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11-05-2010, 08:37 PM   #1
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Is the F number constant or does it change?

I just ordered a Pentax SMCP-DA 17-70mm f/4 AL (IF) SDM for the new K-7. I haven't got them yet but the lens says it's a constant f/4 yet some of the pictures uploaded in that lens review section show a photo to be taken at f/7.1.

How can the f/number change if it's suppose to be a constant f/4??

I'm new at this and obviously, don't understand the aperture ratings of these lens.
Thanks.

11-05-2010, 08:46 PM - 1 Like   #2
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The f/4 is the maximum aperture of the lens, and remains f/4 throughout the zoom range.
Note that a lot of zooms do get slower (smaller maximum aperture) as they are zoomed.
For example, the kit lens may be something like 18-55 f/3.5-5.6.
It will have a maximum aperture of 3.5 at 18mm, but will have slowed down to 5.6 at 55mm.

The f/7.1 is the taking aperture of the lens (IE, this is what the lens was set to when the picture was taken)

http://www.uscoles.com/fstop.htm
11-05-2010, 10:17 PM   #3
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f number

So when they say the f/# is "constant", they mean the "maximum size" (smallest f/#, largest hole) f/# is f/4. The f/# can be larger (smaller hole, f/8) but no smaller f/# than f/4.

The "constant" description of f/4 is (for me) misleading since I thought it could not be changed and never varied. Thanks.... it's still somewhat confusing.
11-05-2010, 10:20 PM - 1 Like   #4
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QuoteOriginally posted by wrconley Quote
So when they say the f/# is "constant", they mean the "maximum size" (smallest f/#, largest hole) f/# is f/4. The f/# can be larger (smaller hole, f/8) but no smaller f/# than f/4.

The "constant" description of f/4 is (for me) misleading since I thought it could not be changed and never varied. Thanks.... it's still somewhat confusing.
"Constant" is not "fixed". Compare it to the kit lens that is "variable" aperture. Meaning the maximum, or lowest number, aperture changes as the zoom increases.

11-06-2010, 10:29 AM   #5
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constant f/#

So the largest aperture (smallest f/#) will be f/4 regardless of whether it's at 17mm or 70mm. It can be changed to a smaller aperture (f/8) if needed. OK, thanks for all your help, now I get it..... I just spent more for this lens than my old camera cost; I see where you can put big bucks in glass.

They should advertise it as "constant f/4 at any zoom" for us neophytes.
11-06-2010, 11:41 AM   #6
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QuoteOriginally posted by wrconley Quote
So the largest aperture (smallest f/#) will be f/4 regardless of whether it's at 17mm or 70mm. It can be changed to a smaller aperture (f/8) if needed. OK, thanks for all your help, now I get it..... I just spent more for this lens than my old camera cost; I see where you can put big bucks in glass.

They should advertise it as "constant f/4 at any zoom" for us neophytes.
It's a very nice lens. I got to play with one today.
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