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Lens Turbo AKA Speed Booster
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PostPosted: Wed Mar 21, 2018 4:51 pm    Post subject: Lens Turbo AKA Speed Booster Reply with quote

I've done a comparison of the combination of a 50mm lens at F4 in combination with Zhongyi's Lens Turbo and a glass-less adapted 35mm lens at F2.8. Both pictures have been shot with tripod from exactly the same distance. Exposure times have been the same; i.e. the focal reduced F4 lens actually needs the same exposure time like the F2.8 lens. Camera was the NEX-C3 16MP APS-C.

Minolta MD 50mm/F1.4 at F4 on Lens Turbo:


Minolta MD 35mm/F2.8 at F2.8:


In the second comparison I compared a 50mm lens + LT on the NEX with the same lens on the FF Sony A850 (both fully open):

Voigtlaender Color-Ultron 50mm/F1.8 + LT on NEX-C3:


Voigtlaender Color-Ultron 50mm/F1.8 on A850:


The same comparison with the Color-Ultron outdoor at infinity landscape (LT/NEX against A850) was likewise clearly in favor of the FF picture as the picture corners from the LT have been really lousy; i.e. totally unsharp. At closer distance this isn't that obvious.

However, even the old Lens Turbo seems to have some advantages, particularly if there is neither a FF camera nor a real wide angle lens on hand.

BTW, I made this comparisons because of heavy discussions in an other forum. There the theory came in that a so called "focal reducer" is able to change a 50mm lens into a BETTER and REAL 35mm lens. Besides that I doubt that a focal reducer is able to make from a mediocre lens a better one, I really oppose the definition that it will change any existing lens into a new one with shorter focal length. I would likewise not call a 50mm lens with a 2X converter a 100mm lens or any lens a "macro lens" just because it was used with extension rings.

Would be nice if some of the folks here let me know their point of view about the definition of a focal reduced lens; i.e. will it change it's focal length to be a new 35mm lens (to stay with this example) or will it still be simply a focally reduced 50mm lens (my preferred definition)?

Cheers,


PostPosted: Wed Mar 21, 2018 5:25 pm    Post subject: Reply with quote

Technically, the focal length is the lens' FL multiplied by the adaper's factor (often 0.71 for reducers, 1.4 or 2* for tele converters)

As for preferred vocabulary for describing such a combination, I have no real opinion. We have freedom of speech Smile

It could be helpful to refer to "lens x mm with y* converter" when describing the combination.


PostPosted: Thu Mar 22, 2018 1:48 am    Post subject: Reply with quote

Gosh how I hate to say that AGAIN: the lens focal length IS NOT CHANGED when using a speed booster / focal reducer, only the angle of view AS SEEN BY THE SENSOR is, as the FF coverage is reduced to a (say) 0.7x sensor size (actually the projected image diameter is. It just APPEARS as if the focal length has changed ...


PostPosted: Thu Mar 22, 2018 2:22 am    Post subject: Reply with quote

Yes. A focal reducer works exactly opposite as a tele-converter. They don't change the real focal length of the lens but reduce/magnify the image circle.


PostPosted: Thu Mar 22, 2018 2:27 am    Post subject: Reply with quote

kds315* wrote:
Gosh how I hate to say that AGAIN: the lens focal length IS NOT CHANGED when using a speed booster / focal reducer, only the angle of view AS SEEN BY THE SENSOR is, as the FF coverage is reduced to a (say) 0.7x sensor size (actually the projected image diameter is. It just APPEARS as if the focal length has changed ...


I really tried hard to convince the people at DPR's "Adapted Lens Talk" but I simply had no chance alone in the desert.

At least here I'm not alone.... Smile


PostPosted: Thu Mar 22, 2018 5:30 am    Post subject: Reply with quote

The image circle effect is exactly the point.

I'm quite impressed by my cheapo focal reducer actually.
Very handy, makes a 50mm more useful for max bokeh - with a bigger image you get more of it!
And its great for available light. The cheap 50/1.4 becomes quite a lot more like a 35mm f/1.2 - hey, I have to buy that 7Artisans thing!

That is, if all you've got is an APS-C camera. And you are ok with bulky lenses. And you dont care about corners.

Talking about bulky lenses, when I get time I need to try my 180 Sonnar on that thing.


PostPosted: Thu Mar 22, 2018 7:43 am    Post subject: Reply with quote

kds315* wrote:
Gosh how I hate to say that AGAIN: the lens focal length IS NOT CHANGED when using a speed booster / focal reducer, only the angle of view AS SEEN BY THE SENSOR is, as the FF coverage is reduced to a (say) 0.7x sensor size (actually the projected image diameter is. It just APPEARS as if the focal length has changed ...


The lens system (front-lens + booster) focal length is changed, and the speed too.


PostPosted: Thu Mar 22, 2018 9:41 am    Post subject: Reply with quote

luisalegria wrote:
The image circle effect is exactly the point.

I'm quite impressed by my cheapo focal reducer actually.
Very handy, makes a 50mm more useful for max bokeh - with a bigger image you get more of it!
And its great for available light. The cheap 50/1.4 becomes quite a lot more like a 35mm f/1.2 - hey, I have to buy that 7Artisans thing!

That is, if all you've got is an APS-C camera. And you are ok with bulky lenses. And you dont care about corners.

Talking about bulky lenses, when I get time I need to try my 180 Sonnar on that thing.


Good idea. It never came into my mind to use any tele lens in a speed booster combination. However, the shorter exposure times may make sense to use it. Particularly with the excellent MD 200mm/F4 lens which is "boosted to F2.8" equivalence.

I'm really impressed by the result although it's only the first version of the cheap Chinese Zhongyi "Lens Turbo".

Here we go (all shot at maximum aperture):

Minolta MD 200mm/F4 direct:


Minolta MD 200mm/F4 on LT:


Minolta MD 300mm/F4.5 IF direct:


Minolta MD 300mm/F4.5 IF on LT:


PostPosted: Thu Mar 22, 2018 10:45 am    Post subject: Reply with quote

Finally two other rather extreme examples; again same distance and fully open.

Minolta RF 500mm/F8 direct:


Minolta RF 500mm/F8 on LT:


Minolta MC 58mm/F1.2 direct:


Minolta MC 58mm/F1.2 on LT:


After all, I now have a lens faster than F1.0. Smile Smile

Seriously, I think that when people claim that any speed booster is able to make any lens better that this is only an impression. Logically, when used on tele lenses, the smaller magnification in combination with a speed booster looks a little bit sharper. If watched at same size at 100% crop, i.e. in pixel peeping mode, this isn't any longer the case.

BTW, I think it's time to clean the RF500. Wink


PostPosted: Thu Mar 22, 2018 1:06 pm    Post subject: Reply with quote

ZoneV wrote:
kds315* wrote:
Gosh how I hate to say that AGAIN: the lens focal length IS NOT CHANGED when using a speed booster / focal reducer, only the angle of view AS SEEN BY THE SENSOR is, as the FF coverage is reduced to a (say) 0.7x sensor size (actually the projected image diameter is. It just APPEARS as if the focal length has changed ...


The lens system (front-lens + booster) focal length is changed, and the speed too.


Yes, correct for the SYSTEM, but not for the LENS Wink


PostPosted: Thu Mar 22, 2018 2:57 pm    Post subject: Reply with quote

Good point!
Make some tele lenses more hand-holdable by increasing shutter speeds.

I was thinking of the larger defocused field of view, as you show with the 58mm. More bokeh, and for that purpose who cares about corners.


PostPosted: Thu Mar 22, 2018 5:16 pm    Post subject: Reply with quote

kds315* wrote:
ZoneV wrote:
kds315* wrote:
Gosh how I hate to say that AGAIN: the lens focal length IS NOT CHANGED when using a speed booster / focal reducer, only the angle of view AS SEEN BY THE SENSOR is, as the FF coverage is reduced to a (say) 0.7x sensor size (actually the projected image diameter is. It just APPEARS as if the focal length has changed ...


The lens system (front-lens + booster) focal length is changed, and the speed too.


Yes, correct for the SYSTEM, but not for the LENS Wink


The Lens & booster combined system is as far as the camera is concerned a lens, and most photographers would use it in this way.

Being pedantic & splitting the two is a little like claiming the focal length of the lens is actually that of it's first element (which is itself a lens).

This is nothing like the situation with a cropped sensor, where the focal length remains the same.


PostPosted: Thu Mar 22, 2018 10:53 pm    Post subject: Reply with quote

Focal length of a lens does not change but the field of view does. The same has always been true whether using on a larger format where field of view becomes wider or a crop camera where it becomes more narrow. The whole crop factor thing strikes me as an arbitrary decision made to help people relate to what most were used to with 35mm film cameras.

I've used a 1.5 crop for so long I suppose today with FF it becomes a .66 crop Smile


PostPosted: Fri Mar 23, 2018 3:05 pm    Post subject: Reply with quote

I have a question about focal reducers: will a focal reducer, when converting a given lens to an obvious wide angle field of view, does it increase the apparent depth of field in the same way a "native" wide angle lens would? Let's say that the lens with reducer is set so that its increased aperture ratio is set to duplicate the same aperture as is used with the 'native' wide angle? So you would, what? Stop down the lens with reducer one stop more? And at that point, is there a difference in depth of field?


PostPosted: Fri Mar 23, 2018 4:12 pm    Post subject: Reply with quote

Cooltouch,

The difference in depth of field comes in when you use the same framing for a subject. With a focal reducer, for the same frame, you can come much closer (limited by how close the lens focuses of course), reducing the effective DOF and getting more bokeh.

Same as if comparing full frame sensor vs crop sensors, or for that matter Large format vs medium format vs 35mm.

DOF is affected by focal length, aperture and distance.


PostPosted: Fri Mar 23, 2018 6:26 pm    Post subject: Reply with quote

Michael, if you compare the first two pictures of this thread; i.e. the 50/1.4 stopped down to F4 against the 35/2.8 one fully open (exactly the same exposure time and same distance), you'll barely see any difference. Therefore I tend to say that the focal reduced lens is performing as if it would be the wide angle one in every respect incl. depth of field.


PostPosted: Sat Mar 24, 2018 9:05 am    Post subject: Reply with quote

Thanks guys. This is good to know.