3x teleconverter f-stop question.

[susan12]

I just picked up a Vivitar Automatic Tele Converter, 3x-1. The price was right, but it didn't come with instructions. My question is that, if the lens says f/2.8, would the real f-stop be f/8, and if the lens says f/5.6, would the real f-stop be f/16? I'm assuming that '3x' means that it stops down 2 stops, or a factor of 8. Thanks.

 

[tirediron]

You are correct; that's the whole "you've got to give something to get something' side of life. You'll get 3x the focal length; in other words, it will give a 200mm lens the FL of a 600mm lens, BUT it will turn that f2.8 lens into an f8 lens, meaning that it really isn't usable with modern auto-focus systems on anything except the fastest lenses when used wide open.

 

[SCraig]

f/2.8 to f/8 is THREE stops, not two stops.

 

[Helen B]

Yes, and three stops is about correct. The TC multiplies the f-number by the same factor as the focal length, so 2.8 becomes 8.4 - actually a little more than three stops.

The TC does not change the size of the entrance pupil if it is behind the lens. The formula

f-number = focal length / diameter of entrance pupil

does not change.

TCs that go in front of the lens do increase both the focal length and the entrance pupil size.

 

[tirediron]

f/2.8 to f/8 is THREE stops, not two stops.

:thumbup: - since the OP got the math right, I sort of assumed that they meant "an additional two stops for a total of three".

 

[gsgary]

but you will have to stop down even more to get a half usable shot

 

[SCraig]

but you will have to stop down even more to get a half usable shot

With a 3x converter it's probably not going to matter a lot. A 2x loses enough IQ, I can't imagine what a 3x is going to do.

 

[gsgary]

but you will have to stop down even more to get a half usable shot

With a 3x converter it's probably not going to matter a lot. A 2x loses enough IQ, I can't imagine what a 3x is going to do.

I would say its going to turn the shot to ****

 

[TheBiles]

I would not expect good results from a 3x TC, especially an off-brand one.

 

[SCraig]

I would say its going to turn the shot to ****

I would most assuredly agree with that. My 2x is bad enough.

 

[gsgary]

I would most assuredly agree with that. My 2x is bad enough.

2x on my 300f2.8l is great

 

[TheBiles]

I would most assuredly agree with that. My 2x is bad enough.

2x on my 300f2.8l is great

It's great on the 70-200 2.8 II also. You just need to start with great glass.

 

[jake337]

Yes, and three stops is about correct. The TC multiplies the f-number by the same factor as the focal length, so 2.8 becomes 8.4 - actually a little more than three stops.

The TC does not change the size of the entrance pupil if it is behind the lens. The formula

f-number = focal length / diameter of entrance pupil

does not change.

TCs that go in front of the lens do increase both the focal length and the entrance pupil size.

This is just for light loss, correct? Would a 300mm f2.8 with a 3x TC be a 900mm f8(roughly) but have the DOF characteristics of a 900mm f2.8?

 

[Helen B]

Yes, and three stops is about correct. The TC multiplies the f-number by the same factor as the focal length, so 2.8 becomes 8.4 - actually a little more than three stops.

The TC does not change the size of the entrance pupil if it is behind the lens. The formula

f-number = focal length / diameter of entrance pupil

does not change.

TCs that go in front of the lens do increase both the focal length and the entrance pupil size.

This is just for light loss, correct? Would a 300mm f2.8 with a 3x TC be a 900mm f8(roughly) but have the DOF characteristics of a 900mm f2.8?

No, the new f-number is the true f-number for DoF calcs - it really is the f-number for the newly created lens. In fact there will probably be light loss in addition to the change in the f-number, because of the extra air-glass surfaces. However, that does not affect the f-number, which is strictly geometrical. It affects the T-stop.

Best,
Helen

 

[jake337]

Yes, and three stops is about correct. The TC multiplies the f-number by the same factor as the focal length, so 2.8 becomes 8.4 - actually a little more than three stops.

The TC does not change the size of the entrance pupil if it is behind the lens. The formula

f-number = focal length / diameter of entrance pupil

does not change.

TCs that go in front of the lens do increase both the focal length and the entrance pupil size.

This is just for light loss, correct? Would a 300mm f2.8 with a 3x TC be a 900mm f8(roughly) but have the DOF characteristics of a 900mm f2.8?

No, the new f-number is the true f-number for DoF calcs - it really is the f-number for the newly created lens. In fact there will probably be light loss in addition to the change in the f-number, because of the extra air-glass surfaces. However, that does not affect the f-number, which is strictly geometrical. It affects the T-stop.

Best,
Helen

Thanks for the clarification. But that is how it works with macro and extension tubes though, correct? Maybe I was thinking of extension tubes.