[Na Horuk]

Oh yeah, using a lens with a much bigger image circle can cause an overall loss of sharpness due to extra light bouncing around the mirror box. This can generally be fixed by using a tighter (equivalent) hood on the lens. And with modern Pentax lens coatings this isn't a big issue anyway. People had this problem when adapting old 645 lenses on APSC cameras, if I remember right

[Buffy]

No specific advantage, just that there are two good FA lenses, the FA 35/2 and 50/1.7 and the limiteds. But the DA 35 and 50 have come to replace them on aps-c.

That's a rather broad statement and I just can't agree at all. All my Pentax branded lenses are FA series and I am happy with all of them.

[pakinjapan]

Is there any advantage to using an FA lens on an APS-C body so far as image quality is concerned?
Thanks

I can't say for "all" DA vs FA lenses but I used to own a FA20 and now DA21, so I think I will jump into this conversation based on what I see in the field btw 20 and 21 only.

Sharpness: When look at a real-life images, I don't see a gain or loose between FA and DA. Both are very sharp on digital APSC. The DA seem to have a bit more color and contrast.
Speed: FA is faster and I like it.
Chromatic Aberration: this problem can be fixed in post, but Yes, it is more pronounced in FA.
Flare: This one too, more pronounced in FA. @photoptimist already mentioned a hood can help.
Size: FA is bigger but I like its size more than the DA pancake. It fits better on my hand. I like small lenses but not a pancake small.
Price: Limited DA 21 is a lot cheaper + it offers way better build quality than secondhand FA20. note that FA20 is a ff lens but DA is not.

Are you looking to buy a lens and thinking about getting a FA or DA version? let's us know what lenses are you comparing because their are a lot of FA and DA with the same or similar focal length. Some cheap DA won't do a better job than well design Limited or legendary FA and vice versa.

[GlassJunkie]

If you like light weight and acceptable images on an APSc camera, the DA/DC/etc. class lenses will do fine. If you want really good images on an APSc camera buy the better APSc glass (DA*, pro/ better quality Siggies and Tamrons, or Zeiss with MF). I was a diehard DA* user and defender. At issue were the 4 motors I put in a 16-50 DA* (albeit with very heavy commercial level usage). My wife's "pry my cold dead hands off it" lens is the DA 18-270. She is a well published and award winning pic taker, and for other than long critters, her go to lens. Before that it was the 18-250, not as good a build. DA*s are GREAT lenses, as are the limiteds...

On FF on APSc, I had heavy CA on a FA 85 f1.4, some on F300 F4.5 and a bit on FA*300 F2.8. My wife has little issue with her Siggy 150-500 DG (GEN2, a key difference). Well controlled lens, and FF. I recently switched to the DFA Trio (15-30, 24-70, 70-200 DA*) and have been using the DA 150-450 on a K3 for years. I switched for 2 reasons, build quality and newer coatings, both helpful in the field. Backpack weight went up 2 lbs across the 3 lower range and the DA solution. This is MORE TO LUG. I got 1/2 stop from 135-200mm (F2.8 vs F4), important in what I shoot. Newer coatings perform better, better controlled flare and CA. Have the option for FF if I want a different DoF profile, but due to wildlife, I'm a dedicated APSc shooter. Yes, a tiny bit more crisp at the edges, less vignetting in the corners with wider glass...

No downside to FF on APSc, except weight and cost, older ones will show more CA (even GREAT lenses), upside is bigger apertures (depending on the solution), maybe better on edges and in the corners. Like Norm said, just a smaller image from the center...

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Last edited by GlassJunkie; 06-14-2017 at 08:34 AM.

[Boydmaine]

My sincere thanks to everyone who replied.
boydmaine

[stevebrot]

Oh yeah, using a lens with a much bigger image circle can cause an overall loss of sharpness due to extra light bouncing around the mirror box.

I have read this many times since first going APS-C in 2007. Despite it being convention wisdom, I have nagging doubts. Those doubts are based on the fact that the image circle of most lenses made for rectangular formats will extend to the walls/floor of the mirror box and underside of the mirror on most SLRs. (For 35mm FF SLRs, there will be at least 6-9mm top and bottom and very likely 3mm or more on either side.) This is even more of a concern with perspective control lenses (all have VERY generous image circles) and most primes when doing close focus and macro work. In addition, both film and digital sensors are reflective and even light within the frame will find its way back into the mirror box.

To reduce this problem, some lenses (mostly longer focal lengths) have a frame-shaped baffle built into the rear of the lens. More importantly, a well-designed mirror box will feature baffles, light-absorptive finish, and/or flocking to dampen reflection.*

As for adapted medium format lenses, my memory is that the issue is reflections from the internal surfaces of poorly-designed adapters. Most of the light from the larger image circle hits the adapter surfaces. The same thing can happen when using small format lenses with extension tubes.

This can generally be fixed by using a tighter (equivalent) hood on the lens.

Yes, one may use a deeper hood with FF lenses on crop format with less risk of vignette. I generally do so, though with the intent of flare reduction in general rather a concern about image circle.

And with modern Pentax lens coatings this isn't a big issue anyway.

I am not sure how this relates to image circle and mirror box reflections except, perhaps, in regards to reflection off the film/sensor onto the rear element. A large flat rear element when coupled with a poor on non-existent coatings on that surface may definitely create potential for reflections back onto the film/sensor in the form of a "hot spot" or veiling flare. This is a significant issue with lenses designed for mirrorless cameras as well as some SLR lenses. I have one lens on my shelf (Auto Rikenon 55/1.4, M42) that will sometimes render a "hot spot" into the frame with subjects where most of the frame is bright. Properly coated rear and internal elements will allow that errant light to exit the front of the lens with no harm done.


Steve

* As I am typing this, I have memories of a snow scene photo that made its way into my high school year book. The photo features a bright band across the bottom of the frame courtesy of the underside of a poorly-flocked mirror carrier on my Ricoh Singlex TLS.