Entrance Pupil vs Actual Aperture Size

Started Aug 21, 2011 | Discussions
peter ellner
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Entrance Pupil vs Actual Aperture Size
Aug 21, 2011

I understand that the size of the entrance pupil is what really matters for exposure, since one can have a small aperture size but if the size of the entrance pupil is bigger, it gives the same exposure as if you were using an aperture that was bigger.

Based on this, I just have two questions:

1.) Why is it that the entrance pupil determines exposure, rather than the actual aperture size? If the entrance pupil is just a virtual image of the aperture, and it isn't really as big as it looks, I would think that light would still be physically restricted by the real aperture, since the entrance pupil doesn't really exist. So how is it that the entrance pupil rather than the actual aperture is what matters for exposure?

2.) Since the entrance pupil rather than the actual aperture size determines exposure, does it also determine depth of field, or is depth of field only determined by actual aperture size? In other words, obviously a wider aperture makes a shallower depth of field, but is there any change in depth of field if the actual aperture size remains the same but the entrance pupil gets wider? (For this thought experiment just pretend that the focal length stays the same and the entrance pupil only grows.)

Thank you!

Graystar
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Re: Entrance Pupil vs Actual Aperture Size
In reply to peter ellner, Aug 21, 2011

Light is refracted when it passes through the lens creating a cone. If you want to block the light with a diaphragm that’s set back, you need a smaller hole. That’s all. But really, all the lens diagrams you’ll see are overly simplistic. Modern lenses are extremely complex, and knowledge of the internals doesn’t help your photography one bit. It’s best not to waste brain power wondering about lens design that could be put to much better use expanding your knowledge of lighting.

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Mark Scott Abeln
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Re: Entrance Pupil vs Actual Aperture Size
In reply to peter ellner, Aug 21, 2011

peter ellner wrote:

1.) Why is it that the entrance pupil determines exposure, rather than the actual aperture size? If the entrance pupil is just a virtual image of the aperture, and it isn't really as big as it looks, I would think that light would still be physically restricted by the real aperture, since the entrance pupil doesn't really exist. So how is it that the entrance pupil rather than the actual aperture is what matters for exposure?

Because the path the light goes through is what is important. The entrance pupil size is the diameter that is 'seen' by the light. The optics will constrict the light down significantly, but that doesn't matter. Turn your lens around, and note that the exit pupil may be much smaller than the entrance pupil. All the light from the large front element is squeezed down to go out the back of the lens.

2.) Since the entrance pupil rather than the actual aperture size determines exposure, does it also determine depth of field, or is depth of field only determined by actual aperture size? In other words, obviously a wider aperture makes a shallower depth of field, but is there any change in depth of field if the actual aperture size remains the same but the entrance pupil gets wider? (For this thought experiment just pretend that the focal length stays the same and the entrance pupil only grows.)

Same thing. Apparent aperture size is what matters. As a previous poster wrote, there are lots of lens designs and there may not be a precise relationship between apparent and actual aperture sizes among all models of lenses. The actual aperture size is not really important for a photographer -- unless you plan on going into lens design.

Lens design for DSLRs is quite complicated because of the constraints of the camera systems. You have a mirror box behind the lens, and you also have a fixed maximum width for the back of the lens because of the standard lens mounts. The old bellows cameras had far more flexible designs for adapting lenses, and so the lens designs there tend to be much simpler.

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Great Bustard
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I can answer that!
In reply to peter ellner, Aug 21, 2011

peter ellner wrote:

I understand that the size of the entrance pupil is what really matters for exposure, since one can have a small aperture size but if the size of the entrance pupil is bigger, it gives the same exposure as if you were using an aperture that was bigger.

Not the entrance pupil per se, but the ratio of the focal length and entrance pupil diameter (f-ratio). That is, the exposure is a function of the scene luminance, f-ratio, and shutter speed. However, if we are talking about the total light that falls on the sensor, as opposed to the exposure (the density of the light falling on the sensor), then, yes, it is the entrance pupil diameter that matters.

Based on this, I just have two questions:

1.) Why is it that the entrance pupil determines exposure, rather than the actual aperture size? If the entrance pupil is just a virtual image of the aperture, and it isn't really as big as it looks, I would think that light would still be physically restricted by the real aperture, since the entrance pupil doesn't really exist. So how is it that the entrance pupil rather than the actual aperture is what matters for exposure?

Again, not the entrance pupil, per se, but the ratio of the focal length and the entrance pupil diameter (f-ratio), since this determines the cone of rays that each point on the sensor "sees".

By the way, you are correct to distinguish between the physical aperture and the virtual aperture (entrance pupil), but need to also include the significance (in terms of exposure) of the relative aperture (f-ratio).

2.) Since the entrance pupil rather than the actual aperture size determines exposure, does it also determine depth of field, or is depth of field only determined by actual aperture size? In other words, obviously a wider aperture makes a shallower depth of field, but is there any change in depth of field if the actual aperture size remains the same but the entrance pupil gets wider? (For this thought experiment just pretend that the focal length stays the same and the entrance pupil only grows.)

A good way to think of it is that, for a given perspective and framing, the DOF is determined by the entrance pupil diameter. You may find this answers your question:

h ttp: www.josephjamesphotography.com/equivalence/index.htm#dof

or you may find it a bit too technical. But I'm thinking it might be just your speed.

Thank you!

Hope the above helped!

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