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01-05-2010, 07:44 PM   #1
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Out of Focus Highlight Demo (Focal Length & Aperture)

This is a simple demonstration I threw together real quick to show the effect of focal length and aperture on out of focus background highlight rendering.

Linked into the original Post your ƒ1.2 Photos! (ƒ1.2 ONLY!) thread.

I set up a hiking shoe on a table, and was focusing close to the min focusing distance of all the lenses. The Christmas tree was lit up in the background about 6 or 8 feet behind the shoe. Smaller background lights make for better true circles, because the lens takes that pinpoint of light and disperses it about a circle when it is out of focus. This is how it makes those big out of focus circles. It works really great when you have bright pinpoint lights in the background (like christmas tree lights) because they are bright enough pinpoints to still be bright and visible even when their light is spread out over an area on the sensor.

Hope this helps anyone that was curious about this effect or how to acheive it!

SMC Pentax-A 50mm F1.2


SMC Takumar 50mm F1.4


S-M-C Takumar 35mm F2


SMC Pentax 85mm F1.8


SMC Pentax 135mm F2.5


01-05-2010, 08:10 PM   #2
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Jim, since the 35 has a much wider FOV, it would had been better if you had adjusted the distance between the shoe and the background much closer. I believe that would create a bigger/larger dispersion area/size of light.
01-05-2010, 08:22 PM   #3
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what is also noticeable as you stop further down, the spacing between the light circles become less clustered.

the things that affect such effect are the ff:

1.> focal length
2.> aperture opening
3.> distance between subject and background
4.> lightsource (all sorts including the sun)

Last edited by Pentaxor; 01-07-2010 at 01:27 PM.
01-05-2010, 08:27 PM   #4
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QuoteOriginally posted by Pentaxor Quote
Jim, since the 35 has a much wider FOV, it would had been better if you had adjusted the distance between the shoe and the background much closer. I believe that would create a bigger/larger dispersion area/size of light.
Yes, I could have done that, but I did want to keep the scene the same to that people could see what the same scene looked like through a different lens. If I did move the shoe closer to the lights, the bokeh would have become somewhat smaller, but would have covered more of the frame. Too bad the tree lights are already turned off for the night . Oh well.

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.

01-05-2010, 08:37 PM   #5
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QuoteOriginally posted by arpaagent Quote
Yes, I could have done that, but I did want to keep the scene the same to that people could see what the same scene looked like through a different lens. If I did move the shoe closer to the lights, the bokeh would have become somewhat smaller, but would have covered more of the frame. Too bad the tree lights are already turned off for the night . Oh well.

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.
I dont have the Takumars but I'm wondering how many blades does the 50 and 35 have?

the other 50/1.2, 85 and 135 have atleast 8/9 blades. so it might be due to the number of aperture blades or it is just that the 35mm is just that sharp (harsh bokeh), same to the Pentax 35/2.

anyway, I just recently removed the christmas lights yesterday and about to remove the fir tree out of the house tommorow. so cant do no further testing anymore.
01-05-2010, 08:54 PM   #6
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QuoteOriginally posted by Pentaxor Quote
I dont have the Takumars but I'm wondering how many blades does the 50 and 35 have?

the other 50/1.2, 85 and 135 have atleast 8/9 blades. so it might be due to the number of aperture blades or it is just that the 35mm is just that sharp (harsh bokeh), same to the Pentax 35/2.

anyway, I just recently removed the christmas lights yesterday and about to remove the fir tree out of the house tommorow. so cant do no further testing anymore.
All these were shot wide open so the number of aperture blades should not have had an effect on the bokeh. But to satisfy your curiosity, here are the number of blades for each of the lenses from above, which definitely has an effect when shooting stopped down.

9 blades: SMC P-A 50/1.2
8 blades: SMC T 50/1.4, SMC P 85/1.8, SMC P 135/2.5
6 blades: S-M-C T 35/2

And a few others that I have with 9 blades for what it's worth: DA*16-50 & 50-135, and F135/2.8
01-05-2010, 10:00 PM   #7
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QuoteOriginally posted by arpaagent Quote
All these were shot wide open so the number of aperture blades should not have had an effect on the bokeh. But to satisfy your curiosity, here are the number of blades for each of the lenses from above, which definitely has an effect when shooting stopped down.

9 blades: SMC P-A 50/1.2
8 blades: SMC T 50/1.4, SMC P 85/1.8, SMC P 135/2.5
6 blades: S-M-C T 35/2

And a few others that I have with 9 blades for what it's worth: DA*16-50 & 50-135, and F135/2.8
I see that the TAKs have also the same number of the of the Pentax versions. the Pentax 35 has also 6 blades and also display the same sharpness rendition that is unique to only the 35, except the DA35 which is a stop slower. the 35's optical design and properties could also be the reason behind this.
01-06-2010, 01:04 PM   #8
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QuoteOriginally posted by arpaagent Quote
This is a simple demonstration I threw together real quick to show the effect of focal length and aperture on out of focus background highlight rendering. ...

Linked into the original Post your ƒ1.2 Photos! (ƒ1.2 ONLY!) thread.

I set up a hiking shoe on a table, and was focusing close to the min focusing distance of all the lenses. The Christmas tree was lit up in the background about 6 or 8 feet behind the shoe. Smaller background lights make for better true circles, because the lens takes that pinpoint of light and disperses it about a circle when it is out of focus. This is how it makes those big out of focus circles. It works really great when you have bright pinpoint lights in the background (like christmas tree lights) because they are bright enough pinpoints to still be bright and visible even when their light is spread out over an area on the sensor.

Hope this helps anyone that was curious about this effect or how to acheive it! ...
Wow, thanks. For me a very interesting comparison, to see how the shapes change from circles differently over the visual field.

The first two demonstrate effect of aperture; all demonstrate effect of focal length

In the first the edges of the "circles" are beginning to degrade into another pattern, compared to harder edges in the second (and others). I imagine the bokeh of that 1.2 lens gets swirly. Am I right?


Last edited by rhodopsin; 01-06-2010 at 05:13 PM. Reason: removed images from quote
01-06-2010, 03:59 PM   #9
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you don't need to quote whole group of images

just saying.
01-07-2010, 01:44 PM   #10
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just an addition, COC's shape is also influenced when aperture is closed down. the shape is influenced by the number of aperture blades and the shape of the aperture blades as well.
01-07-2010, 11:12 PM   #11
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Ok, pulling something from another thread...there was a discussion about whether or not sensor format (FF vs APS-C) has an effect on the size of the out-of-focus highlights that are rendered by a lens.

from https://www.pentaxforums.com/forums/872694-post32.html

QuoteOriginally posted by blende8 Quote
This is not correct.
Below is a quick comparison between the DA16-50@50 and the FA50/1.4 (both at f2.8):
Same scene, tripod, focussed on something in the foreground, observed the circle in the background.
Originally I noted it first with the DA70, but other DAs show this, too.
Interestingly the DA55 does not show it. This lens covers FF.

After seeing this, I was very curious why that would occur because I am quite certain that the sensor format that a lens is made for has no effect on OOF highlights. A 50mm F1.4 made for FF will create the same size OOF highlights as a 50mm F1.4 made for APS-C, but the only made for FF will be able to create those same size circles over more area. That is my understanding, so I did a few tests of my own tonight.

Basically had my camera on a tripod a certain distance away from a subject, and had bright lights in the background for OOF highlights. Neither the camera position nor scene arrangement changed at all when comparing similar focal lengths in this test. The setup for my 50mm tests is seen here.



Now, I tested 4 different lenses at 50mm: DA 16-50/2.8, A50/1.2, F50/1.7, and FA50/2.8 Macro. I made shots with each of them at f/2.8 and f/5.6.

Here are the f/2.8 shots, in the order of the lenses listed above:





Now the f/5.6 shots.






There are a few interesting things to note here.
1. Assuming that my 50mm primes are indeed very close to true 50mm focal length, the DA 16-50 does not actually reach all the way to 50mm. I had the lens zoomed as far as it would go, but the field of view was significantly wider than with all the 50mm primes. This leads me to believe that the DA16-50 is more like 45mm on the long end (at least when relatively close focused), and Pentax decided to boost it for marketing.
2. The OOF highlights that the DA 16-50mm renders when set to 50mm are indeed smaller than the primes lenses (although not as much as Weiland seemed to indicate). HOWEVER, realizing the fact that the DA is actually a shorter focal length than the 50mm primes, it is clear that the DA will produce smaller OOF highlights because of the wider field of view. It is not because the DA is a lens made for APS-C. If the DA truly matched the focal length of the 50mm primes, then the OOF highlights would be roughly the same size.

I also did a comparison at 28mm with the DA16-50 against an F28/2.8.

f/2.8



f/5.6



Not much difference. I didn't quite seem to match the FOV perfectly with the zoom, but the OOF highlights are very close to the same size. Had I zoomed in just a tad bit more to match the field of view of the 28mm, they would be even closer in size.

And one more. DA50-135 vs K85/1.8 vs M85/2. All at f/2.8.





Again, the APS-C lens output is basically the same as the FF lenses. The APS-C actually has very slightly larger OOF highlights in this one because I zoomed just a hair too far and the FOV is slightly tighter.

Anyways, my point is that the format that a lens is created for does not have any impact on the size of the OOF highlight circles.
01-07-2010, 11:23 PM   #12
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Ok one other thing to note. I was just playing with the DA*16-50 zoom, to see why it might not be 50mm when zoomed all the way out. It turns out that as you focus closer, the angle of view actually gets bigger (meaning a smaller focal length). You can actually see this effect in the viewfinder, it is pretty obvious. I am guessing this is due to the fact that the DA* is an internal focus lens, and has to play optical tricks when focusing.

Regular focusing lenses (eg A50/1.2) do not exhibit this behavior.

This explains why the DA*16-50 seemed to be a shorter focal length in my examples above...because it was a relatively close subject.
01-08-2010, 01:34 AM   #13
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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.

01-08-2010, 04:52 AM   #14
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The only difference between an FF lens and an APS-C lens is the size of the image circle. Regarding the APS-C portion of the image circle, two respective lenses should throw the same image, everything else being equal between the lenses.

One could turn an APS-C lens into an FF lens by enlarging the image circle by the crop factor, e.g., in the case of the K-7 by a 1.54 x TC. This TC would also enlarge the size of the bokeh circles but we now need to capture that image with an FF sensor to be fair (otherwise we just increased the FL) and when we then look at the size of the bokeh circles relative to the image size, they are the same (relative) size again.

An alternative way of looking at this is that the size of bokeh circles should correspond to circles of confusion which in turn corresponds to DOF. If the bokeh circles had a different size, the DOF should be different as well.
01-08-2010, 06:07 AM   #15
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QuoteOriginally posted by Class A Quote
An alternative way of looking at this is that the size of bokeh circles should correspond to circles of confusion which in turn corresponds to DOF. If the bokeh circles had a different size, the DOF should be different as well.
Yes and no.
DoF is dependent on the aperture and the reproduction scale.
But the "softness" of the background is also dependent on the size of the exit pupil. This is why images with tele lenses give this nice creamy background. It's not just DoF.
That's why I suspect that it has to do with the exit pupil.

But as I said, I am no expert.
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