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FORUMS Canon Cameras, Lenses & Accessories Canon EF and EF-S Lenses 
Thread started 03 Aug 2013 (Saturday) 12:54
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Why not a F1.8-2.8 24-70?

 
Talley
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Aug 03, 2013 12:54 |  #1

I know the technical aspects of creating a F2 constant zoom but why not a variable that is capable of 1.8-2.8. The canon S100/110 cameras can do this. Why not do this for a regular EF lens? I would think it would be easier to do than a constant F2 lens.


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tkbslc
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Aug 03, 2013 13:47 |  #2

I think it is a great idea, but a lot of pros will say they simply will not use a variable aperture lens, even if it is a fast one. I suspect Canon or Sigma marketing knows this.

Although the first 28-70 L was 2.8-4.


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Kolor-Pikker
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Aug 03, 2013 14:49 |  #3

Because of retrofocus.

How do zoom lenses work, and DSLR lenses in general?

For a lens to have a certain focal length, the "aperture" needs to be the same distance away from the sensor that the focal length indicates, so a 50mm lens really just means the the aperture is 50 millimeters away from the sensor. This is approximate as modern optics allow for some leeway.

Now here's the trick, the EOS mount sits 44mm in front of the sensor plane, so how can we have lenses wider than 44mm? You make a lens built up out of two major components, the "base" optic group, and a retrofocus group in front, the base group is really nothing more than a prime lens of a focal length that is physically viable according to the flange distance, while the retrofocus group acts as a wide-angle adapter and compresses a wider field of view into the base group.

So basically, all DSLR wides are really standard lenses with wide angle adapters in front, which is why they tend to suffer optically compared to wides on mirrorless cameras, like Leicas, technical cameras and indeed, even point & shoots. The lens in a PnS has no flange distance to contend with, and so can be as close to the sensor as optical design allows, this is why they can have relatively fast apertures at the wide end.

So here's where we get to the 24-70 f/2.8, which is actually a 70mm f/2.8 with a retrofocus group in front that can widen the field of view depending on it's distance to the lens - the distance of the aperture to the sensor never changes.

Variable aperture zoom lenses use a completely different design formula, in which the distance of the aperture to the sensor plane does change, this typically allows for a slightly faster wide end, but a significantly slower long zoom. The reliance on a quality retrofocus component isn't as important as in a constant aperture lens, and this means the lens can be cheaper, smaller and lighter.

TL;DR: There can't be a variable-aperture 24-70 because it's already at it's fastest aperture.


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Talley
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Aug 03, 2013 14:56 |  #4

Kolor-Pikker wrote in post #16178338 (external link)
Because of retrofocus.

How do zoom lenses work, and DSLR lenses in general?

For a lens to have a certain focal length, the "aperture" needs to be the same distance away from the sensor that the focal length indicates, so a 50mm lens really just means the the aperture is 50 millimeters away from the sensor. This is approximate as modern optics allow for some leeway.

Now here's the trick, the EOS mount sits 44mm in front of the sensor plane, so how can we have lenses wider than 44mm? You make a lens built up out of two major components, the "base" optic group, and a retrofocus group in front, the base group is really nothing more than a prime lens of a focal length that is physically viable according to the flange distance, while the retrofocus group acts as a wide-angle adapter and compresses a wider field of view into the base group.

So basically, all DSLR wides are really standard lenses with wide angle adapters in front, which is why they tend to suffer optically compared to wides on mirrorless cameras, like Leicas, technical cameras and indeed, even point & shoots. The lens in a PnS has no flange distance to contend with, and so can be as close to the sensor as optical design allows, this is why they can have relatively fast apertures at the wide end.

So here's where we get to the 24-70 f/2.8, which is actually a 70mm f/2.8 with a retrofocus group in front that can widen the field of view depending on it's distance to the lens - the distance of the aperture to the sensor never changes.

Variable aperture zoom lenses use a completely different design formula, in which the distance of the aperture to the sensor plane does change, this typically allows for a slightly faster wide end, but a significantly slower long zoom. The reliance on a quality retrofocus component isn't as important as in a constant aperture lens, and this means the lens can be cheaper, smaller and lighter.

TL;DR: There can't be a variable-aperture 24-70 because it's already at it's fastest aperture.

Excellent response! Thanks... learned more than I know in that one right there.


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tkbslc
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Aug 03, 2013 19:07 |  #5

But a 70mm f2.8 would be a 25mm aperture. So if it stayed constant, we'd have a 24mm f0.96 aperture at wide angle.


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Aug 03, 2013 19:37 |  #6

Kolor-Pikker wrote in post #16178338 (external link)
Because of retrofocus.

How do zoom lenses work, and DSLR lenses in general?

For a lens to have a certain focal length, the "aperture" needs to be the same distance away from the sensor that the focal length indicates, so a 50mm lens really just means the the aperture is 50 millimeters away from the sensor. This is approximate as modern optics allow for some leeway.

Now here's the trick, the EOS mount sits 44mm in front of the sensor plane, so how can we have lenses wider than 44mm? You make a lens built up out of two major components, the "base" optic group, and a retrofocus group in front, the base group is really nothing more than a prime lens of a focal length that is physically viable according to the flange distance, while the retrofocus group acts as a wide-angle adapter and compresses a wider field of view into the base group.

So basically, all DSLR wides are really standard lenses with wide angle adapters in front, which is why they tend to suffer optically compared to wides on mirrorless cameras, like Leicas, technical cameras and indeed, even point & shoots. The lens in a PnS has no flange distance to contend with, and so can be as close to the sensor as optical design allows, this is why they can have relatively fast apertures at the wide end.

So here's where we get to the 24-70 f/2.8, which is actually a 70mm f/2.8 with a retrofocus group in front that can widen the field of view depending on it's distance to the lens - the distance of the aperture to the sensor never changes.

Variable aperture zoom lenses use a completely different design formula, in which the distance of the aperture to the sensor plane does change, this typically allows for a slightly faster wide end, but a significantly slower long zoom. The reliance on a quality retrofocus component isn't as important as in a constant aperture lens, and this means the lens can be cheaper, smaller and lighter.

TL;DR: There can't be a variable-aperture 24-70 because it's already at it's fastest aperture.

Welp I just learned a lot.


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Kolor-Pikker
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Aug 04, 2013 02:19 |  #7

tkbslc wrote in post #16178871 (external link)
But a 70mm f2.8 would be a 25mm aperture. So if it stayed constant, we'd have a 24mm f0.96 aperture at wide angle.

In a constant aperture zoom, the aperture does not have an "equivalent" aperture at other focal distances, it'll always be 25mm in this case. If you took, say, a 50mm 1.4 and put a wide-angle adapter on it that made it a 30mm, it would still be an f1.4.

Of course it's not all exactly like that, wide zooms, standard zooms, and tele zooms use different variations on the same concept of keeping the aperture the same size and distance from the sensor plane. For example, the 70-200 f2.8 is the exact opposite in terms of mechanics, it's again a 70mm f/2.8, and instead it has a tele group in front that increases magnification as you zoom in - not unlike moving a magnifying glass further away to get a bigger image.

This is a big simplification, as many zooms have both tele and retrofocus groups that engage at different zoom levels, so that same 24-70 could also be a 50mm that moves the retro group when zooming wide, or the tele group when zooming long.

I'm guessing some of you reading this are wondering "but Kolor, this whole focal-length conversion sounds like a hack!" well... the fact of the matter is that it is. Many early zoom lenses were destroyed by the photographic community for their horrible optics, which ruined their reputation for decades up until recently. Even when it come to "just" wide angle lenses, optics for many other systems run circles over what the typical Canikon user has: The Leica 21mm f/3.4 is sharp as a macro lens wide open, the best large-format/technical lenses are mostly wides, and even the Sony RX1 puts a damn fine 35/2 into its tiny body.


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Aug 04, 2013 03:02 |  #8

Kolor-Pikker wrote in post #16178338 (external link)
TL;DR: There can't be a variable-aperture 24-70 because it's already at it's fastest aperture.

So why was there a 28-70/2.8-4.0 ever built? A 24-85/3.5-4.5?


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Aug 04, 2013 03:41 |  #9

Kolor-Pikker your posts are really interesting, instructive. Do you mind me asking about your background.. are you/were you a lens engineer or similar? Just curious!


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tkbslc
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Aug 04, 2013 04:18 |  #10

Kolor-Pikker wrote in post #16179467 (external link)
In a constant aperture zoom, the aperture does not have an "equivalent" aperture at other focal distances, it'll always be 25mm in this case. If you took, say, a 50mm 1.4 and put a wide-angle adapter on it that made it a 30mm, it would still be an f1.4.
.

How does the wide angle adapter shrink the aperture? I wasn't trying to say "equivalent aperture" only that if the aperture is always 25mm, well 24mm focal length with a 25mm aperture is f0.96.


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Kolor-Pikker
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Aug 04, 2013 05:31 |  #11

tkbslc wrote in post #16179586 (external link)
How does the wide angle adapter shrink the aperture? I wasn't trying to say "equivalent aperture" only that if the aperture is always 25mm, well 24mm focal length with a 25mm aperture is f0.96.

... It doesn't shrink anything, it just changes the field of view. It's not 24mm with a 25mm aperture, it's 70mm with a 25mm aperture, neither the focal length nor aperture ever change, just the field of view, and this requires a lot of quality glass to pull off.

FEChariot wrote in post #16179519 (external link)
So why was there a 28-70/2.8-4.0 ever built? A 24-85/3.5-4.5?

I didn't mean it the way you think, it can't be variable aperture with a smaller wide aperture than f/2.8. These lenses use a different construction that's easier to design for price, size and weight. Back when we didn't have the same modern lens building techniques, variable aperture zoom was a standard feature as there just wasn't the knowhow to do otherwise, constant aperture zooms are a fairly recent development.

Savethemoment wrote in post #16179544 (external link)
Kolor-Pikker your posts are really interesting, instructive. Do you mind me asking about your background.. are you/were you a lens engineer or similar? Just curious!

Nope, a lot of what I know is surprisingly right from here on POTN, but also a multitude of other sources. I just love reading into technical details. I don't believe there's a site on technical photography details that I didn't read yet.


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NotBlake
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Aug 04, 2013 11:10 |  #12

Kolor-Pikker wrote in post #16178338 (external link)
Because of retrofocus.

How do zoom lenses work, and DSLR lenses in general?

For a lens to have a certain focal length, the "aperture" needs to be the same distance away from the sensor that the focal length indicates, so a 50mm lens really just means the the aperture is 50 millimeters away from the sensor. This is approximate as modern optics allow for some leeway.

Now here's the trick, the EOS mount sits 44mm in front of the sensor plane, so how can we have lenses wider than 44mm? You make a lens built up out of two major components, the "base" optic group, and a retrofocus group in front, the base group is really nothing more than a prime lens of a focal length that is physically viable according to the flange distance, while the retrofocus group acts as a wide-angle adapter and compresses a wider field of view into the base group.

So basically, all DSLR wides are really standard lenses with wide angle adapters in front, which is why they tend to suffer optically compared to wides on mirrorless cameras, like Leicas, technical cameras and indeed, even point & shoots. The lens in a PnS has no flange distance to contend with, and so can be as close to the sensor as optical design allows, this is why they can have relatively fast apertures at the wide end.

So here's where we get to the 24-70 f/2.8, which is actually a 70mm f/2.8 with a retrofocus group in front that can widen the field of view depending on it's distance to the lens - the distance of the aperture to the sensor never changes.

Variable aperture zoom lenses use a completely different design formula, in which the distance of the aperture to the sensor plane does change, this typically allows for a slightly faster wide end, but a significantly slower long zoom. The reliance on a quality retrofocus component isn't as important as in a constant aperture lens, and this means the lens can be cheaper, smaller and lighter.

TL;DR: There can't be a variable-aperture 24-70 because it's already at it's fastest aperture.


Came to say that this was an extremely informative post. Thank you for sharing!!




  
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Aug 06, 2013 11:20 |  #13

Kolor-Pikker wrote in post #16178338 (external link)
Now here's the trick, the EOS mount sits 44mm in front of the sensor plane, so how can we have lenses wider than 44mm? You make a lens built up out of two major components, the "base" optic group, and a retrofocus group in front, the base group is really nothing more than a prime lens of a focal length that is physically viable according to the flange distance, while the retrofocus group acts as a wide-angle adapter and compresses a wider field of view into the base group.


It seems like this explains a bunch of questions...

  • Why do most lenses wider than 50 have that big curve on the front ?

  • Why are there no 24/35 1.2's?

  • Why isn't the 24-70II as awesome as the 70-200II?

  • Why is there no 20mm pancake?

  • Why are wides larger/pricey, when they 'should be' smaller/cheaper?

  • Why isn't it easy to find an optically perfect wide?

  • Why can they make such a small/sharp M wide and not do the same on EF?

  • Why do M lenses have a less 'lumpy' outer shape?



BTW, if you wanted to put your knowledge (summaries of what you've put together) into a few guides, I'm sure we'd eat them up!

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Kolor-Pikker
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Aug 06, 2013 12:56 |  #14

ElectronGuru wrote in post #16185700 (external link)
It seems like this explains a bunch of questions...

For the most part, yes.

Why do most lenses wider than 50 have that big curve on the front ?

Likely, though a lens needs to be increasingly spherical to capture a wide field of view in any case.

Why are there no 24/35 1.2's?

Yes. Leica has a 21mm f/1.4, it's ridiculously big and heavy (for a Leica lens), and costs $6500, but it is possible.

Why isn't the 24-70II as awesome as the 70-200II?

I suppose it could be that telephoto lenses as a whole are easier to design than wides. Any camera system has decent 85/100/135/200mm lenses.

Why is there no 20mm pancake?

Likely, the 40mm is probably as wide as Canon could get away with in a pancake.

Why are wides larger/pricey, when they 'should be' smaller/cheaper?

Smaller, definitely, there are some tremendously small and quality wides out there.

Why isn't it easy to find an optically perfect wide?

For sure, a lot of people are having trouble finding good wide angles for the D800, which has 36mp resolution; tech cams can have up to 80mp of resolution and the wide angle lenses are still razor sharp to the pixel.

Why can they make such a small/sharp M wide and not do the same on EF?

Why do M lenses have a less 'lumpy' outer shape?

Can't comment on the M system.

BTW, if you wanted to put your knowledge (summaries of what you've put together) into a few guides, I'm sure we'd eat them up!

That would take quite a while, but there are already great sources that have compiled information with pictures and everything, a good place to start is Cambridge In Colour (external link).


I should add that retrofocus is not all that bad with digital, because unlike film, which could accept light coming in from any angle, sensors like it when light comes straight-on. In a non-corrected wide angle, the light exits the rear of the lens at roughly the same angle it enters, and this could cause color casts on digital sensors, particularly those with micro lenses and/or non-100% fill factor. CCD sensor typically don't have micro lenses and have 100% fill, which is why most medium format back use them, and Leica used CCDs for while, until they developed their own CMOS sensor suitable for the Leica M lenses.

The retrofocus design makes the light rays hit the sensor head-on, which can solve a lot of problems with certain camera/sensor/lens combinations. To put things in perspective, a non-corrected Schneider tech cam wide lens can cost around $1.5~3k, in comparison a Rodenstock retrofocus tech cam lens of the same focal length is anywhere from $4~9k.


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FEChariot
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Aug 06, 2013 14:37 |  #15

ElectronGuru wrote in post #16185700 (external link)
  • Why is there no 20mm pancake?
  • http://www.bhphotovide​o.com …or_Skopar_20mm_​f_3_5.html (external link)


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