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Is a Bahtinov Mask Really Needed?

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#1 PollAirUs

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Posted 21 September 2020 - 06:09 PM

Hi folks,

I'll be starting  astrophotography soon (as soon as I buy a tracker), I already bought a Nikon D5300 and a Rokinon135mm lens to start. However, I have a question: Is there really a need to use a Bahtinov mask in this case? Wouldn't it be enough to focus the lens manually on infinity and that's it? Perfect focus?

 

I'd also like to know if it would work if I made a Bahtinov mask myself and just put it in front of the lens. Is there any distance that should be between the lens and the mask?

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Edited by PollAirUs, 21 September 2020 - 06:11 PM.


#2 wrnchhead

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Posted 21 September 2020 - 06:14 PM

Yes you do need one. Infinity focus on a camera lens is never perfect. Yes you can make one, there are calculators online and you print it out on paper and transfer it to whatever medium you desire. Dr. Benway, a user here 3-D prints them for basically pocket change if you like to have a nice hard plastic one. I think I saw him list those for this lens very recently or just message him. Oh, and the end of the lens hood is fine.

Edited by wrnchhead, 21 September 2020 - 06:15 PM.

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#3 jimr2

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Posted 21 September 2020 - 06:19 PM

PollAir,

 

Hi, yes, a Bahtinov mask does help in focusing, but I have used them only on telescopes to achieve critical focus, not on camera lenses, so I can't say for certain how much they would help in that application. It may be that you can focus just as well using your camera's live-view feature and blowing up the view as much as you can (my Canon's go up to 10X). I would think that a mask would still help a little but the differences might not be very much. And yes, you can make your own mask--I've seen instructions on various websites on how to do this--but I cannot tell you what the optimum distance from the front of the lens to the mask should be. Hopefully someone else can.

 

In any case, good luck!

 

-jim-



#4 jgraham

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Posted 21 September 2020 - 06:19 PM

I use either a Bahtinov mask or a modified Hartmann mask with crossed slits that show a '+' when focused.
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#5 DubbelDerp

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Posted 21 September 2020 - 06:20 PM

+1. F/2 is EXTREMELY unforgiving. I stop mine down to 2.8 just to make focusing a bit easier. Even still, when I want to move it slightly one direction, it’s usually enough just to put my hand on the focus ring and think about moving it.

I got this bahtinov mask.. you can probably find it cheaper:
https://www.amazon.c...ov mask &sr=8-4

It’s not the best quality, but it works. I set the iso to the highest setting, point at a bright star, and can usually see the diffraction spikes in live view. Adjust in live view, and take a 5 second exposure to confirm.

With fast glass, unaided focusing is definitely not accurate enough unfortunately.
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#6 Marcelofig

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Posted 21 September 2020 - 06:24 PM

Watch the latest episode on youtube Cuiv The Lazy Geek. He talks precisely about astrophotography using lenses (and specifically mentions the use of a Bahtinov mask).

 

https://www.youtube....h?v=D78P1h_a50Q



#7 rkinnett

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Posted 21 September 2020 - 06:36 PM

Lots of folks voting for the mask.  Is there any reason he can't use live preview, either on a PC or back of camera, to zoom way in and manually focus to minimize stars?  Or use FWHM methods baked into acquisition software, if he's connected to a computer?

 

If you buy or make a mask, just set it directly against the lens.  Or if you design one, you could make it fit around the hood.  Distance from the lens isn't critical.


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#8 DJL

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Posted 21 September 2020 - 06:36 PM

I have this lens for my Canon DSLR. I would totally recommend a Bahtinov mask because focusing on stars is hard enough and you might as well spend $16 and make sure it's as good as you can get it.

The advice I followed was to get a low cost UV filter and fit the Bahtinov mask into it (I had to to a little filing on the corners of mine to make it fit), then just screw the filter a half turn into the lens to focus and remove it when done.

You will find instructions for making your own, but the make vs buy was not compelling when I bought mine. On the other hand due to the supply and demand issues in astro gear right now, making one may be worth it.

This https://farpointastr...nov-focus-mask/ is $16 from FarPoint and $38 on Amazon - seemingly scalping Bahtinov masks is now a thing.

You can definitely get started without one; either way focus at F2 and then stop down to F2.8 to take pictures. Enjoy!


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#9 kathyastro

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Posted 21 September 2020 - 06:40 PM

Focusing on infinity is harder than it seems, because modern lenses have their "infinity stop" a distance past infinity.  IN the old days, you could turn the focus ring until it stopped, but that doesn't work any more.  I vote for the mask.


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#10 DubbelDerp

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Posted 21 September 2020 - 06:45 PM

Lots of folks voting for the mask. Is there any reason he can't use live preview, either on a PC or back of camera, to zoom way in and manually focus to minimize stars? Or use FWHM methods baked into acquisition software, if he's connected to a computer?

If you buy or make a mask, just set it directly against the lens. Or if you design one, you could make it fit around the hood. Distance from the lens isn't critical.

You probably could with an autofocus lens tethered to a computer, but manually focusing a lens such as this one bounces the mount around far too much for FWHM routines to work well, in my limited experience. If you can focus without touching the lens, it would probably work great, but pending that the mask works best for me with fast optics. Even then, it’s really hard to precisely focus at f/2 or 2.8.

#11 TOMDEY

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Posted 21 September 2020 - 06:52 PM

The mask is cute and it works well, especially for those who are not well-versed in other focusing techniques... aka most all of us here.

 

Also good to know that the exact design and accuracy of the mask are not at all critical. You can just print it on a piece of paper and roughly cut out the spaces between the opaques and it will work every bit as well as a meticulously built one. If you print it on clear plastic... you still need to cut out the spaces. That's all there is to it; go for it!

 

[If you're wondering how a rough-cut mask could perform as well as a perfect one... just execute the Fourier Transforms of both. You will see that the focusability (both accuracy and precision) are virtually identical.]    Tom



#12 PollAirUs

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Posted 21 September 2020 - 07:15 PM

Yes you do need one. Infinity focus on a camera lens is never perfect. Yes you can make one, there are calculators online and you print it out on paper and transfer it to whatever medium you desire. Dr. Benway, a user here 3-D prints them for basically pocket change if you like to have a nice hard plastic one. I think I saw him list those for this lens very recently or just message him. Oh, and the end of the lens hood is fine.

PollAir,

 

Hi, yes, a Bahtinov mask does help in focusing, but I have used them only on telescopes to achieve critical focus, not on camera lenses, so I can't say for certain how much they would help in that application. It may be that you can focus just as well using your camera's live-view feature and blowing up the view as much as you can (my Canon's go up to 10X). I would think that a mask would still help a little but the differences might not be very much. And yes, you can make your own mask--I've seen instructions on various websites on how to do this--but I cannot tell you what the optimum distance from the front of the lens to the mask should be. Hopefully someone else can.

 

In any case, good luck!

 

-jim-

Eric and Jim, thank you guys so much, I found some templates to print and I'll certainly try to do a bahtinov mask myself!

 

 

Watch the latest episode on youtube Cuiv The Lazy Geek. He talks precisely about astrophotography using lenses (and specifically mentions the use of a Bahtinov mask).

 

https://www.youtube....h?v=D78P1h_a50Q

I haven't finished the entire video yet, but I can already thank you and say that it's been very useful for me!

 

 

I have this lens for my Canon DSLR. I would totally recommend a Bahtinov mask because focusing on stars is hard enough and you might as well spend $16 and make sure it's as good as you can get it.

The advice I followed was to get a low cost UV filter and fit the Bahtinov mask into it (I had to to a little filing on the corners of mine to make it fit), then just screw the filter a half turn into the lens to focus and remove it when done.

You will find instructions for making your own, but the make vs buy was not compelling when I bought mine. On the other hand due to the supply and demand issues in astro gear right now, making one may be worth it.

This https://farpointastr...nov-focus-mask/ is $16 from FarPoint and $38 on Amazon - seemingly scalping Bahtinov masks is now a thing.

You can definitely get started without one; either way focus at F2 and then stop down to F2.8 to take pictures. Enjoy!

Thank you! I will try to follow the dimensions of this mask and try to make it myself. Here in Brazil it is not very easy to find these masks to buy, especially the specific size of 77mm!
 



#13 PollAirUs

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Posted 21 September 2020 - 07:22 PM

+1. F/2 is EXTREMELY unforgiving. I stop mine down to 2.8 just to make focusing a bit easier. Even still, when I want to move it slightly one direction, it’s usually enough just to put my hand on the focus ring and think about moving it.

I got this bahtinov mask.. you can probably find it cheaper:
https://www.amazon.c...ov mask &sr=8-4

It’s not the best quality, but it works. I set the iso to the highest setting, point at a bright star, and can usually see the diffraction spikes in live view. Adjust in live view, and take a 5 second exposure to confirm.

With fast glass, unaided focusing is definitely not accurate enough unfortunately.

I'll definitely use F2.8 for shooting, I've been researching and it seems to be the best F number. But for you I would like to ask a specific question.

After seeing your picture taken with the 2" L-eNhace filter in front of the objective lens, I'm seriously thinking about doing the same as you, furthermore, your photo was amazing! But I'd like to ask you, how do you use the mask if you use the filter in front of the lens?
 



#14 Arcamigo

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Posted 21 September 2020 - 07:22 PM

You don't need the mask, but it is useful if you have a bright star in your image. Other times, you might not, and it would be just as effective to use Live View with dimmer stars and the camera set at maximum ISO. In that case, many stars, and a few moons, will only be visible after you have achieved perfect focus, otherwise they will blur into invisibility. Focusing on bright objects without a mask is difficult, because there is a relatively wide focus range during which the object is at its smallest, but typically, there will also be dimmer objects near the bright object with which you could use Live View.

 

As you progress into AP, you will also learn to use Live View to focus on image features that can't be done with a mask, like clouds, valleys, moon craters, or solar prominences. Also, lenses and refractors have chromatic aberration and sometimes you can use that to focus as well by adjusting the intensity of the blue halo.


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#15 DubbelDerp

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Posted 21 September 2020 - 07:43 PM

I'll definitely use F2.8 for shooting, I've been researching and it seems to be the best F number. But for you I would like to ask a specific question.
After seeing your picture taken with the 2" L-eNhace filter in front of the objective lens, I'm seriously thinking about doing the same as you, furthermore, your photo was amazing! But I'd like to ask you, how do you use the mask if you use the filter in front of the lens?

Thanks! I’m sure I’m not doing it the right way, but I focus with the mask and without the filter and step-down ring, get focus right where I want it to be, add the step down rings and filter, and check it again with the mask. With the filter it takes a bit longer of an exposure, but it doesn’t shift for me with this particular lens and this particular filter. It’s mostly to make sure I didn’t bump focus when threading on the rings or filter.

Maybe the mask is cute, but sometimes we find better ways to do things that make the older ways obsolete. Just sayin’.
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#16 ChristopherBeere

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Posted 21 September 2020 - 08:30 PM

I use the Lee 100 Filter system and SharpStar Bahtinov mask.

 

Perfect focus every single time. 

 

I switched to using a Bahtinov mask after I missed focus on a mosaic panel shooting with the Sigma Art 105 @ f/1.4 while I was down in Namibia.

 

When you fly half way across the planet and spend several thousand pounds on an imaging run you want to do everything in your power to guarantee the quality of your data.

 

Introducing the bahtinov mask to my setup eliminated focus error and I consider it an essential part of my setup now.


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#17 DakotaHoosier

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Posted 21 September 2020 - 10:22 PM

New astrophotographer here. I'm using a Sony a6400 with both manual (old Minolta glass adapted to E-mount) and auto-focus lenses. Neither has a 'hard stop for infinity' so you'll need to focus the Rokinon manually. Stars are unforgiving, as is a wide-open aperture. I haven't figured out focusing yet, and the Bahtinov is on my list. I've got a 3D printer so I'll probably do that for myself!


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#18 PollAirUs

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Posted 22 September 2020 - 07:43 AM

You don't need the mask, but it is useful if you have a bright star in your image. Other times, you might not, and it would be just as effective to use Live View with dimmer stars and the camera set at maximum ISO. In that case, many stars, and a few moons, will only be visible after you have achieved perfect focus, otherwise they will blur into invisibility. Focusing on bright objects without a mask is difficult, because there is a relatively wide focus range during which the object is at its smallest, but typically, there will also be dimmer objects near the bright object with which you could use Live View.

 

As you progress into AP, you will also learn to use Live View to focus on image features that can't be done with a mask, like clouds, valleys, moon craters, or solar prominences. Also, lenses and refractors have chromatic aberration and sometimes you can use that to focus as well by adjusting the intensity of the blue halo.

You're right, shooting the moon with some clouds, or any kind of landscape won't work with the mask. I'll definitely give Live Viwe Focusing a try!

 

Thanks! I’m sure I’m not doing it the right way, but I focus with the mask and without the filter and step-down ring, get focus right where I want it to be, add the step down rings and filter, and check it again with the mask. With the filter it takes a bit longer of an exposure, but it doesn’t shift for me with this particular lens and this particular filter. It’s mostly to make sure I didn’t bump focus when threading on the rings or filter.

Maybe the mask is cute, but sometimes we find better ways to do things that make the older ways obsolete. Just sayin’.

I don't see why you're doing it wrong. You focus, put the filter, and check if the focus is still right, nothing wrong with it for me. I will probably do it the same way.

But I'm still thinking about using the filter in front of the objective with these step-down rings. Your image is really convincing me, but I don't know if I'll be able to get rid of vignette as you did. And if it's worth to step-down the amount of light my lens get just to use the filter. But, again, your image is really promissing.

 


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