Relation between dust and aperture setting

Started Mar 30, 2014 | Questions thread
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jfriend00 Forum Pro • Posts: 11,312
Re: Relation between dust and aperture setting

SushiEater wrote:

jfriend00 wrote:

When you shoot at a small aperture, the small aperture is excluding the light rays that come from the outer parts of the lens. As such you end up with only light rays that are of very similar angle through the lens. This means that the light rays hitting a given pixel on the sensor are generally all arriving in at approximately the same angle. This means that a dust particle that's on the glass above the sensor is going to cast a much stronger shadow on that particular pixel and thus it's effect will be quite noticeable.

If you shoot at a large aperture, the entire lens are is used to collect light and light arrives at a given pixel from many different angles. Because the light arrives from many different angles, the dust particle doesn't cast a very pronounced shadow on the pixels and thus you don't notice it as much.

Not even close. Regardless of aperture used the entire lens collects the light. At large aperture light hitting the sensor is straight (er) and at small aperture light is at the angle scattered more thus you get diffraction. It works exactly the same as your eyes. The more you squint the more you see in focus (DOF) but squinting does not prevent seeing less. No matter what your entire pupil collects light you see. The brighter the light the more pupil contracts.

Sorry, dude but you just don't appear to understand here. Why do you think large aperture lenses have much bigger optics. It's because the EXTRA light for the larger aperture comes from collecting more light with the larger lens. When you then take a large aperture lens and use a small aperture with it, you are restricting light so that only light from a narrower set of angles (through the central part of the lens) is actually used, the other light rays are blocked by the aperture.

FYI, squinting DOES restrict the light. It narrows down your eye aperture so not as much light gets in and thus only light from a narrower angle gets in to the eye that is easier to focus and has more DOF. When the aperture of the eye contracts, it's blocking light just like when the aperture in the elns.

It's the same reason why you use a large softbox to make soft shadows or no shadows (light comes from many different angles and thus doesn't make well defined shadows) and why a single point flash makes well defined shadows (light comes from only one angle to each point in the subject and thus casts a well defined shadow).

The soft box makes light to go around the object and it doesn't have to be large but the larger it is the more light goes around. And it is not the only reason. Bouncing light can eliminate shadows too by simply throwing light on the shadows.

You are so wrong here. Light travels in straight lines. It NEVER goes "around" an object (except for diffraction which is not what we're talking about here). The softbox works because the light comes from many different sources at many different angles, thus the light can't form a sharp shadow boundary.

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