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Old 03-06-2021, 06:20 PM
roughy (Mark)
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Imaging Brightness Variation 130mm Triplet APO f7 or f6.5

Whilst there are quite a few tools to calculate the difference in FoV, is there a formula to calculate the percentage difference in brightness between these to focal ratios?
Sensors that will be used are KAF8300 (5.4 microns) and MN34230 (3.8 microns).
Thanks
Mark
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Old 04-06-2021, 07:04 AM
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gregbradley
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https://phlearn.com/magazine/f-stops...-need-to-know/

F7.1 to F5 is double the amount of light.

So F7 to F6.5 then is around 23%. I don't know the scale is linear though it may not be.

Theoretically with the same camera it should need around 20% less exposure time for the same brightness image.

Greg.
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Old 04-06-2021, 07:06 AM
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mura_gadi (Steve)
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at a guess with ep's

Hello,

The total brightness of an object is determined from aperture only, and the f ratio only affecting the magnification when used with eyepieces/lenses.

So, with a given eyepiece used the shorter F ratio will magnify higher and cause the object to be bigger but dimmer. (Same light spread out over a larger area). If you could set the 130 to have the same magnification at f7 and f6.5 you should see no difference in brightness.



Steve
Ps. Sorry was referring to f ratio and not f-stops for lens aperture.

Last edited by mura_gadi; 04-06-2021 at 12:05 PM.
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Old 04-06-2021, 08:28 AM
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The_bluester (Paul)
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For your purposes, Greg is on the money, using a camera the faster ratio will be brighter.

But I would bet you that you will have a hard time measuring a meaningful difference between F7 and F6.5 for the same exposure and gain. I would bet the difference you will see from shot to shot is likely to be more than the difference that F7 to F6.5 will produce.
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Old 04-06-2021, 08:38 AM
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gregbradley
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Quote:
Originally Posted by mura_gadi View Post
Hello,

The total brightness of an object is determined from aperture only, and the f ration only affecting the magnification when used with eyepieces/lenses.

So, with a given eyepiece used the shorter F ratio will magnify higher and cause the object to be bigger but dimmer. (Same light spread out over a larger area). If you could set the 130 to have the same magnification at f7 and f6.5 you should see no difference in brightness.



Steve
Yes the camera reference applies because the aperture does vary when you open up the aperture of a camera lens.

But practically speaking with regards to telescopes with a fixed aperture, if you add a reducer you will reduce exposure times and get a wider view. Hence the popularity of 8-12 inch scope with fast F ratio like F4 or less.
More of the light from the scene is allowed onto the sensor and when you have a long focal length its the same as cropping the field of view.

So the extra light is coming from the wider field of view.

The faster the F ratio the more difficult it is to have the image aberration free and the more sensitive to collimation etc.

Greg.
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Old 04-06-2021, 09:59 AM
JA
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Quote:
Originally Posted by roughy View Post
Whilst there are quite a few tools to calculate the difference in FoV, is there a formula to calculate the percentage difference in brightness between these to focal ratios?
Sensors that will be used are KAF8300 (5.4 microns) and MN34230 (3.8 microns).
Thanks
Mark
Hi Mark,

Yes, it has to do with the way the f-Number is defined and the increase in light per unit area/field, but by way of a worked example:
in your question of an f/7 vs f/6.5 optic, the f/6.5 would be ~16% brighter.

(7/6.5)^2 = 1.15978

In other words the f/6.5 optic is 1.15978 times (~16%) brighter than the the f/7 optic.

Or if you prefer,
if the f/7 brightness were 100 units, then the f/6.5 brightness would be 115.978 units or ~ 16% brighter.
Best
JA

Also note that if the image circle differs between the 2 optics the considerations are more complicated, especially if there is any vignetting.

Last edited by JA; 04-06-2021 at 10:20 AM.
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Old 06-06-2021, 06:56 PM
roughy (Mark)
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Thank you for all the input gents. Much appreciated.
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