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Thread started 29 Jul 2013 (Monday) 18:51
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Sigma 24-70 F2 OS

 
tkbslc
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Jul 30, 2013 17:44 |  #46

Talley wrote in post #16167034 (external link)
A friend of mine who works at NASA, his boy has a friend on his baseball team who gets babysitted by this lady who has a nephew in florida that does top secret testing for the Sigma antartica team.

No Way!!

My Brother's friend has a pet hamster that he bought off craiglist from some guy he met in a parking lot only 3 miles from where that guy had lunch right before leaving for Antarctica

Small world.


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frankchn
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Jul 30, 2013 18:21 |  #47

melauer wrote in post #16167788 (external link)
Thanks, I will definitely look up Hecht's Optics.

Your point about T-stops is more-or-less why I bought this up. I get the feeling that people are arguing that the front element needs to have twice the area (or 1.4 times the radius) so that twice the light can pass through. As you said that's not an argument about f-stops, it's about T-stops.

I don't know hypothetically how small the front element of an f/2 or f/2.8 24-70 zoom can be. Maybe once I get to reading that book I'll figure it out. :) My point is that it does seem wrong to just extrapolate from the dimensions of existing lenses.

Surely modern lenses are built with larger front elements than are technically necessary for performance reasons. For example, the latest 24-70 2.8 zooms all take 82mm filters, but the original Canon 24-70 took 77mm filters. Knowing that, the question becomes how small those front elements can they be, and what other factors (e.g. a shorter "true" zoom range like 25-67mm, accepting more vignetting) might permit an f/2 zoom to be smaller than people think. Still huge no doubt, but not literally twice the size of a current f/2.8.

The 24-70s / 24-105s / 16-35s / 17-40s out there are at least partially retrofocus designs (in order to lengthen the lens so that the flange distance is appropriate for an SLR), which necessitates the large negative front elements to gather light from a wide angle and thus the usual relationship doesn't work too well. The use of large negative front elements in turn introduces aberrations that require a lot of glass to correct properly and a large aperture exacerbates those aberrations even further.

Systems with no mirrors to contend with has much smaller wide-angle lenses because they do not need to be so severely retrofocus (compare the 24L II against the Leica Summilux-M Asph 24). The optical design is often simpler as well.

It is quite an achievement for Canon to make the 24-70 to be as sharp as it is, and it was only achieved through exotic glass including 3 aspherical elements, 2 SuperUD and 1 UD element in a 18 element/13 group design. I personally think Sigma would have a hard time trying to come up with a design that is one stop faster(!) without introducing bigger and more elements, bringing them closer inline with the 70-200s in size and weight.




  
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SMP_Homer
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Jul 30, 2013 18:42 |  #48

Charlie wrote in post #16167663 (external link)
70-300L has good optics

the 28-300L has fairly good optics as well.

if you stick to lenses that offer the same apperture throughout the range, these 2 don't fit in this discussion quite as well
'superzooms' (3.3x or more) with fixed apperture aren't all that common... 24-105 is 4X+ but then not much else around, and most of the 2.8 options are at, below, or just above 3X


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deanedward
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Jul 30, 2013 19:23 |  #49

i'd be happier if it didn't come with IS as it'd make it smaller and more affordable :)


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shinksma
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Jul 31, 2013 07:41 |  #50

frankchn wrote in post #16167556 (external link)
The f-numbers that we all know and love is calculated by dividing the focal length of the lens over the width of the entrance pupil (external link) of a lens. The entrance pupil is the optical image of the physical aperture stop as viewed from the front of the lens, and is limited in part by how big the front element is.

For instance, if the front element is 100mm in diameter, then when you look into the front of the lens at the aperture, the apparent size of the aperture (i.e. size of the entrance pupil) can only be at most 100mm. Hence, in most normal to telephoto designs, the size of the front element is directly related to the aperture (and focal length) of the lens.

This is true in practice - a theoretical 85mm f/1.2 should have an entrance pupil of 70.8mm, and the actual front element of the actual 85LII is approximately that wide (72mm front filter size). A theoretical 400mm f/2.8 should have a entrance pupil size of 143mm and the actual EF 400mm f/2.8L IS II has a lens hood diameter of 155mm (ET-155), which means that the front element should be about 145-150mm, again falling in line with our calculations. This breaks down somewhat when we talk about retrofocus designs (wide zooms like the 16-35 or 17-40, etc...), but that is another story for another time.

Having better coatings and such will improve the ultimate light transmission of a certain lens but has no effect on the the f-number. (If you are interested in the light transmission, then the T-number is what you should be looking up and what most cine lens manufacturers report).

A good reference on optical design is Hecht's Optics if you want to know more about this.

frankchn wrote in post #16167875 (external link)
The 24-70s / 24-105s / 16-35s / 17-40s out there are at least partially retrofocus designs (in order to lengthen the lens so that the flange distance is appropriate for an SLR), which necessitates the large negative front elements to gather light from a wide angle and thus the usual relationship doesn't work too well. The use of large negative front elements in turn introduces aberrations that require a lot of glass to correct properly and a large aperture exacerbates those aberrations even further.

Systems with no mirrors to contend with has much smaller wide-angle lenses because they do not need to be so severely retrofocus (compare the 24L II against the Leica Summilux-M Asph 24). The optical design is often simpler as well.

It is quite an achievement for Canon to make the 24-70 to be as sharp as it is, and it was only achieved through exotic glass including 3 aspherical elements, 2 SuperUD and 1 UD element in a 18 element/13 group design. I personally think Sigma would have a hard time trying to come up with a design that is one stop faster(!) without introducing bigger and more elements, bringing them closer inline with the 70-200s in size and weight.

These are very good explanations. Thank you for summarizing and then expanding so concisely yet completely.

I started to come up with a response yesterday, demonstrating the size of the front element of the 70-200 2.8 as being pretty spot on for the needed f-ratio, but tripped over the very wide front elements of the 17-40 and 24-70 type designs, which, as you point out, are not a classical lens design. "Retro-focus" was the term I couldn't remember, and therefore couldn't explain. And then I needed to get back to work.

The relatively small front element of the 40mm f/2.8 pancake is a good example of the classic lens design front element size requirement for the designed aperture. It's a whole lot smaller than the front of the 17-40 f/4, which makes no sense at first glance.

shinksma


5DII | T3i | EF 17-40 L | EF 24-105 L | EF 24 1.4 L II | EF 28 1.8 | EF 85 1.8 | EF 70-200 2.8 L IS II | EF 100-400 L | EF-S 15-85 IS USM | EF-S 17-55 2.8 IS USM | EF-S 10-22 USM | EF 100 2.8 Macro USM | EF-S 18-55 IS | EF 35-80 III | EF-S 55-250 IS | Rokinon 8mm FE | EF 75-300 non-USM III | SMC Takumar 50mm f/1.4 | Tamron 70-210 | 430EX II | Kenko 2x MC4 and 1.4x Pro300DGX TC

  
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kasey
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Jul 31, 2013 21:51 as a reply to  @ shinksma's post |  #51

There is a pic available in Cameraegg:

http://www.cameraegg.o​rg …dg-os-hsm-lens/#more-7913 (external link)

Looks like a PS job though. Why is there a ridge at the focus ring? We will see. Looks really long though.


  
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frankchn
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Jul 31, 2013 23:07 |  #52

kasey wrote in post #16171338 (external link)
There is a pic available in Cameraegg:

http://www.cameraegg.o​rg …dg-os-hsm-lens/#more-7913 (external link)

Looks like a PS job though. Why is there a ridge at the focus ring? We will see. Looks really long though.

It looks like a composite between a 120-300mm and the 18-35mm (the back of the 18-35 and the front of the 120-300 with some bits chopped off).




  
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Indecent ­ Exposure
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Aug 01, 2013 18:46 |  #53

First possible pic of the lens:

http://www.sonyalpharu​mors.com …0mm-f2-0-full-frame-lens/ (external link)


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frankchn
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Aug 01, 2013 20:08 |  #54

Indecent Exposure wrote in post #16173821 (external link)
First possible pic of the lens:

http://www.sonyalpharu​mors.com …0mm-f2-0-full-frame-lens/ (external link)

Same picture as the cameraegg.com, probably a PS job.




  
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Indecent ­ Exposure
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Aug 02, 2013 01:27 |  #55

No doubt.


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troehr
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Aug 02, 2013 04:28 |  #56

That is not the photo that is posted on camera store websites here.




  
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titi_67207
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Aug 02, 2013 05:24 |  #57

Sigma has made a nice 18-35mm f/1.8 for APS-C, I think the focal for a f/2 zoom on full frame would be something like 24-50mm and not a very heavy 24-70mm.

Titi


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Shadowblade
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Aug 03, 2013 18:37 |  #58

Looks interesting. But I wish it came in 14-24mm focal length instead...




  
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Sirrith
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Aug 04, 2013 06:26 |  #59

Shadowblade wrote in post #16178821 (external link)
Looks interesting. But I wish it came in 14-24mm focal length instead...

I'd be happy if they made a 16-40 with sharp corners wide open.


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Osiriz
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Aug 04, 2013 20:03 |  #60

Shadowblade wrote in post #16178821 (external link)
Looks interesting. But I wish it came in 14-24mm focal length instead...

Sirrith wrote in post #16179685 (external link)
I'd be happy if they made a 16-40 with sharp corners wide open.

I agree with both of you.

Canon has a big hole in their wide angle line up. This is something Sigma should take advantage off.

Just imagine how many photogs would sell their 17-40L and 16-35L, and buy Sigma if they came out with a razor sharp wide angle zoom.




  
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Sigma 24-70 F2 OS
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