Question about lens diameter

Hi all, I ve been obsessed with astronomy when I was a teen. I had an etx125 mead telescope which is awesome by the way. The reason I went with the larger scope diameter was because I read somewhere that the larger the diameter of the scope, the more light it can collect, thus making the image you see brighter.

I assume this rule applies to camera lenses? :idea:

Well, I guess to some degree but mostly its the aperture used. My Tokina 12-24 has a much wider than my 50mm f/1.8 but the 50 will let in a lot more light because it has a wider aperture.
No matter how wide the lens, the light still has to go through that aperture so that's the deciding factor.

The larger the front element the more light it can collect so the larger the maximum aperture. My 28-70mm f2.8 was huge compared with my 28-70 f3.5-4.5.

We certainly use maximum aperture for viewing and focussing so the image through a bigger lens would be brighter.

However the camera sensor/film only needs a fixed amount of light and the camera will usually set an aperture smaller than the maximum. The diaphragm in the lens closes down and shuts out part of the light you collected (astronomical telescopes don't have diaphragms)

So f8 on a f2.8 lens is the same as f8 on a f3.5-4.5 lens. The big lens may even have lower image quality due to the difficulty in making and assembling a bigger device

A big lens is useful when light is low because you can use a higher shutter speed, but the arrival of image stabilising systems on the smaller lenses is reducing this advantage

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Originally Posted by Frog

Well, I guess to some degree but mostly its the aperture used. My Tokina 12-24 has a much wider than my 50mm f/1.8 but the 50 will let in a lot more light because it has a wider aperture.
No matter how wide the lens, the light still has to go through that aperture so that's the deciding factor.

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yes the aperture is the deciding factor to how much light gets to the sensor. But a 77mm front element vs a 58mm front element will allow more light through to the blades that control your aperture.

The F stop is defined as the Lens opening divided by the focal length of the lens. By using aperture priority you set the amount of depth of field (DOF) and useing the DOF preview the placement of the DOF range can be set. Yes there are camera lenses that are faster than F1 ( it's from Canon) and then there was the Nikon 50 mm F1.2 and when it was new it was $1200 1970 dollars.

The smaller the f stop is the more light you have and the faster the shutter speed is. Even shooting at low ISO you can over run the shutter speeds with most cameras.

Thanks everyone. That question was bugging me for quite awhile. It makes perfect sense, our lenses diameter does affect how much brighter the image is only to a limit but our aperture regardless of lens diameter makes the final call. Thanks again!