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Bigger aperture on greater diameter lenses?

by cawahe / March 26, 2013 12:34 PM PDT

so does the diameter of the lens affect the aperture
EX: is 1.4 on a 58mm thread the same as a 1.4 on a 77mm thread?

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All Answers

Best Answer chosen by cawahe

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Aperture is a Ratio
by PistonCupChampion / March 26, 2013 4:23 PM PDT

It has nothing to do with the diameter per se, but with the focal length of the lens, and also what size sensor the lens is designed to cover.

A wide aperture longer focal length lens needs to be physically wider; that's why there are few long lenses with apertures bigger than f/2.8, because a larger aperture lens would become too large and even more expensive than they already cost. For example, the Nikon 300mm f/2.8 is $5900.

A lens also needs to be wider to cover a full frame DSLR sensor compared to cameras with smaller sensors.

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by cawahe / March 27, 2013 10:50 AM PDT
In reply to: Aperture is a Ratio

So the longer the focal length the wider the aperture? so in a 50mm lens with 1.4 is smaller than a 70mm lens of 1.4?

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by PistonCupChampion / March 27, 2013 12:36 PM PDT
In reply to: so..

The aperture is expressed as a ratio. Regardless of focal length, f/1.4 is f/1.4.

From Wikipedia:

"In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture[1]) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil.[2] It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography."

"A 100 mm focal length f/4 lens has an entrance pupil diameter of 25 mm. A 200 mm focal length f/4 lens has an entrance pupil diameter of 50 mm. The 200 mm lens's entrance pupil is larger than that of the 100 mm lens, but given the same light transmission efficiency, both will produce the same illuminance at the focal plane when imaging a scene of a given luminance."

In the above example, the 200mm lens has a larger hole, but the aperture value is still the same as the 100mm lens, f/4.0.

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Yes, but you do realize you've switched measurements, right?
by MarkatNite / March 28, 2013 10:37 AM PDT
In reply to: so..

In your first post you asked about "the diameter of the lens ... 58mm thread ... 77mm thread". But now you're asking about "the focal length ... 50mm lens ... 70mm lens".

For example, a zoom lens can vary its focal length, but will always only have one thread size.

Now, as PCC noted, since the f-number is the ratio of focal length over pupil diameter, if two lenses are set to the same f-number (stop) the one with the longer focal length will have a larger pupil diameter.

e.g. 50mm (focal length) / 35mm (pupil diameter) = f/1.4 (approx) vs 70mm (focal length) / 50mm (pupil diameter) = f/1.4

So a 50mm lens at f/1.4 has an entrance pupil diameter (not to be confused with filter thread or exterior lens diameter) of around 35mm, while a 70mm lens at f/1.4 has an entrance pupil diameter (again, not to be confused with filter thread or exterior lens diameter) of 50mm.

And obviously 50mm is larger than 35mm - Mark

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Same amount of light?
by cawahe / March 29, 2013 9:22 AM PDT

But they do let in the same amount of light correct?

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by PistonCupChampion / March 29, 2013 10:54 AM PDT
In reply to: Same amount of light?

That's the whole point of having aperture values, so you can achieve the same exposure regardless of focal length.

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Thank you!
by cawahe / March 29, 2013 12:07 PM PDT
In reply to: Yes

Alright thanks!!

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