[newarts]

Matt, Thanks for the clarifications.

I didn't think about object space (even though it was clearly stated in the post.)

a fiber optic end is probably a good source.

Dave

d = 1.22 x (1 + M) x F x L in image space where d is the separation of resolvable points in the image,

d = 1.22 x (1 + 1/M) x F x L in object space where d is the separation of resolvable points on the object.

However now that I see what Lowell was getting at neither of these are relevent. As the object distance is constant the magnification varies and so shorter focal length lenses will show more diffraction lenses at the same aperture than longer focal lengths. Also we are not really sampling the airy patern, are we?



My understanding is that in the near field the output of a multi mode fibre is a good approximation to a top hat function. In the far field the approximation is not so good but it should still be better than an LED.

[MattGunn]

First, let me state that I'm a computer engineer trying to find new algorithms for image processing, not an optical engineer. Any optics people out there are encouraged to correct the multitude of errors I make.

What I've been calling PSF is closely related to the Airy Function and it is common to see that type of patterning in PSF of compact cameras. However, to a first-order approximation, the PSF just gets smaller as you near the focus distance. Once the PSF gets small enough, things get much more complex. For example, some colors may be before the focus point while others are after. By then you've also got all sorts of artifacting due to the antialias filter, microlens array, Bayer filter pattern, etc. At that point, you don't reliably get much more than an estimate of spread diameter for x% of the energy, so the PSF detail structure is essentially moot.

Simple diameter measurements, usually made as power in the frequency domain, are commonly used for refocus of nearly-in-focus scenes -- with varying degrees of success. The real value of the PSFs I'm researching is for stuff that's not very close to being in focus, so that recognizing PSF internal structure can help disambiguate the scene.

I think that point spread function is not the correct term for these measurements. The point spread function of an instrument is usually what the instrument sees when it is setup and used under optimum conditions. The point spread function of a lens would therefore be what the sensor sees when a lens is perfectly focussed on a point source. For a normal lens this would be a combination of uncorrected aberations and the airy patern caused by diffraction. As with the bokeh measurements the point spread function will change with aperture due to the outer zones of the lens being cut off and the diffraction patern increasing as the lens is stopped down. The term for the Bokeh is really the Circle of Confusion as it is caused by defocussing the system.

Here are a few more Bokeh measurements. Still taken from 5m but this time the LED was coupled into a 1mm plastic fibre which provides a smaller and more uniform source. Extension tubes were used for these lenses to increase the size of the Bokeh.

SMC Takumar 135mm f/3.5
@ f/3.5


@ f/5.6


@ f/8


@ f/11


@ f/16


@ f/22


Super Takumar 150mm f/4
@ f/4


@ f/5.6


@ f/8


@ f/11


@ f/16


@ f/22


Topman MC 135mm f/2.8
@ f/2.8


@ f/4


@ f/5.6


@ f/8


@ f/11


@ f/16


@ f/22


Cosina Cosinon 200mm f/4 showing a fault at the bottom left hand edge wide open. I don't know what is causing this as I hadn't noticed it untill I took these measurements but I will investigate further.
@ f/4


@ f/5.6


@ f/8


@ f/11


@ f/16


@ f/22

[ProfHankD]

I think that point spread function is not the correct term for these measurements. The point spread function of an instrument is usually what the instrument sees when it is setup and used under optimum conditions. The point spread function of a lens would therefore be what the sensor sees when a lens is perfectly focussed on a point source. For a normal lens this would be a combination of uncorrected aberations and the airy patern caused by diffraction. As with the bokeh measurements the point spread function will change with aperture due to the outer zones of the lens being cut off and the diffraction patern increasing as the lens is stopped down. The term for the Bokeh is really the Circle of Confusion as it is caused by defocussing the system.

I agree that PSF normally are measured approximately in focus, the idea being that in the image plane PSFs can construct an image by simple addition without wave-like interference. However, the image structures we are measuring empirically do seem to add without significant interference at the pixel scale, and I make heavy use of that property. For what it's worth, one of my fellow faculty doing nanofabrication research (involving a lot of traditional PSF issues for lasers and mask imaging) had no problem with my use of the PSF term here.

Circle of confusion is commonly a linear measure (a diameter) and things are not really "confused" within our "PSF" structures, so I don't think CoC is quite right either. I also don't like terms involving the word "blur" because things don't blur.

It is probably appropriate for me to invent a new term.... Bokeh PSF?

Anyway, the new whatever-we-call-thems you posted look good. Thanks.

[MattGunn]

I agree that PSF normally are measured approximately in focus, the idea being that in the image plane PSFs can construct an image by simple addition without wave-like interference. However, the image structures we are measuring empirically do seem to add without significant interference at the pixel scale, and I make heavy use of that property. For what it's worth, one of my fellow faculty doing nanofabrication research (involving a lot of traditional PSF issues for lasers and mask imaging) had no problem with my use of the PSF term here.

Circle of confusion is commonly a linear measure (a diameter) and things are not really "confused" within our "PSF" structures, so I don't think CoC is quite right either. I also don't like terms involving the word "blur" because things don't blur.

It is probably appropriate for me to invent a new term.... Bokeh PSF?

Anyway, the new whatever-we-call-thems you posted look good. Thanks.

I think the term Bokeh covers it. Quote from the wikipedia page on Bokeh (here: Bokeh - Wikipedia, the free encyclopedia) Bokeh - "the way the lens renders out-of-focus points of light."

[falconeye]

I agree that PSF normally are measured approximately in focus

IMHO, the term PSF in optical systems is reserved to characterize the performance of the system. Wikipedia covers this definition:

A more general term for the PSF is a system's impulse response, the PSF being the impulse response of a focused optical system

So, not every impulse response of an optical system is called PSF. Your's is the defocused impulse response. You might call it the confused point image.

I agree that mathematically, the confused point image has all mathematical properties to convolve/deconvolve an integral image. Just like the PSF or any impulse response does have. But this forum is more photographically rather than mathematically oriented.

[ProfHankD]

I think the term Bokeh covers it. Quote from the wikipedia page on Bokeh (here: Bokeh - Wikipedia, the free encyclopedia) Bokeh - "the way the lens renders out-of-focus points of light."

Unfortunately, Bokeh is not a scientifically precise term and has very loose common usage, although it has been used as a modifier to a scientific term before: "Bokeh CA." By analogy, "Bokeh PSF" makes sense, and that's what I'm leaning toward.

Actually, a term like "Bokeh Impulse Response," "Defocused Impulse Response," or "Focus Dependent Impulse Response" (my favorite) wouldn't bother me either, they are just not as intuitive. In any case, there will be a precise definition of what's being done in any publications coming from this work.

The key points are:

1. The measure is of an imaging system, not a lens in isolation. Each physical configuration of that system that may alter the effective aperture (e.g., change of iris or mirror box masking) or sensor properties either can be parametrically modeled or treated as a new imaging system.
2. A single measurement produces a 2D color image or multi-spectral energy distribution pattern resulting from imaging a single point light source on a dark field.
3. Complete characterization should model how the pattern changes as the focus distance and relative position of the point light source are changed, both defocus and vignetting parameters. In-focus, the model should approximate the PSF.

Once again, I'm not expecting this thread to give me data for all the above. I just need to know that what I'm testing and modeling covers the full range of what commonly occurs.

[Matthew]

Here are some Pentax DA*300 OOFH. As the lens is internal focus I have included one shot at minimum focus (as a wedge because the OOFH are huge) and a second at infinity. LED source about 10 metres away and F4 in both cases. Many of the splodges are a dirty sensor. The fine sharp specs are something else.

Attached Images

   

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Last edited by Matthew; 04-04-2010 at 08:57 PM.

[Matthew]

The DA14 is internal focus also but at infinity, 10 metres is in focus and the LED occupied about 4 pixels so only minimum focus is included. This lens focuses down very close so out of focus highlights can be quite extreme wide open but change dramatically with stopping down.

[Lmcfarrin]

I hope your research is ongoing. I am unconventional birds in flight photographer and I use a variety of lenses. I shoot manual focus only and prefer fast aperure lenses.

Titmouse in flight taken with Takumar 50 1.4 at 1.4.

Pentax K-3 II Mounted with SMC Takumar 50MM 1.4 (Titmouse) - PentaxForums.com

[Lmcfarrin]

Mourning Dove from the back using Irix 150MM 2.8 at 2.8.

[Lmcfarrin]

Blue Jay captured with Sony A6000 and 5 element Takumar 135MM 2.5 at 2.5.

[ProfHankD]

It's been a long time on this, with the name OOF PSF (out-of-focus point spread function) now fairly widely accepted for this.

The most accessible overview is A Poorly Focused Talk that I gave to the Lexington Creative Camera Club January 16, 2014.

I published a research paper on this at Electronic Imaging 2014, Out-of-focus point spread functions ( slides, paper ), and that's when the OOF PSF name started to stick. I've personally measured OOF PSF for over 200 lenses now, and slowly keep going. There are still various things being done with this. For example, earlier this year, at EI 2020, I published a paper, Camera support for use of unchipped manual lenses ( website ), that briefly discussed using OOF PSF to identify unchipped manual lenses.

In any case, I am still doing some things with this and, more importantly, the concept of measuring OOF PSF is becoming widely accepted as a useful metric. :-)