Tele converters

Ben johns

Ben johns

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How many extra stops would you have to add for a x2 tele convertor? I just got one for my medium format Camera and looking through it it appears darker than just using the lens alone.
 
Or if you don't mind maths...

A 2x converter multiplies the focal length by 2; a 1.4x converter multiplies the focal length by 1.4.

The f number is the focal length divided by the diameter of the physical aperture. Doubling the focal length means that the f number doubles hence f/2 becomes f/4 and you lose two stops. 1.4 means that the f number is multiplied by 1.4, so f/2 becomes f/2.8 - a loss of one stop.
 
You can use a x2 and 1.4 at the same time .....did it with a 400mm lens.. freaky but works :)
 
KIPAX said:
You can show off all you like..the answer is still 2 ;) :)
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It;s the way my mind works - I'd rather understand the principle so that I can apply it to a new situation than accept a rote answer that leaves me in the dark when something changes.

mark4183 said:
what if you add the math for full frame or crop sensor but then factor that by the new sensor sizes
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The sensor size has no effect on the characteristics of a lens, so it's irrelevant. Lenses are inherently stupid, and don't reconfigure themselves to alter their focal length and size when the camera behind them changes.
 
Ben johns said:
How many extra stops would you have to add for a x2 tele convertor? I just got one for my medium format Camera and looking through it it appears darker than just using the lens alone.
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Medium format? Film or digital?

What camera are you using plus what lens and TC combo?
 
StewartR said:
Why do you care? The answer is still 2.
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Curiosity as I have not read of anyone, film or digital, using a medium format body with any lens plus a X2 tele-convertor attached. The OP picqued my interest!
 
Box Brownie said:
Curiosity as I have not read of anyone, film or digital, using a medium format body with any lens plus a X2 tele-convertor attached. The OP picqued my interest!
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Just for your curiosity then....

I've got a 2x to use with my 300/4 on the Pentax 645Z. There is minor loss of sharpness although it doesn't degrade as much as my Canon 2x on an equivalent lens.....presumably because it's specifically matched to the 300/4 and not formulated to use with other lenses. I've also got the matched 2x for the Pentax 67 300/4 although this is marginally less pleasing.

Bob
 
Ben johns said:
looking through it, it appears darker than just using the lens alone.
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It will do, they all do to a greater or lesser extent, same as if you add a macro tube, probably because it extend the light path to the sensor/film plain/mirror and light fall off isnt linear to distance (as we all know :) ).
 
Eloise said:
Good point but wrong (kind of)...

Yes the answer is 2, but the clue isn't in the name because if you use a 1.4x teleconverter, you're not adding 1.4 extra stops, you're adding 1 extra stop!
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the question was a x2 so my answer is correct and the clue was in the question...... you changing the question is what we call....cheating :)
 
MatBin said:
It will do, they all do to a greater or lesser extent, same as if you add a macro tube, probably because it extend the light path to the sensor/film plain/mirror and light fall off isnt linear to distance (as we all know :) ).
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I have no idea if MF automatically adjusts aperture or not, with DSLRs wouldn’t a bare lens be brighter looking through the viewfinder as the camera holds the aperture wide open (say it’s a f2.8 lens) until the image is taken. With 2x TC on the max aperture would now be f5.6 so it would appear darker as the smaller aperture would let less light in. I would have thought TCs would look like a smaller aperture lenses would compared to a f2.8 fast prime.
 
rob-nikon said:
I have no idea if MF automatically adjusts aperture or not,.
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Way back in the mists of time, possibly not but post 1970's offerings (eg,,,Pentax 6x7 and 645) certainly stop down to shoot in the same way that an SLR/DSLR works.
 
rob-nikon said:
I have no idea if MF automatically adjusts aperture or not, with DSLRs wouldn’t a bare lens be brighter looking through the viewfinder as the camera holds the aperture wide open (say it’s a f2.8 lens) until the image is taken. With 2x TC on the max aperture would now be f5.6 so it would appear darker as the smaller aperture would let less light in. I would have thought TCs would look like a smaller aperture lenses would compared to a f2.8 fast prime.
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The maximum aperture of the lens affects the brightness of the image in the viewfinder (and as an aside it's ability to focus), so if you put a lens of a certain focal length on a camera it will have a certain brightness in the viewfinder, if you had the same focal length lens but with a much smaller max aperture the image would appear much dimmer in the viewfinder e.g. a 200 f1.8 against a 200 f5.6 (if they exist), although the naked eye might not be sensitive enough to see the difference. Alternatively compare an f8 lens against an f1.8 lens (different focal lengths).
I am not sure if putting a TC on a lens darkens the image because it is "lengthening" the lens, making the light travel further to the mirror/film plain or if it is affecting the apparent aperture. But as above during the period before the shutter is released you are almost certainly viewing at max aperture not selected aperture (unless they are both the same of course i.e. wide open).
 
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MatBin said:
I am not sure if putting a TC on a lens darkens the image because it is "lengthening" the lens, making the light travel further to the mirror/film plain or if it is affecting the apparent aperture.
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They're two sides of the same effect. The aperture is the ratio of the diameter of the aperture to the distance from the lens to the film (or sensor, if you have that sort of camera). Exposure should be based on the "effective aperure" rather than the marked one. Since you normally have to rack out a lens to focus closer, the distance increases and the effective aperure decreases, leading to the two stop decrease at 1:1. That requires adding one focal length to the film/lens distance, which at infinity is equal to the focal length of the lens. Clearly, if the focal length has doubled, infinity focus requires the lens to be twice the old focal length away.

But I'm not sure that providing any theory is helpful, when a simple numeric answer with no frills (or explanations) seems to be what is needed.

So my answer to the quoted question is "both" because you can't lengthen the lens without making the light travel further.
 
StephenM said:
But I'm not sure that providing any theory is helpful, when a simple numeric answer with no frills (or explanations) seems to be what is needed.
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I answered the second part of the question as to why the image appears darker in the viewfinder, which couldnt be answered with a simple numeric.
 
Box Brownie said:
Curiosity as I have not read of anyone, film or digital, using a medium format body with any lens plus a X2 tele-convertor attached. The OP picqued my interest!
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Sorry for the late reply, it would be on a mamiya 645 and the lens would be the 80mm 2.8. I got the x2 convertor for cheap when I bought the 45mm 2.8, got so I had a wide, normal and kind of a tele lens with 2 lenses. I’ve yet to use it though
 
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