F question - and not the usual one

[stevieh59]

I brought a 450d and have been bitten by the bug, purchased my 2nd lens now, so it must be love.

I was reading about landscape photography and it mentioned about taken a lens to f16, which got me thinking about my kit lens.

The kit lens is 18-55mm IS f/3.5-5.6 does this mean that it only does between 3.5 and 5.6, because on the camera I can dial right up to f29 if I wish. It wont go any lower than 3.5, so bit confused as to what the f range means??

[ap4a]

That's the maximum. As your lens doesn't have a constant maximum aperture it displays the range of maximums. So at 18mm the max is f/3.5 and at 55mm it's f/5.6. The minimum aperture will be the same across the focal range.

[stevieh59]

Originally Posted by ap4a

That's the maximum. As your lens doesn't have a constant maximum aperture it displays the range of maximums. So at 18mm the max is f/3.5 and at 55mm it's f/5.6. The minimum aperture will be the same across the focal range.

I think I understand what you are saying. Sorry only just started. So when I switch to Av mode and ramp it up to f29, its no different to f/5.6

[ap4a]

It's very different, approximately 5 stops of light different (f/32 is 5 stops lower than f/5.6).

The f/3.5-5.6 part of the lens title is informing you what the maximum apertures are at each end of its zoom range.

Zoom your lens out to 55mm and go to your maximum aperture, it will be f/5.6. Now adjust it to the minimum, it'll be f/29 (based on what you've said, I don't own that lens to confirm it).

Next, zoom in to 18mm and change your aperture from f/29 to the maximum. You'll now find you can get to f/3.5 instead of f/5.6

[matt wilson]

Going smaller than F 16 often leads to poorer image quality .(can't really explain why as it's down to physics and sensors which are not my strong point )

By smaller I mean F 22 or beyond.

Most lenses tend to have a sweet spot or aperture value where they perform best .

Unless you spend an awful lot of money that is .

You will probably find on your kit lens it's somewhere near the middle i.e F 8 or F 11 .At the extremes I.e fully open at F 3.5 or fully closed at F 29 definiton may well be poorer .

A landscape shot at F 16 ,especially if you focus carefully should be sharp from front to back.

[stevieh59]

Ahh thanks for that guys. I always wondered what the f/ numbers mean on a lens. I didnt think it meant that was the only f/ range you could use.

[KeithT]

It's all down to the Circle of Confusion (and many of us have been confused for years) which leads to what is know as diffraction at narrow apertures. At the wide end of the aperture range we get the narrow depth of field phenomonen which also needs careful handling if you want to use it to your advantage. Diffraction at narrow apertures can ruin many an otherwise good landscape shot, so it's best to read up on it and become aware of the pros and cons of your own particular lenses. A good article on diffraction can be read here: http://www.bobatkins.com/photography...ffraction.html

[chris-p]

Originally Posted by matt wilson

Most lenses tend to have a sweet spot or aperture value where they perform best .

Unless you spend an awful lot of money that is .

Every lens has a sweet spot. My microscope optics make top end SLR lenses look like cr*p and they still have a sweet spot... It just gets larger the more you pay (with camera lenses, that is)

[GeoffWessex]

Here's a fairly good article on F Numbers..... you soon realise why a 300mm lens has to have a quite high maximum f number (and is therefore 'slower')