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01-12-2011, 11:59 PM   #1
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lens 'diameter' benefits

Hi again.
Another noob question....

The DA L 28-55 lens that came with my K-x takes a 52mm filter, my 100-300 zoom is 55mm, and I'm looking at a 28-200 72mm Tamron.

What benefits are there for going to larger 'diameter' lenses. I would've thought to do with the amount of light entering, but I tried the 18-55 and the tamron 28-200 both at 28, and the 100-300 and again tamron 28-200 both at 200mm, and got the same automatic shutter speed and aperture.

Why would I/should I/shouldn't I go larger?

Thanks
Andrew

01-13-2011, 12:47 AM   #2
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Diameter = Resolution

More glass means more light................................

Easier shooting under darker conditions..................................

Wider aperature = Narrower depth of field

In optical terms your resolution is purely based upon the diameter of the objective (unstoppered down) and the wavelengths passing through it. That's why larger telescopes produce better images. They collect more light and have better resolution. I'm not sure if that's very helpful with camera lens imaging qualities. However the physics remain the same.....
( something about Rayleigh limit I think?)

Last edited by Postumus; 01-13-2011 at 12:49 AM. Reason: additions
01-13-2011, 03:11 AM   #3
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The only time filter size is relevant to anything is when you are buying a filter to fit a particular lens. Other than that, in and of itself filter size is meaningless for judging anything.

Aperture, on the other hand, is an entirely different matter.
01-13-2011, 04:42 AM   #4
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Size does matter :-) If it wasn't only for the looks, you could try staying with your two-lens kit. AFAIK that Tamron does not have 72mm but 62mm.

There is one thing people tend to forget about filters and that is steprings. Don't get fooled by the mad filter industry, a stepring should set you back no more than say 3,- US-$.

You are eyeing a so-called superzoom and be warned, it lures people into thinking you would be able to bend the laws of physics with a one-lens-for-all (and it doesn't). What this lens can do this combo or even this combo can do much better imo.

If you happen to stumble upon a Pentax-SMC-M 1.7/50 (often cheaper when attached to a camera like the ME super) you might just take it and give it a try (should set you back no more than say 50,- US-$). It'll make your K-X sing.

Best of luck wherever you go.

01-13-2011, 05:27 AM   #5
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As others have said, the issue is one of aperture, not of lens diameter. Now, aperture is dependent on the diameter of your glass, as well as the focal length. A 300mm f2.8 lens is going to have a large diameter, on the other hand, FA 50mm f1.4 has a filter size of 49mm. Superzooms like the 28-200 have variable apertures, but it will be at its fastest at the 28mm mark. On the other hand, I would bet that your kit lens will be quite a bit sharper wide open at 28mm than the Tamron.
01-13-2011, 06:29 AM   #6
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I think some of the previous posts are a little confusing.

there are two driving forces with respect to the thread diameter of filters and attachments, one is quite simple and intuitive the other is not.

First of all, take any lens. and for sake of argument, a 50mm F2 lens.

the definition of f stop is focal length / diameter.

therefore a 50mmF2 needs to be a minimum of 25mm in diameter. Now remember, that is the glass, not the overall lens. There may be a need to have other mechanical things beyond the diameter of the glass itself, including the things that hold the glass in place, so the F stop really only defines the minimum diameter a lens could be. In reality, lens diameter really only starts defining the attachment diameter in telephoto lenses.

the second issue is that the filter has to go in front of the lens, and the attachment ring, which is protruding out in front of the lens cannot enter into the field of view, otherwise it would cause vignetting. Therefore, as lenses get wider, the filter diameter gets bigger and bigger just to ensure that it is out of the field of view.

the location of the front element relitive to the accessory ring is a variable determined by each maker, therefore two similarly specified lenses could have considerably different accessory diameters, all as a function of how close to the front of the lens body the front element is located.
01-13-2011, 07:28 AM   #7
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QuoteOriginally posted by Stray Dingo Quote
Hi again.
Another noob question....

The DA L 28-55 lens that came with my K-x takes a 52mm filter, my 100-300 zoom is 55mm, and I'm looking at a 28-200 72mm Tamron.

What benefits are there for going to larger 'diameter' lenses. I would've thought to do with the amount of light entering, but I tried the 18-55 and the tamron 28-200 both at 28, and the 100-300 and again tamron 28-200 both at 200mm, and got the same automatic shutter speed and aperture.

Why would I/should I/shouldn't I go larger?

Thanks
Andrew
To answer your specific question about why, when you set your lenses all at 28mm and you got the same automatic shutter speed reading for all of them is that you were obviously shooting with adequate lighting so that the camera set all of the lenses at the same aperture setting, therefore it would have also have set the "automatic" shutter speed the same for all of the lenses.

It's a combination of shutter speed and aperture setting that makes for proper exposure. Also if you had your ISO setting on automatic and had included the entire range of ISO settings in your automatic settings, the camera could also have automatically adjusted your ISO to compensate for any variance in exposure caused by a miss-matched aperture and shutter speed combination.

The fact that you set your lenses to 28mm had no affect on your shutter speed settings. Other than the 28mm focal length was one of the convenient focal lengths that was shared by all of your lenses. Clear as mud eh?
01-13-2011, 11:19 AM   #8
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OK, I'll add a little confusion to the discussion. . .

The diameter of the glass determines the entrance pupil -- for spherically ground lens elements. Aspherical elements can allow for smaller diameter front elements, and thus smaller and lighter lenses of the same spec.

A good example of this is the Tamron 28-75/2.8 XR Di Aspherical which takes a 67mm filter compared to just about any other lens with this spec which will use 77-82mm filters.

. . .size isn't everything. . .

Scott

01-13-2011, 05:48 PM   #9
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Ok...so my original belief was correct - the larger the glass diameter, the more the light coming in.....but thats only part of the big picture (excuse the pun)

The lens itself is a Tamron 371D, which is the centre one at Tamron 28-200 differences

So if I understand correctly, the larger glass diameter on this is fairly superfluous as the aperture range is 3.8-22. The larger glass would be better suited to a lower aperture? Which could be why the follow model was reduced to 62mm?

Rondec - yes, I was finding better images on the other lenses than this - sharper, improved colour

Andrew
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