as this one:

Manual Focus with AF DSLRs
03-09-2010, 10:45 PM   #13
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QuoteOriginally posted by Marc Sabatella Quote
It's the design of the focus screen itself, and the little microlenses that form the image. These are what cause the DOF to appear to great at large apertures. At leats that's the explanation I've read in articles such as this one:

Manual Focus with AF DSLRs
That article explains why my K3 focusing screen improved the usability of my faster lenses, especially the Super Takumar 50mm F1.4.

So there's your solution smokeydan, buy an expensive focusing screen
Focusing Screen
03-10-2010, 05:54 PM   #14
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Cheers guys- very enlightening! I guess I'll have to read through all the posts on focusing screens now, and decide which one to get...
03-10-2010, 10:11 PM   #15
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QuoteOriginally posted by smokeydan Quote
.... which was my question in the first place- why isn't the depth of field the same?

The link you provided pretty much says exactly the same as my previous post- L1 must = L2 for the camera to function properly.

I take your point and will now stop wishing for the impossible. But I still want to understand why it behaves the way it does.
The depth of field preview (optical) is not the same because you are using a focusing screen with the brightening that Pentax applied to its newer screens, such as the factory screen for my K10, which also completely screws up the exposure metering with a manual aperture lens. Both problems disappeared when I installed the screen from the *ist D series - the LL-60 grid screen in my case.
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