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08-21-2019, 05:00 AM   #106
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QuoteOriginally posted by Fogel70 Quote
If you had read my whole post it was about compressing FF image circle into an APS-C image circle.
This will make the image on APS-C sensor brighter as the same amount of light is focussed on a smaller area
I am reading every post on here as carefully as I am able. I am not a lens engineer, just a photographer.

What I did not understand was the term "compressing". How does a lens compress light ?

Surely if a lens has say a baffle that restricts the spread of the light so only aps-c area is covered, all that is happening is the "spread" of light is affected. I do not understand how its intensity or brightness can be affected at all. My window example a few posts up demonstrates that restricting the passage of light does not change it's intensity/brightness for an area that is unrestricted in both cases (centre).

---------- Post added 08-21-19 at 01:04 PM ----------

QuoteOriginally posted by Fogel70 Quote
This will make the image on APS-C sensor brighter as the same amount of light is focussed on a smaller area.
If this were the case, why do I get the same reading with my FF camera as my aps-c camera ?

08-21-2019, 05:39 AM - 2 Likes   #107
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I suspect we're moving into that part of any equivalence thread now where those who have been trying to explain how cameras actually work will start giving up out of sheer weariness. So it's possible that the last couple of posts will come from equivalence true believers, which is unfortunate because they tend to think that repeating things that are false again and again until people walk away counts as winning.

Either that or two people will end up shouting at each other and the mods will shut it down. That often happens too.

Anyway, I'm done with this one.
08-21-2019, 05:58 AM   #108
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QuoteOriginally posted by pschlute Quote
I am reading every post on here as carefully as I am able. I am not a lens engineer, just a photographer.

What I did not understand was the term "compressing". How does a lens compress light ?

Surely if a lens has say a baffle that restricts the spread of the light so only aps-c area is covered, all that is happening is the "spread" of light is affected. I do not understand how its intensity or brightness can be affected at all. My window example a few posts up demonstrates that restricting the passage of light does not change it's intensity/brightness.
It is like an inverted teleconverter. But instead of enlarging the projected image like a TC do, focal reducer compress the projected imaged to a smaller area.

It takes the captured image from a 43 mm image circle and compress it down to a 28mm image circle.
As then the same amount of light cover half as large area the intestity of light doubles.

It is kind of like using a magnifying glass out in the sun to set stuff on fire.

QuoteQuote:
If this were the case, why do I get the same reading with my FF camera as my aps-c camera ?
Because the intestity of the light is not changed. But if you FI use a teleconverter that change the projected image you will get different readings with or without the teleconverter.

Last edited by Fogel70; 08-21-2019 at 06:23 AM.
08-21-2019, 06:01 AM   #109
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QuoteOriginally posted by pschlute Quote
I am reading every post on here as carefully as I am able. I am not a lens engineer, just a photographer.

What I did not understand was the term "compressing". How does a lens compress light ?
It compresses the light onto a smaller surface, just like holding a magnifying glass into the sun to light a fire.

That is what a wide angle converter does. Turn a FF Image circle lens into an APSC image circle lens.

A teleconverter does the exact opposite. It „enlarges“ the image on the sensor plane, effectively throwing away all the light around the sensor.

08-21-2019, 06:28 AM   #110
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QuoteOriginally posted by beholder3 Quote
It compresses the light onto a smaller surface, just like holding a magnifying glass into the sun to light a fire.

That is what a wide angle converter does. Turn a FF Image circle lens into an APSC image circle lens.

A teleconverter does the exact opposite. It „enlarges“ the image on the sensor plane, effectively throwing away all the light around the sensor.
'Concentrates' might be a bit more conceivable than 'compresses' in terms of light-compression. In terms of image, however, compression makes more sense perhaps?
08-21-2019, 07:10 AM   #111
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I think the key term would be „shrinking“ the image on the sensor plane while still using all captured photons, so that the photons per unit of area increase.

And not only that. By shrinking the image you get smaller actual circles of confusion. So perceived sharpness will increase in theory. Again just the opposite effects of a teleconverter.
08-21-2019, 10:14 AM - 1 Like   #112
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QuoteOriginally posted by dbs Quote
I better get some more popcorn
😉 I should feel bad, but I’m sure learning different opinions
08-21-2019, 10:34 AM   #113
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Am i understanding this correctly?

Speedbooster is marketing. Should be called a retrofocus converter. It can change a 50mm simple lens which by definition is 50mm long into a 35mm retrofocus lens about 45mm long. It will maintain effective aperture diameter at the cost of image circle. 50/aperture changes to 35/aperture which is stop larger?

This is like how a 2x tc converts a 50mm simple lens into a 100mm lens about 65mm long (if tc is 15mm) with the same effective aperture diameter. Which loses 2 stops because 50/aperture is half 50 (2)/aperture?

Finally all this is in the properties of the lenses and has nothing to do with the sensor.
I think I am pretty close but am sure some subtlety is needed.

08-21-2019, 12:43 PM - 1 Like   #114
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QuoteOriginally posted by swanlefitte Quote
Am i understanding this correctly?

Speedbooster is marketing. Should be called a retrofocus converter. It can change a 50mm simple lens which by definition is 50mm long into a 35mm retrofocus lens about 45mm long. It will maintain effective aperture diameter at the cost of image circle. 50/aperture changes to 35/aperture which is stop larger?

This is like how a 2x tc converts a 50mm simple lens into a 100mm lens about 65mm long (if tc is 15mm) with the same effective aperture diameter. Which loses 2 stops because 50/aperture is half 50 (2)/aperture?

Finally all this is in the properties of the lenses and has nothing to do with the sensor.
I think I am pretty close but am sure some subtlety is needed.
Yes you are echoing what i was saying.
Fogel does not seem to understand the relationship between f-stop , focal lens and iris diameter; f-stop = focal length/iris diameter.
As Metabones own site states, the speedbooster has turning a 50 into a 35 but kept the same iris diameter.
No optical laws being broken - no equivalence required - what is the point of the discussion?
08-21-2019, 12:57 PM - 1 Like   #115
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QuoteOriginally posted by pschlute Quote
If this were the case, why do I get the same reading with my FF camera as my aps-c camera ?
In the situation of the speedbooster it will be brighter because the added glass has shortened the focal length but kept the iris diameter the same so as in my post above a 50 becomes a 35 and f1.4 becomes f1.0.
There is no magic nor is there relevance to the thread.
08-21-2019, 01:32 PM   #116
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QuoteOriginally posted by GUB Quote
Yes you are echoing what i was saying.
Fogel does not seem to understand the relationship between f-stop , focal lens and iris diameter; f-stop = focal length/iris diameter.
As Metabones own site states, the speedbooster has turning a 50 into a 35 but kept the same iris diameter.
No optical laws being broken - no equivalence required - what is the point of the discussion?
What are you babbling about? I already pointed this out.

It is simple physics. The speed booster convert the FF image to APS-C with same AOV and aperture diameter on both.
This is done by changing the projected FF image circle from the lens to an projected APS-C image circle on the sensor.

It is not only changing focal length, the image circle must also be changed. Otherwise It would break the optical laws.

Last edited by Fogel70; 08-21-2019 at 01:54 PM.
08-21-2019, 02:58 PM   #117
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QuoteOriginally posted by Fogel70 Quote
What are you babbling about? I already pointed this out.

It is simple physics. The speed booster convert the FF image to APS-C with same AOV and aperture diameter on both.
This is done by changing the projected FF image circle from the lens to an projected APS-C image circle on the sensor.

It is not only changing focal length, the image circle must also be changed. Otherwise It would break the optical laws.
And why are you disputing the very article you referred us to?
https://www.metabones.com/assets/a/stories/Speed%20Booster%20White%20Paper.pdf
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08-21-2019, 03:11 PM   #118
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QuoteOriginally posted by Photobill Quote
😉 I should feel bad, but I’m sure learning different opinions
Hi Photobill

I'm a "happy snapper " not a photographer and any seemingly 'easy' question just evolves into a lively discussion.
I have noticed that the question is answered on the first page the rest.... well...
It does make for some interesting reading though....hence the popcorn comment.
Enjoy your photography persuits I certainly am.
This forum is a good educational tool.
I hope you have enjoyed youre last year here and many more to come.

Dave
08-21-2019, 03:14 PM   #119
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QuoteOriginally posted by Fogel70 Quote
What are you babbling about? I already pointed this out.

It is simple physics. The speed booster convert the FF image to APS-C with same AOV and aperture diameter on both.
This is done by changing the projected FF image circle from the lens to an projected APS-C image circle on the sensor.

It is not only changing focal length, the image circle must also be changed. Otherwise It would break the optical laws.
Or it is creating a vignetting 35mm fullframe lens from a 50mm non vignetting fullframe lens in terms of keeping the "FF equivalence"
08-21-2019, 03:15 PM   #120
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QuoteOriginally posted by GUB Quote
And why are you disputing the very article you referred us to?
https://www.metabones.com/assets/a/stories/Speed%20Booster%20White%20Paper.pdf
I don't. But you do not even seem to have bothered to read the "basic design" section on page 2 - 4.
QuoteQuote:
Another unavoidable consequence of adding a focal reducer is that the new image is
reduced in size compared to the original image formed by the objective. So, if the
combined optical system must cover a certain size format, then the objective must cover a
larger format.

Last edited by Fogel70; 08-21-2019 at 03:23 PM.
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