Doug Kerr
Well-known member
Here's the "little map" showing where in the frame the object was in the "off axis" test:
Best regards,
Doug
Best regards,
Doug
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The out of focus blur figure of a point source
- Thread starter Doug Kerr
- Start date
Doug Kerr
Well-known member
I measured the actual diameter of the pinhole object used in the test of the 18-200 mm lens at about 0.89 mm. That would theoretically project onto the focal plane (with the object "in focus") with an image diameter of about 0.39 pixel pitch.
Thus the spot of diameter about 11 mm I observed with the object supposedly "in focus" was obviously the result of misfocus (even though I had tried to focus on the object).
I need to look into how that happened. But in any case, I think it means that the spot diameters observed in the "misfocused" cases were meaningful as observed.
Best regards,
Doug
Thus the spot of diameter about 11 mm I observed with the object supposedly "in focus" was obviously the result of misfocus (even though I had tried to focus on the object).
I need to look into how that happened. But in any case, I think it means that the spot diameters observed in the "misfocused" cases were meaningful as observed.
Best regards,
Doug
Doug Kerr
Well-known member
Well, it looks like a number of things went wrong with these latest tests.
I thought I had chosen an exposure such that the brightest part of the figure was a little below saturation (in the defocused case). But I used that same exposure for the in-focus case, and thus was well over saturation.
When I backed that exposure down to where it should have been, the diameter of the object in the focused case was much more what was expected. Apparently there is an area of non-trivial luminance around the pinhole (not sure how that can be, but...). Maybe lens flare.
But, while working on that problem, I discovered that my tests were not run at f/4.0 as I had intended but rather at f/5.6. This was probably what the camera did when I increased the focal length to do some observing (the lens aperture declines with increasing focal length).
Then, I discovered that in fact the out-of-focus exposures were all substantially overexposed as well.
So this thing is as screwed up as Hogan's goat. I will start over.
For now, never mind.
Best regards,
Doug
I thought I had chosen an exposure such that the brightest part of the figure was a little below saturation (in the defocused case). But I used that same exposure for the in-focus case, and thus was well over saturation.
When I backed that exposure down to where it should have been, the diameter of the object in the focused case was much more what was expected. Apparently there is an area of non-trivial luminance around the pinhole (not sure how that can be, but...). Maybe lens flare.
But, while working on that problem, I discovered that my tests were not run at f/4.0 as I had intended but rather at f/5.6. This was probably what the camera did when I increased the focal length to do some observing (the lens aperture declines with increasing focal length).
Then, I discovered that in fact the out-of-focus exposures were all substantially overexposed as well.
So this thing is as screwed up as Hogan's goat. I will start over.
For now, never mind.
Best regards,
Doug
Doug Kerr
Well-known member
Well, we'll try this again. From the top.
************
Today we were on the indoor test range, this time testing with the EOS 40D and the EF-S 18-200 f/3.5-5.6 IS.
The object today was a pinhole on an LED flashlight, not of infinitesimal size, and was far closer than Alpha Orionis. There were in fact many compromises in this test series, but there are still some interesting results.
The test were done at an (indicated) focal length of 18 mm. The object was at a distance of about 7.2 m. The near focus distance was at about 1 m, but uncertainty about where the first principal point of the lens is makes that subject to some uncertainty (and of course with an f/3.5 aperture and a focal length of 18 mm, manual focus is not too "snappy" anyway").
Here we see the result. This is a 40 px × 40 px crop of the image at original resolution.
On the left panel is a yellow circle with diameter 25 px, for reference. I would characterize the diameter of this figure as about 18 px.
I do not understand the distribution of luminance across the figure. It is probably a creature of my pinhole object system (which was rather agricultural).
On the right, we have the same figure, but I have shown (as a red circle) the calculated spot diameter, which is approximately 14 mm. (But this is not highly reliable, owing to the uncertainty as to the actual focus distance.)
One might be tempted to say that the enlargement of the spot diameter over the theoretical value may due to spherical aberration, but we must be cautious owing to the uncertainty in the focus distance.
In the second shot, the object was substantially off-center. This shows the location of the object in the frame:
Here we see the actual result. Same drill as before:
Here, it is difficult to characterize the "diameter" of the figure. I suspect that the main reason for the shape here is the clipping of the entrance pupil as we saw in earlier tests.
Very interesting.
Best regards,
Doug
************
Today we were on the indoor test range, this time testing with the EOS 40D and the EF-S 18-200 f/3.5-5.6 IS.
The object today was a pinhole on an LED flashlight, not of infinitesimal size, and was far closer than Alpha Orionis. There were in fact many compromises in this test series, but there are still some interesting results.
The test were done at an (indicated) focal length of 18 mm. The object was at a distance of about 7.2 m. The near focus distance was at about 1 m, but uncertainty about where the first principal point of the lens is makes that subject to some uncertainty (and of course with an f/3.5 aperture and a focal length of 18 mm, manual focus is not too "snappy" anyway").
Here we see the result. This is a 40 px × 40 px crop of the image at original resolution.
On the left panel is a yellow circle with diameter 25 px, for reference. I would characterize the diameter of this figure as about 18 px.
I do not understand the distribution of luminance across the figure. It is probably a creature of my pinhole object system (which was rather agricultural).
On the right, we have the same figure, but I have shown (as a red circle) the calculated spot diameter, which is approximately 14 mm. (But this is not highly reliable, owing to the uncertainty as to the actual focus distance.)
One might be tempted to say that the enlargement of the spot diameter over the theoretical value may due to spherical aberration, but we must be cautious owing to the uncertainty in the focus distance.
In the second shot, the object was substantially off-center. This shows the location of the object in the frame:
Here, it is difficult to characterize the "diameter" of the figure. I suspect that the main reason for the shape here is the clipping of the entrance pupil as we saw in earlier tests.
Very interesting.
Best regards,
Doug
Asher Kelman
OPF Owner/Editor-in-Chief
Zeiss went to a lot of trouble in optimizing for perfect bokeh and clean circles i their 135mm lens.
There's "the double iris construction, from which the front one is designed to produce perfectly round shaped circles of confusion of hightlights outside the focal plane. Between the sets of aperture blades this lens has a special "apodization element", a kind of circular ND filter that reduces to its borders the intensity of light rays the more they are out of focus. This design gives an extremely smooth transition to the blurred out-of-focus regions and boosts sharpness inside the focal plane. If you are interested in more details about this masterpiece with a weight of 730g, you should take a look into this review: Sony 135 F2.8 [T4.5] STF SAL-135F28 review." More here
Enjoy at your own risk, you may end up with a lighter wallet!
Asher
There's "the double iris construction, from which the front one is designed to produce perfectly round shaped circles of confusion of hightlights outside the focal plane. Between the sets of aperture blades this lens has a special "apodization element", a kind of circular ND filter that reduces to its borders the intensity of light rays the more they are out of focus. This design gives an extremely smooth transition to the blurred out-of-focus regions and boosts sharpness inside the focal plane. If you are interested in more details about this masterpiece with a weight of 730g, you should take a look into this review: Sony 135 F2.8 [T4.5] STF SAL-135F28 review." More here
Enjoy at your own risk, you may end up with a lighter wallet!
Asher
Doug Kerr
Well-known member
Hi, Asher,
Thanks.
Best regards,
Doug
Sounds like quite a machine. I'll look into it.
Thanks.
Best regards,
Doug
Jerome Marot
Well-known member
The Sony 135 F2.8 [T4.5] STF SAL-135F28 was not designed by Zeiss, but by Minolta.
Asher Kelman
OPF Owner/Editor-in-Chief
……………………….and the mouse was designed at Xerox!
Jerome Marot
Well-known member
Doug Kerr
Well-known member
Hi, Jerome,
I think Asher forgot to set the "irony here" flag.
Best regards,
Doug
I think Asher forgot to set the "irony here" flag.
Best regards,
Doug