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Lens SIZE question (diameter)
Old 09-04-2005, 10:14 PM   #1 (permalink)
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so im finally understanding different focal lengths and how they compress the background and such. but now my question is regarding the actual size (diameter) of the glass. for example needing a 55mm filter vs. a 72 mm filter.

i will see a 70-200 in both sizes and i wonder what the difference is? does a larger opening allow more light therefore easier to be faster??

very curious any answers greatly appreciated!!!
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Re: Lens SIZE question (diameter)
Old 09-04-2005, 10:48 PM   #2 (permalink)
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[ QUOTE ]
does a larger opening allow more light therefore easier to be faster??

[/ QUOTE ]

A larger diameter glass, all things being equal, allows more light in than a smaller diameter lens of the same focal length.

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Re: Lens SIZE question (diameter) - Aperture Definition
Old 09-05-2005, 12:52 AM   #3 (permalink)
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The technical defintion of aperture refers to the ratio of focal length of the lens divided by the effective diameter of the aperture. A smaller f-number implies a larger diameter lens and therefore more light available for high-speed photography, or for work in poorly illuminated areas. However, small f-numbers involve small depths of focus.

This definition works well for normal and telephoto lenses; however, for wide angle lenses there are some tricky optics going on. If you use the above ratio, a 20mm lens would only require a 20mm front element to give an aperture of f/1.0. In addition, if you measure from the film plane, your camera is probably a lot wider than 20mm... So what gives... All SLR's use a reverse telephoto design since there is a mirror box located between the rear lens element and the film plane or CCD/CMOS sensor.

Here's something new to consider... if you are interested in truely spectacular wide angle lenses you will need to try out a rangefinder or viewfinder camera. If you get to examine one (i.e. Leica M series, Contax G1/G2, Sinar, Hasselblad X-Pan, etc) you'll notice that the rear element sits extremely close to the film plan or CCD/CMOS sensor. It is my understanding that current digital technology will not allow these type of lenses to work, since the light would intersect with the sensor at too acute an angle at the corners.

Aloha from Hawaii,
Michael
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Re: Lens SIZE question (diameter)
Old 09-05-2005, 02:04 AM   #4 (permalink)
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Well,....all 4 zooms I own use the 77mm filter ring.. I can use 77mm filters on my 52mm filter threaded 100mm f2.8 Macro if I want because I can always "step down" to 52mm if I so desire thanks to a cheep screw on "step up" ring.....that can fit the two together..

I know from experience that it is much much better to get a 77mm filter right off the bat because you can never have too much filter, although you can always get into trouble if you don't have a large enough filter to cover your lens.. which can really suck!

JP

 
 
Re: Lens SIZE question (diameter)
Old 09-05-2005, 09:14 AM   #5 (permalink)
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Tim, I've seen the same thing as well. It was explained to me this way. Lets say a 70-200mm 3.5-5.6f has a diam. of 55mm and you have one that is 2.8f @ 77mm; the end glass size does not alone determine the amount of light coming in, there is also the diaphragm, iris, whatever you want to call it that comes into play giving you the true f-stop factor. Generally the faster lenses have larger glass on the front-but dont let that by itself miss lead you. I've seen Pheonix brand lenses (and others) with 72-77mm glass on the front that are the same "f" range as the cheapy brand lenses(4.0-5.6f with a 52-58mm for example)
Some might disagree or not but I hope this answers your question.
Have a Great holiday!
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