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Lens porn: Sigma 50-100mm F1.8 Art

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19 minutes ago, Grim Fandango said:

Then you will have literally no problem proving that! Get two cameras with different size sensors, stick two lenses on the front of them set to the same f-stop number, and try to take identical photographs with them. When you give up because the bokeh in one is far more out of focus than the other for the same frame, apply my formula and then take those same photos. Or just watch the video I posted above where they perform that precise test.

Btw, background blur (bokeh) = physical aperure size and focus distance

                                        DoF = focus distance, f-stop and focal length

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1 hour ago, Cinegain said:

Exactly, had the same argument with Tony myself, but these people can't be convinced.

I mean, I can crop in on a photo taken with a fullframe sensor and it's just as bright as the rest. Why would a crop sensor work differently? Hint: it doesn't.

We are not talking about leaving a lens in place and putting a crop sensor behind it, we are talking about putting a different lens that throws a smaller circle. But regardless, lets talk through your scenario, step by step. You put a lens on a tripod. It creates an image in a circle, lets say 44mm behind the lens. You place a sensor behind that lens. It is a 1m by 1m sensor. Luckily, it's a really large format lens. You measure the number of photons striking the sensor and ignoring the quantum problem of changing the energy by measuring it, you find that exactly 1,000,000 photons strike the sensor every second. Each photon carries 1 unit of energy, you you have 1,000,000 energies of light striking your sensor, energy that activates it. Interestingly, if you isolate a 1cm square area, you would find that as there are 10,000 square cm in 1m square, there are 10,000 times less photons striking that small isolated area, no matter where you measure that cm squared.

Great!

Now you remove that sensor, and put one there that is only 1cm by 1cm. You could place 10,000 of them on the big sensor to cover it exactly. So naturally, it is being hit by 10,000 less photons every second, that is 100 every second. So the energy being recieved by the smaller sensor is, drum roll, 100 energy units, 10,000 less. Thats a lot less light striking your sensor.

Fortunately, you are a good engineer, so you don't lug that gigantic lens around to cover that tiny sensor, you designed a much smaller, lighter lens for that camera, that still lets 100 photons strike it every second. But because it's only got to cover a 1cm area, it is so tiny, it only lets 100 photons in every second, being a great designer, they are all passed through straight to the sensor. But the big lens is letting 1,000,000 photons in every second. To provide the same level of illumination on the sensor. The big lens is letting in a lot more light.

It's so simple. The big sensor has a lot more light to work with.

Why is this even worth worrying about. You're making a film, you're halfway through a piece of dialogue. You're filming at 100mm on a full frame sensor, open at 2.8. Your camera just stops working. You've got actors, crew etc all standing around, not doing anything. You reach for your spare, and guess what, it's a m4/3 camera. You know you need this to match as closely as possible or it's going to be really jarring, and difficult to edit. You get your 50mm lens because like a trooper, you are aware that the crop sensor needs a wider lens. Then comes the sticky question, what f stop do you set this new lens to?

....

....

If you said f2.8 of course, (BUZZZZZZZ) Wrong, you notice that the background which was a beautiful creamy blur, is now clearly showing cars in your victorian costume drama.

So what gives! It let's the same amount of light through, you incorrectly bemoan to all and sundry around you. Suddenly, I pop up, I'm your ruggedly handsome lead. In my deep, inspiring voice, I declare my truth. "Good Director of Photography, please, allow me. I know a thing or two about these matters. You chose this lens very wisely, for it has an apeture which opens much wider than your setting here of f2.8. It's a m4/3, a 2 times crop. If you apply that factor to the aperture and open it all the way up to f1.4, you will make a picture the equal of your ruined kit"

"Nonsense" you hiss "This cannot be so! Get back to your place, sullen curr, and approach me not again!"

Eventually, after much wailing and decrying of the gods, men and sheep, you, in a temper grasp the lens and make to hurl it from your visage.

But, whats this, your fingers knock the aperture dial to 1.4, and in front of you, the beauty of my face, creamy, carless bokeh behind. It is the picture you longed so desperately for all along. It's overexposed now, but no matter, you have an ISO control in your picturebox.

"To your places!" you shout rumbuctiously "Put down those nachos and clean the cheese from your chin. We resume shooting!"

.

.

.

Again, if you really think I am wrong on this, then do post two identical pictures, from different sized sensors demonstrating that the relationship between the differing f stops you used was not directly related to the differences in the sensor sizes.

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7 hours ago, Grim Fandango said:

And don't forget that while it has the DOF of a 1.8, in light terms, it is illuminating like a 2.8 on a crop frame sensor - making the speed boosted 70-200 faster :)

I won't read through your posts. If you take a 70-200 2.8, put a 0.7x speedbooster behind it will give you a 49-140mm f1.96 lens, with an APS-C sized image circle. The sigma is 50-150 f1.8, so it will be brighter. End of story.

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2 hours ago, bamigoreng said:

Btw, background blur (bokeh) = physical aperure size and focus distance

                                        DoF = focus distance, f-stop and focal length

Bokeh - a term used to subjectively describe the way a lens renders an out of focus area. Is not measured by aperture size or focal distance, but in rather more fanciful ways, such as "jittery", "creamy", "lively"

Circle of Confusion - The flattened spherical area rendered acceptably sharp by a lens to be described as in focus. Determined by a complicated function unique to each lens, commonly able to be moved and resized/shaped by a user using the aperture and focus controls on a camera

Background Blur - Any part of the image that is in the background of an image and outside of the circle of confusion

Depth of field - a laymans description of a linear line through the circle of confusion

10 minutes ago, araucaria said:

I won't read through your posts. If you take a 70-200 2.8, put a 0.7x speedbooster behind it will give you a 49-140mm f1.96 lens, with an APS-C sized image circle. The sigma is 50-150 f1.8, so it will be brighter and render a different picture, making such a slapdash comparison useless to someone who wants to buy a lens knowing what result it can achieve, like say, a film maker looking to match a lens to his existing gear. End of story.

Fixed that for you ;)

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5 minutes ago, araucaria said:

So you were in fact wrong, right?

No, if you stick that sigma on a crop frame camera and open it up to 1.8, you will get the precise same image at f2.88 on the canon with the speed booster on that camera for any given focal length. That gives you a 0.8 twist on the aperture dial of extra speed that is not avaliable to the sigma. Not a life changing amount I grant you, but mathmatically there.

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The thing is... you said:

7 hours ago, Grim Fandango said:

And don't forget that while it has the DOF of a 1.8, in light terms, it is illuminating like a 2.8 on a crop frame sensor - making the speed boosted 70-200 faster :)

and it's actually the other way around. In light terms, illumating a 2x crop frame sensor by e.g. a f/2.8 lens results in a 35mm equiv. depth of field that would be shot at f/4. So, if you want to shoot for the same look you have to shoot the FF at f/4 and the 2x crop frame sensor at f/2.8. Sure, I think we can agree on that. Now... if you don't give a jolly gosh darn about equiv. depth of field and just shoot the lens as is, with the exact same settings, except you switch the lens in between shots with each lens at its widest aperture: the Sigma at f/1.8 and the Canon at f/2.8, the result will be that the Sigma illuminates a brighter image onto the sensor. The Sigma is clearly the faster lens. Then again, no surprise as it's f/1.8 VS f/2.8. Like TRM said 'Light gathering, not bokeh'. Besides, 2x crop isn't APS-C, so what stops you from speedboosting the Sigma as well?

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3 minutes ago, Cinegain said:

The thing is... you said:

and it's actually the other way around. In light terms, illumating a 2x crop frame sensor by e.g. a f/2.8 lens results in a 35mm equiv. depth of field that would be shot at f/4. So, if you want to shoot for the same look you have to shoot the FF at f/4 and the 2x crop frame sensor at f/2.8. If you don't give a jolly gosh darn about equiv. depth of field and just shoot the lens as is, with the exact same settings, except you switch the lens in between shots with each lens at its widest aperture: the Sigma at f/1.8 and the Canon at f/2.8, the result will be that the Sigma illuminates a brighter image onto the sensor. The Sigma is clearly the faster lens. Then again, no surprise as it's f/1.8 VS f/2.8. Like TRM said 'Light gathering, not bokeh'. Besides, 2x crop isn't APS-C, so what stops you from speedboosting the Sigma as well?

Yes, well I'm new here and I honestly thought this stuff was basics 101 and you would all understand what I meant. Clearly, I misjudged! Of course nothing stops you from speedboosting the sigma onto a m4/3, but the conversation was talking about emount sonys.

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5 minutes ago, Cinegain said:

Nah, we just look at a lens in terms of brightness. You like to compare for a same depth of field situation. Which is fine... if you say that's what you want to do. Which you didn't in that quote.

It's like if I told you I was shooting my lens wide open and you all jumped on me telling me that if I used screwdrivers on my lenses then my warrenty would be void. It's obvious that despite the words I used, I just used shorthand to refer to how I had set my iris.

And likewise, to me the idea that you would buy a lens without understanding the picture it will give you on your cameras compared to your other lenses, well, lets just say, my jaw has hit the floor and it won't be being picked up for quite a long time. I literally didn't think that this needed to be explicitly stated.

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20 minutes ago, Grim Fandango said:

No, if you stick that sigma on a crop frame camera and open it up to 1.8, you will get the precise same image at f2.88 on the canon with the speed booster on that camera for any given focal length. That gives you a 0.8 twist on the aperture dial of extra speed that is not avaliable to the sigma. Not a life changing amount I grant you, but mathmatically there.

Lol, what speedbooster on canon cameras, what are you talking about. The 70-200 2.8 is a fullframe lens, you can only use it with speedboosters on aps-c sensors. Do you understand what a speedbooster/focal reducer is/does?

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9 minutes ago, araucaria said:

Lol, what speedbooster on canon cameras, what are you talking about. The 70-200 2.8 is a fullframe lens, you can only use it with speedboosters on aps-c sensors.

My apologies then, you were correct! In truth, my first contact with you was in the piracy thread, where you were needlessly rude and caustic and I really only skim through your posts giving them minimal attention because, quite frankly, I don't like you.

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Just now, Grim Fandango said:

My apologies then, you were correct! In truth, my first contact with you was in the piracy thread, where you were rude and caustic and I really only skim through your posts giving them minimal attention because, quite frankly, I don't like you.

That's not my problem, get your methylation working again.

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21 minutes ago, Grim Fandango said:

It's like if I told you I was shooting my lens wide open and you all jumped on me telling me that if I used screwdrivers on my lenses then my warrenty would be void. It's obvious that despite the words I used, I just used shorthand to refer to how I had set my iris.

And likewise, to me the idea that you would buy a lens without understanding the picture it will give you on your cameras compared to your other lenses, well, lets just say, my jaw has hit the floor and it won't be being picked up for quite a long time. I literally didn't think that this needed to be explicitly stated.

Your statement was about brightness and illumination. And that statement is just plain wrong, nothing we can do about that.

But if you want me to break it down for you

Quote

And don't forget that while it has the DOF of a 1.8

You're contradicting yourself. You say it has a depth of field of a f/1.8... yet we agree that the f/1.8 depth of field would be the associated depth of field for that crop sensor. So if you do want to bring in the 35mm equivalent, on a 2x crop sensor it falls just below f/2.8 (whole stops are: f/1.4 - f/2 - f/2.8 - f/4 - f/5.6 - f/8), perhaps if you take into account the video crop, sure: ~ f/2.8. Nothing to do with brightness.

Quote

in light terms, it is illuminating like a 2.8 on a crop frame sensor

No. In light terms, it is illuminating like a f/1.8 regardless of sensor. But perhaps casting an equivalent depth of field associated with f/2.8 on fullframe. Again. That's not what you said.

But even if up to that point everything was correct...

Quote

making the speed boosted 70-200 faster

How exactly is a f/2.8 (even speedboosted) ever faster/brighter then a f/1.8 lens? And the f/1.8 lens might act as a f/2.8 depth of field equivalent to 35mm, but the f/2.8 would be f/4. Even speedboosted wouldn't come anywhere near f/2.8 again.

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Yikes...what a train wreck. 

Back to the subject at hand, what makes this lens more exciting than a speed boosted 70-200 is that it basically comes "pre-boosted," so it can be boosted again for m4/3 and BM cameras. 75-150 f/1 on the speed booster XL, and even faster on the BMPCC booster (too lazy to do that math right now). 

Of course, the practicality of using a lens this huge, expensive, and heavy on a GH4 or BMPCC leaves something to be desired...

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3 hours ago, Grim Fandango said:

Then you will have literally no problem proving that! Get two cameras with different size sensors, stick two lenses on the front of them set to the same f-stop number, and try to take identical photographs with them. When you give up because the bokeh in one is far more out of focus than the other for the same frame, apply my formula and then take those same photos. Or just watch the video I posted above where they perform that precise test.

Dude, you're mixing light gathering and bokeh with your long winded wikipedia cut and paste.

I just put my 16-35 on the a5100 and the A7rII, set the aperture to F/4 and left everything else to the camera - they both picked the exact same exposure - 1/60, ISO 3200. F/4 is F/4 no matter what the sensor size is, and a F/1.8 lens is faster than a F/2.8 on a focal reducer since you only gain one stop.

Why is this so difficult for you to understand?

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#delurk

my brain is melting trying to follow this, mostly because that video makes perfect sense - but you all are saying it's wrong! The Cris, can you post your pics, you just took, do they have the same amount of blur in the background? I only have a 16-35 on my bmpcc with the dedicated speed booster and am really interested in this lens, but it will or won't give me the same performance as a theoretical lens with the same mm range, the same f stop but covers a full frame - and would that be the case if I put that theroetical lens on a theoretical ff camera it would perform differently to the sigma on a crop frame....

ARGH! I'm in the market for a new camera and some that I'm looking at include the A7r II, Ursa Mini, so while I could just buy the lens and be happy, I do want to make an informed choice.

#relurk

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