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Just want to know if the AF inaccuracies on both OEM & third party lenses can be overcome by manually focusing the lens thru focus confirmation?
The sigma 50mm 1.4 does have FF & BF issues like what users have reported. To overcome these issues, should the photographer use manual focus to get the precise focusing??? Or does it still FF & BF even if you manual focus them??
Please help me out. Im getting confused![8-)]
Good question and I guess it depends a lot on what copy you get. Believing Bryans review on the sigma 50mm it had front and backfocus issues. Anyway I still wanted to try it out because I saw more potential in it than in the Canon version. To be honest my lens needed microadjustment of +9, which ment it was front focussing. But after that I really enjoy it and I didn't miss any shots due AF yet. Yes I missed some shots, but mostly because movement of me or the subject, depth of field is really really shallow at 1.4 on FF.
If you buy from a local business and you could take it back within let's say 1-2 weeks if it doesn't suit you...I say it is definitely worth trying! At least I'm very happy that I did!
Oh and manual focussing at 1.4 isn't as easy as it seems. If your subject is close, sure manual focus isn't a problem. But let's say you photograph your girlfriend who is standing at about 10m/100ft and you want to shoot at 1.4 it is really hard to do that whit your bare eyes. AF or liveview manual focussing really come in handy in those things.
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Originally Posted by sirhc_1
That would be up to you [:P] Short answer: no it won't. Longer answer...if it does, it's not the camera nor the lens [;)]
Jan
Thanks Sheiky for the advice! I'll definitely try that. I just installed an Ef-S screen on my 40D & its much easier doing manual focus with it. The reason why I was asking the above is bcoz Im planning to go for a Zeiss Planar 50mm 1.4 ZE or a Zeiss Makro Planar 50mm 2 ZE. Both of the are manual focus. So its gonna be difficult to recognize if my copy will be crappy. Don't you think so?
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Originally Posted by sirhc_1
No it won't matter to you in any way. AF only has to deal with focus-issues. With manual focus, the focus is entirely up to you: what you see is what you get! So don't worry about that.
I only stated that manual focus might be more difficult at big apertures <f2 and with a reasonable subjectdistance. Like more than 5m. For closedistance-focussing, I'm thinking of headshots/headshoulder portraits and full body shots, manual focus is really do-able and perhaps more consistent and easier to do than AF. You can compare these shots with macro-photography where AF really is something you barely will use, because the shots are too difficult and change too much. At least my AF is off like 99% of the time on my 100mm macro. The other 1% is where I might use the lens for other purposes than macro-photography.
Can I ask why you want the Zeiss so badly? They only offer manual focus (alright I suppose you can live without it), what do they offer more than say a sigma or a canon 50mm 1.4? I see prices are much more expensive, do they really offer that much more image-quality or build-quality? Or is it just the name?
Jan
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Originally Posted by Sheiky
I myself can't explain as well. When I visited one of my local stores, I tried the "legendary" Distagon 21mm/2.8 & the Makro Planar 50mm/2. I must say that the images are really impressive & gives that "Zeiss" color rendition alongside its "3D" IQ. The build of these lenses are on par or even better than the Canon's L series that are not weather-sealed. I also had the same dilemma between the Canon & Sigma. I did own 2 Canon 50 1.4 & their micro USM gave up on both occasions. While Sigma is sharp wide open, sometimes it FF & BF as well. And quite heavy as well.
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Originally Posted by Sheiky
So, you're saying that lens-sepcific front- or back-focus AF issues will not be a factor if you manually focus. Ok, that makes sense if you're relying on your eyeball view of the subject, or you've changed out your focusing screen for one that provides more visual assistance (if possible on your body, which it's not on my 7D).
But, whatsirhc_1 originally asked was:
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Quote:
Originally Posted by sirhc_1
I'm not sure the answer there is the same, thatfront- or back-focus AF issues will have no effect. Since the focus confirmation (i.e. the green dot in the VF) 'knows' that correct focus has been manually achieved by (passively) using the data from AF system, it may be that any FF or BF issues would also affect focus confirmation, right? I don't know the answer - I suppose it depends on why FF or BF occurs with a particular lens, i.e. does it focus inaccurately,imprecisely, or both. (Note: those two terms have different meanings, see this graphical illustration or this wiki article). A lens that exhibits both noticeable FF and BF is imprecise (but may be accurate). AF Microadjustment corrects for inaccuracy, but not imprecision. It may also depend on how the misfocusing occurs, i.e. does the AF system 'know' the correct focus and the lens' motor just misses that mark, or is the AF system 'wrong' meaning focus confirmation would also be 'wrong'.
Thoughts, anyone?
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Originally Posted by neuroanatomist
Yes I do think so. I'm thinking about how I manually focus with my macro lens. You clearly see what is and what isn't in focus. Once you push the trigger, the part that you see in focus, will also be in focus on your photo. Nothing is going to change that, that's what I meant to say. I didn't change focussing screens either, that's also why I said that manual focussing isn't to hard at shorter distances, where you can easily separate the infocus parts from the out of focus parts. Once your subject gets further away from you (like more than 10feet/3m), I believe precise manual focus gets a lot harder. A different focus screen might help you out with that, I don't know, don't have that experience. Hope that clears my statement a bit?
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Originally Posted by neuroanatomist
Yes I do think I misinterpreted this one. You're right about that. If your lens is inaccurate in AF, so FF or BF or both. I think it will also misinterpret thru focus confirmation when focussing manually.
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Originally Posted by neuroanatomist
Nope I guess you are very right about this one. Thanks for bringing that up. I don't know if I'm right about it, but I guess that if your lens has purely inconsistent AF, perhaps it will focus correctly from time to time using manual focus with focus confirmation. But is your lens inaccurate, your focus will always be off for a specific amount of back or frontfocussing, depending on your lens. This was the case with my Sigma 50mm 1.4 and this case is easily solvable with AF micro-adjustment. Inconsistent AF isn't I think.
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Originally Posted by neuroanatomist
So yes I'm with you on this one John.
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Originally Posted by neuroanatomist
Good question and it practically all depends on this one.
I know it's only wikipedia, but still it might have something:
[url="http://en.wikipedia.org/wiki/Autofocus]http://en.wikipedia.org/wiki/Autofocus[/url]
I read that there are multiple ways to achieve AF, so it's just the questions which type canon uses? Man this question is bigger than I thought [:P]
I guess this one is appliable, but I'm not 100% sure: quote from wikipedia
"
<h2><span id="Passive_autofocus" class="mw-headline"]Passive autofocus</h2>
Passive AF systems determine correct focus by performing passive analysis of the image that is entering the optical system. They generally do not direct any energy, such as ultrasonic sound or infrared light waves, toward the subject. (However, an[url="http://en.wikipedia.org/wiki/Autofocus_assist_beam]autofocus assist beam[/url]of usually infrared light is required when there is not enough light to take passive measurements.) Passive autofocusing can be achieved by[b]phase detection[/b]or[b]contrast measurement.[/b]
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<div class="thumbinner"][url="http://en.wikipedia.org/wiki/File:US_pat_5589909_fig_2.png]http://upload.wikimedia.org/wikipedi...9909_fig_2.png[/url]
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<div class="magnify"][url="http://en.wikipedia.org/wiki/File:US_pat_5589909_fig_2.png]http://bits.wikimedia.org/skins-1.5/...gnify-clip.png[/url]</div>
Phase detection system</div>
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<h3><span id="Phase_detection" class="mw-headline"]Phase detection</h3>
Phase detection is achieved by dividing the incoming light into pairs of images and comparing them.SIR TTLpassive phase detection (secondary image registration,through the lens) is often used in film and digitalSLR cameras. The system uses abeam splitter(implemented as a small semi-transparent area of the main reflex mirror, coupled with a small secondary mirror) to direct light to an AF sensor at the bottom of the camera. Twooptical prismscapture the light rays coming from the opposite sides of the lens and divert it to the AF sensor, creating a simplerangefinderwith a base identical to the lens's diameter. The two images are then analysed for similar light intensity patterns (peaks and valleys) and the phase difference is calculated in order to find if the object is infront focusorback focusposition. This instantly gives the exact direction of focusing and amount of focus ring's movement.
Although AF sensors are typically one-dimensional photosensitive strips (only a few pixels high and a few dozen wide), some modern cameras (Canon EOS-1V,Canon EOS-1D,Nikon D2X) featureArea SIRsensors that are rectangular in shape and provide two-dimensional intensity patterns for a finer-grain analysis.Cross-type (CT)focus points have a pair of sensors oriented at 90° to one another, although one sensor typically requires a larger aperture to operate than the other. Some cameras (Canon EOS-1V, Canon EOS-1D,Canon EOS 30D/40D) also have a few 'high precision' focus points with an additional set of prisms and sensors; they are only active with 'fast lenses' of certainfocal ratio. Extended precision comes from the increased diameter of such lenses, so the base of the 'range finder' can be wider."
So if this is the way, I believe it is true that the FF and BF also exist with focus confirmation, sincePassive AF systems determine correct focus by performing passive analysis of the image that is entering the optical system.
Thoughts?? [:P]
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Originally Posted by Sheiky
Both, actually. Primarily, phase detection AF is the one that's used - that's the camera's AF system. In bodies with Live View, you have the option of using contrast detection AF (with or without face detection) or the 'quick mode' where the mirror flips down briefly so the phase detection system can be used.
Inaccurate but precise focusing with a given lens (consistent front-focusing, for example) is due to a misalignment of the lens optics, which project the focal plane of the lens either in front of or behind the plane of the AF and image sensors (the reflex mirror reflects most light up to the VF, some light down to the AF sensor, which should be in the same optical plane as the image sensor, and if it's not, that's what you'd use the [Adjust all by same amount] in the AF Microadjustment C.Fn to correct). Since the AF calculations are based on this same image, I'd expect this problem to affect focus confirmation as well. AF microadjustment will correct this inaccuracy, and thus if you AF microadjust a MF lens (yes, that sounds like an oxymoron), focus confirmation should be corrected as well.
So, if a lens has a simple and consistent inaccurate AF, my guessis that both AF and manual focus confirmation would be affected, and that performing an AF microadjustment would correct both.
On the other hand, if a lens has randomly imprecise AF (sometimes FF, sometimes BF), that's a mechanical issue with the lens and nothing is going to correct that, but MF with a focusing screen will be fine.
Then there's the additional case of a lens with inconsistently inaccurate AF - meaning that at some subject distances it consistently front-focuses and others it consistently back-focuses. I hadn't considered this possibility until I ran across it inRoger's (lensrentals.com)description of the Sigma 50mm f/1.4 lens- hecalls this 'schizophrenic AF' ("Closer than 5 feet it will front focus, further than 20 feet it will backfocus."). There's no way to adjust for that, it must be lived with. If he's correct, it may well account for the varying reports of AF issues or non-issues with this lens - more obvious on FF than crop due to the greater DoF with crop, and highly dependent on what you shoot. (Also highly dependent on how you test - for example, the LensAlign Pro instructions suggest testing a 50mm lens at 4 feet from the target, where the Sigma would apparently front-focus, but Canon's guidance would suggest testing it at 8 feet, where it would apparently be fine.)
Wow! This is an informative topic. That's why when I tried the Sigma 50/1.4, on AF its missing the target. When I switched to MF (w/ AF confirmation), its still not hitting the mark. But when I neglected the AF confirmation on MF & strictly trusted what my eyes see thru the VF, it started showing significant sharpness in pics shot within 5-15 feet.
Thanks a lot guys for the inputs! I'll keep those ones you posted in mind most of the time.[;)]