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08-09-2010, 01:51 PM   #1
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Why DA lenses are designed that way?

This is possibly a physics question, why do DA prime lenses as they become wider also become slower? For example: DA70 - F2.4, DA40 - F2.8, DA21 - F3.2, DA15 - F4, etc.

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08-09-2010, 02:05 PM   #2
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I've wondered this too, it seems like a trend in general that the wider a lens is the slower it is, which doesn't make sense to me since f/number is a ratio of focal length, it seems like the wider a lens is it should be easier to make it faster
08-09-2010, 02:11 PM   #3
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The DA14 is a f/2.8 lens and look how large it is compared to the DA15/4...
08-09-2010, 02:25 PM   #4
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QuoteOriginally posted by future_retro Quote
I've wondered this too, it seems like a trend in general that the wider a lens is the slower it is, which doesn't make sense to me since f/number is a ratio of focal length, it seems like the wider a lens is it should be easier to make it faster
I think different forces are at work here.... the thinnest fast lenses are all in the 50mm range, I'm guessing it has something to do with the 43-50mm range being the "normal" range and thus is the smallest that can be made with wide apertures. If we look at fast lenses of the same aperture, the 50mm range is usually the thinnest; as you go wider or longer than 50, the size of the lens increases. If you compare a collection of fullframe lenses of the same aperture:

Lens / Diameter / Length
Sigma 20/1/.8 3.5 x 3.43 in
Sigma 24/1.8 3.3 x 3.1 in (just for comparison, Pentax 24mm/2 2.8 x 2.6 in)
Canon 28/1.8 2.8 x 2.2 in
Pentax 31/1.8 2.6 x 2.7 in (is this with or without hood? can anyone confirm?)
Pentax 43/1.9 2.5 x 0.95 in
Pentax 50/1.8 2.5x1.2 in
Pentax 77/1.8 2.5x1.8
Nikon 85/1.8 2.8 x 2.3 in
etc

The goal of the (DA) limiteds, however, seem to be to make it as small as possible while maintaining approximately the same shutter speed when they're used interchangeably in the same lighting condition (15mm gathers way more light than 40mm, so it can afford to be f/4). This results with 40mm becoming our smallest lens, since it's the one that falls closest to the 43-50mm range


Of course the lens design and quality will affect its size, but I'm just trying to show a trend (unfortunately with only with one sample point)

Just my 2c


Last edited by Andi Lo; 08-09-2010 at 02:33 PM.
08-09-2010, 02:32 PM   #5
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Is it to do with wider angle needing a larger front element(s) and for larger apertures larger again?
08-09-2010, 05:52 PM   #6
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Possibly to keep cost relatively down maybe? I could be way off, but I think a DA15 f1.8 would be much more $$$.
08-09-2010, 07:09 PM - 1 Like   #7
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It's not just DA's -- all lens lines in this realm have the same characteristics.

QuoteOriginally posted by kiwi_jono Quote
Is it to do with wider angle needing a larger front element(s) and for larger apertures larger again?
Probably not. My two widest lenses are fishy -- the DA10-17/3.5-4.5 objective is 55mm in diameter, the faster Zenitar 16/2.8 is 53mm across. My kit 18-55 is 37mm across, and a slower Tokina 21/3.8 has 72mm threads but the glass is just 47mm across. Anyway, the the front glass size doesn't control lens speed -- that's due to the aperture, which is at the other end.

I suspect that what limits wide-lens speed are 1) necessary optical corrections, and 2) the retrofocus element (like an inverted teleconverter) that mates short focal lengths with a SLR's swinging mirror. Simple short lenses are possible, but for APS-C or FF they'll have terrible optical aberrations. Yes, 10/1.5 (and shorter and faster) glass exists, but for 16mm or video, with much smaller frames, so such short lenses aren't wide-angle.

QuoteOriginally posted by Andi Lo:
the thinnest fast lenses are all in the 50mm range, I'm guessing it has something to do with the 43-50mm range being the "normal" range and thus is the smallest that can be made with wide apertures.
Blame those retrofocus elements (RFE's). We see fairly fast glass around 35-45mm on 135 rangefinders, where the rear elements can intrude almost all the way to the film frame if needed, because they have no mirrors to dodge. (I recall one old Canon HF-RF with a 28/1.7 lens.) For glass wider than 50mm on a SLR whose register (working distance) is around 45mm, an RFE is needed. And they seem to slow down the lens, as do corrective elements.

I don't think we can attribute Fast Fifty's to 'normality' -- it that were the case, f/1.4 glass around 70-100mm should abound for MF cams. Awhile back I asked why Nifty Fifty's are so common, and the general response (besides I DUNNO) was that optics are just easy to calculate in the 50-60mm range. So it goes.
08-10-2010, 04:28 AM   #8
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The issue with Pentax has to do with their pursuit of small size. They consciously made a decision with primes to make them as small as possible, even if that meant sacrificing speed. The DA 40 in particular could have been made a stop faster without increasing the size to more than about the DA 70 size; however, they wanted that ultra slim size and so it ended up being f2.8 (I think they also started with the original 40mm pancake desigen, which was also f2.8).

Lenses on both ends (really long or really wide) tend to increase in size. It just requires more glass to get a sharp image without a lot of distortion. The solution is to either increase the physical size of the lens, or to make it slower. Clearly Pentax decided that wide lenses were for landscapes and people would be shooting them stopped down anyway and so they made them slower, but also smaller in size.

08-10-2010, 05:13 AM   #9
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It is a matter of choice, mainly regarding lens size.

You can design any lens with any aperture, within some rational limitations of course. But a faster lens is larger (just look at the Sigma 20 mm f1,8 when compared to the DA 21 f3,2).

So the Pentax engineers, in accordance with their marketing team, decided in favor of size instead of speed.

Also keep in mind that a wider lens doesn't have to be as fast to be handheld without motion blur.
08-10-2010, 06:25 AM   #10
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QuoteOriginally posted by Rondec Quote
The issue with Pentax has to do with their pursuit of small size. They consciously made a decision with primes to make them as small as possible, even if that meant sacrificing speed. The DA 40 in particular could have been made a stop faster without increasing the size to more than about the DA 70 size; however, they wanted that ultra slim size and so it ended up being f2.8 (I think they also started with the original 40mm pancake desigen, which was also f2.8).
That has been a big part of the Pentax philosophy ever since the M lenses appeared in the '80s. It really is their niche.
08-10-2010, 06:37 AM   #11
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Compare the FA 85mm to the FA 77mm size-wise and you'll see why Pentax opts for smaller apertures in consumer/mainstream lenses

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