It isn't quite that simple.
Light transmission
- Thread starter Spike A
- Start date
Fred Bohl
Gold $$ Contributor
I think you misunderstand equating f/# with amount of light when applied to scope objective. When used in camera lenses as in the typical layout shown, the focal length is from H' to the image plane and D is the diameter of the image of the diaphram on the front lens element. Within a particular system (with a particular imager size and mount configuration), a range of lens focal lengths is designed so that the diaphram openings for a given f/# will all result in the same light on the imager.
In either case (scope objective or camera lens) the amount of light passing through the clear aperture, D, is proportional to area of the aperture or D squared. See also Scope Brightness – Perception vs Measured.
Last edited:
I read that; several times. It contradicts the physics of light and lenses regarding image planes and lens focal lengths. As any of several optical formulas used in design of binoculars, telescopes, microscopes, camera lenses and rifle scopes to determine each lens element's focal length and image plane sizes and positions easily demonstrate.
Last edited:
Light rays are not converged in lenses; they're spread out all over it. They converge at the image plane; 5" behind a lens' optical center with a 5" focal length focused on a target at infinity.
Light rays from a point on target 100 yards down range spread out in a 2 MOA cone as they enter a 2.095 inch diameter lens with a 5 inch focal length in a rifle scope. They're focused (converged) to a point about 5.01 inches back from its center at its first image plane. Rays at the lens edge are bent in about 11.5 degrees at the lens' edge then lesser angles towards lens center where they're not bent at all.
Light rays from a point on target 100 yards down range spread out in a 2 MOA cone as they enter a 2.095 inch diameter lens with a 5 inch focal length in a rifle scope. They're focused (converged) to a point about 5.01 inches back from its center at its first image plane. Rays at the lens edge are bent in about 11.5 degrees at the lens' edge then lesser angles towards lens center where they're not bent at all.
Last edited:
Thanks for that image. I now understand your reasoning. What link is that Minolta 28mm AF lens' image at? Would like to see it to understand what it's trying to convey as it shows the field of view angle across the image, not light ray focusing to a single point.
Those parallel light rays entering that lens focus at a single point on the film/sensor plane to make the image there. Too bad that picture doesn't show that. If light rays from a distant point entering the lens really spread out as shown, the image will be very much out of focus.
Those parallel light rays entering that lens focus at a single point on the film/sensor plane to make the image there. Too bad that picture doesn't show that. If light rays from a distant point entering the lens really spread out as shown, the image will be very much out of focus.
Last edited:
That image has been in my photo file since 1990.
It must have originated somewhere in Minolta. But it still doesn't show those parallel light rays entering then converging from the lens focusing at a single point on the film/sensor plane.
Hold a semitransparent plastic jar lid behind your camera lenses. Move it to where the lens image focuses something sharply on it as viewed behind it. That point is where the lens' image plane is at. It happens only when all light rays from points on subject focus at single points. When the image is blurry, those light rays are spread out and diverge in front of or behind the lens' image plane.
Hold a semitransparent plastic jar lid behind your camera lenses. Move it to where the lens image focuses something sharply on it as viewed behind it. That point is where the lens' image plane is at. It happens only when all light rays from points on subject focus at single points. When the image is blurry, those light rays are spread out and diverge in front of or behind the lens' image plane.
Camera lenses contain a number of convex elements (lenses). A convex lens is a converging lens.
Edit: A magnifying glass contains a convex lens. Though the light rays enter the entire element, those rays converge to a single point, allowing you to take advantage of the sun, and let you burn a small hole in a piece of paper, rather than a hole the size of the element. The greater the distance, the smaller the point.
Edit: A magnifying glass contains a convex lens. Though the light rays enter the entire element, those rays converge to a single point, allowing you to take advantage of the sun, and let you burn a small hole in a piece of paper, rather than a hole the size of the element. The greater the distance, the smaller the point.
Last edited:
Right.
They also have concave lenses that diverge light rays; the 2nd and 3rd lenses back from the front of that lens. That's done so the iris diaphragm leaves can fit in a smaller space. Their physical diameter for a given f-stop is smaller than that of the front lens diameter for that stop, but does the same thing optically.
Did you ever put a telescope converter on the back of a camera lens? Marvelous image quality. Stop the lens down to see all the microscopic dust and lint on lens elements appear.
They also have concave lenses that diverge light rays; the 2nd and 3rd lenses back from the front of that lens. That's done so the iris diaphragm leaves can fit in a smaller space. Their physical diameter for a given f-stop is smaller than that of the front lens diameter for that stop, but does the same thing optically.
Did you ever put a telescope converter on the back of a camera lens? Marvelous image quality. Stop the lens down to see all the microscopic dust and lint on lens elements appear.
Last edited:
There is actually 5 convex elements before the point of convergence. This is the element group in my new Canon 11-24:
Last edited:
I see them, I count 6. Some are part of doublet and triplet compound lenses.
The convergent point for light rays is 44 mm back from the lens mount flange that fits against the body flange. That's the image plane on the pixeled sensor (and viewfinder's ground glass screen) all the modern Canon SLR camera lenses focus at.
Older Canon film SLR's flange-image distance is 42 mm. That Minolta 28mm AF lens mount flange is 43.5mm forward from its image plane.
The convergent point for light rays is 44 mm back from the lens mount flange that fits against the body flange. That's the image plane on the pixeled sensor (and viewfinder's ground glass screen) all the modern Canon SLR camera lenses focus at.
Older Canon film SLR's flange-image distance is 42 mm. That Minolta 28mm AF lens mount flange is 43.5mm forward from its image plane.
Last edited:
Wow, I didn't think I would come back to this thread, but I thought I would throw in my two cents and say simply that you can compare a riflescope and a camera lens only so far. A camera lens' purpose is to project an image circle on the sensor/film that presents the focused image of the object with the proper perspective and amount of light (illuminance,) that can be controlled by the camera/photographer.
Camera lenses are sold, bought, marketed, discussed using two numbers, focal length and f-number. The first one tells you the perspective of the lens and the second one gives you the amount of light available at the widest aperture. The lower the f-number, the faster the lens or the more light that it can collect and ultimately transmit to the sensor. You will always talk about a lens by mentioning the focal length and the maximum F-number; to do otherwise makes no sense. The f-number is a dimensionless number that represents the ratio of the objective lens to the focal length. A bigger objective lens at the same focal length just means more light is able to come in.
Riflescopes on the other hand, are sold, bought, marketed and discussed using two number, magnification and objective lens diameter. Unlike camera lenses, riflescope are used only for magnification, never for wide-angle perspective. Also, unlike camera lenses, riflescopes present an image to the eye of the user that uses lenses to magnify a portion of the image that is formed by the objective lens onto the first focal plane.
The diameter of the first focal plane, the equivalent of the image circle of the camera lens onto the sensor/film, is actually the inner diameter of the main tube. The erector tube is move up and down and side to side to select the portion of this first focal plane image that will be magnified by the erector zoom lenses and the ocular or eyepiece. The more illuminance available from the objective the brighter that image is and certainly when you magnify a portion of it, that becomes important.
In other words, if the riflescope cannot collect light, it can't transmit it.
When riflescopes become digital, the paradigm shift will be staggering, because at that point, the front part of the scope, the one in front of the erector cell, will become just like a camera lens getting the image circle to the CMOS sensor at the first focal plane, and the erector cell and zoom lenses will disappear as they are replaced by an OLED screen on which the computer can zoom digitally.
Also, we will be able to play with ISO values and other things. Should make for some interesting debates.
Camera lenses are sold, bought, marketed, discussed using two numbers, focal length and f-number. The first one tells you the perspective of the lens and the second one gives you the amount of light available at the widest aperture. The lower the f-number, the faster the lens or the more light that it can collect and ultimately transmit to the sensor. You will always talk about a lens by mentioning the focal length and the maximum F-number; to do otherwise makes no sense. The f-number is a dimensionless number that represents the ratio of the objective lens to the focal length. A bigger objective lens at the same focal length just means more light is able to come in.
Riflescopes on the other hand, are sold, bought, marketed and discussed using two number, magnification and objective lens diameter. Unlike camera lenses, riflescope are used only for magnification, never for wide-angle perspective. Also, unlike camera lenses, riflescopes present an image to the eye of the user that uses lenses to magnify a portion of the image that is formed by the objective lens onto the first focal plane.
The diameter of the first focal plane, the equivalent of the image circle of the camera lens onto the sensor/film, is actually the inner diameter of the main tube. The erector tube is move up and down and side to side to select the portion of this first focal plane image that will be magnified by the erector zoom lenses and the ocular or eyepiece. The more illuminance available from the objective the brighter that image is and certainly when you magnify a portion of it, that becomes important.
In other words, if the riflescope cannot collect light, it can't transmit it.
When riflescopes become digital, the paradigm shift will be staggering, because at that point, the front part of the scope, the one in front of the erector cell, will become just like a camera lens getting the image circle to the CMOS sensor at the first focal plane, and the erector cell and zoom lenses will disappear as they are replaced by an OLED screen on which the computer can zoom digitally.
Also, we will be able to play with ISO values and other things. Should make for some interesting debates.
Good write up.
Years ago, there was an idea that rifle scopes, binocs and spotting scopes should have an "f" number for their objective lenses. A 16X rifle scope with a 180mm focal length objective lens having a 40mm diameter clear aperture would be marketed with an f/4.5 lens. Same scope with a different erector lens at a different place but the same eyepiece lens would be a 10X one, but noticeably brighter in dim light with the same objective lens and 4.5 f-stop.
Both scopes' first image is equally bright. Images at the second plane are not equal brightness.
Years ago, there was an idea that rifle scopes, binocs and spotting scopes should have an "f" number for their objective lenses. A 16X rifle scope with a 180mm focal length objective lens having a 40mm diameter clear aperture would be marketed with an f/4.5 lens. Same scope with a different erector lens at a different place but the same eyepiece lens would be a 10X one, but noticeably brighter in dim light with the same objective lens and 4.5 f-stop.
Both scopes' first image is equally bright. Images at the second plane are not equal brightness.
Last edited:
I can see why riflescope makers do not use focal length and f-number because in reality, the only place where that is important is between the objective lens and the FFP. You don't buy a riflescope for photographic purposes, it's for magnification purposes only. So the focal length would be meaningless since a good part, if not most of the magnification comes from enlarging the image at the FFP and then at the ocular. Knowing the objective size gives you an idea how bright the scope and that's what counts, and as you say, that's how bright it is at the FFP. What happens to it after that is a subject to the vagaries of lenses, coatings and other fun stuff. By the time you get to the SFP, it have gone through most of it, the ocular remains.
Almost true. Except each time a given diameter objective lens increases 41% in focal length, brightness of its focal plane (1st one in scope) is reduced 50%. A 50mm diameter obj. lens with a 180mm focal length has its image plane equal in brightness as the image plane of a 25mm diameter obj. lens with a 90mm focal length.
It's an optical nightmare. Best resolved by simply calculating exit pupil diameter.
Last edited:
This is why I always say that for the SAME FOCAL LENGTH, a bigger objective lens brings in more light, it does not magnify the image. You just changed focal lengths on me. Naughty, naughty.
Yes, I did. 'Twasn't mentioned in the phrase I questioned, but you're right.
That old inverse square law rules many things; bore capacity is another one besides lens stuff.
Similar threads
Upgrades & Donations
This Forum's expenses are primarily paid by member contributions. You can upgrade your Forum membership in seconds. Gold and Silver members get unlimited FREE classifieds for one year. Gold members can upload custom avatars.
Click Upgrade Membership Button ABOVE to get Gold or Silver Status.
You can also donate any amount, large or small, with the button below. Include your Forum Name in the PayPal Notes field.
To DONATE by CHECK, or make a recurring donation, CLICK HERE to learn how.
Click Upgrade Membership Button ABOVE to get Gold or Silver Status.
You can also donate any amount, large or small, with the button below. Include your Forum Name in the PayPal Notes field.
To DONATE by CHECK, or make a recurring donation, CLICK HERE to learn how.