I would like to use my Nikon D3100 for astrophotography. I have attached the camera to my telescope with a T-ring, after which the camera recognized the absence of the lens, which is perfectly normal. However, without the lens on, I am unable to adjust the aperture, which is extremely important for astrophotography. I can set the ISO, the shutter speed, and some other, minor things but not the Aperture.

I’m using the camera in manual mode.

Is setting the aperture without the lens even possible?

  • I had misunderstood how aperture works big time. Thank you guys for clearing that up. It makes sense now. Apr 21, 2019 at 6:09

2 Answers 2


Is setting the aperture without the lens even possible?

Considering that the aperture is a part of the lens, not the camera body, no, it is not possible.

Telescopes do not typically have variable apertures — there's no need to stop down to limit incoming light (which is absolutely the opposite of what is wanted for star photography), and depth of field control is meaningless when all objects are no closer than the moon.

The aperture of the telescope is the diameter of the front element. In ƒ-number terms, it is the ratio of the focal length of the telescope divided by the diameter of the front element. Be sure to use the same units (i.e., inches, or mm/cm/m).

  • I'm not sure about OP's camera - but I know some may refuse to shoot when no aperture is selected in certain modes. Do you know if this affects the D3100?
    – OnBreak.
    Apr 19, 2019 at 22:12
  • 1
    @Hueco not sure precisely. It never occurred to me to use a t-mounted camera in anything other than manual mode. I had no problems shooting manual with a D90 mounted on a telescope, nor with my D800E on a scope.
    – scottbb
    Apr 19, 2019 at 22:31
  • This is not entirely true--you could use an additional aperture inline between the objective and the flange. Apr 20, 2019 at 1:29
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    @chrylis Well, I did use the weasel phrase "telescopes do not typically have variable apertures"... =) But regardless, the bigger point is that apertures are not a part of the camera.
    – scottbb
    Apr 20, 2019 at 1:33
  • WHAT aperture? A lens has focal length and aperture. The telescope is now the lens. As I understand telescopes, you pay money to get as LARGE an aperture as possible! Why is it important to adjust it?
    – Laurence
    Apr 20, 2019 at 11:55

Check your telescope manual and find the telescope's aperture. Assuming no telescope eyepiece is utilized, the camera is said to be at the "prime focus" position. Find the scope's diameter. Find the scope's focal length. Divide focal length by diameter. The results of this math is the working f number of the system. Example: 1000mm focal length 4 inch diameter. Convert all units to millimeters. Inches to millimeters conversion is 25.4. Thus 1000 / (4 X 25.4) = 1000 / 101.6 = 9.8. Round to f/10. By the way, f/10 is typical of most systems.

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