Reconciling the Thick Lens Model with P2P Optical Bench

Started 8 months ago | Discussions
Bernard Delley Senior Member • Posts: 1,860
Re: just go with the thick lens model

bclaff wrote:

Bernard Delley wrote:

...

Below is a comparison of P2P data with my measurement using 0 and 27.5 mm extension ring.

I see two main differences compared to the P2P simulation based on patent data. The camera with this lens reports a less fast lens at 1:1. The measured internodal distance is quite a bit smaller. However it is plausible by guessing from the eyeballed pupil positions seen from front and back.

That example is the Nikon AF-S Micro-Nikkor 60mm f/2.8G ED

You stumbled onto a patent where I did not enter all of the pertinent information.
I have now done so and updated the files.
The new files will show a Specified value under NA for 1:2 and 1:1

F# values reported by the Optical Bench are now:

Infinity 2.88 (Scenario 1)
1:4 3.15 (using Focus slider)
1:2 3.55 (Scenario 2)
1:1 4.98 (Scenario 3)

These values are in agreement with the NA values stated in the patent.

Note patent species NA of 0.10 at 1:1 and the Optical Bench now agrees

Thanks a lot for fixing this !

This is indeed the correct full name of the lens that I measured.

I still read  H = 22.17  H'=-37.75 in your example. I  interpret this as  i = h = 59.92mm . Am I misunderstanding   H,H', or is there a remaining discrepancy between measured and simulated h ?

 Bernard Delley's gear list:Bernard Delley's gear list
Olympus TG-6 Nikon D7200 Nikon D500 Nikon D850 Nikon AF-S Nikkor 14-24mm f/2.8G ED +11 more
OP Garry2306 Regular Member • Posts: 132
Re: just go with the thick lens model

Bernard Delley wrote:

bclaff wrote:

Bernard Delley wrote:

...

Below is a comparison of P2P data with my measurement using 0 and 27.5 mm extension ring.

I see two main differences compared to the P2P simulation based on patent data. The camera with this lens reports a less fast lens at 1:1. The measured internodal distance is quite a bit smaller. However it is plausible by guessing from the eyeballed pupil positions seen from front and back.

That example is the Nikon AF-S Micro-Nikkor 60mm f/2.8G ED

You stumbled onto a patent where I did not enter all of the pertinent information.
I have now done so and updated the files.
The new files will show a Specified value under NA for 1:2 and 1:1

F# values reported by the Optical Bench are now:

Infinity 2.88 (Scenario 1)
1:4 3.15 (using Focus slider)
1:2 3.55 (Scenario 2)
1:1 4.98 (Scenario 3)

These values are in agreement with the NA values stated in the patent.

Note patent species NA of 0.10 at 1:1 and the Optical Bench now agrees

Thanks a lot for fixing this !

This is indeed the correct full name of the lens that I measured.

I still read H = 22.17 H'=-37.75 in your example. I interpret this as i = h = 59.92mm . Am I misunderstanding H,H', or is there a remaining discrepancy between measured and simulated h ?

Bernard I got confused by looking at the measured and position lines.

I believe h is 48.96-22.17=26.79

It’s those pesky optical sign conventions that confuse things on the measured line.

It’s always best to use the positions data, but Bill will confirm, I’m sure.

bclaff Forum Pro • Posts: 12,940
Re: just go with the thick lens model

Garry2306 wrote:

Bernard Delley wrote:

bclaff wrote:

...

These values are in agreement with the NA values stated in the patent.

Note patent species NA of 0.10 at 1:1 and the Optical Bench now agrees

Thanks a lot for fixing this !

This is indeed the correct full name of the lens that I measured.

I still read H = 22.17 H'=-37.75 in your example. I interpret this as i = h = 59.92mm . Am I misunderstanding H,H', or is there a remaining discrepancy between measured and simulated h ?

Bernard I got confused by looking at the measured and position lines.

I believe h is 48.96-22.17=26.79

It’s those pesky optical sign conventions that confuse things on the measured line.

It’s always best to use the positions data, but Bill will confirm, I’m sure.

Correct, the values in the Positions line are all relative to the first vertex of the lens.

While P' and H' in the Measured line are, by convention, relative to the last vertex of the lens.

So one must use the Position values to calculate HH'

-- hide signature --

Bill ( Your trusted source for independent sensor data at PhotonsToPhotos )

Bernard Delley Senior Member • Posts: 1,860
DoF estimates

Garry2306 wrote:

Garry2306 wrote:

Bernard Delley wrote:

Garry2306 wrote:

I’m keen to try this approach and wondered if you could share the process steps that you undertook. Reading the various posts here has confused me a bit as to how one uses extension rings to work out the inter nodal etc of a lens at, say, the minimum focus distance and ‘infinity’.

You gave the the correct thick lens equation in your opening post. I am slightly elaborating on it. I somehow prefer to use the symbol h instead of your t for the internodal distance. When you turn the focus ring of the lens, you modify the focal length, so f is a function of the 'distance' setting on the lens: f(s) . Also the internodal distance is a function of this setting in general: h(s)

Rewriting, the (your) thick lens equation:

d_i = (2 + 1/m_i + m_i) * f(s) + h(s)

I have also introduced a subscript _i to index the measurements that you do with the same (!) setting of the lens. So each lens measurement yields a pair (triplet) of values:

d_i a sensor - object distance

m_i magnification, derived from the portion of the ruler visible in the image or in LV.

x_i extension ring thickness used, as additional parameter for Method_1

interestingly, the extension ring thickness does not appear in the equation for d_i. But, for each different extension you get another d_i , m_i pair of values. If you have two such measurements, defining 2 equations, you can solve them in Method_1 by realizing the proportionality of m_i with the total extension (ring + internal) of the lens, ex_tot=x_i+e(s):

x_i + e(s) = (1 + m_i)*f(s)

eliminating the unknown internal focus extension e(s) depending on setting s, you find f(s) from

x_1 - x_2 = ( m_1 - m_2 ) *f(s)

In closeup settings of the lens, there may be no extension ring in measurement 2, then x_2=0.

Once you have f(s) , you find h(s) as you already wrote.

If you have several measurements using different extension, then you can find an average f(s) from the different pairs of measurements that can be formed. And you can determine the RMS deviation of each f(s) evaluation from the mean. I would argue the the RMS uncertainty for the mean of f(s) is smaller by a factor sqrt(n-1) than the RMS for the single evaluation. n is the number of measurements. No RMS single can be determined with just two measurements.

I will not go into details of Method_2, which considers a linear regression for d_i and v_i = (2 + 1/m_i + m_i) .

d_i = v_i *f(s) + h(s)

Bernard

Many thanks: very clear.

Now to find some time.

Cheers

Garry

Bernard

I’ve been thinking, which isn’t alway good

My use case is to code in my CHDK Lua script a ‘better’ lens model. Currently I assume a fixed lens thickness, ie that at MFD. This is a conservative view that introduces positive focus lap insurance when focus bracketing, ie the DoF will likely be reduced because of the fixed hiatus. Based on assuming hiatus at MFD is greater than that at infinity.

If I work out the internodal distance, hiatus, using the method you described, it will be right using the focal length you calculate from knowing the two extensions and magnification.
However, in CHDK and Magic Lantern Lua, the focal length reported is ‘just’ the infinity focal length, ie it doesn’t vary with focus.

I’m therefore thinking that knowing the focal length and thus hiatus at two actual extremes, eg at MFD and, say, around the hyperfocal, won’t be that useful, if the camera only reports a fixed focal length, ie focal length reporting doesn’t vary with focus.
I would welcome an6 thoughts you may have, or anyone else.
Cheers

Garry

The function f(s) shows a lot of variation. For example the 70-300mm zoom shown in my first post goes down from 300mm to 174mm at MFD. So one would have to parametrize that function f(s) properly, gleaning from information that the camera reports.

As I showed in a thread with illustrations gone now estimating DoF becomes more simple when calculating it from the magnification ratio, rather than from the distance. It turns out that the focal length matters little once you settled for a magnification at the focal plane and thus for a field of view at that plane. The separation of the H H'  planes, ~ the internodal, does not matter at all in this approach. There is a universal transition into the hyperfocal regime when you go top sufficiently short focal lengths.

see text under gallery image for details

However DoF from geometrical optics in paraxial approximation is not the whole story. Diffraction and aberrations of the real lens contribute to overall un-sharpness. In practice I take memorized or quicky calculated settings from the DoF model as guidance and may shoot two or three different apertures, to pick later for best effect.

guidance comes from simple formulas:  magnification m = 1 / r , r reduction factor

max Focal length for hyperfocal : Fh = c * f# *r

where f# is always the effective aperture number

closeup limit DoF to either side : dg0 = Fh * r

blur circle for (highlight) point a infinity referred to sensor: CoCi =  f * m / f#  , this one strongly depends on the focal length.

The DoF considerations show that the precise value of the focal length is  not so important in many practical contexts.

 Bernard Delley's gear list:Bernard Delley's gear list
Olympus TG-6 Nikon D7200 Nikon D500 Nikon D850 Nikon AF-S Nikkor 14-24mm f/2.8G ED +11 more
Bernard Delley Senior Member • Posts: 1,860
Re: just go with the thick lens model

bclaff wrote:

Garry2306 wrote:

Bernard Delley wrote:

bclaff wrote:

...

These values are in agreement with the NA values stated in the patent.

Note patent species NA of 0.10 at 1:1 and the Optical Bench now agrees

Thanks a lot for fixing this !

This is indeed the correct full name of the lens that I measured.

I still read H = 22.17 H'=-37.75 in your example. I interpret this as i = h = 59.92mm . Am I misunderstanding H,H', or is there a remaining discrepancy between measured and simulated h ?

Bernard I got confused by looking at the measured and position lines.

I believe h is 48.96-22.17=26.79

It’s those pesky optical sign conventions that confuse things on the measured line.

It’s always best to use the positions data, but Bill will confirm, I’m sure.

Correct, the values in the Positions line are all relative to the first vertex of the lens.

While P' and H' in the Measured line are, by convention, relative to the last vertex of the lens.

So one must use the Position values to calculate HH'

Thanks a lot for the clarification . So there is no significant difference left between your kindly provided simulations and my measurement for this lens.

 Bernard Delley's gear list:Bernard Delley's gear list
Olympus TG-6 Nikon D7200 Nikon D500 Nikon D850 Nikon AF-S Nikkor 14-24mm f/2.8G ED +11 more
bclaff Forum Pro • Posts: 12,940
Re: DoF estimates

Bernard Delley wrote:

...

The function f(s) shows a lot of variation. For example the 70-300mm zoom shown in my first post goes down from 300mm to 174mm at MFD.

I assume specifically the Nikon AF-S Nikkor 70-300mm f/4.5-5.6G VR in your gear list?

...

As I showed in a thread with illustrations gone now estimating DoF becomes more simple when calculating it from the magnification ratio, rather than from the distance. ...

Totally agree. Magnification is how I derive the equations and use DOF in the field.
It's not only focal length independent but I find estimating magnification much easier than estimating distance.

Regards

-- hide signature --

Bill ( Your trusted source for independent sensor data at PhotonsToPhotos )

bclaff Forum Pro • Posts: 12,940
Re: just go with the thick lens model

Bernard Delley wrote:

bclaff wrote:

Garry2306 wrote:

Bernard Delley wrote:

bclaff wrote:

...

These values are in agreement with the NA values stated in the patent.

Note patent species NA of 0.10 at 1:1 and the Optical Bench now agrees

Thanks a lot for fixing this !

This is indeed the correct full name of the lens that I measured.

I still read H = 22.17 H'=-37.75 in your example. I interpret this as i = h = 59.92mm . Am I misunderstanding H,H', or is there a remaining discrepancy between measured and simulated h ?

Bernard I got confused by looking at the measured and position lines.

I believe h is 48.96-22.17=26.79

It’s those pesky optical sign conventions that confuse things on the measured line.

It’s always best to use the positions data, but Bill will confirm, I’m sure.

Correct, the values in the Positions line are all relative to the first vertex of the lens.

While P' and H' in the Measured line are, by convention, relative to the last vertex of the lens.

So one must use the Position values to calculate HH'

Thanks a lot for the clarification . So there is no significant difference left between your kindly provided simulations and my measurement for this lens.

The Optical Bench values (using the Focus slider) are

0:1 58.01mm 24.01mm f/2.88
1:4.05 50.98mm 24.45mm f/3.15
1:2 45.34mm 25.41mm f/3.54
1:1 37.60mm 26.79mm f/4.98

So your 30mm measurement is certainly closer to 24.45mm than to your wrongly calculated 62mm.

-- hide signature --

Bill ( Your trusted source for independent sensor data at PhotonsToPhotos )

OP Garry2306 Regular Member • Posts: 132
Re: DoF estimates

bclaff wrote:

Bernard Delley wrote:

...

The function f(s) shows a lot of variation. For example the 70-300mm zoom shown in my first post goes down from 300mm to 174mm at MFD.

I assume specifically the Nikon AF-S Nikkor 70-300mm f/4.5-5.6G VR in your gear list?

...

As I showed in a thread with illustrations gone now estimating DoF becomes more simple when calculating it from the magnification ratio, rather than from the distance. ...

Totally agree. Magnification is how I derive the equations and use DOF in the field.
It's not only focal length independent but I find estimating magnification much easier than estimating distance.

Regards

Bill/Bernard

I understand what you both are saying, however, my use case is rather unique.

That is I’m using the Canon EXIF reported sensor to object distance, in fact the lower value as I wish to ensure a positive DoF overlap when focus bracketing with my in camera Lua script.

My most stressful use case is when I focus bracket from the MFD of the lens out to a multiple of the hyperfocal, ie an implied infinity defocus blur.

Life is further complicated as Canon reports a fixed focal length value with focus.

It’s easy to ensure that my split/thick lens model is correct at the MFD, as I can work out the hiatus at that location, ie from a measure magnification. Thus like a stopped watch, that is precisely correct twice a day, I know my DoF calculations are ‘right’ at the MFD, plus I can even throw in a pupil mag correction term at this position.

It all goes ‘wrong’ once I start focusing away from the MFD. That is I have no chance or desire to functionalise focal length, hiatus and pupil mag.

But I’m saved by the following logic.

I’m not that interested in the DoF at a single location, as I need to ensure I don’t have a focus gap as I focus bracket, and believe this is best achieved by choosing a suitable defocus (overlap) blur criterion, using the Canon lower value for focus distance and assuming the hiatus at the MFD is fixed through the focus range. Plus noting my focus bracketing algorithm is not fit for macro use, and best for WA to normal lenses, ie not long focal tele lenses.

My approach may/will result in a few extra focus brackets, but it has the value of not creating focus gaps. That is I will predict less DoF than if I knew the lens internals at a particular focus, eg focal length, hiatus and pupil mag etc

Bottom line: I think I will give up trying to refine my lens model, and I’ll stick with the logic above, which seems to work robustly. I would like to say, however, both of you have contributed to my education, and for that I thank you both.

Cheers

Garry

Bernard Delley Senior Member • Posts: 1,860
Re: DoF estimates

bclaff wrote:

Bernard Delley wrote:

...

The function f(s) shows a lot of variation. For example the 70-300mm zoom shown in my first post goes down from 300mm to 174mm at MFD.

I assume specifically the Nikon AF-S Nikkor 70-300mm f/4.5-5.6G VR in your gear list?

I can only see the AF-S 80-400mm f/4.5-5.6 G in my gear list. I have at least one image with the AF-S VR Zoom-Nikkor 70-300mm f/4.5-5.6G IF-ED in my gallery, but it is now with my daughter in law. The recently measured lens, also pictured in this thread is the AF-P 70-300mm f/4.5-5.6E ED VR.  - Sorry to say that it does not load in your optical bench simulator, when I try with my old iMac OS 10.11.6 .  On another zoom the AF-S 70-200mm f/28 G VRII I did not see how to focus to MFD (at the long end). -- It must be an admirable amount of work setting up and maintaining such a site. Bravo !

...

As I showed in a thread with illustrations gone now estimating DoF becomes more simple when calculating it from the magnification ratio, rather than from the distance. ...

Totally agree. Magnification is how I derive the equations and use DOF in the field.
It's not only focal length independent but I find estimating magnification much easier than estimating distance.

Regards

 Bernard Delley's gear list:Bernard Delley's gear list
Olympus TG-6 Nikon D7200 Nikon D500 Nikon D850 Nikon AF-S Nikkor 14-24mm f/2.8G ED +11 more
bclaff Forum Pro • Posts: 12,940
Gear

Bernard Delley wrote:

bclaff wrote:

Bernard Delley wrote:

...

The function f(s) shows a lot of variation. For example the 70-300mm zoom shown in my first post goes down from 300mm to 174mm at MFD.

I assume specifically the Nikon AF-S Nikkor 70-300mm f/4.5-5.6G VR in your gear list?

I can only see the AF-S 80-400mm f/4.5-5.6 G in my gear list. I have at least one image with the AF-S VR Zoom-Nikkor 70-300mm f/4.5-5.6G IF-ED in my gallery, but it is now with my daughter in law.

Strange. I must have accidentally looked at someone else's gear list !

The recently measured lens, also pictured in this thread is the AF-P 70-300mm f/4.5-5.6E ED VR. - Sorry to say that it does not load in your optical bench simulator, when I try with my old iMac OS 10.11.6 .

So the link Nikon AF-P Nikkor 70-300mm f/4.5-5.6E ED VR does not work for you?
Strange as it works for me. Perhaps we can sleuth this off-line.

On another zoom the AF-S 70-200mm f/28 G VRII I did not see how to focus to MFD (at the long end).

The patent ( US 8,416,506 ) for the Nikon AF-S Nikkor 70-200mm f/2.8G ED VR II doesn't contain any numerical focusing information.

The patent figure does indicate which group moves (G3) and in what direction.

I have considered supplementing such patents with implied focus information but haven't tackled that project yet.

-- hide signature --

It must be an admirable amount of work setting up and maintaining such a site. Bravo !

Thanks. It is non-trivial but I've established a pretty efficient workflow that works with most patents.

-- hide signature --

Bill ( Your trusted source for independent sensor data at PhotonsToPhotos )

OP Garry2306 Regular Member • Posts: 132
Re: DoF estimates

@Bill

I’ve come to a generalised conclusion that I would welcome your views on.

Using your Optical Bench Hub I’ve explored various lenses that allow focus exploration.

My general conclusion is that, the position of the front principal, relative, to the sensor, will always be closer to the sensor as you focus away from the MFD, towards infinity.

Assuming I’m right, this supports my focus bracketing logic to use a fix hiatus (i) throughout the focus range, calculated at the MFD, where we at least have manufacturers data as well as a measurement we can make.

Adopting this approach we ensure a positive focus overlap, as knowing the sensor to object distance, we can position H at (1+m)f+i, because the distance we then use in the DoF calculations will always be less than the actual distance from H to the object.

BTW I can’t find a general rule for pupil mag, ie knowing the pupil mag at the MFD can we say something general about the way the pupil mag changes, higher or lower, as we focus towards infinity, ie for a retro and tele lens.

Cheers

Garry

bclaff Forum Pro • Posts: 12,940
Re: DoF estimates

Garry2306 wrote:

@Bill

...

My general conclusion is that, the position of the front principal, relative, to the sensor, will always be closer to the sensor as you focus away from the MFD, towards infinity.

I haven't done this study but that conclusion doesn't surprise me.

...

BTW I can’t find a general rule for pupil mag, ie knowing the pupil mag at the MFD can we say something general about the way the pupil mag changes, higher or lower, as we focus towards infinity, ie for a retro and tele lens.

Pupil magnification can vary greatly based on lens design.

It's very rough to say that unity is more typical between 30 and 60 degrees angle of view.
Below 30 degrees will be over unity and over 60 degrees is likely to be below unity.
But the variation is so great that this generalization is of no practical value.

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Bill ( Your trusted source for independent sensor data at PhotonsToPhotos )

OP Garry2306 Regular Member • Posts: 132
Re: DoF estimates

Bill thanks.

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