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Old Mar 18, 2010, 10:03 AM   #21
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Within their respective angles of view, absolutely. What were you thinking?



Light gathering ability is a function of focal length and pupil diameter. The aperture (expressed as an f-number) is a function of just those two values (f-number = focal length / pupil diameter.) All f/2.8 lenses have the same light gathering ability regardless of their focal length or the crop factor of the body they're mounted on.
This is misinformation. And I have to clear it up.

The total amount of light that is captured by a lens is it's t-stop. f stop, refers to the intensity of the light, not the actual amount of light.

All lenses at a particular f stop have the same intensity of light, the actual amount of light is measured by the t-stop. It's an esoteric number, and we don't need to know it in order to take pictures.

The relationship of light gathering ability and focal lenght is how real magnification is acquired.

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Old Mar 18, 2010, 12:31 PM   #22
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T-stops are a measure of the ability of a lens to transmit light, as opposed to absorb it. All optical mediums (glass being the most common) absorb some light, so the amout of light that passes through a lens is always less that the amount of light that enters it. The actual amount of light that passes through a lens is expressed in T-numbers. T-numbers are direct measurements of the amount of light that pass through lenses, and are not affected by focal length.

F-numbers are not direct measurements, but are ratios of two independant variables: focal length and pupil diameter.

Aperture affects depth of field, so the F-number is necessary for more than just exposure, and exposure systems directly measure the amount of light that passes through a lens anyway, so "T-stop" is not pertinent. In effect, "T-Stop" is analogous to "Exposure Value (EV)".

So, the total amount of light that is transmitted by a lens is it's t-stop, but so what? The total amount of light that is transmitted by a lens is also adequately described by its EV.

And just for the sake of clarity, could you please define "real magnification"?
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Old Mar 18, 2010, 1:25 PM   #23
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T-stops are a measure of the ability of a lens to transmit light, as opposed to absorb it. All optical mediums (glass being the most common) absorb some light, so the amout of light that passes through a lens is always less that the amount of light that enters it. The actual amount of light that passes through a lens is expressed in T-numbers. T-numbers are direct measurements of the amount of light that pass through lenses, and are not affected by focal length.

F-numbers are not direct measurements, but are ratios of two independant variables: focal length and pupil diameter.

Aperture affects depth of field, so the F-number is necessary for more than just exposure, and exposure systems directly measure the amount of light that passes through a lens anyway, so "T-stop" is not pertinent. In effect, "T-Stop" is analogous to "Exposure Value (EV)".

So, the total amount of light that is transmitted by a lens is it's t-stop, but so what? The total amount of light that is transmitted by a lens is also adequately described by its EV.

And just for the sake of clarity, could you please define "real magnification"?
T stops are a measurement of the absolute amount of light gathered, f-stops are a measurement of the relative amount of light gathered.

All things being equal 450mm lens gathers more light then a 300mm lens, and gives us greater reach, greater real magnification.

And yes, it's only becomes relevant in discussions like this; discussions where we make decisions on what kind of system we wish to purchase.

In normal day to day photography, it's irrelevant because the relative amount of light is all that we are concerned with.

A 50mm lens, with a fifty megabyte sensor will not resolve two objects that are close together at 1/4 of a mile, whereas an 800mm lens will. This is simply because the absolute amount of light gathered by the 800 far surpases that of the 50mm.

The misinformation that you post is that an f-stop is a measurement of the light gathered.

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TCav: All f/2.8 lenses have the same light gathering ability regardless of their focal length or the crop factor of the body they're mounted on.
True magnification is the ability to resolve the angular seperation of objects, and can only be a side effect of the ability to gather light and the focal length.

Now resolving power is also a product not just of the lens, but of the entire system being used. Wont do much good to have an 800mm lens and a 500K sensor, just as it wont do much good to have a 50mm lens and a fifty megabyte sensor.

But all things being equal, a FF camera with a 450mm lens will outresolve one with 1.5 crop and a 300mm sensor. It will outresolve it because the lens gathers more light. What IS true, is that anything the 1.5 crop sensor can resolve, AND has more pixels on the target, will have a clearer version of the subject. Which is why the cropping factor has some value.

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Old Mar 18, 2010, 3:49 PM   #24
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The actual physical aperture of the pupil does matter.

For instance a 300mm f2 lens will have an aperture of 150mm. So will a 600mm f4 lens.

The sensor size matters because of the actual number of photons captured. A FF sensor will capture more photons than a crop sensor with the same exposure and thereby gets its low-light advantage.

I have found this essay very interesting:

http://www.josephjamesphotography.co...ence/light.htm
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Old Mar 18, 2010, 4:13 PM   #25
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The actual physical aperture of the pupil does matter.

For instance a 300mm f2 lens will have an aperture of 150mm. So will a 600mm f4 lens.

The sensor size matters because of the actual number of photons captured. A FF sensor will capture more photons than a crop sensor with the same exposure and thereby gets its low-light advantage.

I have found this essay very interesting:

http://www.josephjamesphotography.co...ence/light.htm
Interesting read, I'll finish it later. FF sensors have an edge over cropped sensors for everything BUT telephoto. Which is why when I chose a camera, I choose the Nikon D2x over the Canon1Ds Mark II. I simply said, to heck with the FF and got myself both 500, 800 and 1100 lenses and 'scopes. I can't get more powerful lenses, unless I want to go up to the 20K or more mark, which ain't gonna happen.

Here's an interesting demonstration of what my 800 can do compared to a normal view. In the first I have have outlined roughly the area shot by the 800

I'm posting three images, taken years apart, and in different seasons. What they share in common is being taken from the same spot. The two hawk images represent almost a full frame and a tight crop.







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Old Mar 18, 2010, 4:23 PM   #26
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T stops are a measurement of the absolute amount of light gathered, f-stops are a measurement of the relative amount of light gathered.
Just to be clear, f-stops aren't a measurement of anything.

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Originally Posted by Chato View Post
All things being equal 450mm lens gathers more light then a 300mm lens, and gives us greater reach, greater real magnification.
Actually, a 450mm lens has a narrower angle of view than a 300mm lens, is therefore more selective about the light it receives, and thus gathers less light, though it does give greater magnification.

But increasing the magnification doesn't increase the number of photons entering a lens. Photography is an entirely passive activity. Changing the focal length of a lens doesn't make a subject give off more light. And by narrowing the angle of view, you're reducing the number of similarly sized subjects from which light is being reflected, thereby reducing the light entering the lens.

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And yes, it's only becomes relevant in discussions like this; discussions where we make decisions on what kind of system we wish to purchase.
Actually, as I said in my previous post, a camera's autoexposure system compensates for variations in the absolute amount of light gathered, so it is irrelevant in discussions like this.

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Originally Posted by Chato View Post
In normal day to day photography, it's irrelevant because the relative amount of light is all that we are concerned with.
Actually, the absolute amount of light gathered is all we're ever concerned with, whether it is described in terms of t-stops or EV.

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Originally Posted by Chato View Post
A 50mm lens, with a fifty megabyte sensor will not resolve two objects that are close together at 1/4 of a mile, whereas an 800mm lens will. This is simply because the absolute amount of light gathered by the 800 far surpases that of the 50mm.
The resolving power of a lens is always measured relative to the size of the projected image, not the absolute size of the subject. A good 800mm lens will out-resolve a bad 50mm lens, but a good 50mm lens will out-resolve a bad 800mm lens. What makes a lens good or bad is, among other things, it's ability to resolve.

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The misinformation that you post is that an f-stop is a measurement of the light gathered.
No. I never said that.

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True magnification is the ability to resolve the angular seperation of objects, and can only be a side effect of the ability to gather light and the focal length.
Magnification has nothing to do with resolution. The ability to resolve the angular seperation of objects is resolution, not magnification, and the angular seperation of objects has nothing to do with the ability to gather light or the focal length.

You said it. "... the ability to resolve the angular seperation (sic) of objects. ...", not the linear separation of objects. A lens with a longer focal length may be better able to resolve the linear separation of objects than a lens with a shorter focal length, but it's the angular separation of objects by which we judge the resolution of a lens.

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Now resolving power is also a product not just of the lens, but of the entire system being used. Wont do much good to have an 800mm lens and a 500K sensor, just as it wont do much good to have a 50mm lens and a fifty megabyte sensor.
The resolving power of a lens is dependant on a number of factors, none of which have anything to do with the image sensor. The resolution of an image sensor is dependant on a number of factors, none of which have anything to do with the lens. To be sure, the resolving power of a camera is dependant on the resolving power of these two major components. But focal length all by itself doesn't change the resolution of a lens, much less an image sensor.

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But all things being equal, a FF camera with a 450mm lens will outresolve one with 1.5 crop and a 300mm sensor. It will outresolve it because the lens gathers more light. What IS true, is that anything the 1.5 crop sensor can resolve, AND has more pixels on the target, will have a clearer version of the subject. Which is why the cropping factor has some value.


There are a number of fallacies all stacked up here, so I'm going to point them out one at a time:
  1. As I've already said, a 450mm lens has a smaller angle of view and therefore, receives and transmits less total light than a 300mm lens, all other things being equal.
  2. The ability of an image sensor to gather light is dependant not it's size, but on the number and size of its photoreceptors. If we presume that a 'Full Frame' image sensor has the same number and size photoreceptors as an APS-C image sensor, they will both gather the same amount of light.
  3. The ability of a lens to resolve an image has nothing to do with its focal length, and it also has nothing to do with the size of the image sensor the image is projected onto.
  4. The 'Crop Factor' does not alter the resolution of the image sensor. While it does place more emphasis on a portion of the projected image, and optical flaws in that portion of the lens would be more prominent, that portion of the lens is in the center and is less likely to contain significant optical flaws anyway. In that way, the 'Crop Factor' does indirectly increase the resolution of a camera. (You happen to be correct here. Just not for the reasons you stated.)
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Old Mar 18, 2010, 4:36 PM   #27
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The resolving power of a lens is always measured relative to the size of the projected image, not the absolute size of the subject. A good 800mm lens will out-resolve a bad 50mm lens, but a good 50mm lens will out-resolve a bad 800mm lens. What makes a lens good or bad is, among other things, it's ability to resolve.
Quote:
TCav: All f/2.8 lenses have the same light gathering ability regardless of their focal length or the crop factor of the body they're mounted on.


Quote:
Originally Posted by Chato
The misinformation that you post is that an f-stop is a measurement of the light gathered.
No. I never said that.

[/QUOTE]

I should have quit this thread when I said I would. There is no arguing with someone who deletes his own words whenever convenient.

I'm done with it...

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Old Mar 18, 2010, 4:57 PM   #28
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All f/2.8 lenses do have the same light gathering ability (and it's not a measurement.) How well they use it is where your T-stop comes in, and as I said, exposure systems handle the extremely slight variations in light transmission from lens to lens anyway.

But if that's the sum total of your objection to my posts, then I concede your point.

Thanks. It's been fun.
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Old Mar 18, 2010, 9:32 PM   #29
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Originally Posted by Chato View Post
All things being equal 450mm lens gathers more light then a 300mm lens, and gives us greater reach, greater real magnification.



Dave
I think I begin to understand the confusion. Your statement is true if one is talking about diameter of the lens, but not focal length.

Your earlier references to telescopes should have tipped me off sooner.
It seems you need to check some basic references and definitions.
F-Stop is often as source of confusion. It is correctly written as f/(number). IOW, the focal length of the lens divided by a number. The maximum theoretical f/number of a lens is the focal length divided by it's entrance pupil.

Magnification, as I seem to recall, is determined by the focal length of the objective lens divided by the focal length of the eyepiece (in a telescope). In a camera, the eyepiece fl is a fixed quantity, so we refer to lens focal length.

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Old Mar 19, 2010, 10:57 AM   #30
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I think I begin to understand the confusion. Your statement is true if one is talking about diameter of the lens, but not focal length.

Your earlier references to telescopes should have tipped me off sooner.
It seems you need to check some basic references and definitions.
F-Stop is often as source of confusion. It is correctly written as f/(number). IOW, the focal length of the lens divided by a number. The maximum theoretical f/number of a lens is the focal length divided by it's entrance pupil.

Magnification, as I seem to recall, is determined by the focal length of the objective lens divided by the focal length of the eyepiece (in a telescope). In a camera, the eyepiece fl is a fixed quantity, so we refer to lens focal length.

brian
Light gathering ability - i.e. diameter is the key aspect of resolving power, Focal lenth is the key aspect of magnification. Put them together and you have your final result.

Eye pieces on telescopes are actually often interchangable and magnify the final result of all that glass. They are an add on to the resolving power of the telescope. If the eyepiece exceeds the ability of the telescope to resolve two objects, then the result is "empty magnification," which is more or less what the cropping factor does.

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A telescope's focal length is the distance light travels from a telescope's lens or mirror to the point inside the telescope where it is focused.
Focal lengths for commercial telescopes vary from 15.8 inches to 118 inches (400 millimeters to 3000 millimeters). The longer the focal length, the larger the image at the focal point. Think of it like the distance between a slide projector and the screen. Move the screen and slide projector further apart, and the image gets larger and dimmer.
"Focal ratio" is the ratio of the instrument's focal length to its aperture. It's found by dividing focal length by objective diameter. A telescope with a mirror of 8 inches across and a focal length of 48 inches has a focal ratio of f/6. (Notice that you can also find a telescope's focal length by multiplying focal ratio by aperture.)
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