Apertures at different focal lengths

[Janice]

To save me spending ages testing this, does anyone already know?

If I find that the scratches dont show up in my shots at apertures narrower than say, F11 at a focal length of 50mm

Is that going to be the same aperture when using 28mm or 75mm etc.
Or will the point where it notices change at all with these different lengths?

 

[I MAG images]

I think that the only difference would be DOF and the longer the focal length the more compression you will have to the finished shot. The aperture would remain the same.

 

[Janice]

So would I be able to use different apertures then without showing the scratches is really what I mean.

 

[I MAG images]

I should imagine that no matter what the focal length you would still have a wide ish aperture to make sure that the scatches are not too visable...Ian

 

[apodo]

That is correct, the focal length does not matter, just the aperture.

And just in case anyone is interested in the reason for this, with a larger aperture light enters through the lens from more angles and so it more light "get around" dust spots on the filter. With a smaller aperture the light rays from more obtuse angles are prevented from passing through the lens and so there is less light at angles that are able to get behind the dust spots so they cast a noticeable shadow. It is the same effect that gives greater depth of field at smaller apertures.

The narrower angles at which the light reflected from a specific point hits the sensor means it will have a smaller circle of confusion. This circle being the maximum area within which all the light rays coming from a single point will hit the sensor, and what causes out of focus areas to appear blurred. As a smaller circle is cast at smaller apertures it increases the distance from the focal point from which the light rays will be smaller than the acceptable circle of confusion to appear to be in focus.

Michael.

 

[RobertP]

As said the range of angles the light can come from is the key. Smaller aperture = narrower angles = harder shadows.

The F number is the ratio of the focal length to the size of the opening (aperture) and so defines the angle the light can travel through as well.

A 200mm F4 lens will have a 50mm objective glass as 200 / 50 = 4.

So short answer all lenses should be the same.

Caveat : answer fuelled by a few small french beers and a half bottle of fizzy wine

 

[Janice]

I shouldve known I wouldnt understand a bloody word when I asked this question!

Thanks anyway guys. I have found that the scratches are there at F11 as well, so its pretty large apertures for me for a while.