Can you change aperture on a DSLR when it’s attached to a telescope?
Asked 4/19/2019
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I’m using a Nikon D3100 for astrophotography and attaching it to a telescope with a T-ring at prime focus, so there’s no camera lens mounted. In Manual mode I can change ISO and shutter speed, but I can’t adjust aperture. Is that normal, and is there any way to set aperture when shooting through a telescope instead of a lens?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
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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).
Originally by user11924. Source · Licensed CC BY-SA 4.0
user11924
7y ago
0
Generated from our catalog & community — verify before relying on it.
Yes, that’s normal. The aperture is part of the lens or optical system, not the camera body, so with no lens attached the camera has no adjustable diaphragm to control.
When your DSLR is attached directly to a telescope at prime focus, the telescope itself determines the effective aperture. In practice, that means the telescope’s front opening is the aperture, and the working f-number is:
f-number = focal length ÷ aperture diameter
Use the same units for both. For example, a 1000mm telescope with a 101.6mm opening works out to about f/10.
Most telescopes used this way do not have a variable aperture, and for astrophotography you usually want as much light as possible anyway. So instead of changing aperture, you control exposure with shutter speed and ISO, and by choosing a telescope with different focal length/aperture characteristics.
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