How does a lens work to form an image on the camera's sensor...

Started Mar 7, 2014 | Discussions thread
57even Forum Pro • Posts: 11,610
Re: How does a lens work to form an image on the camera's sensor...

GBJ wrote:

Wow, I am very thankful and impressed with the clarity of your explanation! You very quickly have showed me that the lens does not focus an image on the sensor in one point only...which should be obvious, but was not to me.

I can see by your well layed out diagram, that for example, a tree has elements that are closer and further away from the sensor. Each branch of the tree has a varying distance away from the sensor, and must be recorded on the sensor, but only the parts of the tree that are on one parallel plane from the sensor can be in complete focus, and the parts of the tree that are closer, or further away from that parallel plane of focus must be out of focus to the degree to which their distance varies from this plane of focus. Now I can see the development of the idea of depth of field.

From your diagram, I can imagine how each needle of the tree branch reflects it's own light ray towards the lens, and gets focused by the lens onto one pixel, or photon, of the sensor? 12 megapixel camera has 12 million pixels, or photo receptors, I think, and I can imagine each part of the tree having a designated pixel on the sensor to capture the image of the tree as a whole.

I know this can become a larger discussion.

Anyways, if you would be willing to write more on this topic, I would be very very keen to learn more.

Thankyou, 57 even, for the time you have taken to post these diagrams and explain them!

Couple more points on optics....

Lens focal length = distance from lens optical centre to point of focus for parallel light rays (from an object at effective infinity). As you increase focal length, you reduce the range of angles at which light can enter the lens, which reduces the field of view and makes objects appear closer/larger (smaller field projected onto the same size sensor).

Sensor crop factor = ratio of field of view (in degrees) between FF sensor and smaller sensor for a given focal length.

Focal length multiplier = equivalent FF focal length for a given focal length on a smaller sensor.

For instance, with a 200mm lens, an APSC sensor will be able to record only 2/3 of the image (by width or height) of the FF sensor, hence the crop factor is .67.

This means it gives the same field of view as a 300mm lens on an FF body, hence the focal length multiplier is 1.5X. The two are often confused.

Note in neither case has the focal length changed.

F number. F number is a rather complex measurement of aperture which takes focal length into account. Because the field of view is so much smaller on long focal length lenses, the physical aperture has to be bigger to get the same image brightness  - hence long fast lenses are wide too, whereas a fast 50mm lens can be quite small.

F stop is the ratio between focal length and aperture diameter. F 2.8 means the focal length is 2.8X larger than the aperture diameter.

However actual light gathering depends on the area, hence 1 stop in aperture terms is not linear with F number. F2.0 is 1 stop slower than F1.4 because it is half the area. Confusing I know...

T number. F numbers are useful for DOF calculations, but all that glass actually cuts down the amount of light transmitted by the lens. T stops/numbers compensate for this. Hence a T1.4 lens may actually have a slightly larger physical aperture than an F1.4 lens. Movie makers rely on this to calculate accurate exposure.

Why are wide angle lenses so huge?

If wide angle lenses have a very short focal length, why are they so huge? The answer is because you cannot place the lens directly up against the sensor. All cameras have a limit for this.

This means you have to add concave elements to the front part of the lens to diverge the rays which are then focused internally by another group of lenses which (on their own) actually have a longer FL. This concave lens is normally quite bulbous to accommodate the wide field of view, and the further from the sensor the lens has to be, the larger the front element has to get (because the real FL of the internal group has to get longer). DSLRs have to clear the internal mirror, so wide angle lenses are a particular issue.

The combined lens has a focal length much shorter than it appears, but the optical centre of the lens is actually behind the rear element (DSLR registration distance is about 40mm, but lenses may have an effective FL of 12mm or even smaller....).

The problem is much less complex for mirrorless cameras as the lens can be much closer to the sensor.

OK have to check out now. If you have direct questions email me privately.

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