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04-20-2018, 10:06 AM   #1
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Wide lenses with small apertures, why?

Hi

I'm curious to hear whether anyone hear can explain this one to me.

Aperture is usually given as proportion of focal length and as such it seems rather obvious why telephoto lenses would have smaller maximum aperture numbers, as 400mm lens would have to have 142mm wide aperture at f2.8, 300mm would be 107mm at f2.8 and so forth (even though I suppose lenses with this focal lengths and apertures may exist)

But why would wide angle and ultra-wide angle lenses (8mm to 18mm) be limited to f4 or f4.5 by design as these lenses have usually body diameters that are more than 10-fold the size ofthe maximum aperture, even at f2.8 .

interested to hear from those who know.

regards
K

04-20-2018, 10:18 AM   #2
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QuoteOriginally posted by krazny Quote
But why would wide angle and ultra-wide angle lenses (8mm to 18mm) be limited to f4 or f4.5 by design
They are not, unless I misunderstand you. Pentax has the DA 14mm f/2.8. So do many other brands. Rokinon / Samyang has a number of fast ultra-wides. Sigma has a 14mm f/1.8, 20mm f/1.4 and a 14-24 f/2.8. Not all for Pentax of course.

The new Pentax DA*11-18 is to be f/2.8
04-20-2018, 10:38 AM - 3 Likes   #3
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1. Accurately refracting the light rays from both the center of the image and the corners gets increasingly hard as the angle gets wider.

2. Also, "simple" wide angle lenses have severe vignetting and designing a lens to avoid that is very challenging.

3. A "simple" wide angle lens must be mounted extremely close to the sensor but that is not possible given the distance to the mount. Creating a so-called retrofocus to accommodate the mount is complicated.

(Note: in theory, mirrorless cameras have an advantage on #3 but putting the lens close to the sensor creates more problems with issue #2 which takes the designer back to using a bulky, small aperture retro-focus design)

P.S. Those photons are go 300,000 km per sec! You try deflecting them!!!!
04-20-2018, 10:54 AM - 3 Likes   #4
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In a nutshell, physics. Let's take two comparably priced 15mm wide angle primes as an example:

Pentax DA 15mm f/4 ED AL Limited vs. IRIX 15mm f/2.4 Firefly Lens for Pentax K

For the Pentax to achieve f/4, 15mm/4=an aperture diameter of 3.75mm. Because of this small diameter the lens is only:
1.6" long
49mm filter size
190 grams

For the Irix to achieve f/2.4, 15mm/2.4=an aperture diameter of 6.25mm. Because of this larger diameter the lens is now:
3.94" long
95mm front filter thread
581 grams (and this is the lighter Firefly, not Blackstone)

To have gained 1.3 EV more light, we have a lens that is double in length and girth, and nearly triple in weight.

At the edge of the envelope would be something like the Sigma 14mm f/1.8 DG HSM Art (for Nikon, Canon, or Sigma) for 4x the price and:
4.96" long
150mm square filter attachment required
1170 grams (6x the weight of the Pentax) 16 elements in 11 groups!!

The bottom line is that with a wide to ultra wide angle lens:
a) Great depth of field is inherent. So a larger aperture for bokeh is not going to happen unless maybe you're at minimum focus.
b) Camera movement induced blur is reduced and even without SR, you should still be able to handhold down to 1/15". So why all the expense and bulk for a larger aperture?
c) Generally fast action is not being shot with an ultra wide angle lens. Thus a need for larger apertures to afford higher shutter speeds is rare.

04-20-2018, 11:03 AM   #5
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QuoteOriginally posted by jatrax Quote
They are not, unless I misunderstand you. Pentax has the DA 14mm f/2.8. So do many other brands. Rokinon / Samyang has a number of fast ultra-wides. Sigma has a 14mm f/1.8, 20mm f/1.4 and a 14-24 f/2.8. Not all for Pentax of course.

The new Pentax DA*11-18 is to be f/2.8

I guess I should have been clearer, but yes you are misunderstanding.

I am mainly wondering from the point of design I guess. Why go for f4.5 vs f2.8

QuoteOriginally posted by photoptimist Quote
1. Accurately refracting the light rays from both the center of the image and the corners gets increasingly hard as the angle gets wider.

2. Also, "simple" wide angle lenses have severe vignetting and designing a lens to avoid that is very challenging.

3. A "simple" wide angle lens must be mounted extremely close to the sensor but that is not possible given the distance to the mount. Creating a so-called retrofocus to accommodate the mount is complicated.
Thank you
04-20-2018, 11:10 AM   #6
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QuoteOriginally posted by krazny Quote
I guess I should have been clearer, but yes you are misunderstanding.

I am mainly wondering from the point of design I guess. Why go for f4.5 vs f2.8



Thank you
Its smaller, lighter, cheaper, and easier to design a slower f/4.5 lens. Wide angles tend to be used to landscapes where you want a large amount of your image in focus so a narrower aperture like f/11 - f/22 is going to be used anyway. However, there are many available wide angles that have f/2.8 and wider apertures if you need it for indoor low light situations like weddings.
04-20-2018, 11:18 AM   #7
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QuoteOriginally posted by krazny Quote
Why go for f4.5 vs f2.8
Generally wide angle lens are used for landscape where you might be stopping down to f/8 or f/11 anyway. So no reason to design it faster which will increase the size and cost. That seems to be changing somewhat perhaps because of the interest in astrophotography. There are quite a few very fast wide angles available now.
04-20-2018, 01:46 PM   #8
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In fact, it is easier (and cheaper) to build a fast tele lens than a fast wide angle lens.
It is due to reasons explained above (photoptimist and Alex645 posts).

Cheap wide angle lenses (i.e. Sigma 24mm F1.8) are not well corrected, I sold mine because of that.
If you wanna create a fast wide angle with excellent MTF, it won't be cheap! Who is gonna buy it?
Who said "niche market"?

04-20-2018, 02:51 PM - 1 Like   #9
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QuoteOriginally posted by krazny Quote
But why would wide angle and ultra-wide angle lenses (8mm to 18mm) be limited to f4 or f4.5 by design as these lenses have usually body diameters that are more than 10-fold the size ofthe maximum aperture, even at f2.8 .
As noted above, there are design challenges that are particularly prominent with wide angle lenses. Those include vignette, geometric and volumetric distortion (assuming rectilinear projection), optical aberration (all types), flatness of field, and accommodation of the camera with which they are mated. I don't have any rectilinear ultra-wides, but I do have a small collection of 35mm, 28mm, and 24mm lenses as well as 15mm and 8mm fisheyes.* A few generalities may be helpful:
  • Physical size does not correlate with maximum aperture. My largest lens is also the slowest.
  • Size of front lens element does not correlate with maximum aperture
  • Focal length does not correlate with maximum aperture. The shortest is also one of the two slowest at f/3.5.
  • Image circle does not correlate with maximum aperture
  • Performance does not correlate with physical size, front element size, focal length, or image circle. None of my wide angles suck and all but one (a 28mm taking 67mm filters) offer very decent performance.
I take the variety on my shelf to be evidence that there is more than one way to meet design goals. I also take it to mean that compromise is a fact of life at the short end of things. Now about the practical aspects driving and influencing design decisions:
  • Almost all wide-angle lenses employ some degree of retro-telephoto (aka retro-focus) design. This comes with the price of complexity and erosion of performance along with difficulty supporting wider maximum aperture. This is even true for mirrorless camera lenses where symmetrical designs are practically non-existent at shorter than 35mm due to the lens center being well within the camera body.
  • There is little incentive for designers to offer the fast/wide combination, even for crop-sensor. Consider:
    • The use cases for fast and wide are limited to low magnification compositions having limited DOF. How many photos of a king and queen on a chess board with rooks either side and nothing sharp beyond one's own pawns does the world need? BTW...flatness of field counts double here.
    • The common use cases involve a need for deep DOF, something that is difficult even at f/2.8 at 16mm. Why waste good glass making an f/1.7 lens when the shooting will be done at f/11 or f/16?
    • Fine focus at 28mm (20mm on APS-C) and wider using manual technique is not easy even with live view or focus aides. Everything in the frame is simply too tiny to detect out-of-focus. It is even harder for AF to consistently perform when magnification is low and the sensor has a "baseline" tuned for f/2.8 and narrower. The machine needs something to work with. Anticipating the loud chorus of objection...no, ultra-wide does not provide infinite DOF, it just makes missed focus harder to see.** I have to work double hard with my fisheyes to get attain critical focus than with longer focal lengths.
    • Ditto the preceding point
  • There is a strong incentive to simplify design where possible to keep size/weight/costs down while maintaining adequate flatness of field, resistance to vignette, and sharpness across the frame. A wider maximum aperture works against all of those.
Reasons to not go slow at the wide end?
There is a strong case for giving manual and AF systems something to work with. Increased DOF, full open, cuts both ways. Simply put, f3.5 and f/4 are both unacceptably dim for many lighting conditions, particularly when the view is wide. There is a reason why most wide-angles are clustered around f/2.8. I own the highly-regarded S-M-C Takumar 28/3.5 and it is not a fun lens to use in other than bright light. There, I said it.

Summary...there may well be design constraints resulting in slower wide-angles, but if there is a design benefit, it is not being widely leveraged.


Steve

* I suppose I could extend the discussion to include my 90mm f/6.8 for 4x5" film, but I won't.

** The unlimited DOF discussion is a difficult one, particular since shooting the hyperfocal is both valid and useful. Some lens designs complicate the matter. My Rokinon 8/3.5 Fisheye is a prime example. DOF rules for the focal length apply (go figure), but focus throw is incredibly short and hard to work with. The rule of thumb for owners of that lens is to set the distance scale at 6' with aperture at f/8 and shoot at will...works like a charm OTOH, my Zenitar 16/2.8 Fisheye snaps focus easily and rewards efforts at proper focus.

Last edited by stevebrot; 04-20-2018 at 03:06 PM.
04-20-2018, 02:56 PM   #10
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i'd kill for a 15-30 f4 weather sealed zoom it'd be sharper, lighter and cheaper than 2.8, and with simple screw-in filters (150mm filters are expensive, as well as their mounting brackets).
04-20-2018, 03:13 PM - 1 Like   #11
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QuoteOriginally posted by Alex645 Quote
a) Great depth of field is inherent. So a larger aperture for bokeh is not going to happen unless maybe you're at minimum focus.
Yep...very true, though not fully appreciated unless one has such a lens in hand.

QuoteOriginally posted by Alex645 Quote
c) Generally fast action is not being shot with an ultra wide angle lens. Thus a need for larger apertures to afford higher shutter speeds is rare.
Those that shoot skate park would strongly disagree, though fast ISO may provide an out.


Steve
04-20-2018, 03:29 PM - 1 Like   #12
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QuoteOriginally posted by stevebrot Quote

Those that shoot skate park would strongly disagree, though fast ISO may provide an out.

Steve
At skate parks I've seen lots of GoPros or ultra wides. But I've only seen outdoor SoCal and Hawaii skate parks (bright and sunny) and I suppose indoor skate parks would require faster glass if they can't use strobes with 2nd curtain sync.

Would you risk a Sigma Art to eat a flying board? I once had a basketball slammed into my lens under the boards at a Lakers game. Wasn't my camera, but the viewfinder (video) gave me a black eye.
04-20-2018, 03:37 PM   #13
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QuoteOriginally posted by Alex645 Quote
Would you risk a Sigma Art to eat a flying board?
Not me, but I know photogs that might. As for dim light, here in my area some of the skate parks are under bridges to allow all-weather skating and even when its not raining, cloudy skies are pretty much the rule for about 7-8 months out of the year.


Steve
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