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UltravioletPhotography

Focus shift as a function of wavelength - advice needed


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I could do with some advice from the collective brain power of the site please.

 

With all the lens transmission measurement stuff myself and others have been doing, the question came from Andrea about other aspects of lens behaviour and how they influence UV imaging. It got me thinking about trying to measure how focus point varies with wavelength.

 

Initially I am thinking something simple - ruler at 45 degrees, and image it with different filters and see how the focus point changes on the ruler scale.

 

What I'm not sure of is this - how to standardise the distance between the ruler and the camera? If I want to compare the behaviour of lenses of different focal length, does the distance from the camera to the ruler need to change to reflect the focal length difference?

 

Any advice welcome.

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Jonathan, excellent question. I certainly don't know the answer. So we start by experimenting and reporting results.

 

I would suggest trying 3 (obvious) distances first: infinity, very close, and some middling distance. The ruler set-up would really only work for near distances. But you could put up some kind of a target for middling distances. A middling distance would be what - about 10 feet? Not sure where I would set that. For infinity, perhaps focusing on some distant building would work. A distant landscape feature as a infinity focus point might be too variable if conditions are windy.

 

FWIW, you can and will get some minor wavelength based focus shift even with corrected lenses. Correction is not perfect. I've seen this with the UAT.

 

I don't think "equivalence" would play a particular role in wavelength based focus shift except at perhaps the extremes, so I wouldn't initially worry about matching up angle of view (or distance from the camera) for different focal lengths until you have some preliminary results. For example, at infinity -- if infinity is really far far away (la!) -- UV wavelengths scatter a lot and distant horizons become very hazy looking while IR gets through to produce more detail. It becomes difficult to even determine whether you have focused in UV. I have some photos made from atop a (smallish) mountain which show this problem quite well.

 

One thing I don't know how to account for is the filter thickness. Suppose you had a well-corrected lens like the CO60/4.5. How would you compare its correction under a 2mm thick UV-pass filter versus a 1mm thick IR-pass filter, for example? So there's another good question. But as mentioned, all you can do is start experimenting and see where it goes.

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P.S. Remember that your targets need to be UV-visible and IR-visible. Not always a given with commercial targets designed for visible tests.
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I think a custom designed ruler-pattern at 45° is a good idea.

That is easily printed on a laser printer and the paper can be taped to a flat surface.

It is a good idea to make the spacing of the lines into a suitable distance to get nice numbers when viewed at 45°.

At even distances some of the lines can be made thicker to simplify identification.

If the pattern is printed reasonably wide, the field curvature can also be detected.

 

I think that focus shift can vary with different focus distances, at least for some lenses.

To better pinpoint the test wavelength I would / (will eventually) illuminate the test pattern with power LEDs of different suitable wavelengths.

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Ok, I've seen the commercial ones, but I have a ruler to hand and the ability to make a 45 degree stand (and I begrudge spending £60 on a 45 degree angle ruler).

 

The filter question I hadn't thought about, but as the filter would be needed in photography at those wavelengths, I would initially assume that 'relevant filter + lens' is what I'd be examining.

 

I too have seen focus shift with the UAT and even the UV Nikkor when up close, especially when going deeper in the UV.

 

I suppose a set of visible 'red', 'green' and 'blue' band pass filters could be useful here too - break the visible region down a bit.

 

As for different distances, I can imaging needing larger and larger ruled scales. I might just chose a close distance (say 1m) initially. Most wide and macro lenses will be fine with that, and I can use extension tubes or helicoids if needed for others.

 

I like the idea of a tunable light source, like I use for my camera sensor sensitivity calibration, but it would need a huge amount more light to illuminate a ruler, and I'd need to do everything in pitch black.

 

At the moment I will stick to focus shift. Other lens parameters can come later.

 

Thanks all, plenty to think about.

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enricosavazzi

Answering from a mobile phone, so no long texts and fancy formulas.

The answer to comparing lenses of differdnt focal lengths is simple if what you want to measure is DOF (depth of field) or something related to DOF.

 

First of all, DOF does not depend on FL. Period.

 

DOF depends on magnification on sensor and aperture. So compare at the same magnification and aperture. A convenient magnification for closeup lenses is 0.1x. For macro lenses, 1x is convenient.

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Enrico is correct. At 1:1 magnification the minimum focus distance compensate for the different lens focal lengths.

Thus at 1x magnification a 60mm will have the same depth of field as a 30mm. The difference in the image is just the compression of the background and the field in view.

 

However, Andrea brings up the critical point that will drive you crazy at 1:1 and that is filter thickness and placement. Moving from 5mm thick to 2mm to 1mm filter will affect significantly the focus point.

 

I wasvusing the UAT and am more front focus at 313nm, due to filter thickness, then back a bit at 365nm, then front again in Visible and IR. And trying to align things in higher more live view friendly wavelengths and then going down to deep UV without considering the filter thickness will drive you crazy.

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Thinking this through, I will have to measure the thickness of my B&W 486 filter but maybe possible to standardize a test.

At 4mm:

UV (340 peak) 2mm U340 with 2mm S8612

UV (360 peak) 2mm UG1 with 2mm S8612

Visible 4mm BG38

Ir lee 87 or 87C film between 2 2mm glass filters.

I have these filters

 

At 1mm

UV: Baader venus U filter

Visible: 486 filter if 1mm, need to measure mine.

Ir lee87 film over 1mm glass.

 

I need to measure the true thickness of my filters. Now that my digital caliper arrived with 0.01 accuracy, this should be fun.

 

All using two UVb Exxo terra light bulbs should work as these bulbs output UV, visible and IR

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Ok, I've seen the commercial ones, but I have a ruler to hand and the ability to make a 45 degree stand (and I begrudge spending £60 on a 45 degree angle ruler).

 

I like the idea of a tunable light source, like I use for my camera sensor sensitivity calibration, but it would need a huge amount more light to illuminate a ruler, and I'd need to do everything in pitch black.

 

 

Not in pitch black, only reasonably darker than the illumination. A low working light environment will be enough.

If you are not looking for vignetting at the same time the illumination on the surface can be a bit uneven.

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If using LEDs I would aim at these wavelengths or colours:

 

UV 365-370nm (Convoy?)

Purple 405nm power LED

Green ?nm power LED

Red 660nm Power LED

IR 840nm Power LED

IR 950nm Power LED

 

Something like this might work well and can be powered directly with around 10-12V, or preferably a constant current driver:

https://www.ebay.com...872.m2749.l2649

 

The IR variants is missing, but the heatsink/fan and LEDs can be bought and easily assembled.

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enricosavazzi

I thought that my earlier post failed to go through because my mobile phone went crazy after I hit the Post button, but now I can see my post.

 

I have no good suggestion about compensating for filter thickness, other than the obvious: use no filter on the lens, and use reasonably monochromatic light sources like LEDs to illuminate the test pattern.

 

My statement on the irrelevance of focal length needs a couple of qualifications. One is that effective aperture (not nominal aperture) controls DOF. Effective aperture is a function of nominal aperture, magnification, and pupil ratio, which is a constant of the lens (or a variable for some lenses like zooms, internal focusing lenses and lenses with floating groups). Pupil ratio may change the effective aperture by up to a stop or so, so it should not be ignored.

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