Why does a 2x TC lose 2 stops?

[Dylan777]

I don't think that was a stupid question at all. We got some good answers that educated some of our readers. I am sure that others besides you had wondered about this.

We need more posts like this.

+1

 

[JumboShrimp]

The reverse is also true: A 0.5x TC gains 2 stops.

Those are usually sold as speed boosters to use a full frame lens on a crop camera. If i recall correctly, some lens designs with a wide angle and a large aperture have such an element built in.

In astronomy terms, I think these gadgets are called "telecompressors". I used one on my Celestron 8-inch to reduce the EFL from 2000/11 to 1000/5.6, or something like that.

 

[Click]

I don't think that was a stupid question at all. We got some good answers that educated some of our readers. I am sure that others besides you had wondered about this.

We need more posts like this.

I totally agree with you.

 

[Scumbag]

The second reason applies even if the system was intended to adapt an EF lens to an EF-S DSLR: physically inserting the converter would likely require a shorter backfocus distance, thus impinging on the mirror box.

If Canon did some way of refocussing such that they could do an EF->EF-S speed booster, then I'm sure it would be a huge seller - even with the high prices that Canon would likely charge for it.
You would gain a bit over 1 stop - I think. But the primary motivation would be allowing you to use your wide EF lenses effectively.

 

[rs]

Such an adapter would not be practical for two reasons: first, because the image circle is reduced in proportion to the conversion scaling factor, and because most if not all EF lenses are designed to only project an image circle roughly large enough to cover the 24x36mm imaging sensor, the result of such a converter in an EF-to-EF system would be large sections of black, unexposed sensor in the image periphery.

Naturally, but note that many people don't mind cropping (D800/600, probably many sony models will do it in camera), and may be willing to give up peripheral pixels in exchange for brightness.

That said, there likely isn't a major market for it.

Same quantity of light (if you exclude transmission losses), resulting in a brighter projection on a smaller area. If you take advantage of the extra light by lowering the ISO for a higher S/N ratio, remember that the cropped image requires magnified further to match the size of the full frame, which lowers the S/N ratio. Back to square one.

Instead, what you've ended up with is a smaller viewfinder, worse AF, less light transmission, more aberrations, less resolution, more post processing, and of course more weight, size and expense.

 

[dgatwood]

I've not seen one that is, for example, EF to EF. It would be kinda cool I suppose to use one to brighten up and widen EF lens for use on a crop body. Maybe there's something out there.

Such an adapter would not be practical for two reasons: first, because the image circle is reduced in proportion to the conversion scaling factor, and because most if not all EF lenses are designed to only project an image circle roughly large enough to cover the 24x36mm imaging sensor, the result of such a converter in an EF-to-EF system would be large sections of black, unexposed sensor in the image periphery.

That's only correct if you used one on a full-frame body or used an EF-S lens with it, which isn't what the original question asked.

The second reason applies even if the system was intended to adapt an EF lens to an EF-S DSLR: physically inserting the converter would likely require a shorter backfocus distance, thus impinging on the mirror box. Unlike a teleconverter, wide-angle converters are typically designed to be closer to the image plane--this is how they achieve the focal length reduction. So, for a mirrorless system with a lens that was designed to work on a mirrored system, there's enough space to insert something in between. But with a system that has a mirror box, such a converter would be unlikely to have satisfactory performance.

It is possible to shift the focal point of a lens with additional optics. Kodak did it twenty years ago. See also US Patent #5,499,069.

 

[tron]

The reverse is also true: A 0.5x TC gains 2 stops.

Those are usually sold as speed boosters to use a full frame lens on a crop camera. If i recall correctly, some lens designs with a wide angle and a large aperture have such an element built in.

In astronomy terms, I think these gadgets are called "telecompressors". I used one on my Celestron 8-inch to reduce the EFL from 2000/11 to 1000/5.6, or something like that.

I do not know if there is such a term but there isw a term reducer. For example I do have a Celestron 0.63 reducer.

 

[jrista]

The reverse is also true: A 0.5x TC gains 2 stops.

Those are usually sold as speed boosters to use a full frame lens on a crop camera. If i recall correctly, some lens designs with a wide angle and a large aperture have such an element built in.

In astronomy terms, I think these gadgets are called "telecompressors". I used one on my Celestron 8-inch to reduce the EFL from 2000/11 to 1000/5.6, or something like that.

In astrophotography, they are called focal reducers. Focal reducers do exactly that, reduce the focal ratio by a certain factor. So, if you have a 0.71x FR, your and you start with an f/8 f-ratio, then you end up with an f/5.68 f-ratio. Your aperture remains the same, so your focal length drops. If you have an 11" f/10 SCT, and apply a 0.71x reducer, you end up with an 11" f/7.1 SCT. As far as astrophotography goes, those two numbers are more important...physical aperture and f-ratio (we don't usually talk about focal length, not nearly as much as we do with normal photography.)


It's just common knowledge that an f/7.1 scope is shorter than an f/10 scope when the aperture is the same. What the exact FoV is usually doesn't matter all that much. One of the primary reasons we use focal reducers is to get a proper image scale. We generally try to oversample by 2-3x for best results. If your oversampling by 4-5x, then adding a focal reducer brings the sampling ratio to a more useful level where stars are well sampled, round, but still bright (oversampling too much, and you end up having a tougher time actually getting any exposure on dim nebula...focal reducers can GREATLY help get more light onto each pixel in a given exposure time.)