Good M42-based iris

[ulf]

I have found a good adjustable aperture mounted in a housing with M42-threads, male and female in the ends.

https://www.ebay.com/itm/324294448250

The mechanical quality is good and the price is quite reasonable.

It can easily be mated, in both ends, with suitable helicoids, extension tubes and step rings.

 

I have successfully used this type of aperture, for different setups while measuring with my spectrometer and also when experimenting with fused silica lenses.

 

The maximum aperture is ca 25mm, way too much for single-lens experiments as those normally needs to be stopped down considerably.

With ca 1mm minimum aperture the iris still works quite well for such usage, even if the practical adjustment range is just a part of the full range.

 

The adjustment is click-less you can set the opening freely.

For close up usage the typical aperture opening is f/8 and smaller for sufficient FOV.

 

When I want a known opening diameter I often use a reference part to set the opening diameter.

Any round object can be used to very carefully close the iris around, to note the adjustment ring's position.

The shaft of a drill bit has normally a well defined diameter.

[colinbm]

I can understand using a drill shank to measure the opening, is there a way to guess the aperture f # or is it too complex ?

[ulf]

If you know the focal length of the lens it is very simple.

 

Here is some examples:

Assume you have a 80mm lens.

If the opening is 25mm the f # would be 3.2 ( 80/25=3.2 )

If you want 1/f to be 11 you need to set the aperture to ca 7mm ( 80/11=7.2727 )

 

1/f => aperture diameters for a 80mm lens:

3.2 => 25 mm

4.0 => 20 mm

5.6 => 14 mm

8.0 => 10 mm

11 => 7.2 mm

 

16 => 5 mm

22 => 3.6 mm

 

A PCX-lens needs to be stopped down to at least 1/f = 8 for reasonable image quality.

If you are seeking dreamy artistic effects with low contrast and lots of spherical aberration, then wide open is the way to go.

 

There is very little transmission losses with an uncoated fused silica lens.

it is well above 90% for most of VIS and UV:

https://www.thorlabs...mission_780.gif

[colinbm]

Too easy, thanks Ulf

[dabateman]

Yes I have too many of these now as I have made a bunch of fused silica lenses. You can get M52 to M42, M42 to M42 and M42 to M52 aperture control rings. I have many of the M42/M42 ones and at least one of the others. They also come in larger openings options. So my 39mm fused silica element is with a m43rds to M42 adapter, then M42 to M52 aperture control ring, then the 39mm element mounted in a M52 ring. The opening allows for at least F1 for crazy thin field and down to at least f16 if I remember correctly. With this I can get infinity focus now.

 

You can also get 10mm to 15mm helicoids, which help for some elements.

 

[Stefano]

One day I will try to build a lens like this. Looks fun.

[Andrea B.]

me too, Stefano !!!

 

********

Ulf, thank you for making this topic. It is something really good to know about.

 

I don't think I knew that an aperture/iris thingie like this could be bought as a separate thingie. But then I have not ever looked either.

[colinbm]

Does it matter how far the aperture is behind the lens please ?

[ulf]

Does it matter how far the aperture is behind the lens please ?

Yes, if it is getting to far behind it starts to become visual but it is not critical as long as the iris is reasonably close to the lens.

In a normal reasonably symmetric camera lens the iris is, as you know, placed rather central.

By moving the iris away from that position can create interesting effects.

 

Also compare central shutters with film plane shutters.

[colinbm]

Thanks Ulf

[dabateman]

Colin if you have a really large front element and adjust the aperture distance from it, than you can create a hypercentric lens like this.

 

https://petapixel.com/2020/05/01/hypercentric-optics-this-camera-lens-can-see-behind-objects/

[colinbm]

Thanks Dave, I have found a 70mm quartz lens & thinking about setting it up later.

Not sure of the focal length yet, maybe 50-10mm I hope ?

[ulf]

Hypercetricity is fun, but telecentricity might have been even more useful for us.

At around 12.30 in the movie the guy is explaining a technique I used a bit for about 15 years ago for VIS Macro.

 

By placing an aperture at exactly the image plane with the lens fixated at infinity and then add more extension you get a macro lens with telecentricity.

That can be useful in some cases with stacking.

 

The problem for UV photography with this, is that you need a rather fast lens with a big front element and those are normally not good for UV.

A second problem is that really fast lenses often are designed with floating lenses and when set to infinity they do not perform optimally for macro.

A third problem is that rather fast lenses with really big front elements have a big focal length making them very difficult to use with the massive extension needed to get close up for macro.