Physics of light.

Culture · 09-05-2019, 09:25 AM

How is the light converged on the sensor?
For example is some of the light coming through an FF lens lost when it falls on a crop sensor?
And vice versa, is an FF sensor not seeing/receiving enough light to fill the sensor area when a cropped lens is mounted? I am guessing this is the reason for vignetting strong or weak depending on the lens.

How does this affect real aperture then?
Because a cropped sensor is seeing all the light that it needs if an FF f1.7 lens is mounted or APS-C 1.7 lens is mounted given an f-stop. The reverse I am not sure of.

Culture.

**AstroDave** · 09-05-2019, 10:12 AM - **2 Likes**

Originally posted by Culture

For example is some of the light coming through an FF lens lost when it falls on a crop sensor?

No - it falls outside (around the edge) of the sensor (where it may or may not be absorbed by something else or just bounce around inside the camera until it gets absorbed somewhere).

Originally posted by Culture

And vice versa, is an FF sensor not seeing/receiving enough light to fill the sensor area when a cropped lens is mounted? I am guessing this is the reason for vignetting strong or weak depending on the lens.

No - the image circle (the image area) created by the light passing through the APS-C lens is smaller than the size of the FF sensor. Imagine taking out the FF sensor and replacing it with an APS-C sensor - the smaller sensor would be fully illuminated. It is just that the size (the angular spread) of the image light cone coming through the APS-C is smaller than the FF sensor. This is determined by the various lenses/elements within the lens itself.

Wasp · 09-05-2019, 10:58 AM

There are full frame to crop frame adapters out there that will shrink the image. In the process things gets brighter as well. Typically, focal length is multiplied by 0.7 and you gain one f/stop. These things only work with mirrorless bodies, though.

stevebrot · 09-05-2019, 11:14 AM - **1 Like**

Originally posted by Culture

For example is some of the light coming through an FF lens lost when it falls on a crop sensor?

The key is the word "crop". For a given focal length, a smaller sensor is illuminated by the same light within its surface area as a larger sensor within the same boundaries. This is easily demonstrated empirically on a camera such as the K-1 where the same sensor may be used for both 24x36 FF and smaller crops.

Originally posted by Culture

is an FF sensor not seeing/receiving enough light to fill the sensor area when a cropped lens is mounted?

It depends on where one measures. The image circle of a crop-sensor lens is usually inadequate to provide sufficient coverage for the larger format to avoid significant vignette outside the "crop" area the lens was designed for with a boundary that is almost never perfectly distinct; the fall-off is incremental by radius. This is often immediately obvious in the viewfinder and in live view when the image circle is gravely inadequate (e.g. my Rokinon 8mm fisheye) and somewhat less so with other lenses where the vignette is more subtle, but still enough to affect the image quality.

Steve

Dartmoor Dave · 09-05-2019, 11:23 AM

Please, please, please read very carefully what AstroDave and stevebrot have written, and accept that they are correct in what they say. Then try to forget anything else that you might have read on the internet.

Soon, people will show up here and start saying things that are factually incorrect about something that they call "equivalence", and then we'll have the usual 20 pages of nonsense.

pschlute · 09-05-2019, 11:31 AM - **2 Likes**

i am not getting involved in this debate again

Well...maybe a little bit.....

Originally posted by Culture

How is the light converged on the sensor?
For example is some of the light coming through an FF lens lost when it falls on a crop sensor?

Imagine you are standing right in front of a window looking out and it is a bright day outside. You need to put on your sunglasses. Ahhh, that is better.

Now imagine that instead of a window, you are standing in front of floor-to-ceiling glass patio doors that cover the entire surface area of that side of the room. Do you need stronger sunglasses ?

Dartmoor Dave · 09-05-2019, 11:55 AM

Originally posted by pschlute

i am not getting involved in this debate again

Maybe we can save everyone the trouble by linking back to the last epic saga: lens light-gathering abilities - PentaxForums.com

Or would that mean depriving the popcorn industry of some much-needed sales?

stevebrot · 09-05-2019, 12:10 PM - **1 Like**

Originally posted by Dartmoor Dave

Soon, people will show up here and start saying things that are factually incorrect about something that they call "equivalence", and then we'll have the usual 20 pages of nonsense.

Are you saying that upsampling a patch of gray does not turn it black?

Steve

**GUB** · 09-05-2019, 01:38 PM

With an OP like that one would almost wonder whether Culture had a popcorn franchise

stevebrot · 09-05-2019, 03:12 PM

Originally posted by GUB

With an OP like that one would almost wonder whether Culture had a popcorn franchise

I have am acquiring positions for both salt and butter. $$

Steve

Culture · 09-06-2019, 12:12 AM - **1 Like**

This post was made just before I went to sleep.
Well I will say that I am smiling going through the posts.

I guess it makes sense what @AstroDave and @stevebrot have mentioned.

Shhh.... about the popcorn franchise. I do not have one. But I did bring a sachet to work hoping for a longer fight to enjoy this.

I will take my popcorn to the other linked post by @Dartmoor Dave.

Thanks guys.
Culture.

**GUB** · 09-06-2019, 01:20 AM

Originally posted by Culture

This post was made just before I went to sleep.
Well I will say that I am smiling going through the posts.

I guess it makes sense what @AstroDave and @stevebrot have mentioned.

Shhh.... about the popcorn franchise. I do not have one. But I did bring a sachet to work hoping for a longer fight to enjoy this.

I will take my popcorn to the other linked post by @Dartmoor Dave.

Thanks guys.
Culture.

Happy reading. Every page -every post -- just don't bring the beast back to life!!

09-05-2019, 09:25 AM	#1
Culture Veteran Member Join Date: Sep 2011 Location: Vaasa Photos: Gallery \| Albums Posts: 818	Physics of light. How is the light converged on the sensor? For example is some of the light coming through an FF lens lost when it falls on a crop sensor? And vice versa, is an FF sensor not seeing/receiving enough light to fill the sensor area when a cropped lens is mounted? I am guessing this is the reason for vignetting strong or weak depending on the lens. How does this affect real aperture then? Because a cropped sensor is seeing all the light that it needs if an FF f1.7 lens is mounted or APS-C 1.7 lens is mounted given an f-stop. The reverse I am not sure of. Culture.

09-05-2019, 11:31 AM - 2 Likes	#6
pschlute Moderator Loyal Site Supporter Join Date: Mar 2007 Location: Surrey, UK Photos: Gallery Posts: 8,210	i am not getting involved in this debate again Well...maybe a little bit..... Originally posted by Culture How is the light converged on the sensor? For example is some of the light coming through an FF lens lost when it falls on a crop sensor? Imagine you are standing right in front of a window looking out and it is a bright day outside. You need to put on your sunglasses. Ahhh, that is better. Now imagine that instead of a window, you are standing in front of floor-to-ceiling glass patio doors that cover the entire surface area of that side of the room. Do you need stronger sunglasses ? Last edited by pschlute; 09-05-2019 at 01:24 PM.

09-06-2019, 12:12 AM - 1 Like	#11
Culture Veteran Member Join Date: Sep 2011 Location: Vaasa Photos: Gallery \| Albums Posts: 818 Original Poster	This post was made just before I went to sleep. Well I will say that I am smiling going through the posts. I guess it makes sense what @AstroDave and @stevebrot have mentioned. Shhh.... about the popcorn franchise. I do not have one. But I did bring a sachet to work hoping for a longer fight to enjoy this. I will take my popcorn to the other linked post by @Dartmoor Dave. Thanks guys. Culture. Last edited by Culture; 09-06-2019 at 12:57 AM.

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09-05-2019, 10:12 AM - 2 Likes	#2
AstroDave Loyal Site Supporter Join Date: May 2007 Location: Flagstaff, Arizona Posts: 1,644	Originally posted by Culture For example is some of the light coming through an FF lens lost when it falls on a crop sensor? No - it falls outside (around the edge) of the sensor (where it may or may not be absorbed by something else or just bounce around inside the camera until it gets absorbed somewhere). Originally posted by Culture And vice versa, is an FF sensor not seeing/receiving enough light to fill the sensor area when a cropped lens is mounted? I am guessing this is the reason for vignetting strong or weak depending on the lens. No - the image circle (the image area) created by the light passing through the APS-C lens is smaller than the size of the FF sensor. Imagine taking out the FF sensor and replacing it with an APS-C sensor - the smaller sensor would be fully illuminated. It is just that the size (the angular spread) of the image light cone coming through the APS-C is smaller than the FF sensor. This is determined by the various lenses/elements within the lens itself.

09-05-2019, 10:58 AM	#3
Wasp Pentaxian Join Date: Mar 2017 Location: Pretoria Photos: Gallery Posts: 4,661	There are full frame to crop frame adapters out there that will shrink the image. In the process things gets brighter as well. Typically, focal length is multiplied by 0.7 and you gain one f/stop. These things only work with mirrorless bodies, though.

09-05-2019, 11:14 AM - 1 Like	#4
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by Culture For example is some of the light coming through an FF lens lost when it falls on a crop sensor? The key is the word "crop". For a given focal length, a smaller sensor is illuminated by the same light within its surface area as a larger sensor within the same boundaries. This is easily demonstrated empirically on a camera such as the K-1 where the same sensor may be used for both 24x36 FF and smaller crops. Originally posted by Culture is an FF sensor not seeing/receiving enough light to fill the sensor area when a cropped lens is mounted? It depends on where one measures. The image circle of a crop-sensor lens is usually inadequate to provide sufficient coverage for the larger format to avoid significant vignette outside the "crop" area the lens was designed for with a boundary that is almost never perfectly distinct; the fall-off is incremental by radius. This is often immediately obvious in the viewfinder and in live view when the image circle is gravely inadequate (e.g. my Rokinon 8mm fisheye) and somewhat less so with other lenses where the vignette is more subtle, but still enough to affect the image quality. Steve

09-05-2019, 11:23 AM	#5
Dartmoor Dave Pentaxian Join Date: Aug 2012 Location: Dartmoor, UK Photos: Gallery Posts: 3,888	Please, please, please read very carefully what AstroDave and stevebrot have written, and accept that they are correct in what they say. Then try to forget anything else that you might have read on the internet. Soon, people will show up here and start saying things that are factually incorrect about something that they call "equivalence", and then we'll have the usual 20 pages of nonsense.

09-05-2019, 11:55 AM	#7
Dartmoor Dave Pentaxian Join Date: Aug 2012 Location: Dartmoor, UK Photos: Gallery Posts: 3,888	Originally posted by pschlute i am not getting involved in this debate again Maybe we can save everyone the trouble by linking back to the last epic saga: lens light-gathering abilities - PentaxForums.com Or would that mean depriving the popcorn industry of some much-needed sales?

09-05-2019, 12:10 PM - 1 Like	#8
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by Dartmoor Dave Soon, people will show up here and start saying things that are factually incorrect about something that they call "equivalence", and then we'll have the usual 20 pages of nonsense. Are you saying that upsampling a patch of gray does not turn it black? Steve

09-05-2019, 01:38 PM	#9
GUB Loyal Site Supporter Join Date: Aug 2012 Location: Wanganui Photos: Gallery \| Albums Posts: 5,760	With an OP like that one would almost wonder whether Culture had a popcorn franchise

09-05-2019, 03:12 PM	#10
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by GUB With an OP like that one would almost wonder whether Culture had a popcorn franchise I have am acquiring positions for both salt and butter. $$ Steve

09-06-2019, 01:20 AM	#12
GUB Loyal Site Supporter Join Date: Aug 2012 Location: Wanganui Photos: Gallery \| Albums Posts: 5,760	Originally posted by Culture This post was made just before I went to sleep. Well I will say that I am smiling going through the posts. I guess it makes sense what @AstroDave and @stevebrot have mentioned. Shhh.... about the popcorn franchise. I do not have one. But I did bring a sachet to work hoping for a longer fight to enjoy this. I will take my popcorn to the other linked post by @Dartmoor Dave. Thanks guys. Culture. Happy reading. Every page -every post -- just don't bring the beast back to life!!