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01-08-2010, 06:25 AM   #16
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QuoteOriginally posted by arpaagent Quote
What is interesting to me about the 35mm is the harsh edges of the bokeh circles. Much more pronounced than the other lenses that I tested.
The 35/2 has bad bokeh wide open. It's amazing, if you click that thing down to f2.8 things get much smoother...

01-08-2010, 11:52 AM   #17
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QuoteOriginally posted by blende8 Quote
I agree that the shorter focal length is probably due to internal focussing.

I already took this into account by enlarging the DA image by 10%.
Below is a comparison of your test with mine (both roughly corrected).
There may be small differences, but the effect is clear I think.

Unfortunately I am no optics expert. My idea is that this has to do with the size of the exit pupil. As one can see from your test, even the various 50mm lenses show different behaviour. So it seems that the size of the circle is also dependent on the optical construction.
Unfortunately I don't have enough DA lenses to make a systematic test, but it would be interesting to investigate this further.

I was curious about this, so I set up a tripod with the DA*16-50 and the A 50 1.2 (both at f/2.8).

To account for the internal focusing messing up the focal length, I decided to focus at infinity and have the point light source in the foreground.

Two things I noticed. The exit pupil hypothesis is not supported. And the 16-50 at 50mm at infinity is still not the same FOV as the 50mm prime!

Thoughts? No expert here either!
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01-08-2010, 02:12 PM   #18
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QuoteOriginally posted by farfisa Quote
I was curious about this, so I set up a tripod with the DA*16-50 and the A 50 1.2 (both at f/2.8).

To account for the internal focusing messing up the focal length, I decided to focus at infinity and have the point light source in the foreground.

Two things I noticed. The exit pupil hypothesis is not supported. And the 16-50 at 50mm at infinity is still not the same FOV as the 50mm prime!

Thoughts? No expert here either!
What this does show here is that the OOF hightlights are just about equal in size (which I was attempting to show in my posts, but was foiled by the internal focusing...). And the focal lengths are very close (not exact, but closer than in my example). I think that the exit pupil only affects DOF when focused at near-macro distances. It is my understanding that it doesn't matter for normal shooting.
01-08-2010, 02:21 PM   #19
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QuoteOriginally posted by arpaagent Quote
What this does show here is that the OOF hightlights are just about equal in size (which I was attempting to show in my posts, but was foiled by the internal focusing...). And the focal lengths are very close (not exact, but closer than in my example). I think that the exit pupil only affects DOF when focused at near-macro distances. It is my understanding that it doesn't matter for normal shooting.
Right! The main point was to show that the OOF highlights were the same size! By saying "the exit pupil hypothesis is not supported" I was looking for a nice way to shoot down Blende8's theory--sorry!

I was surprised though that the 16-50 isn't a true 50 at, um, 50, but yes, it's close enough.

I also noticed that the DA 40 and the Voigtlander 40 are different lengths too. Which one is actually 40mm, who knows? Wieland?

01-08-2010, 05:17 PM   #20
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I've worked a bit on the geometric optics that describes out of focus disks. The great data given here allowed me to check the theory predictions. Things agree reasonably well.

Theory predicts the disk diameter increases in proportion the actual aperture diameter and to distance in front of or behind the in-focus subject. This is countered by the perspective effect which makes the disk apparently decrease in size as the point creating it gets farther behind the subject.

Here's the data in graphics form:

In the equation, So is the camera-subject distance and S is the camera-background distance; Mo is the optical magnification. The Focal_length/F-Number term is equal to the actual aperture opening so in a sense, the equation is independent of focal length (it cancels in the ratio so all that's left is the aperture size.)

Notice that the out-of-focus disk diameter approaches the actual aperture size times the magnification as the point approaches infinity.

Dave

Last edited by newarts; 01-08-2010 at 08:56 PM.
01-08-2010, 07:56 PM   #21
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QuoteOriginally posted by blende8 Quote
This is why images with tele lenses give this nice creamy background. It's not just DoF.
I should not have made the link to DOF, I agree.
But the reason why tele lenses give a creamy background is because of their higher magnification (they make the background and thus the blur look bigger).

QuoteOriginally posted by newarts Quote
Notice that the out-of-focus disk diameter approaches the actual aperture size times the magnification as the point approaches infinity.
Which means it is independent of the image circle size and thus the OOF disk diameter should be the same for APS-C and FF lenses (everything else being equal).

P.S.: Thanks for the math, Dave, I'll have a closer look later.
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