Is there an aperture difference between lenses?

[Waddizzle]

Happy Holidays.

I've been thinking too much again.  Let me start with what is probably a false assumption.  Suppose I have a lens that uses a 58mm filter, and another that uses a 77mm filter.  For argument's sake, I'm assuming that the lens elements in the 58mm filter lens are 58mm, and that the lens elements in the 77mm filter lens are 77mm.

For argument's sake, let's assume that both lenses have a 50mm focal length.  The point being, all things are equal except for the diameter of lens elements in the two lenses.  Suppose that both lenses are both rated with a f/1.8 aperture.

My question is this.  Wouldn't the larger lens let in more light than the smaller lens when both lenses are set to the same aperture setting?

[ebiggs1 · Accepted solution]

The short answer is, no.  F1.8 or f2 or whatever f-stop, is exactly that.  It doesn't matter what size the lens is.  Since this is a ratio, the actual size of the aperture blades will be different but f1.8 will always be f1.8.  It will let in exactly the same amount of light.

Not even two 50mm lenses that have different size elements will be identical but the f-stop ratio will be.  Exactly the same amount of light will pass through it.

If this were not so metering in a camera would be impossible.  I hope that is clear to you.

[RobertTheFat · Accepted solution]

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@ebiggs1 wrote:

The short answer is, no.  F1.8 or f2 or whatever f-stop, is exactly that.  It doesn't matter what size the lens is.  Since this is a ratio, the actual size of the aperture blades will be different but f1.8 will always be f1.8.  It will let in exactly the same amount of light.

Not even two 50mm lenses that have different size elements will be identical but the f-stop ratio will be.  Exactly the same amount of light will pass through it.

If this were not so metering in a camera would be impossible.  I hope that is clear to you.

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The long answer is that it depends on the light gathering power of the elements in front of the aperture diaphragm. At least in principle, those elements, if they're large enough, could concentrate enough extra light to affect the amount of light passing through the aperture. That lens would be slightly faster, at every aperture setting, than a corresponding lens with smaller front elements. The effect would likely be pretty small; but in any case it shouldn't invalidate the camera's metering, since the light the meter sees has already passed through the lens. The camera would just interpret the scene as slightly brighter than it "actually" is and set the aperture (and/or the shutter speed) accordingly.

[ScottyP]

Welllllll.....  Hmmm.

if the lens elements are bigger, then the aperture iris would have to be bigger so it could open wide enough to use the whole extent of the glass.  If the two lenses are both the same length (50mm or whatever), then the one with the bigger iris opening (aperture) would actually be a bigger max aperture, so the question would fall apart.  The ratio of aperture diameter to focal length would change. One lens simply would be a lower f/stop bigger max aperture, right?

Like one would be f/1.8 and the other would be f/1.2 or something so it is apples and oranges. 

[Waddizzle]

My understanding of aperture settings is that f/stop is the ratio of the focal length of the lens to the size of the aperture.  I would think that the size of the opening for the two hypothetical lenses should be identical, but my brain keeps wanting to make me believe that the bigger glass would have a bigger aperture.  I don't know. 

[TTMartin]

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@Waddizzle wrote:

My understanding of aperture settings is that f/stop is the ratio of the focal length of the lens to the size of the aperture.  I would think that the size of the opening for the two hypothetical lenses should be identical, but my brain keeps wanting to make me believe that the bigger glass would have a bigger aperture.  I don't know. 

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The f/number ratio of the lens's focal length to the diameter of the entrance pupil.

The entrance pupil is the optical image of the physical aperture stop, as 'seen' through the front of the lens system..

[Waddizzle]

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@ScottyP wrote:

Welllllll.....  Hmmm.

if the lens elements are bigger, then the aperture iris would have to be bigger so it could open wide enough to use the whole extent of the glass.  If the two lenses are both the same length (50mm or whatever), then the one with the bigger iris opening (aperture) would actually be a bigger max aperture, so the question would fall apart.  The ratio of aperture diameter to focal length would change. One lens simply would be a lower f/stop bigger max aperture, right?

Like one would be f/1.8 and the other would be f/1.2 or something so it is apples and oranges. 

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I think you and I may have cross posted during your edit.

Yes, I have been thinking too much.  If the 58mm glass can only open up its' aperture to a ratio that is roughly half, 1.8, of the 50mm focal length, then that would work out to about a 25mm diameter for the aperture.....of both(?) lenses.

But, if the 77mm glass can open up to 50mm, then in theory it would be an f/1.0 lens.  And, if it opened up wider than 50mm, then it could go less than to f/1, to say f/0.9 for example.  Right?

[EDIT]   This is where my confusion begins.  Okay, both lens are 50mm, and both apertures are opened up to 25mm, f/2..  But, one lens has a primary objective of 58mm, while the other has a primary objective of 77mm.  So, wouldn't the bigger objective gather more light than the smaller one?

[ScottyP]

But you would be stopping the bigger one down.  That eliminates its potential max aperture advantage, and so if both are now at the same f/number the light gathered would be the same.  Right?

[Waddizzle]

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@ScottyP wrote:

But you would be stopping the bigger one down.  That eliminates its potential max aperture advantage, and so if both are now at the same f/number the light gathered would be the same.  Right?

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Negative. Thanks, for your input .... and PATIENCE.

I'm assuming that both lenses are set to the same f/stop, f/2.  The only difference is to be the diameter of the objective lens and the internal lens elements.  My head wants to keep comparing the advantages of having bigger glass to the advantages of having a bigger image sensor.  It seems intuitive that bigger glass should be better, but I cannot find any reason to confirm or deny that conclusion.

[ScottyP]

But I was also assuming both lenses stopped down to the same f/stop.  

I see see the intuitive feeling to your logic, but although I can't express why, it seems like if a fatter lens could gain you a couple stops of light then the industry would go that way instead of pushing the limits of physics to squeeze another 1/3 stop of ISO performance out of a sensor?  Like how golf drivers have all evolved into ginormous hollow light bulbs; a big Bertha lens.

[TCampbell]

While there is a physical difference in aperture sizes between lenses of different types, the filter size is not an indicator of aperure size.

You can calculate the aperture size by dividing the focal length of the lens by the lowest possible focal ratio for that lens.

For example... if you use a 50mm f/1.8 lens then the math is 50 ÷ 1.8 = 27.8  That means the aperture diameter (at wide-open) is 27.8mm wide.  The "STM" version of that lens uses a 49mm diameter filter.  

If you pick on a different lens... such as the 70-200mm f/2.8 and you use the 200mm end then it's 200 ÷ 2.8 = 71.4.  That lens uses a 77mm diameter filter.  

You can see from these two examples that while the 70-200mm has an aperture which is only slightly smaller than the 77mm filter size, the 50mm f/1.8 STM lens has an aperture which is considerably smaller than it's 49mm filter size.  So the filter isn't really a meaningful indicator of aperture size.

For exposure purposes, physical size isn't really important... it's all about the focal ratio.

[Waddizzle]

Yes, thanks for the reply. I get the part about focal ratio. I only referred to filter size as a reference, to distinguish between two hypothetical lenses. The two 50mm lenses in my example would be identical except for the physical size of the glass. If both lenses are set to f/2, then their apertures should both be open to identical sizes. My question was the following: since the larger lens has a larger opening at the front end, is it letting in more light than the smaller lens?