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12-07-2017, 10:36 PM   #1
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front element size - does size matter?

yes, i totally asked that.

anyways, just wondering if some of you can shed some light to on this for me.
i've seen some lenses that are TINY, and others that are massive. yet their size doesn't related to their focal length. IE: the Tamron 90mm macro lens is pretty big lens when you compare it to the Pentax 100mm WR macro lens. all the while the 85mm by any company is usually much bigger than both of them.

yes, i get that the f-stops do vary significantly. and i get that wider open lenses tend to be bigger lenses, but it still doesn't corealte to the focal length.

case in point: pentax DA 50mm F1.8 vs pentax-FA 50mm F1.4
the 1.8 has a front element of 52mm vs 49mm of the 1.4.

and i get that often times, the bigger the front element, the more expensive the lenses are..usually. but aside from being more expensive, heavier, and bigger, what does a front element size do?

12-07-2017, 10:41 PM   #2
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Sure, reducing the size of a lens is a design compromise.

Take away glass and it can't cover the frame completely, it distorts or there are aberrations robbing sharpness.

Then you have to compensate with other features of your design.
12-07-2017, 11:14 PM   #3
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QuoteOriginally posted by hadi Quote

case in point: pentax DA 50mm F1.8 vs pentax-FA 50mm F1.4
the 1.8 has a front element of 52mm vs 49mm of the 1.4.
what does a front element size do?
The DA 50mm f/1.8 is designed to create a smaller image circle for APS-C sensors, whereas that lens would vignette with a larger FF sensor. The FA 50mm f/1.4, in part, is larger because of the larger aperture and the larger image circle needed to not vignette for the FF sensor.

Your link for the DA 50mm f/1.8 on Adorama shows that they are incorrect in listing that lens as Full Frame. It's an APS-C lens.

If you compare the Canon 50mm f/1.8 (49mm) vs 50mm f/1.4 (58mm) vs. 50mm f/1.2 (72mm), it also shows how just half stops/EV impacts size.
12-08-2017, 01:17 AM - 2 Likes   #4
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The filter size does not necessarily indicate how big the elements (especially front) are. Take the A and M series of 50mm lenses. The f2; f1.7; and f1.4 all have 49 mm filter threads, but the 1.4 lenses have more glass. M 50/2 | The K-Mount Page M 50/1.4 | The K-Mount Page

12-08-2017, 02:44 AM   #5
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Front element size only a lens design property. For larger format (image circle) with larger max aperture it's easier (and cheaper) to design high quality lenses with larger front element in general.
I'am trying to get lenses with 49mm filter thread because I can use my filter set. (Smaller filters are somewhat cheaper in addition.) Pentax has a great line up of fast small primes with 49mm filter thread: DA15, A28, DA35, FA43, FA50, FA77, DFA100.
12-08-2017, 03:32 AM   #6
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QuoteOriginally posted by hadi Quote
yes, i totally asked that.

anyways, just wondering if some of you can shed some light to on this for me.
i've seen some lenses that are TINY, and others that are massive. yet their size doesn't related to their focal length. IE: the Tamron 90mm macro lens is pretty big lens when you compare it to the Pentax 100mm WR macro lens. all the while the 85mm by any company is usually much bigger than both of them.

yes, i get that the f-stops do vary significantly. and i get that wider open lenses tend to be bigger lenses, but it still doesn't corealte to the focal length.

case in point: pentax DA 50mm F1.8 vs pentax-FA 50mm F1.4
the 1.8 has a front element of 52mm vs 49mm of the 1.4.

and i get that often times, the bigger the front element, the more expensive the lenses are..usually. but aside from being more expensive, heavier, and bigger, what does a front element size do?
I've got both lenses. You are referring to the filter thread sizes where the 1.4 is 49mm and 1.8 is 52mm. The 50 1.4 has a larger front element by at least 1/3rd larger than the 50 DA 1.8. Although the DA 50mm 1.8 is claimed to be an apsc lens, if you look at its lens review here on PF, people do use it successfully on the K-1. I'm not gonna put a ruler to the front elements, but yes the 50mm 1.4 front element is significantly larger.

I believe the FA50mm 1.4 is based on the old design from manual era lenses (don't quote me), and a lot of those used 49mm filter threads. I guess it made life easier if most of your lenses took 49mm filters. Numerous zooms also had 49mm filter threads over the decades.

Why did front element size get larger? To improve corner sharpness. The older film era lenses were great, but as photographers became more fussier, they wanted better corner sharpness...........

But do they get this? Not on every lens. Because "Portrait" lenses are supposed to have softer edges and corners, that's the whole point of a portrait lens. I think I've seen a review of either the Canon 50mm 1.2 or 85mm 1.2, and the edges and corners are significantly softer than the center, disappointingly so ......but thats a portrait lens.

I also have the Sigma 50mm 1.4 with a massive front element, and against the FA50mm 1.4, the Sigma lets in more light, how much, I still haven't bothered to check. But in manual and on the same settings, the Sigma will overexpose slightly. I haven't done a show down with both lenses, because first and foremost, the Sigma is probably triple the weight of the FA 50mm 1.4.

If you are wondering which lens is better, the 1.4 or 1.8, then thats hard to determine. Like everyone else on this forum, we have a 50mm addiction, I have 5 fast 50's. The FA 1.4 will let in more light in certain situations that can really help. The DA 1.8 has really nice Bokeh and rounded aperture blades for round specular highlights (Bokeh light balls). The F and FA 50 1.7 is really sharp at 1.7 and has great bokeh.

Get a 1.4 for when you need more light. A 1.7 when you want sharpness wide open. A DA 1.8 for better specular highlights. (Sigma doesn't have the best color rendition IMO but still a great lens). Its something you really need to research for yourself, but I would not be swayed by the canikon crowd who crave this edge to edge sharpness in a portrait lens - they're missing the point. If you really want edge to edge sharpness and a humongous lens to drag around, you won't want to use it as much as one which is discrete and lightweight.
12-08-2017, 04:11 AM - 1 Like   #7
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QuoteOriginally posted by Alex645 Quote
The DA 50mm f/1.8 is designed to create a smaller image circle for APS-C sensors, whereas that lens would vignette with a larger FF sensor. The FA 50mm f/1.4, in part, is larger because of the larger aperture and the larger image circle needed to not vignette for the FF sensor.

Your link for the DA 50mm f/1.8 on Adorama shows that they are incorrect in listing that lens as Full Frame. It's an APS-C lens.
It's Full Frame, Alex, this has been known for years and reported on this forum.

There is a thread where I posted pics of a day in a park where I used only the K-1 and the DA50 and DA35.

Before the K-1 was even released, I had used the DA50 on my Sony A7.

These plastic fantastics are a great pair of sharp, cheap primes Pentax can be proud of.

They of course have 35mm heritage in the M50 f1.7 and FA35 f2.
12-08-2017, 04:28 AM - 1 Like   #8
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As others have noted, the diameter of the front element is a function of focal length and maximum aperture. If the focal length is fixed, the diameter must increase substantially to add one larger f-stop. Another effect is diffraction. Theoretically (emphasis: IN THEORY) a "perfect" lens will have its best performance wide open because as you reduce the diameter of a circular opening through which light is passing, the amount of diffraction increases. This is most apparent in photographic lenses when they are stopped down to about f11~16. Almost all lenses (almost) have substantially reduced IQ at f22 because of diffraction, commonly unacceptable for most photographers.

BUT, BUT it isn't quite that simple. A 400mm f2.8 lens has a huge front element, but doesn't necessarily have better IQ, than a 50mm f4 macro with a tiny front element. And another effect - camera manufacturers seem to put special effort into lenses with a large f-stop because those are "pro" lenses. If you read through lens tests, you'll commonly find that the larger the maximum f-stop for a lens of particular focal length, the better the IQ rating - - 28mm f1.4 better than 28mm f2,8; 85mm f1,4 better than 85mm f1.8; 300mm f2,8 better than 300mm f4; 70~200mm f2.8 zooms better than 70~200mm f4 zooms PROVIDED the lens pairs are from the same manufacturer. Go to the DXO Mark website and look through the list of highest rated lenses and take note of their maximum f-stop. How many are f1.4 to f2 versus how many are f2.8 to f4?

BUT, BUT there are many exceptions. The modest focal length, modest aperture, bargain-priced 40mm f2.8 has excellent IQ, but it isn't, well, as "sexy" as a 50mm f1.2. Macro lenses most commonly have a maximum aperture of f2.8, but they are generally regarded as among the sharpest lenses available.


Last edited by WPRESTO; 12-08-2017 at 04:35 AM.
12-08-2017, 04:32 AM - 1 Like   #9
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I think a better example of what OP is asking would be lenses like FA 50mm f1.4 and Sigma 50mm f1.4 art (or the new DFA* 50mm f1.4, by the looks of it). Both are 50mm, both are f1.4. But one is much bigger, which makes a lot of people wonder how they can still have the same focal length and aperture. In this case, its not just filter threads, but actual amount of glass

The answer is that we are not dealing with simple lenses, but very complex ones with many elements, special materials.

Last edited by Na Horuk; 12-08-2017 at 08:05 AM.
12-08-2017, 04:53 AM - 1 Like   #10
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QuoteOriginally posted by clackers Quote
Before the K-1 was even released, I had used the DA50 on my Sony A7.

These plastic fantastics are a great pair of sharp, cheap primes Pentax can be proud of.

They of course have 35mm heritage in the M50 f1.7 and FA35 f2
How did you manipulate the aperture on the Sony? Did the physical adapter (for mount and register distance) have an aperture clicker?

As for the 35mm heritage, whenever I put my A-era deglassed teleconverter behind the DA50/1.8 as an extension tube, it reports the DA50/1.8 wide open as an f/1.7 lens. Perhaps it remembers the good old days?
12-08-2017, 04:55 AM   #11
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QuoteOriginally posted by pathdoc Quote
How did you manipulate the aperture on the Sony? Did the physical adapter (for mount and register distance) have an aperture clicker?
Irritatingly, the Novoflex is clickless.



12-08-2017, 04:59 AM - 1 Like   #12
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QuoteOriginally posted by clackers Quote
Irritatingly, the Novoflex is clickless.
That's probably because it was designed by and for videographers.
12-08-2017, 06:54 AM - 1 Like   #13
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@pathdoc it isn't just video that drives this, the way Pentax aperture works requires a complicated lever action to accurately reflect f stops; in part this is due to the fact that the travel distance to aperture ratio changed between K/M series and A series so accuracy is complicated by this. Most - if not all adapters are stepless. The m43 adapter I have for DA lenses is also stepless and while it makes picking a particular aperture a bit more difficult it works ok.
12-08-2017, 07:21 AM   #14
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QuoteOriginally posted by UncleVanya Quote
@pathdoc it isn't just video that drives this, the way Pentax aperture works requires a complicated lever action to accurately reflect f stops; in part this is due to the fact that the travel distance to aperture ratio changed between K/M series and A series so accuracy is complicated by this. Most - if not all adapters are stepless. The m43 adapter I have for DA lenses is also stepless and while it makes picking a particular aperture a bit more difficult it works ok.
It would still be nice to be able to consistently pick some aperture with a given lens, e.g. 5 clicks with the da50/1.8 always gives f/3.1, on the da35/2.4 it might always give f/4.02.
12-08-2017, 07:31 AM - 1 Like   #15
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QuoteOriginally posted by dcshooter Quote
If a lens has no chip or is used on a pre-AF body (or as in your case, the chip does not communicate with the body via the tube), the aperture range is encoded by the series of conductive and non conductive surfaces on the lens mount that align with the contacts on the camera mount, and there is no combination to encode for f/1.8, so it uses the closest value. Bojidar Dmitrov has a listing of the various combinations on his page:

Features and Operation of the Ka Mount

---------- Post added 12-08-17 at 07:12 AM ----------



It's not that it's particularly complicated, it's that it's very precise (and as of the A series, directly proportional to the arc of travel of the aperture lever). That's why a slightly bent aperture lever can have large effects on the accuracy of image exposure. For Pre-A lenses as well as later lenses when not in A mode, the aperture lever is just on/off, with the distance traveled having no consistent relationship to the amount the lever closes. It just travels until it hits a hard stop as determined by the position of a cam inside the lens.
I can't find the info now but the site I used for information claimed that pre-A series lever distance traveled was proportional to aperture diameter, whereas lever distance traveled in the A series is proportional to aperture area. When I said complex that's a slight misuse of the word, the physical action is simple; calibration and correlation to exact aperture is complicated.

---------- Post added 12-08-17 at 09:36 AM ----------

QuoteOriginally posted by BrianR Quote
It would still be nice to be able to consistently pick some aperture with a given lens, e.g. 5 clicks with the da50/1.8 always gives f/3.1, on the da35/2.4 it might always give f/4.02.
Sure, sadly few offer this. One approach is to dedicate an adapter for each lens and slowly map the aperture to the specific lens and adds marks on the adapter to indicate the stops. One way to do this is to use a consistent light source lighted scene, meter wide open, then slowly stop down until you see a 1 stop difference in the meter result, then repeat this until you have enough stops marked. Of course this still doesn't give tactile feedback and the need to look to see what aperture is set is annoying.

Last edited by UncleVanya; 12-08-2017 at 07:37 AM.
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