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02-20-2009, 11:28 AM   #1
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What goes into deciding lens diameter?

just thinking...would you pay more for an 77-82mm diameter lens with the same specs of the usually 58-62mm variety?

tradeoff would be size and weight versus light gathering power in the night.

02-20-2009, 11:33 AM   #2
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QuoteOriginally posted by Reportage Quote
just thinking...would you pay more for an 77-82mm diameter lens with the same specs of the usually 58-62mm variety?

tradeoff would be size and weight versus light gathering power in the night.
Light gathering power implies that the specifications are not the same.
02-20-2009, 11:57 AM   #3
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The article I posted in film cameras from the time of the K mount introduction made the point that the screw mount had limited the maximum aperture to about f/1.4 at 50mm - now faster lenses were possible.

With the same f/stop speed, I don't think 'light gathering' is necessarily the primary gain with larger diameter glass. Rather, there's more to throw out - the edge messiness - and therefore the manufacture can be cheaper due to lower tolerances required.

However, I have the impression that manufacturing tolerance being equal, a larger diameter lens (at least in the past) could out perform a smaller diameter one. Within reasonable limits and the geometries imposed by the mount and film size.
02-20-2009, 01:32 PM   #4
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the most of 28mm lenses have filter-ring diameter od 49mm, while mine 28mm lens (Pallas Magenta) have 62mm diameter - it's huge! it easily dwarfs

so, does that makes bigger lens superior in light gathering and/or IQ?

02-20-2009, 02:39 PM   #5
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Easy enough to check:
assuming their wide open aperture is the same, do you get a similar shutter speed?
in low light, do they produce equally 'light gathered' photos?
Which has better IQ?

The huge diameter probably has two reasons: they could be sloppier in manufacture, and the big lens impresses the rubes... erm... poorly informed consumers. The IQ is likely worse than a Takumar.

(although, I still suspect the wider lens produces wider bokeh... as the angle of light converges from further out... but that's probably a false thought.)
02-20-2009, 08:01 PM   #6
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QuoteOriginally posted by Reportage Quote
just thinking...would you pay more for an 77-82mm diameter lens with the same specs of the usually 58-62mm variety?

tradeoff would be size and weight versus light gathering power in the night.
with telephotos, the diameter is directly related to maximum apature. for example a 200mm F2.8 needs 71.7 mm diameter lens. by the time you add the mounting etc, you might be just ok with 72mm but 77 is much more typical.

when you go to wide angle, clearly an F2.8 20 mm lens only needs a 7 mm diameter, BUT when you consider the field of view, you can wind up with very large filters to insure that vignetting does not occur.
02-20-2009, 09:12 PM   #7
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And you can see this with a lot of wides - just compare the size of the front element with respect to the lens's filter size, like the DA 12-24, most of the 28s and the DA 14.

Then again, the DA 16-50's front element is quite sizeable - vignetting may start to occur with those larger filters.
02-20-2009, 10:30 PM   #8
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QuoteOriginally posted by elkarrde Quote
the most of 28mm lenses have filter-ring diameter od 49mm, while mine 28mm lens (Pallas Magenta) have 62mm diameter - it's huge! it easily dwarfs

so, does that makes bigger lens superior in light gathering and/or IQ?
I sold an old Sears M42 28mm f2.8 lens that took 58mm filters. The front element (whole lens too) is pretty big. They maybe could have gone with 55mm but not smaller.

The older Super-Takumar 35mm f2 took a 67mm filter (I think), while later versions took 49mm. Clearly there were improvements.

02-21-2009, 04:01 AM   #9
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"What goes into deciding lens diameter?"

If you are talking about just gross over-all physical size then lens diameter does not mean all that much. This dimension is determined by manufacturing concerns as much as anything else.

What you should be concerned about is clear aperture. Clear aperture is the ultimate limit, imposed by physics, on the amount of light that can pass through a lens system.

It is defined as the focal length of the lens divided by the "speed" of the lens system in f stops.

Thus a 50mm f/1.4 lens requires a clear aperture of at least 36mm. This means that with the diaphragm wide open it should measure pretty close to 36mm across the diaphragm opening . Try it sometime.

Of course, in this case, the actual physical size of lens would have to be greater to accommodate mounting rings, diaphragms, type of mount, focus mechanism etc.

This goes a long way in explaining the high cost of prime telephotos. Assume you wanted a 500mm f/1.4 prime telephoto. This would require a clear aperture of 357mm (9 inchs)! You can imagine the cost, size, and weight of such a lens not to mention filters for such a cannon. It would not be practical.

Last edited by wildman; 02-21-2009 at 04:13 AM.
02-21-2009, 05:38 AM   #10
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QuoteOriginally posted by Reportage Quote
just thinking...would you pay more for an 77-82mm diameter lens with the same specs of the usually 58-62mm variety?

tradeoff would be size and weight versus light gathering power in the night.
I think, I sum up, what the other already wrote:

1. light gathering power is a function of diameter and focal length, which translates into the maximum aperture (smallest aperture number)

2. the diameter of the front lens has nothing to do with light gathering power, but with other lens properties (like angle of view, which is the reason, that wide angle lenses have a proportonially much bihher front lens, than tele lenses). Sometimes manufacturing considerations also influence the size of the lens.

3. wide angle lenses of "iold times" usually had bigger front elements, than more modern counterparts, which only proves the manufacturing point and is no hint of superior quality.

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02-21-2009, 06:42 AM   #11
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so for lens, there is no physical science or industrial rating to determine just how much light goes into the various lens at the chosen aperture?

if that is the case...is there a possibility that the same model F/2.8 lens could be "brighter" then another of the same model?
02-21-2009, 08:23 AM   #12
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QuoteOriginally posted by Reportage Quote
so for lens, there is no physical science or industrial rating to determine just how much light goes into the various lens at the chosen aperture?

if that is the case...is there a possibility that the same model F/2.8 lens could be "brighter" then another of the same model?
Your first sentence is not correct. The answers above only say, that you cannot simply translate a big diameter front lens into light gathering power. The answers do nowhere indicate, tha one cannot apply basic physics. The decisive measure is the aperture. And any lens, may it be a 28mm wide angle, a 100mm macro or a 300mm tele with a maximum aperture of 2.8 will gather the same amount of light, aka give you the same shutter speed. And aperture is really easy to calculate (focal length divided through clear aperture or entrance aperture) and there is no guesswork involved.

Now the caveats. Of course lens construction (many lenses in side or only a few) and the quality of the coating have an influence on the trnasmission of the light. So a super-zoom with 15 or 17 lenses inside will absorb a bit more light, than a simple 5 element standard lens. But this is an individual thing and cannot be calculated generally - and on top the differences in light transmission are very small, may be half an aperture stop at max. The physical important parameter is the aperture and this is given by simple geometry.

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02-21-2009, 06:24 PM   #13
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Given similar construction and equivalent glass, I would expect a larger diameter lens to have more light gathering capability. At least this was true of astronomical refractor telescopes.
02-21-2009, 06:28 PM   #14
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QuoteOriginally posted by Reportage Quote
so for lens, there is no physical science or industrial rating to determine just how much light goes into the various lens at the chosen aperture?

if that is the case...is there a possibility that the same model F/2.8 lens could be "brighter" then another of the same model?
For photography we are not concerned about how much light enters a lens but how much light reaches the focal plane of the camera. Conventionally this is measured in f stops with respect to lens'.

Simply put f/2.8 means f/2.8. That is f/2.8 is an absolute number and not a relative number. Thus the amount of light available to the focal plane of a camera at f/2.8 is the same for the opticians at Pentax as for those at Zeiss. If this was not so how else could all the various manufacturers calibrate their light meters?

I don't know how this is measured or what units are used but it is a real number just like 100 gallons/per min is a real definite number. So when Zeiss says a lens is an f/2.8 and Pentax says a lens is an f/2.8 they both mean that more or less the same amount of light is available to the camera's focal plane, within very narrow limits, in both cases.

Last edited by wildman; 02-21-2009 at 06:42 PM.
02-22-2009, 12:34 PM   #15
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QuoteOriginally posted by mithrandir Quote
Given similar construction and equivalent glass, I would expect a larger diameter lens to have more light gathering capability. At least this was true of astronomical refractor telescopes.
If construction was similar, that would mean, the focal length to be similar. In that case a bigger diameter would also mean larger aperture and more light gathering power.

Astronomical telescopes must be viewed completely different from photographic lenses, because (apart from the basic geometric optics) they have different applications.

Light gathering power in an astronomical telescope is sure defined by its sheer diameter. But what does that mean? It would mean, that a scope with 30 cm in diameter gathers more light, than one with 20cm diameter. BUT now the focal length comes into the play. If the 30cm scope has 300cm focal length (a 1:10 refractor for example or the typical Schmidt-Cassegrain), but the 20cm scope only has 80cm focal length (1:4 focal ratio, typical for the current fast Newtonians), the astronomer will view these scopes differently:

The bigger scope has a slower (much slower) f-number. So, despite having obviously the larger diameter, it will show much less of a faint astronomical nebulae, which the smaller 1:4 instrument makes easily visible.
The situation is different for point sources (= stars), where the pure diameter, independent of the f-number plays its strength. Here the 30cm scope will show much fainter stars (and accordingly much more stars), than the 20cm scope, because the focal length needs not to be taken into account with these point sources.

In photographic lenses, the light gathering power is ALWAYS determined by the focal ratio or f-number or max. aperture, because we simply do not image point sources, but always objects spread about a certain area. Thus a 300/2.8 with is app. 10cm front lens will exactly show as much, as a 50/2.8 standard lens, with ist tiny app. 2cm diameter.

There is one other and - for astronomers above a certain beginners stage - much more important thing connected with diameter - and that is resolution. The resolving power of a telescope ort lens is directly determined by its diameter. The bigger the lens' diameter, the smaller the objects I can resolve. This is true for the telescope, where a 40cm instrument will show you much more detail on the moon's surface, than a 10cm beginners scope and this is also true for photographic lenses. But the latter will usually be limited in their resolving power by aatmospheric distrubances etc. anyway.

Ben
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