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How Do I Stop Down Lenses?

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#1 Jim Waters

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Posted 19 February 2019 - 03:44 PM

I hate diffraction spikes...  My lenses are good but I need to stop them down to make them better and to reduce star bloat. I have read that putting a reducer on the from of lenses it not the best way to go.


Edited by Jim Waters, 19 February 2019 - 03:45 PM.


#2 Richard O'Neill

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Posted 19 February 2019 - 03:48 PM

Construct a circular aperture stop and leave the iris fully open. A more perfect solution would be to replace the mechanical iris with a circular stop in the same plane. I've done this with old surplus military camera lenses and it works very well.


Edited by Richard O'Neill, 19 February 2019 - 03:53 PM.


#3 happylimpet

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Posted 19 February 2019 - 03:50 PM

Paint thick tar around the outside few inches of the lens.


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#4 Jim Waters

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Posted 19 February 2019 - 03:52 PM

Construct a circular aperture stop and leave the iris fully open.

Where would I place it?



#5 ngc7319_20

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Posted 19 February 2019 - 03:55 PM

Where would I place it?

In front of the lens where the filters go...

 

bare.jpg

 

Margarine container lid with some black paint....  I prefer plastic -- if it gets wet with dew, nothing bad happens.  I would avoid black paper / cardboard.

 

mask.jpg

 

Whether it works well or not depends on optical details of the lens design.  So far placing masks over the front has worked fine on the lenses I've tried (180mm F/2.8 and 300mm F/2.8). Where I think this would go to trouble are designs where the front element is MUCH larger than the effective aperture of the lens -- for example, wide angle lenses.  I expect it would work poorly on say a 16mm F/2.8 lens where the front element is 70mm in diameter but the effective aperture is 6mm.


Edited by ngc7319_20, 19 February 2019 - 08:32 PM.

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#6 Jim Waters

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Posted 19 February 2019 - 03:59 PM

I was told that this isn't the best place to put the aperture stop.  I could just use standard step down rings.



#7 Rudy Pohl

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Posted 19 February 2019 - 04:11 PM

I was told that this isn't the best place to put the aperture stop.  I could just use standard step down rings.

Hi Jim,

 

I have been using an aperture mask made from a standard set of Fotodiox step-down rings ($20) for 2 years now on my Nikon 300/f4 ED IF lens. Not only does it radically reduce coma and tighten up the stars, but it also gets rid of all diffraction spikes.

 

My post of this thread explains it: https://www.dpreview...s/post/61784275  It's the 5th post from the bottom.

 

Cheers,
Rudy


Edited by Rudy Pohl, 19 February 2019 - 04:26 PM.


#8 Jim Waters

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Posted 19 February 2019 - 04:13 PM

Paint thick tar around the outside few inches of the lens.

Funny



#9 freestar8n

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Posted 19 February 2019 - 04:13 PM

Putting an aperture stop directly in front of the lens isn't ideal but it may well have a net improvement because the slower f/ratio may have more net benefit than any aberrations introduced by the new stop location.

 

I think the longer the focal length, the less likely there is to have a problem.

 

I stop down my canon 135 f/2 to f/4 with an aperture in front and it works very well.

 

It's important that the hole is very smooth and round or it will show diffraction effects.

 

It's easy and inexpensive to try - so if it works and you see an improvement - then it works.

 

There are many ways to make the aperture.  I cut a crude hole in a lens cap and then glued a smooth washer in place to provide a smooth round hole.

 

You could also 3D print something.

 

Frank


Edited by freestar8n, 19 February 2019 - 04:14 PM.

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#10 Jim Waters

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Posted 19 February 2019 - 04:17 PM

Putting an aperture stop directly in front of the lens isn't ideal but it may well have a net improvement because the slower f/ratio may have more net benefit than any aberrations introduced by the new stop location.

 

I think the longer the focal length, the less likely there is to have a problem.

 

I stop down my canon 135 f/2 to f/4 with an aperture in front and it works very well.

 

It's important that the hole is very smooth and round or it will show diffraction effects.

 

It's easy and inexpensive to try - so if it works and you see an improvement - then it works.

 

There are many ways to make the aperture.  I cut a crude hole in a lens cap and then glued a smooth washer in place to provide a smooth round hole.

 

You could also 3D print something.

 

Frank

Thanks Frank - I will try some step-down rings.



#11 mxpwr

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Posted 19 February 2019 - 04:24 PM

I suppose you could even go for some more advanced stuff and use a serrated aperture to reduce Fresnel rippling effects. Just saying.

#12 Poynting

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Posted 19 February 2019 - 04:25 PM

Jim, I use a 72mm to 52mm stepdown ring on my Canon 200mm L I lens. It was a couple bucks and really helps with the CA and tighten the stars a bit. I personally much prefer the star spikes on your version II versus my version I which are hideous. Here's an example done with the step down ring on my 6d. I think the vignetting from the ring is negligible, and could easily be corrected with flats.


Edited by Poynting, 19 February 2019 - 04:27 PM.

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#13 Jim Waters

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Posted 19 February 2019 - 05:16 PM

Thanks all.



#14 Jim Waters

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Posted 19 February 2019 - 06:08 PM

I suppose you could even go for some more advanced stuff and use a serrated aperture to reduce Fresnel rippling effects. Just saying.

I have access to a CNC shop.  I may give that a try.



#15 whwang

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Posted 19 February 2019 - 07:32 PM

Keep in mind that stopping down the lens from the front aperture only works on telephoto.  It starts to become problematic at <100 mm, and does not work at all on wide-angle lenses.


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#16 spokeshave

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Posted 19 February 2019 - 07:41 PM

The problem with using step-down rings (or some other stop at the front of the lens) is that the limiting aperture of the lens is defined by the aperture stop internal to the lens, not the lens aperture (diameter of the front element). You can check this by measuring the diameter of the front element and multiplying it by the wide-open f-number. You'll probably get a number longer than the stated focal length.

Using step down rings will have no effect until you reach the point where the aperture of the step-down rings becomes limiting instead of the internal aperture stop. So there is nothing wrong with using them, but there is no easy way to tell how much the front element needs to be stopped down to get the desired result.

Tim

#17 Jim Waters

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Posted 19 February 2019 - 07:54 PM

I want to do this on my Rokinon 135mm ED UMC and Canon 200mm f/2.8L II lenses.  It sounds like there's no easy solution to eliminate diffraction spikes.  Maybe I should learn to live with them.


Edited by Jim Waters, 19 February 2019 - 07:59 PM.


#18 Poynting

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Posted 19 February 2019 - 08:10 PM

I want to do this on my Rokinon 135mm ED UMC and Canon 200mm f/2.8L II lenses.  It sounds like there's no easy solution to eliminate diffraction spikes.  Maybe I should learn to live with them.

I feel like the couple bucks spent on two step down rings for both lenses is worth a try. If you are willing to wait a few weeks you can get them for literally a couple dollars on ebay or aliexpress.



#19 freestar8n

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Posted 19 February 2019 - 08:21 PM

The problem with using step-down rings (or some other stop at the front of the lens) is that the limiting aperture of the lens is defined by the aperture stop internal to the lens, not the lens aperture (diameter of the front element). You can check this by measuring the diameter of the front element and multiplying it by the wide-open f-number. You'll probably get a number longer than the stated focal length.

Using step down rings will have no effect until you reach the point where the aperture of the step-down rings becomes limiting instead of the internal aperture stop. So there is nothing wrong with using them, but there is no easy way to tell how much the front element needs to be stopped down to get the desired result.

Tim


As long as the aperture in front is limiting the light it is indeed the true aperture of the system - and the focal length is unchanged.

You know it is the limiting aperture because when the lens operates wide open the internal stop limits the aperture. So when you stop it down in front you know it is the new aperture stop. And since there are no lenses in front of it you know it is the entrance pupil and true aperture.

As for what lenses this will or won’t work with, it’s more of a problem with shorter focal length but will depend on the design and the final f stop.

It is easy to try different things and find out.

Frank

#20 Carl N

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Posted 19 February 2019 - 10:48 PM

With Nikon AIS lenses, or any that have a mechanical operated iris, you can get a cable control to manually operate the f stop.

#21 Samir Kharusi

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Posted 20 February 2019 - 02:16 AM

I want to do this on my Rokinon 135mm ED UMC and Canon 200mm f/2.8L II lenses.  It sounds like there's no easy solution to eliminate diffraction spikes.  Maybe I should learn to live with them.

No idea about the 135mm but I expect step down rings will also work well. As for the 200mm/2.8 I used it with standard 2" astro filters. Here's a useful tidbit. Rather than buying some random step down rings, buy a step down ring from 72mm(?or whatever is the filter diameter of your 200mm lens) to the filter diameter of your 135mm lens (55mm? 58mm?), then buy a second step down ring from that diameter to 48mm (this is the astro standard for 2" astro filters). By using the 2 step down rings in tandem you end up with the 200mm lens at about f4.1 (and also be able to use standard NB filters. With a filter the diameter is cut down further to 46mm?). The smaller step down ring to 48mm will also close down your 135mm to f2.8 (and take NB filters wink.gif ).


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#22 Poynting

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Posted 20 February 2019 - 09:57 AM

No idea about the 135mm but I expect step down rings will also work well. As for the 200mm/2.8 I used it with standard 2" astro filters. Here's a useful tidbit. Rather than buying some random step down rings, buy a step down ring from 72mm(?or whatever is the filter diameter of your 200mm lens) to the filter diameter of your 135mm lens (55mm? 58mm?), then buy a second step down ring from that diameter to 48mm (this is the astro standard for 2" astro filters). By using the 2 step down rings in tandem you end up with the 200mm lens at about f4.1 (and also be able to use standard NB filters. With a filter the diameter is cut down further to 46mm?). The smaller step down ring to 48mm will also close down your 135mm to f2.8 (and take NB filters wink.gif ).

Samir, this is exactly what I plan on doing with 2" filters. I'm glad to hear it is not a crazy idea :)



#23 xiga

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Posted 21 February 2019 - 09:03 AM

Step-down rings work fine. I recently picked up a cheap (£20!) vintage Tamron 135mm F2.8 lens to have a play about with. After testing it out with a 37mm step-down ring (F3.65) it had pretty bad CA. Before I could order a small one, we had a clear night so I quickly butchered an old DVD case and glued a homemade 24mm one to the 37mm ring. It worked fine. I have a gif which shows the before and after but it's too big to post here, so here's 2 screen grabs instead:

 

Capture1.JPG

 

Capture2.JPG

 

I'm using a 58mm-48mm ring initially (so I can use my 2" mounted filters) then the smaller rings come after.


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#24 calypsob

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Posted 21 February 2019 - 09:18 AM

I want to do this on my Rokinon 135mm ED UMC and Canon 200mm f/2.8L II lenses.  It sounds like there's no easy solution to eliminate diffraction spikes.  Maybe I should learn to live with them.

step down rings vignette horribly.

 

Diffraction spikes are a characteristic of the lens, nature of the beast so to speak.

 

You shouldn't need to stop down the Rokinon, the canon will definitely need to be stopped down.



#25 Rudy Pohl

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Posted 21 February 2019 - 09:19 AM

Step-down rings work fine. I recently picked up a cheap (£20!) vintage Tamron 135mm F2.8 lens to have a play about with. After testing it out with a 37mm step-down ring (F3.65) it had pretty bad CA. Before I could order a small one, we had a clear night so I quickly butchered an old DVD case and glued a homemade 24mm one to the 37mm ring. It worked fine. I have a gif which shows the before and after but it's too big to post here, so here's 2 screen grabs instead:

 

attachicon.gif Capture1.JPG

 

attachicon.gif Capture2.JPG

 

I'm using a 58mm-48mm ring initially (so I can use my 2" mounted filters) then the smaller rings come after.

Hi Ciarán,

 

That's very impressive. A couple of question pls....

1. What is the model number of the Tamron 135/2.8, I'm looking for something similar.

2. What was your final effective aperture before you got rid of the coma?

 

UPDATE: Sorry about the gazillion typos in the previous version of this post... I guess not enough coffee today!

 

Thanks,
Rudy


Edited by Rudy Pohl, 21 February 2019 - 12:10 PM.

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