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02-25-2008, 04:39 AM   #1
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Circle of Confusion: DA versus FF lenses

Hey folks,

A question for you optical experts out there. Will the circle of confusion (appearing in the rendering of out-of-focus areas) for a DA lens be the same size as that of an FF lens of the same focal length, focus distance, and all other variables being equal?

02-25-2008, 07:07 AM   #2
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assuming you are of equal distance from the subject, and all other variables such as number and design of the aperture blades, quality and design of glass, and focal length, then yes... the bokeh will be equal
02-25-2008, 07:11 AM   #3
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QuoteOriginally posted by d.bradley Quote
Hey folks,

A question for you optical experts out there. Will the circle of confusion (appearing in the rendering of out-of-focus areas) for a DA lens be the same size as that of an FF lens of the same focal length, focus distance, and all other variables being equal?
What you are referring to is the bokeh. Circle of confusion is something else.

Circle of confusion - Wikipedia
02-25-2008, 07:15 AM   #4
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QuoteOriginally posted by Jerry Thirsty Quote
What you are referring to is the bokeh. Circle of confusion is something else.

Circle of confusion - Wikipedia
no, circle of confusion is bokeh...

from the same site that you linked to...

"Bokeh (from the Japanese boke ぼけ, "blur") is a photographic term referring to the appearance of out-of-focus areas in an image produced by a camera lens.[1] Different lens bokeh produces different aesthetic qualities in out-of-focus backgrounds, which are often used to reduce distractions and emphasize the primary subject. The effect itself is the circle of confusion, an image of the aperture convolved by the image itself."

02-25-2008, 07:21 AM   #5
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QuoteOriginally posted by Gooshin Quote
assuming you are of equal distance from the subject, and all other variables such as number and design of the aperture blades, quality and design of glass, and focal length, then yes... the bokeh will be equal
So the actual physical diameter of the aperture is the same for an FA 200/2.8 as it is for a DA 200/2.8...because that would change the circle of confusion, right?

I ask because of this discussion:
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/21500-comparis...8-200-4-a.html

and this image linked in that post:

02-25-2008, 07:33 AM   #6
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QuoteOriginally posted by d.bradley Quote
So the actual physical diameter of the aperture is the same for an FA 200/2.8 as it is for a DA 200/2.8...because that would change the circle of confusion, right?

I ask because of this discussion:
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/21500-comparis...8-200-4-a.html

and this image linked in that post:

judging by the two F4.0 images, they have different aperture construction, one has 8 blades while the DA has 9

esp since at F4 one is fully open (hence the full circle) and the other is partially closed (hence the hexegon)
02-25-2008, 07:38 AM   #7
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also, i'm at work and dont have photosoftware

but can u try and combine the F4.0 images of the two lenses, i have a sneaky suspecion that the DA circle will fit perfectly into the FA
02-25-2008, 07:59 AM   #8
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QuoteOriginally posted by Gooshin Quote
no, circle of confusion is bokeh...

from the same site that you linked to...

"Bokeh (from the Japanese boke ぼけ, "blur") is a photographic term referring to the appearance of out-of-focus areas in an image produced by a camera lens.[1] Different lens bokeh produces different aesthetic qualities in out-of-focus backgrounds, which are often used to reduce distractions and emphasize the primary subject. The effect itself is the circle of confusion, an image of the aperture convolved by the image itself."
Okay, I fixed that.

02-25-2008, 08:47 AM   #9
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The question is why the circles at f4 are of equal size, but those at f5.6 are not.
02-25-2008, 08:54 AM   #10
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QuoteOriginally posted by blende8 Quote
The question is why the circles at f4 are of equal size, but those at f5.6 are not.
the DA could have had a problem reading the camera inputs, the dial said 5.6 but the camera clicked it more

if we look and see that the F4.0 apertures are identical in size, one would guess that the FA is properly stepped down while the DA stopped down too much, either from mechanical error or electronic.
02-25-2008, 09:07 AM   #11
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A faulty readout? I don't think that's overly likely...

It might make sense the physical aperture sizes are different, and the aperture size changes on logarithmic (?) scale ...in which case the difference at a large aperture will be less visible than at a smaller value...

Besides...DA lenses are smaller than their full frame counterparts...wouldn't it make sense that the physical aperture size is smaller as well? I'm not going to be convinced by repetition...I would like to see some references.
02-25-2008, 09:42 AM   #12
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QuoteOriginally posted by d.bradley Quote
A faulty readout? I don't think that's overly likely...

It might make sense the physical aperture sizes are different, and the aperture size changes on logarithmic (?) scale ...in which case the difference at a large aperture will be less visible than at a smaller value...

Besides...DA lenses are smaller than their full frame counterparts...wouldn't it make sense that the physical aperture size is smaller as well? I'm not going to be convinced by repetition...I would like to see some references.
aperture diameter increase and decrease in a linear fashion:




since the focal length in the two lenses is the same, we have to assume that the apertures should be identical in size as well

if we extrapolate converging lines from the top and bottom of the circles you will see that we create two different forms.

so, either the DA's 5.6 is wrong, or the FA's 5.6 is wrong.
02-25-2008, 09:51 AM   #13
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here...

using some extrapolation....

the top is the FA

the bottom is the DA

as you can see, there is a stop missing in the DA line

OR

the "5.6" in the FA is actualy something like a 5.0 ???

this is why even with the FA lenses i still shoot using the aperture ring, because honestly even something like shutter speed can easily be subjective, so i decrease the chances of error.
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Last edited by Gooshin; 02-25-2008 at 10:14 AM.
02-25-2008, 10:45 AM   #14
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QuoteOriginally posted by Gooshin Quote
as you can see, there is a stop missing in the DA line
This argumentation is flawed.
You can equally well say that the A is lying in the middle of f4 and f5.6.

I have measured the cirlces and the DA follows the square root law.

I will look further into this with some tests if I find time (and Bokeh).
02-25-2008, 10:49 AM   #15
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QuoteOriginally posted by blende8 Quote
This argumentation is flawed.
You can equally well say that the A is lying in the middle of f4 and f5.6.

I have measured the cirlces and the DA follows the square root law.

I will look further into this with some tests if I find time (and Bokeh).
i'm sorry you take offence that decided to denote the DA as the bad one... yes, you can equaly say that the A is not calibrated

but a lens with an aperture ring that clicks into pre-defined (and extensively tested) aperture positions would have much more credibility than a fully electronically controlled lens.

or, the electronic aperture control is poorly interacting with the FA lens.

i would re-do the tests while using the aperture ring first to get a rock solid base.
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