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Aperture : My brain hurts
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Jul 23, 2018 05:21:08   #
Hammer Loc: London UK
 
I understand that when attaching a teleconverter to a lens the aperture is altered by the factor of the converter . ie a 1.4 converter will turn a f2.8 lens into an f4.

The aperture for a lens is altered from within the camera . When I attach a teleconverter the aperture is already is reduced/affected by the converter. What is happening?

I accept that my photography will not be affected by this knowledge but my brain is really hurting and I need to resolve this .

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Jul 23, 2018 05:38:11   #
BebuLamar
 
The aperture number i.e. f/4, f/2.8 is the ratio of focal length / diameter of the aperture opening. Attaching the teleconverter doesn't do anything to the aperture really but it increase the focal length of the lens and thus change the ratio of focal length / diameter of opening.

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Jul 23, 2018 05:39:43   #
DavidPine Loc: Fredericksburg, TX
 
The lens is farther from the camera's sensor and it compensates for the adjusted focal length and added glass would be my guess. An extension tube acts differently because it has no glass so you have a different adjustment by allowing you to move closer to the subject with the camera to focus. My explanation is not scientific and probably a little too simplistic, Someone will come along and straighten us both out. Some things should be simply accepted because they are consistent within the photography world.

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Jul 23, 2018 06:25:11   #
tommy2 Loc: Fort Worth, Texas
 
DavidPine wrote:
... Some things should be simply accepted because they are consistent within the photography world.


Good thought! My music teacher tells me this about music theory - accept it and go on about enjoying the hobby. (Also said that if he was to accept another engineer as a beginning student he would charge double )

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Jul 23, 2018 06:39:56   #
jerryc41 Loc: Catskill Mts of NY
 
Hammer wrote:
I understand that when attaching a teleconverter to a lens the aperture is altered by the factor of the converter . ie a 1.4 converter will turn a f2.8 lens into an f4.

The aperture for a lens is altered from within the camera . When I attach a teleconverter the aperture is already is reduced/affected by the converter. What is happening?

I accept that my photography will not be affected by this knowledge but my brain is really hurting and I need to resolve this .


You might be confusing aperture with the reduction of light caused by attaching a teleconverter.

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Jul 23, 2018 06:41:36   #
jerryc41 Loc: Catskill Mts of NY
 
tommy2 wrote:
Good thought! My music teacher tells me this about music theory - accept it and go on about enjoying the hobby. (Also said that if he was to accept another engineer as a beginning student he would charge double )


Right! You can spend hours exploring the physics that cause a rainbow, or you can just enjoy the sight.

A friend (an engineer) told me that salespeople hate dealing with engineers. : )

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Jul 23, 2018 06:45:15   #
Bill_de Loc: US
 
BebuLamar wrote:
The aperture number i.e. f/4, f/2.8 is the ratio of focal length / diameter of the aperture opening. Attaching the teleconverter doesn't do anything to the aperture really but it increase the focal length of the lens and thus change the ratio of focal length / diameter of opening.

This

The extra glass may reduce the light transmission some, but that would change the T stop, not the F stop.

https://petapixel.com/2016/12/30/f-stops-vs-t-stops-difference-explained-plain-english/


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Jul 23, 2018 06:52:47   #
Larryshuman
 
The tele extender will reduce the amount of you see thru the lens. So a 300mmF:4 with a 1.4 extender will reduce the light entering the lens by a factor of 1. So a 300mmF:4 lens with a 1.4 on it will be at 5.6. The 300mmF:4 with a 2x on probably will not focus since it will be a t F:8.Only cameras that have that ability will auto focus. My 800mmF:5.6 AF lens cannot use extenders because a 1.4 will take the lens to F:8 but give the equivalent focal length of 1120mm F:8 a 2x on that lens would be F11. I have manual focused the lens at those settings. Since I use D8xx series cameras I use crop mode to get to DX mode. I am using Nikon's 600mmF:4 VR G now. This will give the lens the equivalent focal length of 900mm F:4. I DON'T lose 1 stop of light this way. Using a 1,4 on F2.8 lens will give you F:4, a 2x will be 5.6. You lose light when you use extenders.

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Jul 23, 2018 07:06:12   #
DannyKaye Loc: Sheffield now but soon moving to Blanzay
 
Simply the 1.4 converter spreads the light so that a smaller area of the light that would have formed the image gets to the sensor. Each f-stop alters the area of the hole that passes the light by a factor of 2. To calculate the area of a circle we use pi x r^2. so twice the area is 1.414... x the radius (square root of 2) so starting at 1 the f-stops go

1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22 etc

multiplying by sqrt 2 each time.

If the 1.4x converter is used it spreads the light over a circle that is 1.4 times larger
in radius so it is the same effect as decreasing the aperture by 1 stop. (I think!)

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Jul 23, 2018 07:14:26   #
Bill_de Loc: US
 
Larryshuman wrote:
The tele extender will reduce the amount of you see thru the lens. So a 300mmF:4 with a 1.4 extender will reduce the light entering the lens by a factor of 1. So a 300mmF:4 lens with a 1.4 on it will be at 5.6. The 300mmF:4 with a 2x on probably will not focus since it will be a t F:8.Only cameras that have that ability will auto focus. My 800mmF:5.6 AF lens cannot use extenders because a 1.4 will take the lens to F:8 but give the equivalent focal length of 1120mm F:8 a 2x on that lens would be F11. I have manual focused the lens at those settings. Since I use D8xx series cameras I use crop mode to get to DX mode. I am using Nikon's 600mmF:4 VR G now. This will give the lens the equivalent focal length of 900mm F:4. I DON'T lose 1 stop of light this way. Using a 1,4 on F2.8 lens will give you F:4, a 2x will be 5.6. You lose light when you use extenders.
The tele extender will reduce the amount of you se... (show quote)


While many newer cameras will focus (some only center point) at F/8.0, the current Nikon 800mm comes with a dedicated 1.25 tele extender. It works great on the D4s and D500. On the D500 it's the equivalent of 1500mm @ F/7.1

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Jul 23, 2018 13:21:17   #
Darkroom317 Loc: Mishawaka, IN
 
The lens being a further distance from the film/ sensor is a big part of it. The f-number does not change as I understand it. It is similar to bellows extension factor with view cameras. The further the lens from the film plane the more compensation is needed.

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Jul 23, 2018 13:39:00   #
Rich1939 Loc: Pike County Penna.
 
BebuLamar wrote:
The aperture number i.e. f/4, f/2.8 is the ratio of focal length / diameter of the aperture opening. Attaching the teleconverter doesn't do anything to the aperture really but it increase the focal length of the lens and thus change the ratio of focal length / diameter of opening.



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Jul 23, 2018 15:47:28   #
Larryshuman
 
That's only true for extension tubes. The father away the lens is from the film plane the closer you can focus. With a tele extender of 1.4x the loose of 1 stop of light is because the stack of glass that's in the extender. With a 2x extender the loose is 2 stops due even more glass in the extender. Using crop mode eliminates the loose of light it just crops the sensor giving you the equivalent view. With my 600mm on DX crop mode I have the equivalent view of 900mm with no loose of light.

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Jul 23, 2018 16:07:15   #
Rich1939 Loc: Pike County Penna.
 
Larryshuman wrote:
That's only true for extension tubes. The father away the lens is from the film plane the closer you can focus. With a tele extender of 1.4x the loose of 1 stop of light is because the stack of glass that's in the extender. With a 2x extender the loose is 2 stops due even more glass in the extender. Using crop mode eliminates the loose of light it just crops the sensor giving you the equivalent view. With my 600mm on DX crop mode I have the equivalent view of 900mm with no loose of light.

Light loss is measured as "T stops" and is not part of a f stop calculation. It it were then any time we are setting up a shot and the light changes we would need to recalculate our F number. The change in total focal length affects the f stop because of the ratio change as noted by others here. Since the depth of field is also changed when using an extender something other than light transmission is at play and that is the increase in total lens length.

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Jul 23, 2018 16:12:21   #
Bill_de Loc: US
 
Larryshuman wrote:
That's only true for extension tubes. The father away the lens is from the film plane the closer you can focus. With a tele extender of 1.4x the loose of 1 stop of light is because the stack of glass that's in the extender. With a 2x extender the loose is 2 stops due even more glass in the extender.


While the extra glass probably accounts for some light loss, it has nothing to do with the calculated 1 and 2 F- stop difference. F stops are a ratio between the focal length of the lens and the physical size of the aperture or (I've read}intake pupil.

Your explanation would require that all extenders, regardless of brand, use the same amount of glass. The actual loss of light would have to be measure and is represented in T-Stops.

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