How does Zooming Effect Light Level

[SpiralOcean]

Hoping I can shed some light on a discussion I'm having with another person.

We disagree on how light level is being effected using a zoom lens.

I don't think the zooming in or out on a zoom lens effects light level. It effects the amount of motion in the composition, and to compensate a camera operator will open up the shutter to capture the image. But the amount of light entering the lens doesn't change.

The person I'm discussing this with claims there is more glass in between the lens opening and the CCU or Film. There is no more glass in the lens, the glass is just moving. I don't see how this could effect the amount of light hitting the CCU.

To complicate matters, point and shot cameras will often decrease the aperture as a lens is zoomed in. Allowing less light into the camera. I started another thread about this topic. My assumption is a smaller aperture has a sharper image. As a lens is zoomed the depth of field decreases and the aperture must be closed down to keep the depth of field to a certain length.

[rc53]

Are you considering video or still cameras? See answer to your other question; zooming to telephoto will [frequently] cause the aperture f number to be larger -- a smaller aperture -- unless it is changed.

This might happen automatically in a video camera.

Changing the zoom in a lens means that some of the elements of the lens move forwards or backwards; there are no extra or fewer lens elements.

The 'light level' is determined by the light falling on the subject, not by the lens.

[SpiralOcean]

I am considering video cameras.

I may have been unclear when I said amount of light reaching the lens. What I should have said is, amount of light reaching the CCU or Film.

Zooming in and out will not affect the amount of light that hits the CCU. Changing aperture will change the amount of light reaching the CCU.

Is this correct?

[rc53]

I am considering video cameras.

I may have been unclear when I said amount of light reaching the lens. What I should have said is, amount of light reaching the CCU or Film.

Zooming in and out will not affect the amount of light that hits the CCU. Changing aperture will change the amount of light reaching the CCU.

Is this correct?

Not quite. The amount of light passing to the sensor depends on the aperture and the shutter speed; changing the zoom often optically changes the aperture -- so that, to maintain a correct exposure, the camera's auto-exposure must compensate for the optical change of aperture by increasing or decreasing the aperture.

[SpiralOcean]

Aye... there's the rub.

If the aperture changes during a zoom then the amount of light hitting the CCU would change.

However, in the cameras I've used, the aperture does not change while the lens is zooming. The auto exposure does not change... because I'm not using auto exposure. The only time it does change is if your aperture is wide open, and you get to the last bit of the telephoto, then the aperture will stop down to it's max at the telephoto end.

Could it be that for a lens, the aperture is using either the telephoto or wide focal length as it's measurement? If the focal length of a lens is a 30mm to 50mm lens, maybe the manufacture is calculating the f stop based on the 50mm focal length?

[rc53]

Aye... there's the rub.

If the aperture changes during a zoom then the amount of light hitting the CCU would change.

Yes

However, in the cameras I've used, the aperture does not change while the lens is zooming. The auto exposure does not change... because I'm not using auto exposure. The only time it does change is if your aperture is wide open, and you get to the last bit of the telephoto, then the aperture will stop down to it's max at the telephoto end.

OK

Could it be that for a lens, the aperture is using either the telephoto or wide focal length as it's measurement? If the focal length of a lens is a 30mm to 50mm lens, maybe the manufacture is calculating the f stop based on the 50mm focal length?

The lens will be described, for example, 70-200 mm f4, meaning that the aperture value is constant throughout the zoom range.

Or, say 70-200mm f3.5 - 5.6; meaning that at wide end 70mm, it is a f3.5 lens, and at 200 mm it is f5.6. And for focal lengths between 70 and 200, the f value gradually changes from a maximum of 3.5 at 70 to a maximum of 5.6 at 200mm.

[There may be a further complication for video cameras -- I''m not so familiar with them; the ISO value of the sensor - how sensitive it is - may change to reflect any decrease in light when zooming, if the f number becomes smaller -- this to keep the total exposure constant.]

[SpiralOcean]

The lens will be described, for example, 70-200 mm f4, meaning that the aperture value is constant throughout the zoom range.

Or, say 70-200mm f3.5 - 5.6; meaning that at wide end 70mm, it is a f3.5 lens, and at 200 mm it is f5.6. And for focal lengths between 70 and 200, the f value gradually changes from a maximum of 3.5 at 70 to a maximum of 5.6 at 200mm.

[There may be a further complication for video cameras -- I''m not so familiar with them; the ISO value of the sensor - how sensitive it is - may change to reflect any decrease in light when zooming, if the f number becomes smaller -- this to keep the total exposure constant.]

My experience is from a video camera.

One important point of your statement is 'the f value gradually changes'. However, the physical Iris is not changing.

Example:
I can set my aperture to f5.6, zoom in and out on manual shutter and 0db gain and the light level does not change. The f value does not change.

However, if I set my aperture to f1.6. When I zoom in, the aperture changes to f2 and then to f2.4. When I zoom out, the aperture changes back to f1.6.
From what I can tell, the light level is not changing.

If I set my aperture to f11, when I zoom in and out the aperture does not change.

This leads me to believe that the physical aperture is not changing. Only the ratio is changing to reflect what my f stop is for my focal length.

Back to the question of the thread. Is it a correct assumption that the amount of light hitting the CCU does not change when zooming. The f number changes to reflect the new ratio of focal length divided by aperture diameter.

If I were to change physical lenses that have a fixed focal length (one shorter and one longer), then this may affect the amount of light that is hitting the CCU. But with a zoom lens, the length of the lens is not changing. The focal length is changing within the lens.

How does that sound?

[rc53]

My experience is from a video camera.

One important point of your statement is 'the f value gradually changes'. However, the physical Iris is not changing.

Example:
I can set my aperture to f5.6, zoom in and out on manual shutter and 0db gain and the light level does not change. The f value does not change.

However, if I set my aperture to f1.6. When I zoom in, the aperture changes to f2 and then to f2.4. When I zoom out, the aperture changes back to f1.6.
From what I can tell, the light level is not changing.

If I set my aperture to f11, when I zoom in and out the aperture does not change.

This leads me to believe that the physical aperture is not changing. Only the ratio is changing to reflect what my f stop is for my focal length.

Back to the question of the thread. Is it a correct assumption that the amount of light hitting the CCU does not change when zooming. The f number changes to reflect the new ratio of focal length divided by aperture diameter.

If I were to change physical lenses that have a fixed focal length (one shorter and one longer), then this may affect the amount of light that is hitting the CCU. But with a zoom lens, the length of the lens is not changing. The focal length is changing within the lens.

How does that sound?

I'm not so familiar with video cameras.

Your maximum aperture of f1.6 will be at wide setting; but optically this can't be maintained when zooming to tele; however, it would seem that a smaller aperture can be maintained.

I don't know if the actual diameter of the aperture changes in a video camera when zooming, but I would be surprised if it did. Although the diameter doesn't change when zooming, the focal length will, hence the change in f numbers.

f numbers are absolute in relation to any particular lens, and also relative; in principle, any lens at say f:4 will transmit the same amount of light - but this is complicated by transmission losses depending on the number of elements and their coatings, so may not be exactly true.

If the f number changes when zooming, then more or less light is being transmitted.

The amount of light transmitted when zooming won't change if there is no change in f number [excluding any change in transmission losses]

Some zooms extend when zooming, some don't. The relation between focal length and the actual physical length of the lens is irrelevant. Strictly, a telephoto lens is one whose physical length is less than its focal length.

[rc53]

Let me try a different approach.

It's obvious in some lenses that when zooming the lens gets longer and shorter, and so it would seem obvious that there could be a change in the amount of light transmitted.

It isn't so obvious or intuitive when the lens doesn't extend when zooming, but the effect is the same.

It isn't intuitive that changing the f number changes the size of the image - yet this happens.

It isn't intuitive that changing the f number can change the point of focus, yet this can happen.

It isn't intuitive that our planet rotates on its own axis and simultaneously around the sun -- all common sense says the opposite -- yet this is what happens.

Sometimes, common sense and intuition are wrong.

[Dave Humphries]

Many years ago, I was involved with broadcast TV (video) cameras, here's my experience;

As you say, if you set an aperture of f11 and zoomed, it didn't change, what you mean is the marked aperture didn't appear to change, I think you'll find the physical one was, it's just that the ratio of focal length and aperture remained constant at f11. As you zoomed in, the zoom lens' electronics - quite separate from the camera's exposure control, DID change the physical aperture to compensate.

For a given amount of subject illumination, camera gain (equates to ISO), optical diameter and focal length, there comes a point when the physical aperture cannot get any bigger, so the compensation runs out, at this point, the aperture "ramps" down, so the image gets darker at the tight-in telephoto end. Optionally, an electronic zoom end stop could be set and switched in so it didn't do that, but you didn't have the full zoom range.

As you say, if you start wide open at f1.6, you see the effect from the beginning.

Hope that helps, cheers, Dave

[SpiralOcean]

If the physical aperture is changing while zooming, and I don't see the light level changing, then zooming a lens is changing the amount of light hitting the CCU and the camera is adjusting the aperture automatically to keep the same amount of light hitting the lens.

I don't think this is the case.

When zooming, the only thing changing in the lens is the position of the lenses inside the housing. I don't know how this could affect the amount of light hitting the CCU.

When the f stop number changes on my camera when I get to the end of the telephoto, the light level is not changing. Therefore I assume that the digital readout of the f stop is changing but not the physical aperture.

If I am incorrect then how does zooming in a telephoto lens change the amount of light that is hitting the CCU?

[Raycer]

Hi Spiral Ocean,
When you are zooming in and out, the diameter of the aperture is changing.

Here's an experiment you can do.
Point your view finder towards a soft light source and look through the lens. Basically hold the SLR backwards. Now zoom in and out and see what happens.

What kind of lens do you have?
Does your lens say 70-300 1:3.5-5.6 or something like that?

[Dave Humphries]

Hi Spiralocean, I was simply relating what I recall seeing and experiencing, I am no expert. From a 'google', probably the best explanation is here, (you need to scroll about 2/3 down the page to find the relevant bit). While it doesn't prove me correct in my theory, it does confirm that the light falling on the CCD (or other sensor), will diminish as you zoom in past the ramping point inherent in most practical zoom lenses.

I like Raycer's idea too, it is a practical demo of the effect as described here

[orionthunter]

Zooming actually does affect light reaching the sensor. What zooming does it enlarge an image so only part of it fits on the sensor. If you zoom into 2X you only have half the field of view that you do at 1X, since you only get light from half of the field of view, you get half the light reaching the sensor.

[Raycer]

Zooming actually does affect light reaching the sensor. What zooming does it enlarge an image so only part of it fits on the sensor. If you zoom into 2X you only have half the field of view that you do at 1X, since you only get light from half of the field of view, you get half the light reaching the sensor.

[COLOR=#cccccc][COLOR=#cccccc]the math is not correct but I understand your point.

[COLOR=#cccccc]assuming everything in field of view is evenly lit.

[COLOR=#cccccc]agree - if the diameter of the opening, entrance diameter, remains the same.
[COLOR=#cccccc]disagree - if the aperture (f/#) remains the same.

[SpiralOcean]

. From a 'google', probably the best explanation is here, (you need to scroll about 2/3 down the page to find the relevant bit). While it doesn't prove me correct in my theory, it does confirm that the light falling on the CCD (or other sensor), will diminish as you zoom in past the ramping point inherent in most practical zoom lenses.

Excellent article! Thanks for pointing that one out. I did find out why the aperture changes at the end of the telephoto zoom:

If you have zoomed with a zoom lens open to full aperture, you may have noted a drop in video level at the telephoto end. This is called the F drop or "ramping". The "entrance pupil" of a zoom lens changes in diameter as the focal length is changed. As you zoom toward the telephoto end, the entrance pupil gradually enlarges. When the entrance pupil diameter is equal to the diameter of the focusing lens group, it can not become any larger, so the F-stop drops. That is the reason for the F drop.
To eliminate F drop completely, the focusing lens group, (the elements in the front of the lens), has to be larger than the entrance pupil at the telephoto end of the zoom. It has to be at least equal to the focal length at the telephoto end divided by the F-number. To reduce the size and weight of a zoom lens to make it easy to use for hand held cameras, we have a trade off that makes it common to have a certain amount of F drop or ramping at the telephoto end. For better composition effect, however, in some studio zoom lenses the focusing group is made large enough that no F drop occurs. F drop is a major determinant of the value of zoom lenses used in live on-site sports broadcasts, which require a long focal length and must frequently contend with twilight or inadequate artificial illumination.

As many people know, movie camera lenses are rated by a T-number instead of an F-stop.The F-stop expresses the speed of the lens on the assumption that lens transmits 1OO% of the incident light. In reality, different lenses have different transmittance, so two lenses with the same F-stop may actually have different speed. The T-number solves this problem by taking both the diaphragm diameter and transmittance into account. Two lenses with the same T number will always give the same brightness.

I like Raycer's idea too, it is a practical demo of the effect as described here

I got an error when going to this URL. But the book looks interesting.


I tried looking into the lens of a PD-150, unfortunately, I cannot see the Iris.

Regarding this quote:

Originally Posted by orionthunter
Zooming actually does affect light reaching the sensor. What zooming does it enlarge an image so only part of it fits on the sensor. If you zoom into 2X you only have half the field of view that you do at 1X, since you only get light from half of the field of view, you get half the light reaching the sensor

.

I don't agree with zooming in changes the amount of light hitting the sensor. The same amount of light is still entering the lens, the only aspect of the lens that controls the amount of light to hit the sensor is the Iris. Think of zooming into a picture on your computer... you can zoom into a bright spot on the picture, but the light doesn't change. (terrible analogy but I believe some of the process is the same).

If you got closer to the light source, then the amount of light would change.

Appreciate everyone's help with this. I don't mind being wrong, but I need to see it for myself. The posts have been tremendous in helping me work through it.

[Raycer]

I tried looking into the lens of a PD-150, unfortunately, I cannot see the Iris.

of course not, its not an SLR.

...Think of zooming into a picture on your computer... you can zoom into a bright spot on the picture, but the light doesn't change. (terrible analogy but I believe some of the process is the same).

agreed that it is a terrible analogy and has nothing to do with lens. The question is "how does zooming effect light level." I assume this question is about optical zoom and should not be compared with digital 'zoom.'

If you got closer to the light source, then the amount of light would change.

agreed, light falls off by the inverse square law. wait, I thought we were talking about staying at the same place and use different focal length? let's stay on that topic.

amount of light reaching the sensor/film is given by the following (assuming the object is far away from the lens):
E = pi*r^2*t*L*F^-2

where
E = illuminance reach the sensor
pi = 3.14159...
t = efficiency of lens(es)
L = light from the object
r = radius of the entrance pupil
F = focal length

express the above using aperture

E = pi*t*L*4^-1*(f/#)^-2

where f/# = aperture

you are correct by saying the amount of light reaching the sensor is a function of (f/#). However, lens design limits the maximum diameter of the entrance pupil. Note if the aperture is to stay constant, the diameter of the entrace pupil needs to increase as the lens moves from wide angle to telephoto.

Most consumer lenses are not designed to have constant aperture at the wide open setting. That's why when you zoom in, the aperture starts to drop. So answering your original question; yes, when your camera tells you that the aperture is droping, there's less light reaching your sensor.

hope that clears it up.

[orionthunter]

I thought of a good analogy, shine a flashlight at the wall, you get a spot of light, back away you get a bigger dimmer spot of light, zooming essentially does this with your image, so in order to keep the brightness the same, you need a bigger aperture (physical size not f/#)

[SpiralOcean]

Good analogy... I think it needs some adjustment.

The light source isn't moving... so we would have to say the flashlight isn't moving.

There is a piece of glass in between the flashlight and the wall that is bending the light, focusing, zooming, to be projected onto the wall.

The glass can concentrate all the light into a single point, in which case, there will be more light at that single point.

This may have answered my question... if the zooming is bending more light to a focused point, then there is an increase in luminance on the wall. I'm not convinced this is how the zooming is working.

It may be false to think of a flashlight shining through glass, because this is more like a projector.

Probably a better image is, a flashlight shining onto a wall. And a viewer (me) holding up a magnifying glass to view the light on the wall. Only... the CCU on the camera is smaller than the image I am looking at. Even when I am zooming in and out, the image on the CCU is smaller. This link was excellent in helping me visualize this:
http://micro.magnet.fsu.edu/primer/j...s3d/index.html
The only problem with that link and what we are discussing is, the object is not moving, the lens is moving.


Here are some other links:
http://photography-on-the.net/forum/...975#post777253

some cool visual tools about different type of lenses:
http://micro.magnet.fsu.edu/primer/j...s3d/index.html
http://micro.magnet.fsu.edu/primer/j...s3d/index.html
http://micro.magnet.fsu.edu/primer/j...ors/index.html
http://micro.magnet.fsu.edu/primer/j...ors/index.html

[SpiralOcean]

Even better links with regards to lenses:
http://micro.magnet.fsu.edu/primer/j...i-concave.html
http://micro.magnet.fsu.edu/primer/j...bi-convex.html
http://micro.magnet.fsu.edu/primer/j.../p-convex.html

These and more are found on this page:
http://micro.magnet.fsu.edu/primer/l.../javalens.html