Need help with a problem

[duckk]

I have my D40 on manual mode but every time i adjust the focal length, the aperture automatically changes itself. What do I do to stop this from happening? Thanks.

[OsmosisStudios]

Both your lenses are variable aperture. What this means is that at their wide end they have a maximum aperture that is different than at the telephoto end.

For example: your 18-55 f/3.5-5.6

At 18mm, the maximum aperture it has is f/3.5
At 55mm, the maximum aperture it has is f/5.6

If you set the lens to 18mm, f/3.5 and then zoom it to 55mm, the camera automatically adjusts the aperture accordingly.

[duckk]

So in any case, its normal that my camera automatically adjusts the aperture? I don't remember it doing so before, but I could be wrong..

[oldwolf]

Originally Posted by OsmosisStudios

Both your lenses are variable aperture. What this means is that at their wide end they have a maximum aperture that is different than at the telephoto end.

For example: your 18-55 f/3.5-5.6

At 18mm, the maximum aperture it has is f/3.5
At 55mm, the maximum aperture it has is f/5.6

If you set the lens to 18mm, f/3.5 and then zoom it to 55mm, the camera automatically adjusts the aperture accordingly.

+1

The only way you'll avoid your aperture or f/stop from changing is if you get a lens with a constant f/stop. Something like a 24-70mm f/2.8. If the lens specs are like that then it means that it can maintain that 2.8 f/stop at 24mm or at 70mm. Just be ready to spend a lot of money for a lens like that.

[OsmosisStudios]

Originally Posted by duckk

So in any case, its normal that my camera automatically adjusts the aperture? I don't remember it doing so before, but I could be wrong..

Not ANY case.

You have to have set the aperture to the maximum (3.5 in this case) at the wide end and be zooming in to 55. You'll notice that it actually changes along the way at, IIRC, 28, 35 and 42 or so.

It'll also do this in A, if you set it to 3.5 at 18mm, and then zoom to 55mm

On your 55-200, youve got a f/4-5.6, so it's actually faster at 55mm than your other lens.

[oldwolf]

but if you set it to 5.6 at 18mm and zoom to 55mm then it won't change because that aperture is available that whole range.

[duckk]

Thanks a lot Osmosis and oldwolf! Those were very helpful explanations and I get it now. But just to ask another question about aperture..the wider the maximum f-stop is for a particular lens, lets say a f/1.8, the faster that lens is?

[Snowdust]

Depending on the mode you are shooting in, it can change also because at a wide focal length, you have have alot of highlight, then at a tighter zoom, if you zoomed in on a darker area, exposure would change.

[oldwolf]

yes, you've got it.

it's faster at f/1.8 because you can use a "faster" shutter speed to properly expose the image.

[OsmosisStudios]

Originally Posted by Snowdust

Depending on the mode you are shooting in, it can change also because at a wide focal length, you have have alot of highlight, then at a tighter zoom, if you zoomed in on a darker area, exposure would change.

Not in full manual. It'll tell you it's going to over/under expose, but it wont change it

Fast lenses (primes especially) are a ton of fun! You should definitely look into the 35mm f/1.8 if you have the spare change for it.