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INFRARED AND ASTROPHOTOGRAPHY WITH FAST APERTURE LENSES

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#1 RoyMT

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Posted 12 June 2024 - 05:18 PM

Hello everyone… I have a mirrorless Canon M6MKii full spectrum modified by Life Pixel and realized that my fast aperture lenses delivery very unsharp images (specially on the brightest areas of the image) when used wide open… I start getting better results when used above 4.5 for instance…. So I’m just trying to be very sure on the subject: is this caused because the infrared light is focused on a different point on the FS sensor right? So:

1.- Will I always end up using them on a “more closed” aperture like 5.6 or so? Or is there something I may be missing here to improve the image sharpness?

And,

2.- Does that same logic apply for “earth” infrared photography and astrophotography/nightime photography as well?

Thanks and cheers!

#2 Scdouglas

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Posted 12 June 2024 - 05:40 PM

Without knowing 100% for certain, I would say you're probably right. Infrared light will focus at a slightly different point than visible light and at wider maximum apertures that issue is exaggerated. When you stop down the aperture the focus plane is much more forgiving and more of your scene is in focus, giving you the appearance of a sharper image overall. Basically the deeper your depth of field, the less noticeable the issue is, though it will always be there to some extent. 


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#3 ccaissie

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Posted 12 June 2024 - 06:19 PM

Hello everyone… I have a mirrorless Canon M6MKii full spectrum modified by Life Pixel and realized that my fast aperture lenses delivery very unsharp images (specially on the brightest areas of the image) when used wide open… I start getting better results when used above 4.5 for instance…. So I’m just trying to be very sure on the subject: is this caused because the infrared light is focused on a different point on the FS sensor right? So:

1.- Will I always end up using them on a “more closed” aperture like 5.6 or so? Or is there something I may be missing here to improve the image sharpness?

And,

2.- Does that same logic apply for “earth” infrared photography and astrophotography/nightime photography as well?

Thanks and cheers!

IR foucuses differently...there's usually a little mark on the lens barrel to set infinity focus for IR...(or there used to be for film)... 

 

Sure, fast lenses are special lenses, and compromises are made.  Nothing tests an optical system like a bright astronomical point source.  You can access optical test reports for your lens. 

https://www.the-digi...Help/MTF.aspx  


Edited by ccaissie, 12 June 2024 - 06:42 PM.


#4 KLWalsh

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Posted 12 June 2024 - 09:14 PM

Right.
Infrared light is not refracted as strongly as red light, so when visible light is focused on the sensor, the IR image comes to focus slightly beyond the sensor. The ‘just beyond infinity’ setting on the lens focus ring allows IR photons from stars to come to focus on the sensor.
One thing you can do is use an IR blocking filter for the visible image, and an IR-only filter for the IR wavelengths, then combine the images in post processing. The image scale will be slightly different, but some processing apps can correct for that.
If you use a reflective imaging system like a Newtonian or Cassegrain, all the wavelengths will come to focus at the same focal plane at the same image scale.

Edited by KLWalsh, 12 June 2024 - 09:17 PM.


#5 michael8554

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Posted 15 June 2024 - 04:32 AM

A Full Spectrum mod presupposes that you will always be using a filter - Light Pollution, RGB, narrow band S, O, Ha, etc.

 

Or at least a UV/IR filter.

 

If you aren't imaging with filters, a Ha mod is all you need, the camera's UV/IR filter is retained.



#6 Dan Watt

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Posted 21 June 2024 - 12:40 AM

As above posters said, you'll still need a uv/ir filter, otherwise you'll get star bloat bad. I believe for ef-m mount the only option is a clip in L2 or L3 filter from Astronomik. This will filter out UV and IR but still includes HA so it's not as restrictive as the stock Canon filter. And if you ever want to do IR photography with something like an R72 filter all you have to do is pop out the clip-in filter and you're good to go. 

 

With that in mind if you want image sharpness wide open you'll have to pay for it. Only lenses I've used that come close to being sharp wide open are the Sigma Art series and the Zeiss Otus. And even then, you'll usually want to stop down to at least f2. 



#7 galacticinsomnia

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Posted 22 June 2024 - 02:34 PM

An avid canon user with Full spectrum mods of both M6ii, and Canon rebels.
HA only mods don't work for me, as I have other uses. 

As noted, the clip in L2 or L3 will help with star bloat considerably with standard canon lenses.
I've found the f2.8l 200mm to be very sharp, as well at the 100mm 2.8L macro.  Don't worry about the macro designation, works a treat for DSO's and wide field.

Don't know what you are using for focusing, but, APT has native canon auto focusing built in and is very useful.

 

Wide open DSLR lenses will vary in sharpness, good focus is critical.

Clear Skies !


 




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