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Funky diffraction spikes

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#1 Hilmi

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Posted 12 February 2013 - 02:12 PM

I'm new to the DSLR and lens imaging scene having only recived my Polarie last night. I'd never bothered to piggy back my DSLR before. So now I am getting started with DSLR and Canon EF 100 F2 USM lens and the I'm getting some really funky diffraction spikes, or maybe I should call them distraction spikes. Is there anything I can do to reduce them?

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#2 bouffetout

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Posted 12 February 2013 - 02:27 PM

I think you have to go to f/4 or f/5.6 to prevent this from happening...

#3 pfile

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Posted 12 February 2013 - 02:28 PM

i'm assuming you were not running at f/2? if you are stopped down, that's normal - it is because the internal diaphragm is not perfectly round but has some number (maybe 4 or 8) curved 'blades'.

you can stop down externally with a step-down ring. that should avoid the spikes.

if you pixel peep those widefields i did in the CCD group, you'll see that the spikes are there even though the lens is wide open. they are not as pronounced, but they are there. apparently the diaphragm does not open all the way up.

#4 nwinston

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Posted 12 February 2013 - 02:29 PM

Don't worry, diffraction spikes like that are normal for camera lenses. They are caused by the aperture blades in the lens.
I'm not sure of any way to reduce the spikes other than use the lens wide open, but I wouldn't recommend doing that because it reduces image quality.

#5 Hilmi

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Posted 12 February 2013 - 02:34 PM

Yeah, I had the lens stopped down to F4, it's got 8 leaves in the diaphragm. I did try to image at F2, but I found it extremely difficult to focus. I started to have very obvious CA unless the focus was just perfect, which I found so difficult to achieve.

I'm finding that other than the noise level on the DSLR, I'm liking the ease of imaging with the Polarie and DSLR. The noise, I'm not too happy about. My STT-8300 cooled to -25 allows me to take 20 minute exposures. I'm finding that with 20 darks my DSLR images are still full of noise even though the exposures are 20 seconds long.

#6 Falcon-

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Posted 12 February 2013 - 02:36 PM

This is normal for a camera lens. It is caused by the slight off-round corners caused by the aperture blade. I expect your EF100 F2 USM must have an 8-blade aperture as I see 8 spikes.

There are two things you can do about it:

- First, and this is *exceedingly rare* but a FEW lenses can be used wide-open. A Canon 300 L f/4 I had a chance to use was like that. You can give it a try with your 200 f/2 (but I would be shocked not to find you gain distortion and/or chromatic aberration wide open vs stopped down)
- Second, you can use an external aperture. Get yourself some step-down rings that thread into the filter threads on the front of the lens to redice the aperture before the lens and leave the internal aperture at wide-open. This should (I think) produce the same improvements in sharpness and aberration reduction but with a perfectly round aperture you have no spikes. I have an old lens with a damaged aperture internally so I am going to be trying this trick myself - hopefully it works as I expect!

But the third option is of course... learn to love the distrac.... er... diffraction spikes! :D

#7 jambroseus

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Posted 12 February 2013 - 02:37 PM

They are caused by the IRIS stopping down the lens. Open the IRIS wide (shoot at your maximum f/stop).

John

#8 guyroch

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Posted 12 February 2013 - 02:38 PM

For what it's worth, I actually like those spikes. But that is a personal taste I guess.

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#9 Falcon-

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Posted 12 February 2013 - 02:43 PM

Wow, it seems I took so long in typing my response 3 people beat me too it! :p

I did try to image at F2, but I found it extremely difficult to focus.


Do you have a computer with you? Have you tried to make a Bahtinov mask for your 100mm lens?

If when precisely focused that lens *will* give good results at f/2 it is well worth spending a bit of extra time getting perfect focus!

I have found you can use a small Bahtinov mask on most camera lenses (though the wider the FOV the less prominent the bahtinov pattern is). As an alternative while not as fast as a bahtinov mask BackyardEOS and Nebulosity both have some easy to use focus-metrics that can help nail the focus even on very wide leses like my 28mm.

At the very least doing a well focused sequence at f/2 once, even if your normal method does not include a computer, would be worth it (in my opinion) to find out just how good that lens is for AP wide open...

#10 Hilmi

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Posted 12 February 2013 - 02:52 PM

As much as I like BYEOS, I had tried the demo once and was great, the whole idea of getting the Vixen Polarie was to be able to get an astro-photography setup that was extremely portable. The idea was that if I wanted to go camping and actually carry more than 1 passenger, I didn't have to fold down the second row of seats to carry my Asto-Gear!

So, I'll give the Bhatinov mask a try. I'll see how steady my hands are with a blade to make one that small. The big one I made worked great on my SCT.

#11 pfile

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Posted 12 February 2013 - 02:55 PM

any laser cutting shops around there? i had a mask cut into a cheap plastic lens cover. works great.

#12 Hilmi

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Posted 12 February 2013 - 02:59 PM

What's laser cutting used for? Normally these specialty shops are not easy to hunt down in Oman. You have to go to the people who typically use these services and ask them where they get their laser cutting done.

#13 Falcon-

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Posted 12 February 2013 - 03:02 PM

BTW - I found with my own experimenting with Bahtinov and lenses in the at 135 and 200mm is that you likely will need short test exposures rather then live-view. Still gets you a good focus in the end though!

#14 fco_star

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Posted 12 February 2013 - 03:10 PM

like spikes too

#15 Sean13

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Posted 12 February 2013 - 03:15 PM

I see the same things with my lens. Personally I also like the look.

#16 achronut

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Posted 12 February 2013 - 03:35 PM

- Second, you can use an external aperture. Get yourself some step-down rings that thread into the filter threads on the front of the lens to redice the aperture before the lens and leave the internal aperture at wide-open. This should (I think) produce the same improvements in sharpness and aberration reduction but with a perfectly round aperture you have no spikes. I have an old lens with a damaged aperture internally so I am going to be trying this trick myself - hopefully it works as I expect!




Wouldn't using an external mask in front of the aperture cause vignetting? Also I believe that the position of the mask should be 'behind'the blades to be able to block the spikes coming from the blades in front of it. In other words an internal mask positioned behind the blades.

#17 Falcon-

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Posted 12 February 2013 - 03:46 PM

Wouldn't using an external mask in front of the aperture cause vignetting?


With a relatively simple optic like a doublet or triplet refractor or an old-style "long lens" (rather then telephoto lens) you should not have any vignetting issues*.

With complex optics like modern lenses the answer is.... I really have no idea! I do have a step-down ring (55mm-37mm) I will be trying out on my Pentacon 135 but I have as yet not had the chance to do so.

Ideally you would want an internal fixed mask of perfect circularity at the SAME position as the current adjustable aperture. That does though require lens surgery at levels most of us are not quite comfortable with... (or able to pull off!).

Also I believe that the position of the mask should be 'behind'the blades to be able to block the spikes coming from the blades in front of it. In other words an internal mask positioned behind the blades.


Most lenses when wide open either have the aperture blades totally out of the light path or present an aperture near enough to a perfect circle that the spikes will be non-existent or negligible. The idea here is if you are using an external aperture stop then the internal aperture will be set to wide-open, so in that regards behind or in front of the blades should* not matter.







* (said the non-expert in refractive optics)

#18 zawijava

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Posted 12 February 2013 - 03:56 PM

I just posted a question related to noise caused by the heat of the DSLR battery. Are you shooting with battery power? My Post is so new I don't think anyone has responded yet, but perhaps soon. -Tim

I'm finding that other than the noise level on the DSLR, I'm liking the ease of imaging with the Polarie and DSLR. The noise, I'm not too happy about. My STT-8300 cooled to -25 allows me to take 20 minute exposures. I'm finding that with 20 darks my DSLR images are still full of noise even though the exposures are 20 seconds long.



#19 pfile

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Posted 12 February 2013 - 11:28 PM

What's laser cutting used for? Normally these specialty shops are not easy to hunt down in Oman. You have to go to the people who typically use these services and ask them where they get their laser cutting done.


well, it's usually used to cut metals, so i guess asking around at different machine shops would work. sometimes people use it to cut acrylics as well, like to make a trophy or a sign.

it's also used to engrave metals. basically you can cut very intricate patterns in metals using a laser cutter.

#20 Hilmi

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Posted 12 February 2013 - 11:37 PM

Thanks, I'll go talk to the people who made my stair railings. They also do intricately designed metal lamp shades so I guess they have a laser cutter.

#21 Hilmi

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Posted 13 February 2013 - 02:53 AM

Tom,

Sorry I missed your post about the battery. I had actually seen your topic, but I'm not keen on using the AC power adapter because the idea is to be as portable as possible. i.e. no dependence on external power sources. Although I do wonder if a battery grip will achieve the same. After all, it does move the battery to outside the camera body and it does carry 2 batteries.

#22 mmalik

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Posted 13 February 2013 - 03:16 AM

I'm finding that with 20 darks my DSLR images are still full of noise even though the exposures are 20 seconds long.


Are you using in or out camera NR; if not then that would be the reason. Thx

#23 Hilmi

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Posted 13 February 2013 - 05:50 AM

I switched off the camera's built in long exposure noise reduction feature. I took plenty of exposures and then I took 20 darks.

The outside temperature was around 20 C, sensor temperature was 26 C. I also have version 2.x.x of the Canon 7D firmware if that makes any difference

#24 mmalik

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Posted 13 February 2013 - 06:16 AM

Try taking couple of in-camera NR control shots (for comparison I mean) in case out-camera NR is not fully eliminating the noise due to whatever reason, temp mismatch, etc. Thx






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