EF-S 18-135mm f/3.5-5.6 IS STM - Best Way To Set Focus on Infinity?

[wchettel]

I am using a Canon EOS Rebel SL2 with a Canon EF-S18-135mm f3.5-5.6 IS STM lens. Is there an easy way to set the focus on infinity when I'm using MF mode on the lens? I just want to set it and forget it.

[kvbarkley]

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

[kvbarkley]

I take some of it back. The wavelength of IR film is in the near infrared, just like the silicon sensor.

[ebiggs1]

"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.

[kvbarkley]

" 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.

[RobertTheFat]

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@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.

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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.

[ebiggs1]

Panchromatic vs Orthochromatic  

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.

[RobertTheFat]

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@ebiggs1 wrote:

Panchromatic vs Orthochromatic  

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.

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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.

[Ray-uk]

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@RobertTheFat wrote:

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

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Ah, but you can use a red light when processing colour prints, I know that because they do it in films and on TV

[ebiggs1]

Today we use mostly B&W panchromatic films but we still have orthochromatic film.

[RobertTheFat]

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@Ray-uk wrote:

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@RobertTheFat wrote:

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

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Ah, but you can use a red light when processing colour prints, I know that because they do it in films and on TV

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My father had a safelight that consisted of a wooden box with an orange glass panel on one side. I don't think I knew what it was designed for, but I used it when making B&W prints. It never caused a problem that I can recall. I tried to keep it as far as possible from the enlarger; but my "darkroom" doubled as a coat closet, so my options were limited.