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03-29-2011, 07:14 AM   #31
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QuoteOriginally posted by rhodopsin Quote
2nd version of 6x7 3.5/55 has 60mm front element, the first version I don't have to measure, but from photos and diagrams its front element appears to be near 80mm.



Sorry I don't see where you also said wide angle lenses. 1.8/55 is a 'normal' lens.
Look

I am not going to argue lens design all day here, but the reference to wide angle for a 55mm is based upon the 6x7 lens.

wide angle lenses, and this is format dependant, have considerations that drive the front element size up to attain the field of view. this can and will change from lens maker to lens maker.

I can show you the same with film lenses, my 28mm vivitar F2.5 has a 47mm front element, yet the tamron 28mmF2,5 has a 23mm front element, the rear elements are 19.0 and 22.2 mm respectively so neither seems to track meeting an image circle projection

so you can see for wide angle there is a high degree of variation and the elements don't necessairly follow the strict rule of aperture is FL over element diameter, BUT

look at 50mm and above, or even 50mm and the ratio of Focal length to front element diameter

my K50F1.4 has a 38mm front element (ratio 1.312) , my M50F1.7 has a 30.1 mm front element (ratio 1.657) and my M50F2 has a front element 28.57 (ratio 1.75)

All the 50's are following pretty close to front element diameter = focal lengrh / max aperture and it follows all tele lenses from there up.

There is nothing stopping larger front elements at any aperture but the light is always restricted somewhere in baffels, and it does nothing to increase the exposure in the image circle. whether that light is dissipated in the lens internals or the mirror box is irrelevant.

Want further proof, just look at a 50mmF4 lens for a speed graphic, it is probably only 25mm in diameter at best, yet it covers 4x5.

the thing to note, and this is really true of reflex cameras and explains why wide angle designs are so expensive, is that the enture lens, including the rear element is further away from the film / sensor than the effective focal length of the lens.

Pentax has a regestry distance of 45.46mm for its SLRs.

the regestry distance for the 6x7 is 84.95mm this forces all sorts of lens design that is not required on a telephoto lens for the 6x7 to make it focus wehen mounted further away from the sensor than the effective focal length of the lens., the same is true for my 28mm lenses mounted on an SLR at 46mm from the sensor.


Last edited by Lowell Goudge; 03-29-2011 at 07:23 AM.
03-29-2011, 09:47 AM   #32
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QuoteOriginally posted by Lowell Goudge Quote
Look

I am not going to argue lens design all day here, but the reference to wide angle for a 55mm is based upon the 6x7 lens.

wide angle lenses, and this is format dependant, have considerations that drive the front element size up to attain the field of view. this can and will change from lens maker to lens maker.

I can show you the same with film lenses, my 28mm vivitar F2.5 has a 47mm front element, yet the tamron 28mmF2,5 has a 23mm front element, the rear elements are 19.0 and 22.2 mm respectively so neither seems to track meeting an image circle projection

so you can see for wide angle there is a high degree of variation and the elements don't necessairly follow the strict rule of aperture is FL over element diameter, BUT

look at 50mm and above, or even 50mm and the ratio of Focal length to front element diameter

my K50F1.4 has a 38mm front element (ratio 1.312) , my M50F1.7 has a 30.1 mm front element (ratio 1.657) and my M50F2 has a front element 28.57 (ratio 1.75)

All the 50's are following pretty close to front element diameter = focal lengrh / max aperture and it follows all tele lenses from there up.

There is nothing stopping larger front elements at any aperture but the light is always restricted somewhere in baffels, and it does nothing to increase the exposure in the image circle. whether that light is dissipated in the lens internals or the mirror box is irrelevant.

Want further proof, just look at a 50mmF4 lens for a speed graphic, it is probably only 25mm in diameter at best, yet it covers 4x5.

the thing to note, and this is really true of reflex cameras and explains why wide angle designs are so expensive, is that the enture lens, including the rear element is further away from the film / sensor than the effective focal length of the lens.

Pentax has a regestry distance of 45.46mm for its SLRs.

the regestry distance for the 6x7 is 84.95mm this forces all sorts of lens design that is not required on a telephoto lens for the 6x7 to make it focus wehen mounted further away from the sensor than the effective focal length of the lens., the same is true for my 28mm lenses mounted on an SLR at 46mm from the sensor.
Okay, one last example to show that more light must enter a lens in order for it to cast a larger image circle of the same brightness as a smaller image circle, i.e. lenses designed for larger formats must let in more light than lens for smaller format, to produce the same f/#.

Suppose a circle of light with even brightness 2' in diameter is projected onto a wall. Mask any portion of the circle of light, the brightness of any portion is identical, but by masking, the total amount of light is reduced by the mask -- the mask subtracts from the total amount of light projected.

Next, cast a 1' circle of light with the same degree of even brightness as the 2' circle. Looks exactly like the 2' circle masked to 1', both the 2' circle masked to 1' and the 1' circle have identical brightness. The smaller image circle requires a less bright source to project the same brightness in 1' circle as the 2' circle has within the masked 1' circle.

In any lens, the extra light required to project a larger image circle must enter the entrance pupil, i.e. to cast the larger image circle the entrance pupil must be larger, if both circles are to be of identical brightness per unit area.

With F=f/D, notice, by definition, D is the effective diameter of the entrance pupil. (ref. F-number - Wikipedia, the free encyclopedia) The effective diameter of the entrance pupil, for example as seen in wide angle and zooms, is not the same as the physical diameter of the front element -- thus we see wide angle lenses with front elements much larger than the effective diameter. The same is true with lenses of any focal length.

Registry distance is the same when 6x7 lens is mounted on 35mm camera -- the 6x7 lens is mounted farther away than 35mm lens. The greater distance requires more light be projected to attain equal brightness, thus the 6x7 lens must let in more light than the 35mm lens. The same applies to different format sizes FF and APS-C, the FF lens lets in more light.
03-29-2011, 10:22 AM   #33
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QuoteOriginally posted by rhodopsin Quote
Okay, one last example to show that more light must enter a lens in order for it to cast a larger image circle of the same brightness as a smaller image circle, i.e. lenses designed for larger formats must let in more light than lens for smaller format, to produce the same f/#.

Suppose a circle of light with even brightness 2' in diameter is projected onto a wall. Mask any portion of the circle of light, the brightness of any portion is identical, but by masking, the total amount of light is reduced by the mask -- the mask subtracts from the total amount of light projected.

Next, cast a 1' circle of light with the same degree of even brightness as the 2' circle. Looks exactly like the 2' circle masked to 1', both the 2' circle masked to 1' and the 1' circle have identical brightness. The smaller image circle requires a less bright source to project the same brightness in 1' circle as the 2' circle has within the masked 1' circle.

In any lens, the extra light required to project a larger image circle must enter the entrance pupil, i.e. to cast the larger image circle the entrance pupil must be larger, if both circles are to be of identical brightness per unit area.

With F=f/D, notice, by definition, D is the effective diameter of the entrance pupil. (ref. F-number - Wikipedia, the free encyclopedia) The effective diameter of the entrance pupil, for example as seen in wide angle and zooms, is not the same as the physical diameter of the front element -- thus we see wide angle lenses with front elements much larger than the effective diameter. The same is true with lenses of any focal length.

Registry distance is the same when 6x7 lens is mounted on 35mm camera -- the 6x7 lens is mounted farther away than 35mm lens. The greater distance requires more light be projected to attain equal brightness, thus the 6x7 lens must let in more light than the 35mm lens. The same applies to different format sizes FF and APS-C, the FF lens lets in more light.
You are missing the point here.

You don't need a larger objective than the effective aperture to obtain the average uniform illumination over the image circle, and the diameter of the image circle is NOT, repeat NOT dependant upon the diameter of the lens, but restrictions either internally or externally in the light path, that is all. Light is let in through the front element, and as I pointed out the limit there is the effective aperture, that is all,


you are confusiong the design issues that are needed to mount a lens 2-3 times further away then its focal length nominally, yet still produce the same angle of view and image magnification.

If your example were true explain why for something like a 300mm telephoto, the filter diameter of the 6x7 300F4 is 82mm yet the SMC 300F4 (K mount) is 77mm

the front elements are virtually the same and the filter diameter is more determined by the lens mechanics than optics. the 6x7 lens covers a circle diameter of about 93mm vs 43. Since immumination falls as the square of the diameter, the image from the 6x7 lens should be 1/4 the brightness, BUT IT IS NOT.

Stop looking at the wide end where designers do different things, look at the tele end. You cannot tell me there is significantly more light entering the front end of the 6x7 300/4 than the K300/4 it is simply not true, both lenses have the same sixe (approximately) front element, and that limits the effective maximum aperture of the lens,

Edit note. If you don;t believe me, just do a simple survey of all the prime lenses in the lens database, for all formats, I don;t care, plot front element size (use front filter size for this as it will be close enough) As you go up in focal length you will see that once the focal length exceeds the regestry distance the front element diameter rapidly approaches a line defined by focal length over aperture.

I'm done here

Last edited by Lowell Goudge; 03-29-2011 at 10:36 AM.
03-29-2011, 01:22 PM   #34
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QuoteOriginally posted by Lowell Goudge Quote
You are missing the point here.

You don't need a larger objective than the effective aperture to obtain the average uniform illumination over the image circle, and the diameter of the image circle is NOT, repeat NOT dependant upon the diameter of the lens, but restrictions either internally or externally in the light path, that is all. Light is let in through the front element, and as I pointed out the limit there is the effective aperture, that is all,


you are confusiong the design issues that are needed to mount a lens 2-3 times further away then its focal length nominally, yet still produce the same angle of view and image magnification.

If your example were true explain why for something like a 300mm telephoto, the filter diameter of the 6x7 300F4 is 82mm yet the SMC 300F4 (K mount) is 77mm

the front elements are virtually the same and the filter diameter is more determined by the lens mechanics than optics. the 6x7 lens covers a circle diameter of about 93mm vs 43. Since immumination falls as the square of the diameter, the image from the 6x7 lens should be 1/4 the brightness, BUT IT IS NOT.

Stop looking at the wide end where designers do different things, look at the tele end. You cannot tell me there is significantly more light entering the front end of the 6x7 300/4 than the K300/4 it is simply not true, both lenses have the same sixe (approximately) front element, and that limits the effective maximum aperture of the lens,

Edit note. If you don;t believe me, just do a simple survey of all the prime lenses in the lens database, for all formats, I don;t care, plot front element size (use front filter size for this as it will be close enough) As you go up in focal length you will see that once the focal length exceeds the regestry distance the front element diameter rapidly approaches a line defined by focal length over aperture.

I'm done here
well, I have tried. I used to think what you are saying is true, but have learned it is not true. Apparently I'm unable to communicate why. I can't believe because I know better.

Why would a lens designer baffle incoming light?

The 6x7 example was meant to exaggerate the differences, not to introduce register distance as an issue.

The wider the lens the more apparent there is more to it than the simple diameter of the front element -- for sure the front element is always larger than the entrance pupil, but it is the entrance pupil diameter not the diameter of the front element that determines f/#. Stop down diaphragm of any lens to f/16 marking. Did the front element change size? No, the entrance pupil is constricted to achieve f/16.

A difference between the 300mm lenses easy to see is the physical size of the diaphragm opening at say f/8 -- the physical opening of the 6x7 lens is larger, because the lens must pass more light to cover the longer register distance and larger format, i.e. the entrance pupil is larger, it lets in more light...

03-29-2011, 02:33 PM   #35
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QuoteOriginally posted by rhodopsin Quote
well, I have tried. I used to think what you are saying is true, but have learned it is not true. Apparently I'm unable to communicate why. I can't believe because I know better.

Why would a lens designer baffle incoming light?

The 6x7 example was meant to exaggerate the differences, not to introduce register distance as an issue.

The wider the lens the more apparent there is more to it than the simple diameter of the front element -- for sure the front element is always larger than the entrance pupil, but it is the entrance pupil diameter not the diameter of the front element that determines f/#. Stop down diaphragm of any lens to f/16 marking. Did the front element change size? No, the entrance pupil is constricted to achieve f/16.

A difference between the 300mm lenses easy to see is the physical size of the diaphragm opening at say f/8 -- the physical opening of the 6x7 lens is larger, because the lens must pass more light to cover the longer register distance and larger format, i.e. the entrance pupil is larger, it lets in more light...
I am willing to make one more attempt to explain why there are baffels in a lens and why all the light hitting a lens is not wanted.

Start with a simple lens, draw a subject, the inverted image, and something to represent the sensor/ film size at about the same height as the image of your subject.. Draw lines from subject to lens and lens to image to show the limits of the subject

Now add a second much taller subject beside the first, and his image beside the first. Notice how he will be projected beyond the sensor. draw his projection lines also

Draw several more, ever larger subjects and their projection lines.

Now, add a tube around the lens, and as you extend the tube towards the sensor. Notice how some of the subject projections hit the tube, this is what causes vignetting. This also will cause internal scattering of light inside the lens barrel, which is why there are baffels and these are intended to reflect the light away from the elements, because otherwise they cause loss of contrast.

Now draw a hood in front, note how this also causes vignetting, except this time, the light is NOT entering the lens, but kept out by the hood. This is why hoods are so important, they reduce the unwanted light that would need to be lost in the internal baffels and therefore improve contrast.

If you continue this model further, and add additional lens elements to modify the light path, you will see that depending on where the elements are located determines their diameter. SO getting back to your origonal question that format size determines the rear element, that is only partially correct, it is a function of the projected image circle onto the rear element, combined with the projection capability of the rear element itself that determine rear element size, not simply the format.

Note that the light from the subject through to the image does not change as you add a hood or barrel, but will change if you increase the diameter of the lens, this is why the front element is the limiting factor in lens design, The size of the front element determines the brightness of an image, the hood and internal baffels determine the projection circle.

Without doing a ton of drawings to show the light progressing through the lens, this is the best I can offer as an explanation.

Unfortunately the reason you cannot explain your point is simple, it is wrong.
03-29-2011, 03:46 PM   #36
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QuoteOriginally posted by Lowell Goudge Quote
I am willing to make one more attempt to explain why there are baffels in a lens and why all the light hitting a lens is not wanted.

Start with a simple lens, draw a subject, the inverted image, and something to represent the sensor/ film size at about the same height as the image of your subject.. Draw lines from subject to lens and lens to image to show the limits of the subject

Now add a second much taller subject beside the first, and his image beside the first. Notice how he will be projected beyond the sensor. draw his projection lines also

Draw several more, ever larger subjects and their projection lines.

Now, add a tube around the lens, and as you extend the tube towards the sensor. Notice how some of the subject projections hit the tube, this is what causes vignetting. This also will cause internal scattering of light inside the lens barrel, which is why there are baffels and these are intended to reflect the light away from the elements, because otherwise they cause loss of contrast.

Now draw a hood in front, note how this also causes vignetting, except this time, the light is NOT entering the lens, but kept out by the hood. This is why hoods are so important, they reduce the unwanted light that would need to be lost in the internal baffels and therefore improve contrast.

If you continue this model further, and add additional lens elements to modify the light path, you will see that depending on where the elements are located determines their diameter. SO getting back to your origonal question that format size determines the rear element, that is only partially correct, it is a function of the projected image circle onto the rear element, combined with the projection capability of the rear element itself that determine rear element size, not simply the format.

Note that the light from the subject through to the image does not change as you add a hood or barrel, but will change if you increase the diameter of the lens, this is why the front element is the limiting factor in lens design, The size of the front element determines the brightness of an image, the hood and internal baffels determine the projection circle.

Without doing a ton of drawings to show the light progressing through the lens, this is the best I can offer as an explanation.

Unfortunately the reason you cannot explain your point is simple, it is wrong.
I had no original question. I said you are wrong about the same amount of light entering the lens; the larger the format, the more total light must enter the lens to project a larger image circle same brightness as the lens for smaller format.

Okay I'll start a new topic. Perhaps some of the other members can explain it to you better, but I suspect they will attempt it by using some of the examples I've used.
03-30-2011, 09:34 AM   #37
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Wow. This thread has become raw scientific intensity! I'm learning a lot too! I say you guys just continue here
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