The idiots guide to lens & F numbers

[Arty]

OK, I've done a search for this & not found exactly what I'm looking for - so don't shoot me down in flames please! :o Now I know what F numbers are, but what do they mean on a lens spec? For instance; 18-125 F3.5-5.6. What does the 3.5-5.6 actually relate to as far as the lens performance?

[COKE CAN]

OK, I've done a search for this & not found exactly what I'm looking for - so don't shoot me down in flames please! :o Now I know what F numbers are, but what do they mean on a lens spec? For instance; 18-125 F3.5-5.6. What does the 3.5-5.6 actually relate to as far as the lens performance?

It's the maximum aperature on the lens. At 18 the maximum is 3.5, and at 128 the maximum is 5.6.

[COKE CAN]

That does not mean you can't stop it down, I usually shoot my KIT lens @ f8

[aggarcia]

No you can stop it down. It only gives the largest aperature ( biggest opening ) for the given focal length.

[britt777]

http://en.wikipedia.org/wiki/F-stop

Maybe this will help

[COKE CAN]

No you can stop it down. It only gives the largest aperature ( biggest opening ) for the given focal length.

That's what I said.

[Citizensmith]

What does the 3.5-5.6 actually relate to as far as the lens performance?

Lenses stay wide open until you either take the photo or press Depth of Field Preview (if your camera has it). Then the camera stops down to whatever appeture (F number) you had selected. Of course if you are just taking photos you don't see it do that due to shutter blackout.

With regard to performance, a lower number is better. It lets through more light when open so your viewfinder is brighter, and you can take photos in less light. Lower numbers (wider appertures) also allow better separation of subject and background as it reduces your depth of field.

So basically the lower the number the better. A 1.4 is better than a 1.8 is better than a 2.8, etc. But as a lower number means more glass it also means higher price.

[Andy_T]

Something like a 'beginner's guide' can be found here (http://photography-on-the.net/forum/showthread.php?t=58028)

Best regards,
Andy

[Arty]

many thanks for the replies. Pretty much as I thought it worked, but it's nice to have confirmation from people who actually know.
One more thing; On say a 28-135, what number would you consider to be too slow for 'indoor' (No flash)use?

[Phil V]

many thanks for the replies. Pretty much as I thought it worked, but it's nice to have confirmation from people who actually know.
One more thing; On say a 28-135, what number would you consider to be too slow for 'indoor' (No flash)use?

Indoor where? it depends where you live, there's less indoor light in Lapland than there is in Ecuador

Seriously at 800iso (usable on modern DSLRs) a slow zoom is usable during daylight hours, in the evening and under house lighting a slow zoom becomes unusable. With colour film, you'll need fast primes for inside most of the time.

Further, you might find 800 iso film with a cheap lens wide open acceptable, or you might find it a hideous mush. So it's also a matter of your standards or intended use.

[JZaun]

Here is all you need ot know..

http://www.uscoles.com/fstop.htm

JZ