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09-07-2008, 11:11 AM   #1
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Best lens for low light AF

After reading a comment about the FA 50 f1.4 not being a good lens for low light AF that took me by surprise, I thought I would look at the lens MTF numbers to see if this comment was warranted.

I've used Photozone.de's wide open MTF data for prime and zoom lenses for Pentax to create a model to show possible AF performance ranking of the twenty four lens that have focal lengths between 18-77mm,

My assumptions are:
  1. AF is done with the lens wide open and MTF data is a measurement of both resolution and contrast of a lens.
  2. As the AF mechanism is optimized for 2.8, I would need a modifier to account for slower lenses. The modifiers are 1 for a lens 2.8 or faster, 0.5 for a f3.2/3.5/4 lens, 0.25 for a f4.5 lens, and 0.1 for a f5.6 lens.
  3. As a zoom lens is probably not as optimized as far as the AF mechanics in the lens, I thought a penalty for being a zoom was needed. I used 0.1 for this. A prime lens will have a 1 modifier and a zoom will have a 0.9 modifier.
  4. The final assumption is that the highest combined Center and Border MTF that takes into account lens speed and lens type would provide a ranking of lens AF performance for the twenty four lenses listed.

I put this together in a google document. The data sorted by Center and Border modified and according to this model, the Pentax SMC-DA 40mm f/2.8 is the best lens for low light AF. The FA 50 f1.4 would be thirteenth on the list of twenty four lenses.

From this I would say the poster comment is valid. Any comments about an issues with this? Is there another model, or another factor, needed that would describe AF performance better?

Thank you
Russell


Last edited by Russell-Evans; 09-07-2008 at 12:02 PM. Reason: grammer - fix broken link
09-07-2008, 05:32 PM   #2
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No discussion on this at all? I was hoping someone would call BS or post it matches their experience. Without a correlation to real world experience, it doesn't mean anything.

Thank you
Russell
09-07-2008, 05:34 PM   #3
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QuoteOriginally posted by Russell-Evans Quote
No discussion on this at all? I was hoping someone would call BS or post it matches their experience. Without a correlation to real world experience, it doesn't mean anything.
looks like a lot of work you did, i guess you're looking for some calibration or more factors in your rankings?

i think the fastest AF lenses on pentax are the DA40 and FA35 (faster than FA50) and it seems like those lenses are right up there.

does this match your own experience?
09-07-2008, 05:40 PM   #4
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I'll comment: I have no idea what you are talking about. Do MTF numbers contain data about AF speed? I had no idea. The only way I would know to test AF performance would be to take a picture of a stop watch to see how long it actually took.

Or, by "performance", do you mean, how *good* a job it does, as opposed to how *fast* ? In that case, I guess resolution / contrast numbers do have *some* relevance, but really, I wouldn't assume anything. I'd just measure resolution myself using AF and MF and see how how much AF suffered by comparison.

Or maybe you're going on the assumption that high resolution and contrast would always lead to "good" AF performance, and that in general these things were well-correlated. But it seems to me the mass of the lens, and the amount of rotation required, would be far more important determining factors in determining focus speed.

So I'm kind of at a loss to understand what you are actually trying to measure here.

To which, I'll add: if your conclusion is that the DA40 focus is both exceptionally fast and accurate, I'd certainly agree based on experience.

09-07-2008, 06:00 PM   #5
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QuoteOriginally posted by Marc Sabatella Quote
I'll comment: I have no idea what you are talking about. Do MTF numbers contain data about AF speed?
Reading the Wikipedia page on autofocus; passive AF is based on contrast or phase detection.
QuoteQuote:
Passive systems may not find focus when the contrast is low, notably on large single-coloured surfaces (walls, blue sky, etc.) or in low-light conditions. Passive systems are dependent on a certain degree of illumination to the subject (whether natural or otherwise)
The idea is that either of these methods will be helped by a lens with better MTF numbers as it means the AF sensor has more contrast to work with. Of coarse little will help in shooting a blank wall.

Thank you
Russell

Last edited by Russell-Evans; 09-07-2008 at 06:22 PM. Reason: clarification
09-07-2008, 06:15 PM   #6
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QuoteOriginally posted by k100d Quote
looks like a lot of work you did, i guess you're looking for some calibration or more factors in your rankings?

i think the fastest AF lenses on pentax are the DA40 and FA35 (faster than FA50) and it seems like those lenses are right up there.

does this match your own experience?
I don't have the FA 50mm f1.4, nor the DA 40mm.

If you have the FA 50mm f1.4 would you mind doing an experiment for me? The FA 50mm f1.4 has a aperture ring. Manually setting the ring to 2.8 will give the FA 50mm f.14 MTF numbers of: center 2088, border 1766.5, and extreme 1766.5. The experiment is to shoot a scene where the FA 50mm f1.4 is slow to focus when set to A on the aperture ring, and then to try the same scene with the FA 50 f.14 with the aperture ring manually set to f2.8.

I'm not looking exact numbers, but just wondering if it has a noticeable impact on speeding focus acquisition. The FA 35mm f2 has a MTF of: center 2040, border 1867, and extreme 1867, so by manually setting the FA 50 f1.4 to f2.8, maybe you'll get the AF performance of the FA 35? There will of coarse be a point where the amount of light getting through the lens means more than the contrast, but I would think that would be at pretty low EV numbers. At least lower than shooting your kids around the house at night.

Thank you
Russell

Last edited by Russell-Evans; 09-07-2008 at 07:06 PM. Reason: adding more thoughts
09-08-2008, 09:45 AM   #7
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BTW, sorry, I didn't mean to sound harsh - I was just extremely confused. But it does seem that one of my guesses was correct - you are using MTF data to attempt to predict how well the AF system will do, on the theory that AF performance will be more or less directly proportional to some function of sharpness & contrast.

Interesting approach, but I still suspect that the mechanics of the issue - the mass of the lens, the degree of rotation necessary - are going to be significant factors as well. Probably the way in which the rotation is damped affects this to, unless that is perhaps disabled for AF. Anyhow, the DA40 of course also wins big time here - tiny mass, very small angle of rotation necessary. Combine that with how well it does in the MTF model suggests that it is probably heads and shoulders above any other lens. Again, nothing in my experience suggests this isn't true - it is indeed pretty amazing at focusing in low light (for a Pentax :-). But I have only a few AF lenses to compare it against.
09-08-2008, 09:54 AM   #8
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QuoteOriginally posted by Marc Sabatella Quote
Interesting approach, but I still suspect that the mechanics of the issue - the mass of the lens, the degree of rotation necessary - are going to be significant factors as well.
Pentax FA50/1.4 can deliver >= 3 FPS under the good daylight... so for as long as the same AF module can get the work done in dim daylight (_OR_ dim tungsten) the same lens can be equally fast in those conditions too... may be slower mecanically vs DA40 - but the problem is in AF module (not the screw drive motor) and how it handles the dim/tungsten light from specific lenses.

09-08-2008, 09:56 AM   #9
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speaking about MTF

http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf

QuoteQuote:
In some cameras (such as the Canon EOS 20D), at one or more particularly important
AF point locations across the frame (typically the one at the center), two
autofocus detectors are provided, one workable for modest aperture lenses and one
(with a “larger baseline”) workable only for larger aperture lenses. The appropriate
one is put into action based on the maximum aperture reported by the lens in place.
This arrangement exploits the greater precision in focus determination available
with a larger aperture lens in place.
but not Pentax...
09-08-2008, 10:13 AM   #10
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I don't know much about the MTF but I always count on my FA 50 f/1.4 for indoor and low light shooting. The shallow DOF in wide open aperture is always a challenge to get a good shot with sharpness and clarity. So I default my use of FA 50 f/1.4 to f/2.0 as a go-to aperture in low light shooting.

And in one time while visiting an aquarium, I have no tripod and I felt the need to go for f/1.6 and f/1.4 to make the shot with moving jelly fish. My other lens with f/2.8 aperture couldn't make the shot at all.


Jelly Fish with Pentax FA 50 f/1.4 - Hin's Tech Corner

1/20 sec, f/1.4, 50mm, iso 200, 0 Ev


1/30 sec, f/1.4, 50mm, iso 200, 0 Ev


I really like the prime with Pentax FA 50 f/1.4, I find it the best value lens for the money spent. The AF is not the fastest but I find it reasonable to use for night time photo. And it is excellent to use for portraits and daytime shooting. The bokeh and color is excellent. There are definitely better lens more suited for low light but for the best value, I will always recommend FA 50 f/1.4.
09-08-2008, 02:03 PM   #11
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QuoteOriginally posted by deejjjaaaa Quote
Pentax FA50/1.4 can deliver >= 3 FPS under the good daylight
That's not really relevant to my point, though. When shooting in burst mode, the mass of the lens and the angle of rotation don't really come into play except for the first shot of the burst. After that, it's normally making only very small adjustments if any. Also, I'm obviously not saying the FA50/1.4 is a particularly massive lens - of course, it isn't - and I have no idea what its angle of rotation is. If it is slow to focus in low light, that could indeed be primarily be due to resolution and contrast. I'm just observing that in general, those other factors could play a role. That is, there might be a lens as sharp and contrasty at the relevant apertures as the DA40, but would probably focus much more slowly if it were also much bigger and required, say, around 270 degrees of rotation like the old M lens generally did, rather than less than 90 like the DA40.
09-08-2008, 02:19 PM   #12
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QuoteOriginally posted by Marc Sabatella Quote
That's not really relevant to my point, though. When shooting in burst mode, the mass of the lens and the angle of rotation don't really come into play except for the first shot of the burst. After that, it's normally making only very small adjustments if any. Also, I'm obviously not saying the FA50/1.4 is a particularly massive lens - of course, it isn't - and I have no idea what its angle of rotation is. If it is slow to focus in low light, that could indeed be primarily be due to resolution and contrast. I'm just observing that in general, those other factors could play a role. That is, there might be a lens as sharp and contrasty at the relevant apertures as the DA40, but would probably focus much more slowly if it were also much bigger and required, say, around 270 degrees of rotation like the old M lens generally did, rather than less than 90 like the DA40.
I do agree that lens construction (focus throw, weight, etc) plays not a small role - however the mechanics of FA 50/1.4 is not an obstacle under the good light, where AF acq./control module can guide the in body motor w/o much hunting... that was my point that AF module is to blame first of all.
09-08-2008, 03:02 PM   #13
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The choice of AF mode may be a factor in low light conditions. I currently use either my FA 50mm f1.4 or Sigma 20-40mm f2.8 set to center focus.
09-10-2008, 03:01 PM   #14
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QuoteOriginally posted by Russell-Evans Quote
I don't have the FA 50mm f1.4, nor the DA 40mm.

If you have the FA 50mm f1.4 would you mind doing an experiment for me? The FA 50mm f1.4 has a aperture ring. Manually setting the ring to 2.8 will give the FA 50mm f.14 MTF numbers of: center 2088, border 1766.5, and extreme 1766.5. The experiment is to shoot a scene where the FA 50mm f1.4 is slow to focus when set to A on the aperture ring, and then to try the same scene with the FA 50 f.14 with the aperture ring manually set to f2.8.

I'm not looking exact numbers, but just wondering if it has a noticeable impact on speeding focus acquisition. The FA 35mm f2 has a MTF of: center 2040, border 1867, and extreme 1867, so by manually setting the FA 50 f1.4 to f2.8, maybe you'll get the AF performance of the FA 35? There will of coarse be a point where the amount of light getting through the lens means more than the contrast, but I would think that would be at pretty low EV numbers. At least lower than shooting your kids around the house at night.

Thank you
Russell
This will have no affect. Focusing and metering is always done at max aperture, the lens only stops down just before and during exposure.
09-10-2008, 03:05 PM   #15
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QuoteOriginally posted by Blue Quote
The choice of AF mode may be a factor in low light conditions. I currently use either my FA 50mm f1.4 or Sigma 20-40mm f2.8 set to center focus.
This is a good point. Most lenses are sharpest (and will have most contrast) in the center, and will have relatively soft borders wide open. Relying on other AF points here may not help the situation.
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