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Old 10-11-2020, 06:07 AM
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OneCosmos (Chris)
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camera lens SIZE vs refractor

The whole of the Internet seems unable to answer the question I'm asking. Any search about comparing camera lenses with refractors produces only the inevitable responses about the differences between the two for astronomy, but that isn't what I'm asking.


I fully understand that camera lenses are defined by their focal length and nothing really to do with aperture (not directly anyway) and telescopes of course are defined by the diameter of the lens/mirror.


All I want to know is what camera lens is the equivalent of an 85mm f/5.3 refractor in terms of light gathering/image size?



Chris
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Old 10-11-2020, 08:16 AM
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Nikolas (Nik)
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That's not the issue
the issue is the diameter of the front lens element.
I have a 135mm f2.8 camera lens with a 50mm front element hence its light gathering ability is limited by its front lens element.
I also have a 100mm front lens element zoom that has a focal length of 120-300mm f2.8 So If I zoom to 135mm with the zoom, all things equal the zoom with the larger front lens element will extract more light then the 135 with the smaller front lens element.

so the light gathering is determined not so much by focal length but rather it's lens aperture size.
Focal length determines magnification.
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Old 10-11-2020, 08:38 AM
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Originally Posted by OneCosmos View Post
All I want to know is what camera lens is the equivalent of an 85mm f/5.3 refractor in terms of light gathering/image size?
Hi Chris an 85mm f/5.3 refractor is a refractor with an effective 85mm front objective and a focal ratio of 5.3, that means it has a focal length of ...

85 x 5.3 = 450.5 mm,

so an equivalent camera lens in camera parlance would be a 450mm f/5.3.

So the focal length as stated will be 450mm, but there is one further proviso that's not quite as obvious and that relates to the image circle size. For the 85mm f/5.3 telescope and the the 450mm f/5.3 lens to be equivalent in light gathering terms they must have the same image circle size. It is often the case that telescopes have 40-43 mm image circle diameters, but some smaller refractors have ~30mm image circle diameters. This compares with the typical full frame camera lens which has a 43.27mm or so image circle to support a 24x36mm sensor diagonal.

To be sure of the light gathering equivalence you need to know the image circle size (in other words that you are comparing optical devices designed to support the same system / sensor). Presuming your 85mm f/5.3 telescope has a full-frame image circle it is equivalent in focal length and light gathering to a 450mm f/5.3 Full frame camera lens.

So the question to you is, if this a real comparison, what does the manufacturer specify as the image circle of the 85mm f/5.3 telescope you are interested in?

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JA

Last edited by JA; 10-11-2020 at 08:54 AM.
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Old 10-11-2020, 08:51 AM
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OneCosmos (Chris)
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Thanks Nik - I did kind of assume the diameter of the front element was the determining factor - just like a telescope really, but the amount of light captured mist still be affected by the focal ratio because f/1.4 still needs less exposure time that f/5.6 irrespective of the size of the front element in camera lens at least.


Also fast lenses do have far bigger front elements than their slower counterparts. Zoom lenses are a further complication.


Coming back to my question then, there is no fixed answer as to which size camera telephoto lens is the equivalent of my Takahashi fsq-85 f/5.3 refractor?


Chris
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Old 10-11-2020, 08:52 AM
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Originally Posted by JA View Post
Hi Chris an 85mm f/5.3 refractor is a refractor with an effective 85mm front objective and a focal ratio of 5.3, that means it has a focal length of ...

85 x 5.3 = 450.5 mm,

so an equivalent camera lens in camera parlance would be a 450mm f/5.3.

So the focal length as stated will be 450mm, but there is one further proviso that's not quite as obvious and that relates to the image circle size. For the 85mm f/5.3 telescope and the the 450mm f/5.3 lens to be equivalent in light gathering terms they must have the same image circle size. It is often the case that telescopes have 40-43 mm image circle diameters, but some smaller refractors have ~30mm image circle diameters. This compares with the typical full frame camera lens which has a 43.27mm or so image circle to support a 24x36mm sensor diagonal.

To be sure of the light gathering equivalence you need to know the image circle size. Presuming your 85mm f/5.3 telescope has a full-frame image circle it is equivalent in focal length and light gathering to a 450mm f/5.3 Full frame camera lens.

So the question to you is, if this a real comparison, what does the manufacturer specify as the image circle of the 85mm f/5.3 telescope you are interested in?

Best
JA

Thanks Ja. It is a Takahashi fsq-85 f/5.3 with a 40mm image circle I believe.



Just checked - it is actually 44mm, it reduces to 40mm with the reducer.
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Old 10-11-2020, 09:00 AM
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Thanks Ja. It is a Takahashi fsq-85 f/5.3 with a 40mm image circle I believe
It fully supports a full-frame image circle (when operating natively, without the reducer) . It has a 44mm image circle.
See...
http://www.takahashi-europe.com/en/F...D.features.php

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Old 10-11-2020, 10:12 AM
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Originally Posted by OneCosmos View Post
Thanks Nik - I did kind of assume the diameter of the front element was the determining factor - just like a telescope really, but the amount of light captured mist still be affected by the focal ratio because f/1.4 still needs less exposure time that f/5.6 irrespective of the size of the front element in camera lens at least.


Also fast lenses do have far bigger front elements than their slower counterparts. Zoom lenses are a further complication.


Coming back to my question then, there is no fixed answer as to which size camera telephoto lens is the equivalent of my Takahashi fsq-85 f/5.3 refractor?


Chris
I assume the 85mm is the front element?
I thought you were referring to an 85mm focal length camera lens.
In those cases the one with a larger front lens will capture more detail yet will be slower in terms of time capturing that light than a faster lens.
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Old 10-11-2020, 10:23 AM
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With the FSQ-85, with any sensor size larger than APS-C, you need to purchase the special Takahashi 1.01x flattener to achieve full frame flat field. Otherwise, edge stars will be slightly elongated in all corners.

Under APS-C size, the field is DEAD FLAT.

The flattener can be tricky to get (I do have one, but don't use it seeing I am under APS-C, only slightly but have no plans selling it) - best to order it and wait if you intend to use a large sensor camera
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Old 10-11-2020, 01:13 PM
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As other poster mentioned the amount of light captured is determined by the clear aperture of the front element.

Faster lenses require less exposure time because they let in light from a wider field of view that the front element allows in.

So 85mm F5.3 scope is around a 120mm lens F1.4 (120/1.4 = 86mm).

Fast lenses with longer focal lengths have large front elements.

Not al the light a scope or lens collects winds up in the image. Some falls outside the sensor and when you have a faster lens the light cone is steeper so more light lands on the sensor and gives a wider field of view.

Greg.
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Old 10-11-2020, 05:44 PM
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Quote:
Originally Posted by LewisM View Post
With the FSQ-85, with any sensor size larger than APS-C, you need to purchase the special Takahashi 1.01x flattener to achieve full frame flat field. Otherwise, edge stars will be slightly elongated in all corners.

Under APS-C size, the field is DEAD FLAT.

The flattener can be tricky to get (I do have one, but don't use it seeing I am under APS-C, only slightly but have no plans selling it) - best to order it and wait if you intend to use a large sensor camera
Thanks Lewis. Actually Claude has one. Iím just working out what other adapters I may need. I am also likely to change to a FT focuser.
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Old 11-11-2020, 10:25 AM
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And of course, all this is before you stop the lens down... few (any?) lenses are perfect, so there's a good chance it needs stopping down, thus reducing the aperture and potentially introducing diffraction artifacts from the aperture blades.
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