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03-19-2011, 11:41 AM   #1
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Real-Life FA*24mm Sharpness Torture Test

I just got this lens on Wednesday and have been playing around with it, trying to learn how to get the most out of it. I've learned through experience that every lens has its own personality that you have to get to know before you really become confident when using it. As part of this process, I've learned that:
1) This copy backfocuses, relative to my other lenses (requires +200um adj vs. +70 for my other lenses).
2) This lens has some field curvature - my first shots of flat subjects were outright blurry in the middle and sharp starting about half way out to the corners.
3) It's spherically corrected such that the foreground "bokeh" is exceptionally smooth, whereas the background bokeh is, um, not.

I've taken the following shots as a sort of sharpness "torture test", using leafless trees instead of a test chart. Despite the boring, colourless scene, there is lots of fine detail throughout the frame to allow sharpness (both resolution and contrast) to be judged. Focus is on the largest tree in the central portion of the frame. All images were shot in rapid succession without adjusting focus. All shots were processed identically in Lightroom 3.3 (WB adj and my default "S" tone curve).


F2:

http://farm6.static.flickr.com/5258/5540044555_a054235159_o.jpg

F2.8:

http://farm6.static.flickr.com/5015/5540051821_68b8c8e13a_o.jpg

F4:

http://farm6.static.flickr.com/5017/5540640610_6b109e7dea_o.jpg

F11:

http://farm6.static.flickr.com/5019/5540663634_faf18bef8d_o.jpg

In these images, you can see:
1) Purple fringing and resolution loss in the upper corners of the frame at larger apertures
2) Lateral CA in the upper corners at all apertures (correctible in PP, but I haven't done this here)

Overall, I'm impressed with this performance except at F2, where there appears to be a "fogginess" throughout the image. It's possible that this may be a focus error on my part, albeit a slight one. The fogginess is most pronounced at further distances from the camera. F2.8 is much "clearer."

Any insights or comments from other FA*24 owners? Is this fogginess I'm seeing here a matter of technique, or an inherent characteristic of this lens? If it's a technique issue, what do you do to avoid this (aside from shooting at smaller apertures)?

Thanks!

03-19-2011, 05:58 PM   #2
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QuoteOriginally posted by Hello_Photo Quote
Any insights or comments from other FA*24 owners? Is this fogginess I'm seeing here a matter of technique, or an inherent characteristic of this lens? If it's a technique issue, what do you do to avoid this (aside from shooting at smaller apertures)?

the "fogginess" is astigmatism even the canon 24mm f/1.4LII suffers from this to a degree. Aspherical and ED glass elements would reduce and depending on their quality, eliminate it however the price for such a lens would sky-rocket because Aspherical lenses and ED glass are expensive.

All you have to know it to stop the lens f/8 or f/11 to counteract this effect and use the appropriate post-processing to eliminate lens aberrations.
03-19-2011, 06:07 PM   #3
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QuoteOriginally posted by Hello_Photo Quote
In these images, you can see:
1) Purple fringing and resolution loss in the upper corners of the frame at larger apertures
2) Lateral CA in the upper corners at all apertures (correctible in PP, but I haven't done this here)
Sharpness looks great from wide open to me.

I think you would see a lot less of these aberrations if you exposed properly for the sky (looks blown out to me).

Looks like a killer lens.
03-20-2011, 02:27 AM   #4
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I've had FA*24 for just over 6 months now. My most used lens in that time. Love the FOV.

IQ of this lens is great if you ask me. Sharpness is very good right from f2 and only gets better as you stop down. I atribute corner softness to field curvature. The "fogginess" is there at f2, underexposing a bit usually helps a smidge but it won't kill it off. The only way to avoid it is to stop down. If you want to know bit more about what I think of this lens, look at my thread called About FA*24.

03-20-2011, 07:50 PM   #5
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QuoteOriginally posted by Digitalis Quote
the "fogginess" is astigmatism even the canon 24mm f/1.4LII suffers from this to a degree. Aspherical and ED glass elements would reduce and depending on their quality, eliminate it however the price for such a lens would sky-rocket because Aspherical lenses and ED glass are expensive.

All you have to know it to stop the lens f/8 or f/11 to counteract this effect and use the appropriate post-processing to eliminate lens aberrations.
I find that by f5.6 this lens gives massive depth of field and great sharpness.

I have used it stopped down to as much as F22 without noticeable problems. But I don't pixel peep, so your mileage will vary.
03-20-2011, 08:14 PM   #6
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QuoteOriginally posted by axl Quote
I atribute corner softness to field curvature. The "fogginess" is there at f2, underexposing a bit usually helps a smidge but it won't kill it off. The only way to avoid it is to stop down.
I agree re: corner softness and field curvature. I'm not sure that field curvature is necessarily a bad thing for a wide-angle lens; makes focus and recompose techniques more reliable. Also, thanks for the tip about under exposing - I did this instinctively at first, thinking that I had messed-up the metering by not dialing-in negative EV compensation for darker subjects. Now I know I'm on the right track. The good news is that by F2.5, the astigmatism is minimal and it's not visible (to me) at F2.8 and beyond.

QuoteOriginally posted by paperbag846 Quote
I think you would see a lot less of these aberrations if you exposed properly for the sky (looks blown out to me).
True. That's part of the reason why this is a torture test. I'm not convinced that any lens could tolerate this type of misuse (subject, exposure and pixel-peeping) without visible anomalies.

QuoteOriginally posted by Digitalis Quote
Aspherical and ED glass elements would reduce and depending on their quality, eliminate it however the price for such a lens would sky-rocket because Aspherical lenses and ED glass are expensive.
The FA*24 already has an aspherical element (that's the "AL" in it's official name). I'll just take this as a sign that the FA*24 pushed the limits of lens design about as far as possible (in its day - circa 1990).
03-20-2011, 08:19 PM   #7
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The strength of the Fa 24 f2 is its large aperture and great center performance, along with accurate colors--no doubt. After that there isn't a lot to write home about---borders are very weak, until 5.6 and finally good @ f8.

Shoot the lanscapes @ f8 or smaller.

http://kmp.bdimitrov.de/lenses/primes/ultra-wide/FA24f2.html

Last edited by Jewelltrail; 03-20-2011 at 08:29 PM.
03-20-2011, 11:54 PM   #8
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Ernest, your mileage may vary but I can honestly tell you that my copy doesn't have very weak corner and that at f2 they are acceptable and they get good way before f5.6!

03-21-2011, 01:42 AM   #9
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I'm pretty happy with my copy..

Pentax FA*24 f2.0 - a set on Flickr

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