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01-14-2018, 12:00 PM   #1
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Why are some lenses compact and other similar focal lengths large?

I've been curious for a while about the answer to this question. When looking at the older 50mm M & A lenses, or even the SMCP-DA 50mm, they are all relatively compact. Their diameters are similar, as are the lengths. But, looking at the new D FA 50 1.4 and comparing its dimensions to the older SMC-A 50mm 1.4 that I own, they are remarkably different in size. Now, I understand that the SDM autofocusing hardware takes up space in the newer lens, but that can't account for the total difference. Are there optical differences that are responsible for the difference? If so, please educate me about it. I've seen quite a few primes from Canon and Nikon that are enormous compared to their Pentax counterparts. Thanks!

01-14-2018, 12:10 PM - 1 Like   #2
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QuoteOriginally posted by neokind Quote
I've been curious for a while about the answer to this question. When looking at the older 50mm M & A lenses, or even the SMCP-DA 50mm, they are all relatively compact. Their diameters are similar, as are the lengths. But, looking at the new D FA 50 1.4 and comparing its dimensions to the older SMC-A 50mm 1.4 that I own, they are remarkably different in size. Now, I understand that the SDM autofocusing hardware takes up space in the newer lens, but that can't account for the total difference. Are there optical differences that are responsible for the difference? If so, please educate me about it. I've seen quite a few primes from Canon and Nikon that are enormous compared to their Pentax counterparts. Thanks!
Design objectives. With pretty much all modern lenses being quite good these days, much of the focus has shifted to making primes that are perfect corner-to-corner (and really fast), which increases their size.

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01-14-2018, 12:17 PM   #3
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QuoteOriginally posted by Adam Quote
Design objectives. With pretty much all modern lenses being quite good these days, much of the focus has shifted to making primes that are perfect corner-to-corner (and really fast), which increases their size.
That makes sense. Thanks. With wider glass, does that necessarily require more space between the elements, thereby making the length of the lenses greater as well?
01-14-2018, 12:24 PM   #4
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QuoteOriginally posted by neokind Quote
That makes sense. Thanks. With wider glass, does that necessarily require more space between the elements, thereby making the length of the lenses greater as well?
Not sure about that, but they definitely do have to do some magic to get the correct focal length out of such a big lens.


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01-14-2018, 12:36 PM   #5
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As Adam noted, the newer lenses are often designed to higher standards of corner-to-corner sharpness than the other lenses.

Autofocusing also tends to force the designer to create a bigger more complicated lens. The older manual-focus and screw-drive AF lenses often moved the entire assembly of the lens in and out to focus. But that design requires either huge focus motors or slower AF. The newer designs tend to have internal focus that only moves a small subset of the elements inside the lens a short to achieve focus. But the overall result is a lens with a lot more elements and larger overall size.
01-14-2018, 01:39 PM   #6
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To get the corner to corner sharpness which most modern lenses have, additional elements (aspheric and low dispersion) are needed (in addition to larger elements) and these take up some axial space, making the lens longer. Some lenses approach apochomatic design these days usually meaning extra glass. Basically, just chalk larger lenses up to differing (usually better) optical designs and some additional mechanical additions.

Last edited by Bob 256; 01-15-2018 at 09:52 AM.
01-14-2018, 03:22 PM   #7
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What everyone said above sounds right. If cost were no object...well, wait, cost is an object since most consumers have limited funds. Therefore, everything becomes a tradeoff. The more elements, the more corrections that can be made. However, if you have too many elements sharpness is lost. The bigger diameter of the glass help with corner-to-corner sharpness is possible (think of a 6" diameter lens on a K-3 for an outrageous example), but then you have more weight and that sucks for travel or street photography. The Pentax pancakes are lightweight and pretty good corner-to-corner but the tradeoff is that they are only fairly fast (not the very best for shooting stars wide open but definitely usable). For an amateur like me, Pentax understands what I need with a lot of lenses that have become very affordable over the years. These lenses have the right balance among sharpness, speed, build, weight, and price.
01-14-2018, 03:29 PM - 1 Like   #8
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It comes down to optical designs. Other than aperture, some of the elements are:
a) Lens complexity. For example, a superzoom lens might need more elements. Or ones with internal focus or Tilt/shift tech. There are also different base designs that come from different optic lineages. Different lineage are like Tessar, Planar and other designs. They define the lens size and affect the output. You can have two 50mm primes, and they have the same FoV, but their actual photos will look different (bokeh, colours, contrast, etc.)
b) Lens quality. A lens with many corrective elements will be bigger. This is why a lens with extreme sharpness, corner to corner sharpness, low CA, will have more elements. Some claim that having many elements is actually bad, but that comes down to preference.
c) Format. FF lenses in theory would be bigger than APSC lenses. And both are bigger than the smaller Q format. 645 medium format lenses are bigger. Part of this is the lens mount itself - Pentax Kmount allows some interesting pancake designs like DA 40mm, DA 21mm. But it makes the camera "bigger" (require space in the mirror box). Some mirrorless camera mounts have much smaller mirror box requirements (Register distance), but this means their lenses are generally larger. The difference between FF and APSC is almost not noticeable, especially since many APSC lens designs are derived from FF ones. Difference between Q and 645 is more obvious.

What do users want? Top quality in a compact package at a low price. Of course, having all 3 is impossible and manufacturers have to choose between them. Lately lens size is increasing. Many manufacturers seem to be ignoring lens size and price just to produce stunningly sharp lenses, which has its own pros and cons


Last edited by Na Horuk; 01-15-2018 at 07:58 AM.
01-14-2018, 03:33 PM   #9
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We might be in the final stages of the Age of Elephantine Lenses. A new paradigm
is on the way.
01-14-2018, 07:06 PM   #10
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It is too simple an explanation, and of course the design objectives vary and thus similar FL designs will differ, but consider that you want to make a lens that has outstanding performance wide open--say at f/1.4.

One way would be to design the lens to have a maximum opening of f/1.0 and then limit the lens to f/1.4. Roughly no vignetting then, and some of the optical aberrations reduced to roughly 1/10 of what they would be at f/1.0.

In past decades perhaps one wouldn't expect great performance (or rather a different kind of rendering) full open, but if one is willing to accept a large lens, and spend a lot, and because the resolution of sensors are so high now, and (for some) the pixel peeping syndrome, then maybe an f/1.0 lens limited to f/1.4 makes sense. Again this is too simple an explanation, but I think there is likely some truth in this kind of design logic.
01-14-2018, 07:33 PM   #11
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Thanks for such a detailed and informative post. This is exactly what I was hoping to find here.

QuoteOriginally posted by Na Horuk Quote
It comes down to optical designs. Other than aperture, some of them are:
a) Lens complexity. For example, a superzoom lens might need more elements. Or ones with internal focus or Tilt/shift tech. There are also different base designs that come from different optic lineages. Different lineage are like Tessar, Planar and other designs. They define the lens size and affect the output. You can have two 50mm primes, and they have the same FoV, but their actual photos will look different (bokeh, colours, contrast, etc.)
b) Lens quality. A lens with many corrective elements will be bigger. This is why a lens with extreme sharpness, corner to corner sharpness, low CA, will have more elements. Some claim that having many elements is actually bad, but that comes down to preference.
c) Format. FF lenses in theory would be bigger than APSC lenses. And both are bigger than the smaller Q format. 645 medium format lenses are bigger. Part of this is the lens mount itself - Pentax Kmount allows some interesting pancake designs like DA 40mm, DA 21mm. But it makes the camera "bigger" (require space in the mirror box). Some mirrorless camera mounts have much smaller mirror box requirements (Register distance), but this means their lenses are generally larger. The difference between FF and APSC is almost not noticeable, especially since many APSC lens designs are derived from FF ones. Difference between Q and 645 is more obvious.

What do users want? Top quality in a compact package at a low price. Of course, having all 3 is impossible and manufacturers have to choose between them. Lately lens size is increasing. Many manufacturers seem to be ignoring lens size and price just to produce stunningly sharp lenses, which has its own pros and cons
01-14-2018, 11:10 PM - 2 Likes   #12
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QuoteOriginally posted by Na Horuk Quote
What do users want? Top quality in a compact package at a low price. Of course, having all 3 is impossible
Add full-featured and fast aperture and choose three. Top quality, compact, and low price are quite doable for manual focus primes. Add in-lens AF (the full-featured part) and the dynamic shifts towards larger and more costly. The effect of fast aperture is similar. One can have fast aperture, full-featured, and top quality, but the result won't be cheap and won't be small.


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01-15-2018, 02:44 AM   #13
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QuoteOriginally posted by Na Horuk Quote
It comes down to optical designs. Other than aperture, some of them are:
a) Lens complexity. For example, a superzoom lens might need more elements. Or ones with internal focus or Tilt/shift tech. There are also different base designs that come from different optic lineages. Different lineage are like Tessar, Planar and other designs. They define the lens size and affect the output. You can have two 50mm primes, and they have the same FoV, but their actual photos will look different (bokeh, colours, contrast, etc.)
b) Lens quality. A lens with many corrective elements will be bigger. This is why a lens with extreme sharpness, corner to corner sharpness, low CA, will have more elements. Some claim that having many elements is actually bad, but that comes down to preference.
c) Format. FF lenses in theory would be bigger than APSC lenses. And both are bigger than the smaller Q format. 645 medium format lenses are bigger. Part of this is the lens mount itself - Pentax Kmount allows some interesting pancake designs like DA 40mm, DA 21mm. But it makes the camera "bigger" (require space in the mirror box). Some mirrorless camera mounts have much smaller mirror box requirements (Register distance), but this means their lenses are generally larger. The difference between FF and APSC is almost not noticeable, especially since many APSC lens designs are derived from FF ones. Difference between Q and 645 is more obvious.
No fully true, as short register distance make it possible to design more compact lenses than on a longer register distance.

For mirrorless, especially wide angle lenses can be made considerable smaller and lighter, but on DSLR longer focal length can be made smaller as they sit further away from the sensor. Once the focal length is longer than 100mm, the difference between them are small. Below is Sony A7rIII with Zeiss Loxia 21/2.8 and Nikon D850 with Zeiss Milvus 21/2.8. Both lenses are top performers in it's class.

If lenses for DSLR are more compact then on mirrorless, in general that mean that the DSLR lens is based on a very old optical formula.
QuoteQuote:
What do users want? Top quality in a compact package at a low price. Of course, having all 3 is impossible and manufacturers have to choose between them. Lately lens size is increasing. Many manufacturers seem to be ignoring lens size and price just to produce stunningly sharp lenses, which has its own pros and cons
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01-15-2018, 03:50 AM   #14
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QuoteOriginally posted by Fogel70 Quote
No fully true, as short register distance make it possible to design more compact lenses than on a longer register distance.

For mirrorless, especially wide angle lenses can be made considerable smaller and lighter, but on DSLR longer focal length can be made smaller as they sit further away from the sensor. Once the focal length is longer than 100mm, the difference between them are small. Below is Sony A7rIII with Zeiss Loxia 21/2.8 and Nikon D850 with Zeiss Milvus 21/2.8. Both lenses are top performers in it's class.

If lenses for DSLR are more compact then on mirrorless, in general that mean that the DSLR lens is based on a very old optical formula.
From what I understand lenses that are close in focal length to the registration distance will be the smallest lenses. This is the reason that the DA 40 limited (and the 40 XSI by extension) can be so small. This is why lenses from mounts with shorter registrations can be smaller. That size advantage does shrink as you get longer. It looks to me like the FA 77 f1.8 is quite a bit smaller than the Sony 85mm f1.8 -- even with the fact that the FA 77 has an all metal build.

A lot depends on what you expect from a lens wide open. The new DFA *50 is supposed to be sharp at f1.4. That takes a pretty large lens. Traditionally, most lens makers were satisfied to have maximum sharpness stopped down a couple of stops. So, with the FA 50, f1.4 was pretty dreamy looking while f2.8 was actually sharpening up nicely.
01-15-2018, 04:00 AM   #15
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There are already several good answers, so I'll add a graphical representation:
Lens review data: Digital Photography Review
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