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Virtual aperture vs effective aperture
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EinsteinsGhost
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What do you mean by virtual and effective aperture?
I take it that means they don't have any particular meaning. That is probably why I am having a hard time understanding the difference.
A dictionary definition of effective aperture is "the diameter of the entrance pupil of an optical system; specif : the apparent diameter of the diaphragm opening in a camera lens as seen through the front of the lens as seen from the front of the lens."
http://www.merriam-webster.com/dictionary/effective aperture
Wikipedia says virtual aperture is the exit pupil of an optical system.
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Maybe that is the actual aperture as opposed to the apparent, effective, relative, or virtual aperture.
EinsteinsGhost
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I think what you are looking at is that an aperture (f/2, for example on a 50mm lens might not be 25mm across but it acts like one. This is possible but the effective aperture is all that you really need (for DOF anyway).
Exactly, that would be my idea of an effect aperture, but couldn't it be called a virtual aperture also? Could another example be where f/2 on a 50mm lens was 25mm across, but not act like it?
Great Bustard
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OK, I assume that this, from the other thread, is the reason for your question:
http://www.josephjamesphotography.com/equivalence/#aperture
Understanding the fundamental concepts of Equivalence requires making important distinctions between various terms which people often take to mean the same thing. It is very much akin to making the distinction between "mass" and "weight", two terms which most people take to mean the same thing, when, in fact, they measure two different (but related) quantities. While there are circumstances where making the distinction is unnecessary, there are other times when it is critical.
The first of these distinctions that needs to be made is between aperture and f-ratio. The term "aperture", by itself, is vague -- we need a qualifying adjective to be clear. There are three different terms using "aperture":
- The physical aperture (iris) is the smallest opening within a lens.
- The virtual aperture (entrance pupil) is the image of the physical aperture when looking through the front element of the lens.
- The relative aperture (f-ratio) is the quotient of the focal length and the virtual aperture.
This link may shed some light on visualizing the three apertures:
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/stop.html
However, as the last paragraph in italics above spells out, the same relative aperture (f-ratio) results in the same exposure for a given scene luminance and shutter speed, regardless of focal length or format, but the same virtual aperture (entrance pupil) results in the same total amount of light falling on the sensor (the exposure is the density of the light falling on the sensor, as opposed to the total amount of light).
In addition, the same virtual aperture (entrance pupil) for a given perspective, framing, display size, viewing distance, and visual acuity will also result in the same DOF.
Hence, in terms of the visual properties of the photo, it is the virtual aperture (entrance pupil) that matters, not the relative aperture (f-ratio).
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Not the reason, but part of the my problem. The reason for my question is more about the definitions of effective and virtual as applied to aperture. Wikipedia talks about a virtual aperture here:
Exit pupil - Wikipedia
The link in this post:
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www.dpreview.com
talks about effective aperture and comparing that with the Wiki article about virtual aperture got me to thinking about the differences among virtual, effective, and apparent as adjectives for aperture. I recalled seeing your (apparently definitive) definition for virtual aperture and that only added fuel to the existing fire.
Great Bustard
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The virtual aperture refers to the entrance pupil, not the exit pupil. In any case, all the apertures and pupils) are discussed and demonstrated in the link I gave:
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/stop.html
Equivalence holds the virtual aperture (entrance pupil) as primary since for the same perspective, framing, and display size, the same virtual aperture results in the same DOF and also the same total amount of light falling on the sensor for a given shutter speed, which will result in the same noise for equally efficient sensors.
I assume you are not saying the Wiki article (1st link above) is wrong, but then what are are you saying?
I have no doubt what you are saying is relevant to my opening post, but to keep it simple for my benefit, what do you consider the difference to be among effective, virtual, and apparent apertures? Is it semantics or is there a significant difference?
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Great Bustard
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Correct.
That in terms of the visual properties of the photo, it is the virtual aperture (entrance pupil) that matters, not the relative aperture (f-ratio).
Both the entrance and exit pupils are virtual apertures -- that is, they are both images of the physical aperture when viewed through the front element and rear element of the lens, respectively.
Thus, I would say that the effective, virtual, and apparent aperture are all different names for the entrance pupil.
Ontario Gone
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This is what bothers me the most about this topic as well, there seem to be too many overlapping terms. If it is the entrance pupil, why not just call it an entrance pupil? And if F stop doesn't necessarily tell us what the real aperture is for non FF systems, why do we even use F stops?
I mentioned recently in a post that in my time researching telescopes, i noticed most AP don't refer to F stops when discussing the light gathering ability of a scope, they only mention the aperture of the front objective lens. According to that world, the total amount of light is decided by that aperture. This makes sense, as light must be gathered at the front before it can reach the sensor. Now they don't have diaphrams, but it should be the same as a regular camera lens shot wide open.
Im curious though if perhaps bustard or anybody else knows, in AP you can find the F stop by the same equation as with lenses, by taking FL/aperture=F stop. However they use the objective lens diameter, so im wondering why things are so different here.
As a reference, the below scope is listed as a 750mm FL F5 telescope. As we would guess, 750/5=150, and this scope/lens is a 150mm (6 inch) unit. So, why can we not just do the same with a camera lens, using the front diameter in the overall model of light gathering, and to find the F stop? Why all the need for entrance pupils and virtual/relative apertures?
--
"Run to the light, Carol Anne. Run as fast as you can!"
I mentioned recently in a post that in my time researching telescopes, i noticed most AP don't refer to F stops when discussing the light gathering ability of a scope, they only mention the aperture of the front objective lens. According to that world, the total amount of light is decided by that aperture. This makes sense, as light must be gathered at the front before it can reach the sensor. Now they don't have diaphrams, but it should be the same as a regular camera lens shot wide open.
Im curious though if perhaps bustard or anybody else knows, in AP you can find the F stop by the same equation as with lenses, by taking FL/aperture=F stop. However they use the objective lens diameter, so im wondering why things are so different here.
As a reference, the below scope is listed as a 750mm FL F5 telescope. As we would guess, 750/5=150, and this scope/lens is a 150mm (6 inch) unit. So, why can we not just do the same with a camera lens, using the front diameter in the overall model of light gathering, and to find the F stop? Why all the need for entrance pupils and virtual/relative apertures?
--
"Run to the light, Carol Anne. Run as fast as you can!"
Great Bustard
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Depends on what you mean by "it":
http://www.josephjamesphotography.com/equivalence/#aperture
Understanding the fundamental concepts of Equivalence requires making important distinctions between various terms which people often take to mean the same thing. It is very much akin to making the distinction between "mass" and "weight", two terms which most people take to mean the same thing, when, in fact, they measure two different (but related) quantities. While there are circumstances where making the distinction is unnecessary, there are other times when it is critical.
The first of these distinctions that needs to be made is between aperture and f-ratio. The term "aperture", by itself, is vague -- we need a qualifying adjective to be clear. There are three different terms using "aperture":
- The physical aperture (iris) is the smallest opening within a lens.
- The virtual aperture (entrance pupil) is the image of the physical aperture when looking through the front element of the lens.
- The relative aperture (f-ratio) is the quotient of the focal length and the virtual aperture.
The same f-ratio results in the same exposure for a given scene luminance and shutter speed, regardless of the focal length or format. This is very convenient when working within a format, but very deceptive when comparing between formats.
Yes. The maximum possible diameter of the virtual aperture (entrance pupil) is the diameter of the front element of the lens. If you work out the diameter of the entrance pupil (divide the focal length by the f-ratio) for longer lenses, you'll notice it's about the same as the diameter of the front element for longer lenses and, of course, telescopes.
I hope the above paragraph has explained that satisfactorily.
For shorter lenses, the diameter of the entrance pupil (virtual aperture) is generally much smaller than the diameter of the front element.
Ontario Gone
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It does keep things simpler, yet i wouldn't mind doing it the other way either.
LOL yes but the below one seems to contradict it. By "maximum possible diameter", do you mean the entrance pupil can't possible be engineered larger than the front element (which makes sense), or that they just sometimes happen to make the pupil smaller than the front element and sometimes the same size?
I guess regardless of the answer to the above question, is it safe to say that whether the entrance pupil is as big as the front element or not, the larger that front element the more light will be gathered and thus the larger the F stop (smaller F stop #)?
If you happen to say yes to this part, it makes a little more sense.
Great Bustard
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The f-ratio was also more convenient when changing the ISO required changing film. Nowadays, with Auto ISO and such, it's not as important.
However, another nice feature of the relative aperture (f-ratio) is that the DOF is the same for a given framing and f-ratio on a given format. For example, a photo 10 ft from the subject at 100mm f/2 will have the same DOF as a photo 5 ft from the subject at 50mm f/2 on the same camera.
On the other hand, the background blur for objects portions of the scene far from the focal plane will be more blurred for the 100mm f/2 photo than the 50mm f/2 photo (twice as blurred for portions of the scene at infinity, since the aperture diameter at 100mm f/2 is twice the aperture diameter at 50mm f/2).
Hmm. You'll have to quote the contradiction, 'cause all seemed in order to me.
I mean the entrance pupil diameter cannot exceed the diameter of the front element, and, as a general rule, the longer the lens, the maximum diameter of the entrance pupil approaches the diameter of the front element.
No. For example, consider the Sigma 35 / 1.4 (67mm filter) vs the Canon 35 / 1.4L (72mm filter) -- both have the same max aperture diameter (35mm / 1.4 = 25mm) which is considerably smaller than the front element in both cases.
Hopefully it makes even more sense, now.
I take it having two different virtual apertures is not a problem because the exit pupil doesn't come into play when talking about exposure.
Yes, you call the entrance pupil the virtual aperture and Wiki says the entrance pupil is an effective aperture, so that makes virtual aperture equal to effective aperture and does that make virtual synonymous with effective?
And then when a macro lens focuses at 1:1 magnification the lens moves significantly away from the sensor so does the relative aperture becomes an effective or virtual relative aperture, or does the virtual aperture become an effective virtual aperture, or does the virtual or relative aperture actually change, or what? I am not finding an agreement.
That's been my confusion and then throw "equivalence" into the mix and it is difficult to get a handle on it all, but I am working on it.
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David Strachan
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Some of these new "focal reducer speed booster" lenses, which make small format same as full-frame image, may be considered virtual aperture??
Using these you get a virtual one f stop increase in light, but not the same shallower characteristic depth of field as a true aperture opened one stop.
Perhaps that's the difference in the nomenclature??
cheers Dave S
Using these you get a virtual one f stop increase in light, but not the same shallower characteristic depth of field as a true aperture opened one stop.
Perhaps that's the difference in the nomenclature??
cheers Dave S
Great Bustard
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Correct.
Not quite. ;-)
The reason the entrance and exit pupils are virtual is because the image of the physical aperture (iris) does not form an image that can be projected:
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/image2.html
The reason Wikipedia calls the entrance pupil the "effective aperture" is because the entrance pupil is the aperture that matters in terms of the visual properties of the photo.
For example, just as 50mm on mFT is "effectively" 100mm on FF, f/2 on mFT is "effectively" f/4 on FF. Specifically, the diameter of the entrance pupil at 50mm f/2 is 50mm / 2 = 25mm, and the diameter of the entrance pupil at 100mm f/4 is 100mm / 4 = 25mm. Thus, the "effective aperture" is 25mm.
Both the effective focal length and effective f-ratio are scaled by a factor of 1+m/p, where m is the magnification and p is the ratio of the diameter of the entrance pupil to the diameter of the exit pupil. For example, 1:1 means m=1, 1:2 means m=0.5, etc. For low magnifications, the correction factor is essentially 1, so we can ignore it.
For larger magnifications, we must consider both m and p, where p=1 for a symmetric lens, and, as a general rule, gets progressively smaller for more telephoto lenses and progressively larger for wider lenses.
The focal length and f-ratio marked on the lens are both for infinity focus (m=0). For larger magnifications, we have:
- Effective Focal Length: EFL = FL / (1+m/p)
- Effective Relative Aperture: ef = f · (1+m/p)
EP = EFL / ef = [FL / (1+m/p)] / [f · (1+m/p)] = FL / f
Again, we see that the entrance pupil is central.
Hopefully, the whole idea of Equivalence makes even more sense now.
Great Bustard
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A focal reducer does exactly what the name implies: reduce the focal length. However, the diameter of the entrance pupil does not change, so the f-ratio scales accordingly.
For example, let's consider a 50 / 1.4 lens. The diameter of the virtual aperture (entrance pupil) is 50mm / 1.4 = 36mm. Now let's slap on a 0.5x focal reducer. The focal length is now 25mm, but the aperture diameter did not change, so we have 25mm / 36mm = 0.7. Thus, the 50 / 1.4 + 0.5x focal reducer becomes a 25 / 0.7.
This is exactly how teleconverters work, except they increase the focal length and thus decrease the f-ratio. For example, a 50 / 1.4 + 2x teleconverter becomes a 100 / 2.8 (100mm / 2.8 = 36mm).
Of course, both teleconverters and focal reducers will introduce optical aberrations of their own, so the quality will go down. How far down depends on the particular focal reducer or teleconverter.
On the other hand, a focal reducer or teleconverter designed for a particular lens can also include optical corrections to known aberrations of the lens, and actually improve performance, so long as those corrections are greater than the aberrations introduced.
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