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.

[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.

---

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.

[ebiggs1]

Bpb from Boston said,

"... those elements, if they're large enough, could concentrate enough extra light to affect the amount of light passing through the aperture."

You are referring to the T-stop of a lens.  No lens is perfect so all lenses lose some light.

F ratio or f-stop indicates how much light a lens can transmit. Remember it is a ratio.  However, there is always some loss when light enters or exits any lens. The more elements a lens has can add up to a considerable loss of light. Zooms are the worse. This, of course, can affect exposure.

T-stop measures how much light a lens can really truly pass. A zoom lens with a f2.8 max aperture may have a T-stop of T-3.5
There is always a loss of light and never a gain of light.  This makes the T-stop of a lens always slower than the F-stop. 
T-stop is only important to the camera's meter.   

[TCampbell · Accepted solution]

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@RobertTheFat wrote:

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.

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Utlimately it's the ratio of the lens' effective aperture as compared to it's focal length and not the true physical size.  

When you stop down a lens, the aperture blades only allow light to use the middle of the glass -- the outer edges are no longer used (light passes through them... but then slams into the aperture blade and never actually enters the camera.)

While it may seem counter-intuitive, if the light is entering through a tiny aperture then that light is actually able to illuminate the sensor more evenly.  If the aperture is wide then the illumination is actually less even.   It helps to have a drawing to see why it works, but let's say we are photographing a tree and we want to trace the path of light leaving just one particular leaf on that tree as it travels from the leaf to the image sensor.  It turns out that the light actually takes lots of different paths to reach the same ultimate destination.    Some light will hit the center of the lens and then travel inward.  Some light will hit the outer-left edge of the lens and get focused back to the right to land on the very same pixel as the other path of light that went through the center.    Because the leaf is composed in the very center of the frame, the entire glass is used to focus the image.

But if that same leaf were composed to be in at the extreme left edge of the frame (instead of in the center) then it turns out that some paths through the glass aren't actually capable of focusing light into that corner... so the light in the corner is dimmer because fewer paths of light can reach it.  This causes vignetting.  But if we restrict the aperture size so that only the center of the glass is used anyway, the vignetting is reduced because we only allow the light to pass through the center of the glass anyway.

Very low f-stops (large apertures) will cause more of a light build-up near the center of the frame.  But this doesn't necessarily require physically large diameter glass to cause the effect... it mostly just requires a very low focal ratio.  When multiple lens elements are in use, the optical designers go out of their way to minimize these effects (but they can never totally eliminate it.)

But there are a few other lens issues that will effect exposures...  

One effect is that all camera manufacters "round" the values.  f/2.8 probably isn't really f/2.8 and the 100mm lens probably isn't really 100mm.  It might only be 95mm.  Also, as you focus the lens elements, the effectve focal length of the lens can (and will) change ("breathing").   Some lenses "breathe" quite a bit... other's not so much (but they breathe at least a little bit.)  This means that the subject's focused distance will influence whether or not a 100mm f/2.8 lens is really still 100mm f/2.8.

[Waddizzle]

"When you stop down a lens, the aperture blades only allow light to use the middle of the glass -- the outer edges are no longer used (light passes through them... but then slams into the aperture blade and never actually enters the camera.)"

That is probably the explanation that I've been looking for, the center of the glass.

[Waddizzle]

Thanks, guys.

Like I said, I think too much sometimes.  It just "feels" intuitive that the larger lens is behaving almost like an aperture in front of the aperture, that it should be pulling in more light towards the aperture, even though I understand the math behind the f/stop values perfectly.  Okay, I have to confess that the results from the pictures [ they're virtually identical exposures ] tells me to sit down and accept it, that there's no difference.