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03-25-2012, 08:49 PM   #1
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what difference does the lens size make

OK the title pretty much says it all and i guess its kind of a silly question. But i was just curious if there is any pros or cons on getting a lens with bigger glass vs smaller glass. for example lets say i have have two lenses one 49mm and one that's 62mm. what kind of difference will there be in the two if any just off the glass size?

03-25-2012, 08:57 PM   #2
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No difference in itself except that it facilitates different types of optical designs- so you can expect fast lenses to have larger front elements than slower lenses.

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03-25-2012, 09:08 PM   #3
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All other things being equal (e.g. focal length and wide open F-stop), the one with the 62mm glass will be a lot heavier and will require more expensive filters.

For longer focal lengths, a bigger glass is required to get a fast aperture.

F-stop = focal length / diameter of the iris. The front element cannot be physically smaller than the iris.

Also for wide angle lenses, the field of view from the iris outward, will dictate the minimum front glass diameter to avoid vignetting.
03-25-2012, 10:04 PM   #4
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QuoteOriginally posted by xpeppyx Quote
...for example lets say i have have two lenses one 49mm and one that's 62mm. what kind of difference will there be in the two if any just off the glass size?
To piggyback on xpeppyx's question - I'm reminded of the Super Takumar 35mm f/2 lenses: S-M-C/Super Takumar 35mm F2 Reviews - M42 Screwmount Wide-Angle Primes - Pentax Lens Reviews & Lens Database.
The early model had a 67mm filter thread, and the later model had a 49mm thread. Aside from the obvious answer of "different optical designs", can anyone shed light on why there's such a difference in size between the two?

03-25-2012, 11:22 PM   #5
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I can't answer about 35/2's but I must mention some odd 28s. I have three fast 28s: Vivitar-Komine 28/2 CFWA, Tamron BBAR 28/2.5, and Vivitar-Kiron 28/2.5. I don't have the latter two with me at the moment so I can't re-measure their objectives, but I remember their sizes because of their striking differences. I do have the threads and weights recorded. They're like this:

* objective= 37mm, thread= 49mm, weight= 270g -- V.Komine 28/2
* objective= 22mm, thread= 49mm, weight= 230g -- Tamron 28/2.5
* objective= 44mm, thread= 62mm, weight= 360g -- V.Kiron 28/2.5

It's damn curious that two 28/2.5 lenses are so different, with one objective twice the diameter of the other. And I know the faster Komine has a narrower body than the other two, although I think it is longer (protrudes further from the camera).

Now, body size doesn't necessarily reflect the size of the contained optics. A body may be large so it is easier to grasp and control. My Industar-50/3.5 and Meyer Primotar-E 50/3.5 have the same size optics, yet the Meyer is maybe 4x the volume of the Industar. I have two CZJ Tessar 50/2.8s with similar disparity -- the black M42 is about 3x the volume of the little silver Exakta-mount, but they contain the same optics (if different irises).

So, faster lenses aren't necessarily larger than slower lenses, and bigger lenses don't necessarily have different glass than smaller lenses. Ah, the wonders of lens design...
03-26-2012, 01:10 AM   #6
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Read Tamron's brief explanation of how their use of special glasses helps them to decrease the size of their lenses.
Tamron technology
in a nutshell, exotic glass allows much greater bending of light per lens element, permitting the design of a smaller diameter lens.
Therefore, a smaller lens may cost more than a larger lens and have the same light gathering power. Better glass and coatings should also allow better transmission of light (t-stops) which is separate from f-stops and usually overlooked when we buy lenses.
The same thing can be observed in, say, binoculars or scopes, with smaller, expensive optics outperforming cheaper optics with a larger diameter objective. Unfortunately, only aperture (which is partially mechanical) is ever listed when you buy a camera lens, but transmission of light (an optical quality) should be specified too.

On an aside, DA limiteds are all 49mm - the advantage here is only having to buy one circular polarizer to fit them all.
03-26-2012, 02:00 AM   #7
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More Stuff?

Hello Xpeppyx,
Although I assume you're asking about optical differences with different (front) optic sizes (also called filter size), there's a practical issue, too. It may seem like a small matter, but bear with me;
Suppose you're looking at a couple of similar-quality lenses, and the prices are in the same ballpark. One uses a 49mm filter size, the other a 52mm filter.
But you have 3 other lenses that use 49mm filters.
If you get the 52mm (filter) lens, you'll need to buy and carry, a 52mm polarizer AND a different size lens hood. Same for ND filters and any other filters you may use.
Now you're looking at a couple of short zooms; One has a 49mm filter size and the other has a 55mm filter size.
More stuff to buy and transport.
Lens hoods, filters and accessories become more important as your lens collection grows.
Now, I'm certainly not suggesting you settle for a lower-quality lens just because the filter size matches your existing equipment. Always buy the best glass you can afford. But if the quality is equal, the filter size should be a factor. You have to buy and carry this stuff!
JMO
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03-26-2012, 04:41 AM   #8
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One thing no one has mentioned. Although you a looking at the filter size, once you move to telephoto lenses there is a relationship, sort of between filter size and lens performance.

The F ratio or aperture value of a lens is the optical glass focal length / diameter

smaller filters on telephotos imply slower lenses

As others have indicated on wide angle lenses the issue is more of lens design and having a filter that will snot cause vignetting based upon the FOV of the lens and how far back the front element is recessed


Last edited by Lowell Goudge; 03-26-2012 at 04:46 AM.
03-26-2012, 07:46 AM   #9
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QuoteOriginally posted by Lowell Goudge Quote
One thing no one has mentioned. Although you a looking at the filter size, once you move to telephoto lenses there is a relationship, sort of between filter size and lens performance.
And even that relationship is rather rough. The 200/3.5 lenses I have owned have front threads ranging from 58-62-67mmm.
03-26-2012, 01:02 PM   #10
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In repairing binoculars (military) I had to learn optical theory. Much of which has been forgotten in 35 years, but the basics are still with me and one thing is related to this. With binoculars and telescopes, and presumably lenses, the larger objective lens allows flatter field of view and fewer optical abberations, like parallax and bending around the outside of the image. This is why 7x50 binoculars are always a better choice than 7x35. 7 is the magnification, 35 or 50 is the diameter, in mm, of the objective lens. (The objective lens is the one on the outside, facing your subject.) 7x50 will also have a wider field of view usually. You can see this if you take a picture of a building with the 18-55 kit lens at the 18mm end. Any light poles and the outside corners of the building will be slanted inward toward the top of the picture. Then take one with a 50mm lens from the same spot, that effect will be much less noticeable or maybe not even there. If you take pictures with two lenses, say 50mm, one with a larger objective, I'd be willing to bet the larger lens will show less slant in the corners. (I can't remember what this slant is called, maybe parallax.)

The larger lens allows more light gathering capability, less slant in a situation like above, flatter field of view and fewer abberations overall. I think it might affect sharpness too but not sure. I have a 200mm Vivitar M42 lens that does a great job, I get excellent shots with it so I bought another one in 300mm with the same mount adapter system, similar to the Tamron Adaptall. It won't give me a sharp picture no matter what, I can't even use it. Almost identical lens, same multiple camera adapter, but the 200mm has a 67mm objective, the 300mm has a 62mm objective. Much smaller. Is that why it won't get sharp focus? I don't know but as far as I can tell that's the only obvious difference, unless it has different internal element groupings. It's visually almost identical.

With my 50mm lenses, the 49mm filter ring and related lens size seems to work quite well. I haven't used one with another size to compare. But if the properties of the objective lens apply the same as with binoculars and telescopes, it should make a difference, although maybe a minor one. Since my 50mm lenses can get excellent pictures, nice and sharp with loads of detail, I wouldn't expect it to affect sharpness a lot, but a larger objective lens should have an effect on other optical aspects of the lens, especially flatness of view and amount of tilt at the outer edges.
03-26-2012, 02:02 PM   #11
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QuoteOriginally posted by RioRico Quote
And even that relationship is rather rough. The 200/3.5 lenses I have owned have front threads ranging from 58-62-67mmm.
Agreed but it is a really good indicator. If you consider for example a 200mm lens, with a 58 mm filter, the glass must come all the way to the edge of the filter thread to have any chance of being truely F3.5, if there is any mounting besel, then the F3.5 is a false claim.
03-26-2012, 02:09 PM   #12
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QuoteOriginally posted by Paleo Pete Quote
In repairing binoculars (military) I had to learn optical theory. Much of which has been forgotten in 35 years, but the basics are still with me and one thing is related to this. With binoculars and telescopes, and presumably lenses, the larger objective lens allows flatter field of view and fewer optical abberations, like parallax and bending around the outside of the image. This is why 7x50 binoculars are always a better choice than 7x35. 7 is the magnification, 35 or 50 is the diameter, in mm, of the objective lens. (The objective lens is the one on the outside, facing your subject.) 7x50 will also have a wider field of view usually. You can see this if you take a picture of a building with the 18-55 kit lens at the 18mm end. Any light poles and the outside corners of the building will be slanted inward toward the top of the picture. Then take one with a 50mm lens from the same spot, that effect will be much less noticeable or maybe not even there. If you take pictures with two lenses, say 50mm, one with a larger objective, I'd be willing to bet the larger lens will show less slant in the corners. (I can't remember what this slant is called, maybe parallax.)

The larger lens allows more light gathering capability, less slant in a situation like above, flatter field of view and fewer abberations overall. I think it might affect sharpness too but not sure. I have a 200mm Vivitar M42 lens that does a great job, I get excellent shots with it so I bought another one in 300mm with the same mount adapter system, similar to the Tamron Adaptall. It won't give me a sharp picture no matter what, I can't even use it. Almost identical lens, same multiple camera adapter, but the 200mm has a 67mm objective, the 300mm has a 62mm objective. Much smaller. Is that why it won't get sharp focus? I don't know but as far as I can tell that's the only obvious difference, unless it has different internal element groupings. It's visually almost identical.

With my 50mm lenses, the 49mm filter ring and related lens size seems to work quite well. I haven't used one with another size to compare. But if the properties of the objective lens apply the same as with binoculars and telescopes, it should make a difference, although maybe a minor one. Since my 50mm lenses can get excellent pictures, nice and sharp with loads of detail, I wouldn't expect it to affect sharpness a lot, but a larger objective lens should have an effect on other optical aspects of the lens, especially flatness of view and amount of tilt at the outer edges.
Pete, I am not sure the analogy holds true comparing binoculars and lenses. binoculars are rated differently, with both magnification and field of view this is because they are making use of the binocular fision of the user as well as just a single lens design. When it comes to a single lens, the FOV is dictated solely by the focal length and format.

First of all, you need to clarify when discussing lenses and the size of the front element, it needs to be the front element and not the filter ring. different lens makers have different filter rings based upon the manufacturing style of the lens, even if the front element is the true same size and both lenses are the same speed (maximum aperture) the fact that the 200mm lens has a 67 mm filter and the 300mm lens has a 62 mm filter may not be relevant at all to why one lens gives a sharper image than the other. there could be something wrong, for example with one lens.
03-26-2012, 03:27 PM   #13
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Thanks for the info guys. That was just one of those things that just puzzled me.
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