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08-06-2009, 03:38 AM   #1
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How to determine the specs of the lens?

Folks,
How to determine the specs of an old Pentax lens...the focal length is easily distinguished.
How to determine the F number? The lenses say 1:2 or 1:1.4 or 1:1.7. Does the number after the : specify the f number for the lens? What do they mean by 1:X (X being any number)?
Also, how to determine if the lens is a macro lens or not? Is the minimum focussing distance inscribed anywhere on the lens?
Thanks a lot in advance for your help

Regards,
KP

08-06-2009, 03:55 AM   #2
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Yes your right about the f number.

As for the minimum focus distance it will be on the focus ring.
08-06-2009, 06:33 AM   #3
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The f number you talk about is the maximum aperture setting you can set the lens to.
So a 1:1.4 lens means that it has a wide-open aperture of f/1.4 - this is a mathematically calculated number, but don't worry too much about that - the close the number is to 1, the larger the maximum aperture setting possible on the lens, meaning it can create very thin depth of field (in-focus areas) in the photos, and can gather more light than those with higher maximum f numbers.

Macro lenses are either dedicated macro lenses (which are usually designated as 1:1 or 1:2 macros), or 'pseudo' macro lenses (which say they are 1:3, 1:4 up to even 1:6 macros). A 1:1 macro can focus in up to life-size proportions, whereas a 1:4 macro only is able to focus in to a quarter of life-size.
08-06-2009, 07:27 AM   #4
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F-stops are ratios

QuoteOriginally posted by skamalpreet Quote
Folks,
How to determine the specs of an old Pentax lens...the focal length is easily distinguished.
How to determine the F number? The lenses say 1:2 or 1:1.4 or 1:1.7. Does the number after the : specify the f number for the lens? What do they mean by 1:X (X being any number)?
Also, how to determine if the lens is a macro lens or not? Is the minimum focussing distance inscribed anywhere on the lens?
Thanks a lot in advance for your help

Regards,
KP
The reason that aperture values are often shown as 1:x, is that they are, in fact ratios. Specifically, they are the ratio of the diameter of the lens to the focal length. For example, a 50mm, f/2.0 lens has a diameter of 25mm at the iris. The same lens at f/8, has a diameter of 6.25mm.

If you calculate the area of the aperture at f/2, you will find that it is 490.87 sq. mm. At f/2.828 (the actual value of the next f-stop; 2.8 is an approximation), the area is 245.511 sq. mm, or exactly half. At f/4, the area will halve, yet again, and so on.

08-06-2009, 07:38 AM   #5
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And ratios are, as you remember from school, just another way of writing a fraction f/2.8 is the same, mathematically, as f:2.8.

The other thing you shold be aware of is that even with zoom lenses where two different f-stops are lsited, this is just specifying the *maximum* aperture. If it says 1:3.5-5.6, that doens't mean the maximum is f/3.5 and minimum is f/5.6 - it means the maximum is f/3.5 at the short end of the zoom and f/5.6 at the long end. The minimum is rarely specified in the name of the lens, although if it has an aperture ring, you can see it printed there.

Also, the word "Macro" will usually be in the name of a macro lens, and will be printed on the lens, but particularly with zoom lenses, sometimes the word "macro" is tacked on for lenses that aren't really macro lenses but just sort of come quasi-clsoe to providing that kind of magnification. Somewhere in the spec sheet for the lens (not printed on the lens) will be the maximum magnification - again, specified as a ratio. If it says 1:1, it's a macro. If it's 1something bigger than one), then it's that much shy of being a macro lens. The kit lens does 1:3, which isn't bad already; it's not really worth spending a bunch of money on another "macro" zoom that only does 1:2.5.
08-06-2009, 02:37 PM   #6
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QuoteOriginally posted by noblepa Quote
The reason that aperture values are often shown as 1:x, is that they are, in fact ratios. Specifically, they are the ratio of the diameter of the lens to the focal length. For example, a 50mm, f/2.0 lens has a diameter of 25mm at the iris. The same lens at f/8, has a diameter of 6.25mm.

If you calculate the area of the aperture at f/2, you will find that it is 490.87 sq. mm. At f/2.828 (the actual value of the next f-stop; 2.8 is an approximation), the area is 245.511 sq. mm, or exactly half. At f/4, the area will halve, yet again, and so on.
..as he said, I hope you got it..
That is why I became a photographer and not a mathemathecian ..
08-06-2009, 11:23 PM   #7
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thanks a lot guys....appreciate your detailed comments...this will help me in taking a better decision
08-07-2009, 12:06 AM   #8
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Also if a lens is a zoom lens, e.g. 17-70mm and has it's aperture listed as F:2.8-4.5, then the listed aperture is the max aperture at each end of the zoom. E.g.

at 17mm, the max aperture is F2.8
at 70mm, the max aperture is F4.5

08-07-2009, 08:46 AM   #9
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Dedicated macro lenses (or even sometimes zooms with close-focus features) will sometimes include the magnification level on the focusing ring, like 1x, 2x, 3x, 4x, etc.
08-07-2009, 02:37 PM   #10
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I don't know exactly at what f-stop fast lenses are considered but if you get lenses with f-numbers of 1.2, 1.4, 1.7, 2, 2.8...these are usually considered fast lenses and are very good for low-light shooting..meaning you can get faster shutter speeds with these lenses than using other lenses where their maximum (wide) open aperture would be, say 3.5 or 4.
These lenses (fast) usually command a higher price.
08-07-2009, 09:33 PM   #11
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QuoteOriginally posted by GerryL Quote
I don't know exactly at what f-stop fast lenses are considered but if you get lenses with f-numbers of 1.2, 1.4, 1.7, 2, 2.8...these are usually considered fast lenses and are very good for low-light shooting..meaning you can get faster shutter speeds with these lenses than using other lenses where their maximum (wide) open aperture would be, say 3.5 or 4.
These lenses (fast) usually command a higher price.
When people say 'fast,' it's often really relative to others of the type of lens, and often, what someone personally considers 'fast.' On Ebay, sellers who don't know are often just full of it about lenses being 'fast,' (Kit lenses are not fast. )

For instance, a 50mm f2, or a 1.8, generally wouldn't be considered a fast 50, but a 100mm or 28mm f2 generally is. A 2.8 zoom of any kind is usually called a fast zoom, but a 28mm or 135mm f2.8, not so much, cause those are pretty standard. It's sort of relative and subjective, at times.

Generally, the further away from 50mm a lens is, (longer or shorter) the higher the number might be considered a 'fast' lens. Where I usually draw the line is 1.4 with 50s and f2 for most everything else.
08-08-2009, 01:42 PM   #12
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I was kinda speaking generally and yes, they are relative to the lens that it would also be kinda hard to find a 300mm lens with an f stop wider than f4...so, 300mm lenses might not be for you?
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