Some eye pieces are dimmer than others

[5haan_A]

Hi,

I notice that when I'm observing and switching between my different eye pieces some give a dimmer image than others. In particular my 9mm eye piece gives a much dimmer image when compared to my 21 mm. 

Why does this happen? 

[johninderby]

The dimming is caused by the reduction in the exit pupil size as the magnification increases. Brightness  goes down by the square of the magnification.

[Demonperformer]

Yes, the object is still emitting the same amount of light, but the increased magnification is spreading that light out over a bigger area. Less light per unit area => dimmer image. This is one reason why the obsession of "department store" telescopes of massive magnification figures is so wrong. It leads the novice to believe that more magnification = better, which it most certainly ain't.

[5haan_A]

Cheers for the responses to that guys makes a lot of sense.

Just a follow up question to this. If there was a 8" aperture scope vs a 6" aperture scope and they both magnified on an object by the same amount would the bigger aperture show a brighter image? 

[johninderby]

Good read on the subject of brightness.

https://www.thefreelibrary.com/Understanding+Surface+Brightness%3A+It+took+a+while%2C+but+the+light+bulb+...-a0513926468

[Stu]

1 minute ago, 5haan_A said:

Cheers for the responses to that guys makes a lot of sense.

Just a follow up question to this. If there was a 8" aperture scope vs a 6" aperture scope and they both magnified on an object by the same amount would the bigger aperture show a brighter image? 

Simple answer is yes.

Exit pupil is calculated as Aperture/magnification so at say x100 a 6" (150mm) scope will have an exit pupil of 1.5mm and an 8" (200mm) will have an exit pupil of 2mm giving a brighter image.

[Demonperformer]

Or to put it another way the 8" mirror collects 64/36 of the light that a 6" mirror does.