[lotech]

I've a theory on my mind may sounds silly, and don't know if that applies to photographic lenses.

We have fixed aperture lenses like the DA* 16-50/f2.8, and the variable aperture 18-55/f3.5-5.6 kit lens, they are in different design can not be directly compared I know, but although the DA* is IF the lens still extends when zoom in, so that the distance between the front glass is further away from the rest, or the other way around, according to the inverse square law there is light fall off over distance, so that the lens will not always be f2.8 right ? I think this is not the same as T no. If the DA* is f2.8 at the far end, then will it be brighter at the short end huh ?? if this is so then it is say a f2-2.8 instead, there is optical design to limit (waste) some usable light ! I know this sound silly but not completely nonsense. I am talking about purely the lens design, not include in camera adjustment/profile preset for each lens to compensate the difference.

[zapp]

TTL metering compensates for this and many other differences. Auto aperture/ stoping down requires an adjustment profile. You will not have to compensate full f-stop, but differences will be visible unless compensated. In the end, don’t bother unless you use external exposure metering…

[pschlute]

I've a theory on my mind may sounds silly, and don't know if that applies to photographic lenses.

We have fixed aperture lenses like the DA* 16-50/f2.8, and the variable aperture 18-55/f3.5-5.6 kit lens, they are in different design can not be directly compared I know, but although the DA* is IF the lens still extends when zoom in, so that the distance between the front glass is further away from the rest, or the other way around, according to the inverse square law there is light fall off over distance, so that the lens will not always be f2.8 right ? I think this is not the same as T no. If the DA* is f2.8 at the far end, then will it be brighter at the short end huh ?? if this is so then it is say a f2-2.8 instead, there is optical design to limit (waste) some usable light ! I know this sound silly but not completely nonsense. I am talking about purely the lens design, not include in camera adjustment/profile preset for each lens to compensate the difference.

The distance from your subject to the camera sensor is the same whatever your zoom setting so there is no light fall off or increase in brightness due to inverse square law, which does not apply to reflected light anyway.

As you change your focal length there would be a light increase/decrease if your aperture were a constant fixed physical size. But a zoom like your DA* 16-50 does not work like that. The f stop of 2.8 is the ratio of the FL to the apparent size of the entrance pupil (iris). As you zoom from 50mm to 16mm the entrance pupil will decrease in apparent size, maintaining the f 2.8 ratio. Have a look through the front of the lens as you zoom and you can see this occur.

You can easily test your theory anyway. Using an evenly lit subject, set your camera in Manual mode to f2.8 and take a picture at both ends of the zoom range. The brightness should be identical.

[lotech]

The distance from your subject to the camera sensor is the same whatever your zoom setting so there is no light fall off or increase in brightness due to inverse square law.

As you change your focal length there would be a light increase/decrease if your aperture were a constant fixed physical size. But a zoom like your DA* 16-50 does not work like that. The f stop of 2.8 is the ratio of the FL to the apparent size of the entrance pupil (iris). As you zoom from 50mm to 16mm the entrance pupil will decrease in apparent size, maintaining the f 2.8 ratio. Have a look through the front of the lens as you zoom and you can see this occur.

You can easily test your theory anyway. Using an evenly lit subject, set your camera in Manual mode to f2.8 and take a picture at both ends of the zoom range. The brightness should be identical.

So that no matter how complicated the lens design is, how each element moves during zoom and focus, there will be no additional light intensity changes other than the ratio of the front lens to the iris ?

[pschlute]

So that no matter how complicated the lens design is, how each element moves during zoom and focus, there will be no additional light intensity changes other than the ratio of the front lens to the iris ?

Light transmission can vary from one lens to another of the same max aperture depending on design, which is why there is the T-stop designation. But for most still photography this will be negligible, and certainly not of the order of 1 f stop (f2.8 to f2.0)

Inverse square law does not apply to reflected light so the movement of the elements will not be affected by that.

The f stop ratio is of the FL to the apparent size of the entrance pupil. As you zoom in to a subject and your lens increases in FL, so your iris enlarges (on a constant aperture zoom), maintaining the f-stop and the amount of light hitting the sensor.

[lotech]

Light transmission can vary from one lens to another of the same max aperture depending on design, which is why there is the T-stop designation. But for most still photography this will be negligible, and certainly not of the order of 1 f stop (f2.8 to f2.0)

Inverse square law does not apply to reflected light so the movement of the elements will not be affected by that.

The f stop ratio is of the FL to the apparent size of the entrance pupil. As you zoom in to a subject and your lens increases in FL, so your iris enlarges (on a constant aperture zoom), maintaining the f-stop and the amount of light hitting the sensor.

IC, as some said TTL will take care of that no need to worry about, it was just a thought pop up on my mind, a fundermental knowledge we photographer should know !

[pschlute]

as some said TTL will take care of that no need to worry about

Well the camera's meter will give you the correct exposure for the reflected light it records coming through the lens, but that actually misses the point.

If I take a reflective light reading from a hand held spot meter of my subject, I need to tell the meter some parameters. I need to tell it the ISO I am using and the aperture (say f2.8). The meter then tells me what shutter speed I need to set for correct exposure. Unlike the camera, the meter has no idea what focal length I am using, or whether it is a zoom or not.

Equally, if I use a hand held incident light meter (this measures the light falling on the subject, not the light reflected from it), I must tell it the same parameters to get an accurate reading. Again, the meter has no idea of focal length and no idea about the camera to subject distance. Whether I take the picture from 5 feet or 10 feet away, the exposure is the same.

A f2.8 constant-aperture zoom, set at f2.8 will record the same amount of light for any given focal length. It is the optical property of the lens that does this, nothing to do with the camera's meter or in-camera profiles.

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Last edited by pschlute; 01-05-2022 at 02:36 AM.

[lotech]

Well the camera's meter will give you the correct exposure for the reflected light it records coming through the lens, but that actually misses the point.

If I take a reflective light reading from a hand held spot meter of my subject, I need to tell the meter some parameters. I need to tell it the ISO I am using and the aperture (say f2.8). The meter then tells me what shutter speed I need to set for correct exposure. Unlike the camera, the meter has no idea what focal length I am using, or whether it is a zoom or not.

Equally, if I use a hand held incident light meter (this measures the light falling on the subject, not the light reflected from it), I must tell it the same parameters to get an accurate reading. Again, the meter has no idea of focal length and no idea about the camera to subject distance. Whether I take the picture from 5 feet or 10 feet away, the exposure is the same.

Yes, I used Minolta and Seckonic light meter when I shoot film back then, the meter does not take into account the lens parameter, it is all rely on the photographer's skill and experience to produce a good photo, but negative is forgiving need not to be perfectly precise, unless you are playing with zone system.

[Dartmoor Dave]

This link tells you how fixed and variable aperture zooms actually work:

https://www.pencilofrays.com/lens-design-forms/#zoom

And here's a TL;DR:

In true constant aperture zooms, the change in focal length is caused by changing the magnification of the lens elements in front of the aperture diaphragm only, while the elements behind the aperture diaphragm don't move. The physical size of the aperture diaphragm doesn't change, but the apparent size of the entry pupil does change(when viewed from the front of the lens) because it appears to be enlarged by the change in magnification of the elements in front of it. So you get a constant F stop.

In variable aperture zooms, the elements both in front of the aperture diaphragm and behind it move as you zoom, while the size of the physical diaphragm stays the same, so you get a variable F stop.

But there are some constant aperture zooms in which both the front and rear groups move, and in those lenses the physical aperture diaphragm does change size as you zoom in order to keep the F stop constant.

(I've left out the difference between true zooms and varifocals for the sake of simplicity.)

Edit, just to add a further clarification in case it's not completely clear: fixed or variable aperture in a zoom has got absolutely nothing at all to do with light fall off. It's entirely down to the relationship between the effective focal length and the apparent size of the entry pupil.

[Not a Number]

Basically a constant or fixed aperture zoom the aperture will open wider at the longer focal lengths.

Test this by setting the aperture wide open. Set the focal length to the shortest and then stop it down on the camera body. Shoot in Bulb mode or set a long exposure or optical preview and look through the front of the lens. You should see the iris is not completely open. Repeat at the longest focal length. You should probably see the diaphragm is completely open.

On a variable aperture lens the diaphragm when set at full open will always be opened fully at all focal lengths.

[lotech]

This link tells you how fixed and variable aperture zooms actually work:

https://www.pencilofrays.com/lens-design-forms/#zoom

And here's a TL;DR:

In true constant aperture zooms, the change in focal length is caused by changing the magnification of the lens elements in front of the aperture diaphragm only, while the elements behind the aperture diaphragm don't move. The physical size of the aperture diaphragm doesn't change, but the apparent size of the entry pupil does change(when viewed from the front of the lens) because it appears to be enlarged by the change in magnification of the elements in front of it. So you get a constant F stop.

In variable aperture zooms, the elements both in front of the aperture diaphragm and behind it move as you zoom, while the size of the physical diaphragm stays the same, so you get a variable F stop.

But there are some constant aperture zooms in which both the front and rear groups move, and in those lenses the physical aperture diaphragm does change size as you zoom in order to keep the F stop constant.

(I've left out the difference between true zooms and varifocals for the sake of simplicity.)

Edit, just to add a further clarification in case it's not completely clear: fixed or variable aperture in a zoom has got absolutely nothing at all to do with light fall off. It's entirely down to the relationship between the effective focal length and the apparent size of the entry pupil.

Got it, thanks for the detailed explanation !

[stevebrot]

From a metering and DOF aspect, yes.


Steve