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Minimal aperture higher if using 35mm equiv lens on APS-C?
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PostPosted: Sat Jan 08, 2022 11:01 pm    Post subject: Minimal aperture higher if using 35mm equiv lens on APS-C? Reply with quote
From what i understand if Full Frame (FF) lens has been mounted on system with APS-C sensor, it going to lack all the light as lens is calibrated to project image on higher sensor surface that is not there.

Full Frame lens 50mm f/2.0 on APS-C system. Orange is area of FF lens that APS-C system actually use.


Only about 66% of the front lens surface is used if mount FF lens on APS-C so actual aperture becomes darker f/2.0 multiplied by crop factor 1.5x equals f/3.0!





APS-C lens 50mm f/2.0 on APS-C system


Using dedicated APS-C lens all light collected by front glass is being projected on the sensor so if we use dedicated APS-C lens 50mm f/2.0 then its indeed f2.0 (not f/3.0 like with FF lens).



1. Above theory is sound and correct?

2. Only need to multiply widest aperture by crop ratio 1.5x only when using the full frame lens on APS-C system, for APS-C lens it gives all light to the sensor so aperture is same as written on the lens?

3. If using full frame lens e.g. 80mm f/5.6 then actual aperture equals is f/8.4?


PostPosted: Sun Jan 09, 2022 12:06 am    Post subject: Reply with quote
lens f/# ratio is expressed as brightness per unit area. Thus no matter what size film or sensor, the lens brightness per unit area is constant.

Different film/sensor size does affect relative depth-of-field; this effect is measured by f-stop equivalence to give identical dof on each size film/sensor, not to be confused with lens f/#.

Reference:
Depth of Field and Bokeh
by Dr. Hubert Nasse, Carl Zeiss AG
http://lenspire.zeiss.com/photo/app/uploads/2018/04/Article-Bokeh-2010-EN.pdf


PostPosted: Sun Jan 09, 2022 1:23 am    Post subject: Reply with quote
visualopsins wrote:
lens f/# ratio is expressed as brightness per unit area. Thus no matter what size film or sensor, the lens brightness per unit area is constant.

Pardon i am a bit confused, you say that brightness of 35mm format lens with f/2.0 when mounted on APS-C system still gives f/2.0?





What i trying to to say is that 35mm film lenses is made for different system and even with adapter for APS-C system, circle of projection that lens cast on film is too big intended for 35mm film when APS-C is 25mm film meaning 33% of light collected by front glass in lens is not projected on the film, hence minimal aperture is darker than specified on the lens regardless of lens being labeled with F-stop or T-stop, that labels are for different format they are wrong just as focal length labeled on lens is wrong when its used on different system.




To prove this concept lets look at "speed booster" is able to increase the brightness of FF lens when used on APS-C system, speed booster does not make aperture of the lens bigger or make more photons of light travel trough lens it simply focuses the circle of projection at the smaller sensor giving it more light and wider angle of view.


PostPosted: Sun Jan 09, 2022 3:39 am    Post subject: Reply with quote
NiceDays wrote:
visualopsins wrote:
lens f/# ratio is expressed as brightness per unit area. Thus no matter what size film or sensor, the lens brightness per unit area is constant.

Pardon i am a bit confused, you say that brightness of 35mm format lens with f/2.0 when mounted on APS-C system still gives f/2.0? ...


Yes, that is correct.

NiceDays wrote:
...To prove this concept lets look at "speed booster" is able to increase the brightness of FF lens when used on APS-C system, speed booster does not make aperture of the lens bigger or make more photons of light travel trough lens it simply focuses the circle of projection at the smaller sensor givilensng it more light and wider angle of view.


That is how speed booster works.

In other words brightness is constant anywhere inside the lens image circle but the speed booster squeezes the image circle light onto a smaller area.


PostPosted: Sun Jan 09, 2022 4:10 am    Post subject: Reply with quote
visualopsins wrote:
NiceDays wrote:
...To prove this concept lets look at "speed booster" is able to increase the brightness of FF lens when used on APS-C system, speed booster does not make aperture of the lens bigger or make more photons of light travel trough lens it simply focuses the circle of projection at the smaller sensor givilensng it more light and wider angle of view.


That is how speed booster works.

In other words brightness is constant anywhere inside the lens image circle but the speed booster squeezes the image circle light onto a smaller area.


Point i am trying to make is that lens has to loose the "speed" in first place in order for speed booster to be able to boost it, otherwise where the additional light comes from?

And lens looses the brightness (speed) because image circle is spread too much, same as with flashlight with adjustable focus if you spread the light it gets dimmer.



PostPosted: Sun Jan 09, 2022 11:53 am    Post subject: Reply with quote
visualopsins is correct;

an f/2 lens on FF 35mm is still an f/2 optic when used on APS-C. This is simply a function of its maximum (effective) aperture and its focal length (ignoring some theoretical and practical nuances here).

I think you are confusing the concepts of f/# and the image circle; they are totally separate parameters of a lens.

Think of it a different way; the APS-C sensor is simply cropping the FF 35mm image. Just like when you crop a shot in PP, the image doesn't get dimmer.


PostPosted: Sun Jan 09, 2022 5:41 pm    Post subject: Reply with quote
RokkorDoctor wrote:
Think of it a different way; the APS-C sensor is simply cropping the FF 35mm image. Just like when you crop a shot in PP, the image doesn't get dimmer.

Problem with that is you talking about image in Photoshop which is already captured and final, its same as saying print image on paper and crop the image and brightness will not change.

In case with circle of projection we are talking about lens system that actually projects image information on to sensor before it gets captured.


Video explanation of how Speed booster works.


Every theory needs to be put to a test, with movie projector further you put it from the wall larger and less bright the image becomes.
Sensor is just a piece of wall that captures whatever image gets projected on it.

If you don't have movie projector try to shine flashlight (even your phone flashlight) trough the front of your lens and it will project the light on the wall (circle of projection), now place a sheet of paper on the wall (sensor) and focus the light on the paper, move lens away from the wall so the 1/3 of projected light is outside the sheet of paper, and you notice that paper is less bright.

Imagine the sheet of paper is your camera image sensor, it will capture only as much light as projected on it.



I only reference to f-stop of lens for ease of understanding sort of as (equivalent measure for understanding how much actual light is lost) not trying to say that f-stop actually changes because we put lens on other system. What changes is the mount of light projected at the sensor due to diffused projection FF lens gives on APS-C system.

Realistically when using FF lens on APS-C system what gets lost is 33% of light lens produces at wide open aperture.


PostPosted: Mon Jan 10, 2022 12:25 am    Post subject: Reply with quote
I'm using the 800mm rokkor on a7II for shooting long distance. I've received some advice to get the kenko mc7 (vivitar) macro duplicator to reach further. My doubt is how would a em5II image shooted with the same lens compare with the a7II and duplicator ? Thanks


PostPosted: Mon Jan 10, 2022 6:29 am    Post subject: Reply with quote
kiddo wrote:
I'm using the 800mm rokkor on a7II for shooting long distance. I've received some advice to get the kenko mc7 (vivitar) macro duplicator to reach further. My doubt is how would a em5II image shooted with the same lens compare with the a7II and duplicator ? Thanks


The field of view (FOV) will be apprx. the same as the MFT sensor is cropping the center which results in FOV 1600mm and the Kenco mc7 on the A7 is magnifying 2x to FOV 1600mm as well.
I've made several tests with tele converters (even stacked) and different sensor sizes.

However, the characteristic of the used lens will never change; i.e. the depth of field (DOF) will always be the same. Therefore it may be an advantage to use a shorter lens on the smaller sensor in order to get a better DOF.
A 200mm lens on MFT will deliver the same field of view like a 400mm lens on FF, but different DOF.
I'm using different cameras with different sensor sizes (MFT, APS-C and FF) on regular basis, sometimes with identical lenses.

As already explained before: Smaller sensor sizes result in cropping like you may crop in PP but they will not change the characteristics of the lenses.


PostPosted: Mon Jan 10, 2022 8:17 am    Post subject: Reply with quote
Thanks for your quick reply. I have moved from m43 to FF and nex ,but for longer focal length ,i wonder what would be the best setup , and I've never used these duplicators before, and to be honest, i see no other reason to get a m43 , well maybe the silent shutter lol.


PostPosted: Mon Jan 10, 2022 8:59 am    Post subject: Reply with quote
kiddo wrote:
Thanks for your quick reply. I have moved from m43 to FF and nex ,but for longer focal length ,i wonder what would be the best setup , and I've never used these duplicators before, and to be honest, i see no other reason to get a m43 , well maybe the silent shutter lol.


Well, I pretty much enjoy the MFT format when travelling and hiking. It's a small and handsome package, particularly for extreme tele shootings (e.g. birds).
The Panasonic 100-300 Zoom delivers excellent quality in a small package.
With my A7R2 this would require the huge and heavy (and extreme expensive) 200-600 Zoom, delivering in comparison much shallower DOF as well.

I also like the automated focus stacking functionality of my Lumix. Unfortunately Sony isn't able to provide any similar functionality.

For me every format has it's advantages and disadvantages. There is no "one size fits all" - solution.


PostPosted: Mon Jan 10, 2022 9:59 am    Post subject: Reply with quote
Guys anyone has something to say on the topic?


Forgot to mention Teleconverters that mount to the back of the lens, those work opposite to Speed Boosters - they sort of "increase focal length of the lens" by enlarging circle of projection (CoP) reflected from the lens to the sensor, it comes at a cost of the brightness (enlarging CoP diffuses light).

2.0x teleconverter reduces light to about similar extent as lowering lens two f-stops down.


Its a well known fact 1.4x teleconverter mounted behind the lens works by projecting larger image circle at the sensor and due to this loose light equal to approximately one f-stop.

Its also a fact that FF lens mounted on APS-C system projects larger image circle on the sensor (about same size circle as when using 1.4x teleconverter), yet some people claim in this case light is not lost.


Its safe to assume using teleconverter to enlarge CoP lowers light sensor see and using FF lens on APS-C that also known to enlarge CoP does not lower light sensor see?


PostPosted: Mon Jan 10, 2022 12:10 pm    Post subject: Reply with quote
NiceDays wrote:
Guys anyone has something to say on the topic?


Forgot to mention Teleconverters that mount to the back of the lens, those work opposite to Speed Boosters - they sort of "increase focal length of the lens" by enlarging circle of projection (CoP) reflected from the lens to the sensor, it comes at a cost of the brightness (enlarging CoP diffuses light).

2.0x teleconverter reduces light to about similar extent as lowering lens two f-stops down.


Its a well known fact 1.4x teleconverter mounted behind the lens works by projecting larger image circle at the sensor and due to this loose light equal to approximately one f-stop.

Its also a fact that FF lens mounted on APS-C system projects larger image circle on the sensor (about same size circle as when using 1.4x teleconverter), yet some people claim in this case light is not lost.


Its safe to assume using teleconverter to enlarge CoP lowers light sensor see and using FF lens on APS-C that also known to enlarge CoP does not lower light sensor see?


It's quite easy: Teleconverters are simply working like a magnifiying glass but like a glass-less macro tube they are enlarging also the distance to the sensor and that is the reason why the needed exposure time extends. There is a simple formula for that (the longer the extension the longer the needed exposure time will be) and the actual f-stop remains unchanged; i.e. F4 remains F4 but the exposure time extends accordingly.
It's a totally different story to use a FF lens on APS-C: Nothing changes, except the size of the negative/sensor, i.e. there is neither loss nor gain of light. It's simple the same effect as to shoot on FF sensor and crop afterwards in PP. That's the reason why APS-C (and MFT) sensors are often called "crop sensors".


PostPosted: Mon Jan 10, 2022 10:08 pm    Post subject: Reply with quote
OK so we have a camera obscura such as the one that for many years was at Land's End by the Cliffhouse in San Francisco,
https://en.wikipedia.org/wiki/Camera_Obscura_(San_Francisco,_California)

(you'll need to copy the complete url and paste into web browser.)

Block the lens, then imagine a huge piece of slide film covers the table where the image is projected. Unblock the lens a short time to properly expose the film.

Next cut a m43 sensor-size piece of slide film. Place on the table. Unblock the lens the same amount of time as before to expose the m43 sensor-size piece of film.

Please explain why, as you say, with all that extra light shining on the back wall, is the m43 sensor-size film not way way overexposed, but instead is perfectly exposed?


PostPosted: Mon Jan 10, 2022 10:38 pm    Post subject: Reply with quote
visualopsins wrote:
Please explain why, as you say, with all that extra light shining on the back wall, is the m43 sensor-size film not way way overexposed, but instead is perfectly exposed?


I have not had first hand experience with film photography.

From what i understand difference is as follows lens for different format's be it Full Frame and APS-C are optically designed and adjusted to focus light on the proper size of the table/film/sensor for that particular format.



PostPosted: Mon Jan 10, 2022 11:09 pm    Post subject: Reply with quote
NiceDays wrote:
visualopsins wrote:
Please explain why, as you say, with all that extra light shining on the back wall, is the m43 sensor-size film not way way overexposed, but instead is perfectly exposed?


I have not had first hand experience with film photography.


OK, for the film, substitute digital cameras with different size sensors without lenses, shutters open.

NiceDays wrote:
From what i understand difference is as follows lens for different format's be it Full Frame and APS-C are optically designed and adjusted to focus light on the proper size of the table/film/sensor for that particular format.



There is no speed booster effect here because the lens angle of view is reduced, i.e. the same amount of light shines on both APS-C sensors.

https://www.cambridgeincolour.com/tutorials/camera-exposure.htm
https://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm


PostPosted: Wed Jan 12, 2022 7:20 am    Post subject: Reply with quote
tb_a wrote:
kiddo wrote:
I'm using the 800mm rokkor on a7II for shooting long distance. I've received some advice to get the kenko mc7 (vivitar) macro duplicator to reach further. My doubt is how would a em5II image shooted with the same lens compare with the a7II and duplicator ? Thanks


The field of view (FOV) will be apprx. the same as the MFT sensor is cropping the center which results in FOV 1600mm and the Kenco mc7 on the A7 is magnifying 2x to FOV 1600mm as well.
I've made several tests with tele converters (even stacked) and different sensor sizes.

However, the characteristic of the used lens will never change; i.e. the depth of field (DOF) will always be the same. Therefore it may be an advantage to use a shorter lens on the smaller sensor in order to get a better DOF.
A 200mm lens on MFT will deliver the same field of view like a 400mm lens on FF, but different DOF.
I'm using different cameras with different sensor sizes (MFT, APS-C and FF) on regular basis, sometimes with identical lenses.

As already explained before: Smaller sensor sizes result in cropping like you may crop in PP but they will not change the characteristics of the lenses.


Except shutter speed on em5II will be faster. 800mm f/8 lens + 2x TC on A7ii equivalent to 1600mm f/16, compared to 800mm f/8 lens on em5II equivalent field of view to 1600mm lens but f/8 of lens doesn’t change.


PostPosted: Wed Jan 12, 2022 11:48 am    Post subject: Reply with quote
visualopsins wrote:

Except shutter speed on em5II will be faster. 800mm f/8 lens + 2x TC on A7ii equivalent to 1600mm f/16, compared to 800mm f/8 lens on em5II equivalent field of view to 1600mm lens but f/8 of lens doesn’t change.


I have already explained this principle above as to say that a 2x TC converts a lens from F8 to F16 maybe quite misleading for beginners; i.e. the lens remains F8 but the exposure time will be extended as if the lens would be F16.
In other words: F16 only for the calculation of the exposure time but not for the calculation of the depth of field, later remains unchanged at F8.


PostPosted: Wed Jan 12, 2022 2:44 pm    Post subject: Reply with quote
RokkorDoctor wrote:
Think of it a different way; the APS-C sensor is simply cropping the FF 35mm image. Just like when you crop a shot in PP, the image doesn't get dimmer.


NiceDays wrote:
Problem with that is you talking about image in Photoshop which is already captured and final, its same as saying print image on paper and crop the image and brightness will not change.

In case with circle of projection we are talking about lens system that actually projects image information on to sensor before it gets captured.


Ah, I feared you might counter-argue that way Wink . Maybe this analogy of cropping for you isn't the most intuitive one to pick, as I suspect you may be confusing the following two concepts:

a) how much light a lens projects in total over the total image circle emanating from the whole full-frame field of view
b) how much light a lens projects onto a "point" on the sensor emanating from a single point source of light

both are affected by the numerical aperture, but simply changing the image sensor (from FF to APS-C) doesn't have any impact on either.

From your illustrations and following statement I suspect your understanding is centered on concept a):

NiceDays wrote:

Realistically when using FF lens on APS-C system what gets lost is 33% of light lens produces at wide open aperture.


But then consider that on APS-C you are also losing 33% of the image, hence the ratio of the amount of light the lens produces / area of image stays the same.

Maybe another experiment; Assume you are using a full frame camera that has the option to drop a black border mask over the sensor to trim it down to APS-C size. The center of the image (APS-C area) then surely won't change in brightness?


PostPosted: Wed Jan 12, 2022 9:52 pm    Post subject: Reply with quote
RokkorDoctor wrote:
...
a) how much light a lens projects in total over the total image circle emanating from the whole full-frame field of view
b) how much light a lens projects onto a "point" on the sensor emanating from a single point source of light

both are affected by the numerical aperture, but simply changing the image sensor (from FF to APS-C) doesn't have any impact on either.
...


Emphasis mine. A brilliant explanation. Thank you so much!


PostPosted: Thu Jan 13, 2022 2:37 pm    Post subject: Reply with quote
tb_a wrote:
visualopsins wrote:

Except shutter speed on em5II will be faster. 800mm f/8 lens + 2x TC on A7ii equivalent to 1600mm f/16, compared to 800mm f/8 lens on em5II equivalent field of view to 1600mm lens but f/8 of lens doesn’t change.


I have already explained this principle above as to say that a 2x TC converts a lens from F8 to F16 maybe quite misleading for beginners; i.e. the lens remains F8 but the exposure time will be extended as if the lens would be F16.
In other words: F16 only for the calculation of the exposure time but not for the calculation of the depth of field, later remains unchanged at F8.



Not to be misinterpreted that DOF of lens alone on FF will be identical to lens with 2x TC on FF, at the same subject distance .
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