12-06-2019 03:21 PM
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.
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12-07-2019 10:50 AM
" I just want to set it and forget it." " I occasionally like to take pics of the moon and stars ..."
That is one way to shoot nighttime stars. Set the lens to MF. Use the widest aperture. Focus in the day time on something you can see that is very far away and put a small piece of tape on the focus ring. In the dark use your Liveview option to focus.
12-07-2019 12:42 PM
But its location is based on the optical properties of the lens at the wavelength of IR film.
12-07-2019 12:46 PM
I take some of it back. The wavelength of IR film is in the near infrared, just like the silicon sensor.
12-08-2019 10:56 AM
"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.
12-09-2019 09:53 AM
" 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.
12-09-2019 12:30 PM
@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.
12-09-2019 04:16 PM - edited 12-10-2019 10:33 AM
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.
12-10-2019 01:31 AM
@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.
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.
12-10-2019 06:36 AM
@RobertTheFat wrote:I think you meant to say, "A red light works only in an ortho darkroom."
Ah, but you can use a red light when processing colour prints, I know that because they do it in films and on TV
12-10-2019 10:40 AM
Today we use mostly B&W panchromatic films but we still have orthochromatic film.
12-10-2019 11:07 AM
@Ray-uk wrote:
@RobertTheFat wrote:I think you meant to say, "A red light works only in an ortho darkroom."
Ah, but you can use a red light when processing colour prints, I know that because they do it in films and on TV
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.
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