Extreme Macro, in search of Structural Colours

[Andy Perrin]

Oh, but you could flock with a paint like Black 3.0 or something. Not like felt or velvet, which would definitely have those problems. I bought a square foot of aktar metal velvet (which is not velvet, it’s adhesive-backed paper) years ago and I usually use that. It’s like $100US for a square foot but since it’s lasted five years it wasn’t really that expensive. My needs were modest! Black 3.0 is more economical though. 

[colinbm]

Thanks Ulf & Andy
I don't have immediate access to flock to test, but I do have some black cardboard that I have used before & is OK for UV too.
Added two more baffles & flare has gone.
With 430mm of M42 tubes I am getting a clear picture of 1.5mm of my ruler on the 36mm sensor.
I need to take some photos & stacks of this beetles shell.

[nfoto]

You are way deep into vibration territory .... which I'm sure you have become more than aware of.

 

The little Canon 20/3.5 is pretty good if you can get the flare propensity in the setup under control. I'm only using it wide-open to alleviate the worst of diffraction side effects,

[colinbm]

@nfoto Yes it is vibrating at the slightest touch.
Flare is under control with three baffles.

[ulf]

When I did these ( scroll down to my first post in this thread)

I used a 10kg X-Y milling stage as a camera support for the stacking movement and had another lighter motif X-Y stage well coupled to the extension tubes with some flexible plastic foam. to keep the whole setup vibration-wise dampened. I had still to rely on flash illumination for the most demanding magnifications, to freeze motion blur.

 

Finding the surface of your bug must be easy.

It was more difficult for me the find the root of the hair with very high magnification and shallow DOF with a working distance of several mm.

 

In your setup everything is separated. 

[colinbm]

@ulf Very nice, something to strive for.

[Lou Jost]

Hi all, I am new to this forum but I have done a lot of extreme macrophotography. This is a fascinating topic, and a rich one. Here are a few thoughts on what has been said on this thread so far.

 

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"The best optical quality for magnifications up to say 30-50X would be with metallurgical objectives which are designed for "long" working distances. "

 

Optical quality of objectives is determined largely by its NA (at least for name brand objectives with good correction). Metallurgical objectives, or any other long-working-distance objectives, generally have lower NA than objectives of the same magnification with short working distances. For a given NA, the front element of the objective must be much bigger when the working distance is longer. So in fact objectives with the best optical quality tend to have very short working distances.

 

The refractive index of air sets an upper limit to the NA (and hence the resolution) of any objective. This limit can be circumvented by filling the space between the objective and the subject with a liquid, usually water, glycerine, or oil, often in conjunction with a cover slip. So the very highest optical quality objectives are these "immersion" objectives, which are more or less the opposite of metallurgical objectives. Still, metallurgical objectives are extremely convenient.

 

Incidentally immersion oil sometimes comes in low-fluorescence formulations, and we should choose those for UV work.

 

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"Unfortunately most if not all of these, such as the Mitutoyo line_up, are of the infinity optical design meaning they require a tube lens of around 200mm focal length in order to produce an image on the sensor. Tube lenses are rarely suitable for UV, sole exception is the Mitutoyo ML-4 which is very expensive. .  There is also the need for special versions of the metallurgical objectives suitable for UV, which drives up the cost even further."

 

There are a couple of misconceptions here. First, most manufacturers (including Mitutoyo and Nikon) make a line of "measuring objectives" which do not require a tube lens. These are ideal for scientific work where measurements need to be made from the image, because they are telecentric ("perspectiveless").

Second, those objectives that do require tube lenses do NOT require a particular focal length. The focal length associated with a family of objectives (200mm in the case of Mitutoyo or Nikon infinity-corrected objectives) is the "reference focal length" which is used to rate the magnification of the objective. So a 20x Mitutoyo infinity-corrected objective will achieve that nominal 20x magnification only when a 200mm tube lens is used. If you use a 100mm tube lens, that objective will work just as well, but it will project a smaller image circle and the magnification will be 10X. A 300mm tube lens will project a larger image circle and give a magnification of30x. The magnification is given by [nominal magnification]* [actual tube lens focal length]/[nominal tube lens focal length]. There is no optical disadvantage to a tube lens larger or smaller than the nominal one.

 

As a corollary, the focal length of the tube lens should change when you change sensor format. If you are happy with a 200mm tube lens focal length on a FF sensor, then you should use a 100mm tube lens focal length on an MFT sensor. You will get exactly the same photo (apart from aspect ratio differences between formats) from these two setups.

 

For a given sensor size, it is common for extreme macro photographers to try to "push down" the magnification of an objective by using a shorter-than-nominal tube lens focal length. As long as the objective image circle is big enough, this increases the resolution of the image (in terms of lines per sensor millimeter) and lowers the effective aperture.

 

This observation means that the number of tube lens choices is vastly larger than one might have thought. Some of these choices could have good UV transmission. If your sensor is small enough and/or  your objective has a big enough image circle, a UV Nikkor could be a great tube lens. So might the old Apo-Nikkor reproduction lenses and the longer EL-Nikkors,

 

It is worth noting that tube lens performance for photography is very idiosyncratic and hard to predict. Coverage, corner sharpness, and chromatic aberration control have to be checked case-by-case. The best sources for this information are www.photomacrography.com and www.closeuphotography.com

 

Regarding the mentioned extra cost of UV-capable microscope objectives, this is not necessarily true. The most expensive objectives are apochromatically corrected, and these very complex optical formulas usually have poor UV transmission. The next-lower cost level is the "Fluor" objective, and these often have (as the name implies) fluorite lenses with good UV transmission. Lots of these objectives are designed for through-the-objective ("epi") UV illumination of a subject, so that its fluorescence can be observed. These objectives are specifically designed to transmit high levels of UV light. (Transmission curves are often available on the manufacturer's or vendor's websites.) These would be excellent choices for reflected-UV photography using light from an external source. With very special care, even UV epi-lighting might be usable for UV reflected photography, but this would require cross-polarizers that worked in the UV, to eliminate flare in the objective due to the illumination light going through it.

 

Quote

Be aware that microscope objectives not of the infinity type always assume the final optical corrections are performed by the ocular eyepiece. Thus using them directly on a bellows will often lead to inferior performance. These objectives have a very short working distance.. They also assume a cover glass is used otherwise spherical aberration is introduced. 

 

Ulf has corrected some of this above. Finite Nikon CF objectives need no eyepiece correction. Also, there are infinity-corrected objectives that DO require eyepiece correction. I think older Olympus infinity objectives (pre- UIS and UIS-2) expect eyepiece correction,

 

In addition, to my knowledge there is no fundamental connection between short working distances and finite objectives. There are low-power finite objectives with respectable working distances. 

 

Quote

As to the reason why bellows and tubes 'run out of light quickly' is that the effective aperture f_eff is a function of image magnification. The simplified equation f_eff=f*(1+M), where f=aperture number and M=magnification, is often used. Thus, at 5X (M=5), a nominal f/2.8 for infinity focus becomes f/17

That formula applies to magnification achieved by extending a lens. When using infinity-corrected objectives on tube lenses, a different formula applies: EA=f*(m). These two formulas are almost equal when m is high, but when m is low the infinity set-up has significantly lower EA (and hence higher resolution) than the corresponding set-up based just on extension.

 

 

[colinbm]

Thanks Lou for this explanation.

It will take me some time to digest all this.

[Lou Jost]

Colin, if some of it is unclear, please don't hesitate to say something, so I can try to refine or add to what I wrote.

[colinbm]

Thanks Lou
I am now 70 years old & sometimes the brain hurts with new tech input.
I come from a trades background, not from a scientific background, but I do like pushing boundary's.
I am very DIY.

 

[ulf]

Thanks Lou for all the comprehensive information above.

My knowledge in the matter is a bit dated and might have been scrambled over time.

 

I did my thing in that area 12 years ago when out of job and with limited funds.

I never considered any UV-work then.

It is interesting to know that "Fluor" objectives might be good for that.

 

I look forward to be enlightened further in this field in the future.

 

BTW warm welcome as a member of the forum

[nfoto]

I'll attest to the description 'idiosyncratic'. Thus, the mentioned UV-Nikkor does not work well as a tube lens in the combinations I have tried so far, as there are loads of spherical aberration and other nasties present. The MT-L4 tube lens on the other hand is perfect.

 

Warmly welcome to the UVP forum, by the way.