Circle of Confusion: DA versus FF lenses

d.bradley · 02-25-2008, 04:39 AM

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?

Gooshin · 02-25-2008, 07:07 AM

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

Jerry Thirsty · 02-25-2008, 07:11 AM

Originally posted by d.bradley

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

Gooshin · 02-25-2008, 07:15 AM

Originally posted by Jerry Thirsty

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."

d.bradley · 02-25-2008, 07:21 AM

Originally posted by Gooshin

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:

Gooshin · 02-25-2008, 07:33 AM

Originally posted by d.bradley

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)

Gooshin · 02-25-2008, 07:38 AM

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

mattdm · 02-25-2008, 07:59 AM

Originally posted by Gooshin

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.

blende8 · 02-25-2008, 08:47 AM

The question is why the circles at f4 are of equal size, but those at f5.6 are not.

Gooshin · 02-25-2008, 08:54 AM

Originally posted by blende8

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.

d.bradley · 02-25-2008, 09:07 AM

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.

Gooshin · 02-25-2008, 09:42 AM

Originally posted by d.bradley

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.

Gooshin · 02-25-2008, 09:51 AM

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.

blende8 · 02-25-2008, 10:45 AM

Originally posted by Gooshin

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).

Gooshin · 02-25-2008, 10:49 AM

Originally posted by blende8

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.

02-25-2008, 04:39 AM	#1
d.bradley Veteran Member Join Date: Feb 2007 Location: Ontario Photos: Gallery Posts: 631	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, 09:51 AM	#13
Gooshin Veteran Member Join Date: Sep 2007 Location: Toronto, the one in Canada. Posts: 5,610	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. Attached Images Last edited by Gooshin; 02-25-2008 at 10:14 AM.

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02-25-2008, 07:07 AM	#2
Gooshin Veteran Member Join Date: Sep 2007 Location: Toronto, the one in Canada. Posts: 5,610	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
Jerry Thirsty Forum Member Join Date: Sep 2007 Location: Bronson, Missouri Posts: 72	Originally posted by d.bradley 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
Gooshin Veteran Member Join Date: Sep 2007 Location: Toronto, the one in Canada. Posts: 5,610	Originally posted by Jerry Thirsty 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
d.bradley Veteran Member Join Date: Feb 2007 Location: Ontario Photos: Gallery Posts: 631 Original Poster	Originally posted by Gooshin 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
Gooshin Veteran Member Join Date: Sep 2007 Location: Toronto, the one in Canada. Posts: 5,610	Originally posted by d.bradley 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
Gooshin Veteran Member Join Date: Sep 2007 Location: Toronto, the one in Canada. Posts: 5,610	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
mattdm Veteran Member Join Date: Feb 2007 Location: Boston, MA Photos: Gallery Posts: 2,948	Originally posted by Gooshin 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
blende8 Veteran Member Join Date: Feb 2007 Location: Bremen, Germany Photos: Albums Posts: 1,521	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
Gooshin Veteran Member Join Date: Sep 2007 Location: Toronto, the one in Canada. Posts: 5,610	Originally posted by blende8 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
d.bradley Veteran Member Join Date: Feb 2007 Location: Ontario Photos: Gallery Posts: 631 Original Poster	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
Gooshin Veteran Member Join Date: Sep 2007 Location: Toronto, the one in Canada. Posts: 5,610	Originally posted by d.bradley 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, 10:45 AM	#14
blende8 Veteran Member Join Date: Feb 2007 Location: Bremen, Germany Photos: Albums Posts: 1,521	Originally posted by Gooshin 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
Gooshin Veteran Member Join Date: Sep 2007 Location: Toronto, the one in Canada. Posts: 5,610	Originally posted by blende8 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.