Originally posted by hadi 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.