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09-05-2019, 09:25 AM   #1
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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, 10:12 AM - 2 Likes   #2
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QuoteOriginally posted by Culture Quote
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).

QuoteOriginally posted by Culture Quote
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
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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
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QuoteOriginally posted by Culture Quote
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.

QuoteOriginally posted by Culture Quote
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.


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09-05-2019, 11:23 AM   #5
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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, 11:31 AM - 2 Likes   #6
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i am not getting involved in this debate again

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

QuoteOriginally posted by Culture Quote
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.
09-05-2019, 11:55 AM   #7
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QuoteOriginally posted by pschlute Quote
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
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QuoteOriginally posted by Dartmoor Dave Quote
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
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With an OP like that one would almost wonder whether Culture had a popcorn franchise
09-05-2019, 03:12 PM   #10
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QuoteOriginally posted by GUB Quote
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, 12:12 AM - 1 Like   #11
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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.
09-06-2019, 01:20 AM   #12
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QuoteOriginally posted by Culture Quote


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!!
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