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Occasional Contributor
Posts: 6
Registered: ‎11-14-2019

Re: Best Way To Set Focus on Infinity?

If the lens isnt marked, point at a star, focus ring all the way out, then bring it back a little.

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Registered: ‎12-07-2012

Re: Best Way To Set Focus on Infinity?

"I also have a Canon EF 75-300mm F4-5.6 III lens ..."

 

That would be a worse choice.  Typically you want a lens with a low f-ratio like the 16-35mm I used.  ALso you typically want a lens that has a fast aperture like f2.8 again like the one I used.  Of course this advice is for stars and the Milky Way type objects.  If on the other hand you wanted to do a Moon shot then you might give your "EF 75-300mm F4-5.6 III lens ..." a try.

 

Stars are so far away that a 300mm camera lens wil not make any difference in magnification. A super great lens for a Rebel camera is the Sigma 18-35mm f/1.8 DC HSM Art Lens. Another is the Canon EF-S 17-55mm f/2.8 IS USM Lens. Not only would they do an admirable job on the night time sky, they are excellent normal general use lenses.

 

EOS 1DX and 1D Mk IV and several lenses!
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Re: Best Way To Set Focus on Infinity?

"But its location is based on the optical properties of the lens at the wavelength of IR film."

 

No not really. I agree, it is a hold over from film days but that is where the coincidence ends. The red dot is used to adjust the focus if you're shooting using an infrared filter.  In infrared photography, the pin point sharpest focus plane is slightly farther than the one in normal visible light  To correct for this shift in focus, you can use the red dot.  Keep in mind not all lenses have that red dot.

EOS 1DX and 1D Mk IV and several lenses!
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Re: Best Way To Set Focus on Infinity?

[ Edited ]

@wchettel wrote:

@ebiggs1 wrote:

"...Canon EF-S18-135mm f3.5-5.6 IS STM lens."

 

Some lenses do not do a very good job at night sky photography and you have one that doesn't. If you really want to get good shots of the sky look for a more suitable lens for that purpose.


Got any suggestions for a more suitable lens? I also have a Canon EF 75-300mm F4-5.6 III lens that came with my kit.


Your 18-35mm can do a perfectly satisfactory job of capturing landscape shots with the Milky Way in the sky.  All you need is [a] sturdy tripod that won't wiggle in a breeze, and maybe a wired remote shutter release.  The shutter delay built into the camera works, [too], instead of buying a remote.  

 

Using Manual shooting mode, you would want to use the the shortest focal length and the widest aperture, 18mm at f/3.5.  Ernie used f/4 in his example.  You can capture a good exposure using a 15 second shutter speed.  The way to determine shutter speed is to use the 500 rule.  This rule is a guide to determining the longest exposre you can capture before you start seeing star trails.  

 

It works by dividing 500 by the focal length you are using.  Except, 500 is what works for a full frame camera body.  With a crop sensor body you could use 300, or you could multiply the 18mm by the camera crop factor and use 500.

 

Setting the ISO is another matter.  Depending upon how much stray light is in your night sky, you can use anywhere from ISO 800 to ISO 6400, or higher.  I recommend starting at ISO 800.  The higher your ISO, the more noise will be present in your images.  Noise can be reduced [through] image stacking in post processing.  But, for a single shot I recommend ISO 800 as a starting point.  Try higher ISO values, and determine if the shots are too noisy for your liking.

 

Good luck.

----------------------------------------------------------------------------------------------------------------------------------------------------------

"Doctor told me to get out and walk, so I bought a Canon."
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Esteemed Contributor
Posts: 4,126
Registered: ‎02-17-2016

Re: Best Way To Set Focus on Infinity?

" The red dot is used to adjust the focus if you're shooting using an infrared filter.  In infrared photography, the pin point sharpest focus plane is slightly farther than the one in normal visible light "

 

And this is because of the optical properties of the lens at the wavelength of IR. The Infrared filter actually blocks *visible* light and allows IR to pass through.

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Honored Contributor
Posts: 5,527
Registered: ‎06-25-2014

Re: Best Way To Set Focus on Infinity?


@kvbarkley wrote:

" The red dot is used to adjust the focus if you're shooting using an infrared filter.  In infrared photography, the pin point sharpest focus plane is slightly farther than the one in normal visible light "

 

And this is because of the optical properties of the lens at the wavelength of IR. The Infrared filter actually blocks *visible* light and allows IR to pass through.


A very good point. Our terminology can seem contradictory at times. A UV filter blocks ultraviolet light, but an IR filter doesn't block infrared. Historically, blocking IR was unnecessary, because most film was insensitive to it. (Some film was even insensitive to the deepest red in the visible spectrum. Remember the Brownie cameras with a red plastic button in the back to let you read the frame number printed on the film's paper backing?) But "IR" film was typically sensitive to the red end of the visible spectrum, so you needed an "IR filter" to block that.

 

But today's digital sensors would be sensitive to IR if they didn't have a special filtering layer to block it. Which is why most digital cameras have to be modified if you want to use them for IR photography.

Bob
Philadelphia, Pennsylvania USA
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Re: Best Way To Set Focus on Infinity?

[ Edited ]

Panchromatic vs Orthochromatic  Smiley Happy

 

Orthochromatic emulsions do not react to red wavelengths. Panchromatic uses sensitizing dyes to extend their sensitivity in the green and red areas. A red light does not work in a ortho  panchromatic darkroom. (connection per Robert)

Today IR prevents the camera from recording visible light correctly. Canon puts an IR blocking filter over the camera's sensor to prevent IR  from causing a problem. This is because it arrives at different wave lengths causing a focus problem.

Back in the good ole days a special dye, I forget what its called, was added to film to make it more sensitive to IR.

EOS 1DX and 1D Mk IV and several lenses!
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Re: Best Way To Set Focus on Infinity?

"... the Brownie cameras with a red plastic button in the back ..."

 

I doubt that little red window would be sufficient to protect with today's modern films.

EOS 1DX and 1D Mk IV and several lenses!
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Posts: 5,527
Registered: ‎06-25-2014

Re: Best Way To Set Focus on Infinity?


@ebiggs1 wrote:

"... the Brownie cameras with a red plastic button in the back ..."

 

I doubt that little red window would be sufficient to protect with today's modern films.


because "today's modern" films would be panchromatic.  Smiley Wink

Bob
Philadelphia, Pennsylvania USA
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Honored Contributor
Posts: 5,527
Registered: ‎06-25-2014

Re: Best Way To Set Focus on Infinity?


@ebiggs1 wrote:

Panchromatic vs Orthochromatic  Smiley Happy

 

Orthochromatic emulsions do not react to red wavelengths. Panchromatic uses sensitizing dyes to extend their sensitivity in the green and red areas. A red light does not work in a ortho darkroom.


I think you meant to say, "A red light works only in an ortho darkroom."

 

Half the people in this forum won't know what we're talking about, Ernie. I don't know about you, but it sure reminds me of how old I am.

Bob
Philadelphia, Pennsylvania USA
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