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Author Topic: Mirrorless/EVF/Focus?  (Read 6548 times)

Playdo

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Mirrorless/EVF/Focus?
« on: January 20, 2011, 10:37:25 am »

I've been looking into mirrorless cameras recently. To my understanding, they use contrast detection to AF. This eliminates inaccuracy in AF as: 1.) CD is more accurate than PD - 2.) there is no issue with focus screen misalignment.

Someone on another forum mentioned that, as there are benefits of using a viewfinder it is likely that in future, mid-level+ mirrorless cameras will use EVFs to look through. My question is, does EVF technology provide an exact replica of the sensor or is there the possibility of misalignment, as found in SLRs?
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ErikKaffehr

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Re: Mirrorless/EVF/Focus?
« Reply #1 on: January 20, 2011, 10:44:01 am »

Hi,

EVF shows actual sensor image. No misalignment between sensor and finder is possible.

All this to my best understanding.

Best regards
Erik


I've been looking into mirrorless cameras recently. To my understanding, they use contrast detection to AF. This eliminates inaccuracy in AF as: 1.) CD is more accurate than PD - 2.) there is no issue with focus screen misalignment.

Someone on another forum mentioned that, as there are benefits of using a viewfinder it is likely that in future, mid-level+ mirrorless cameras will use EVFs to look through. My question is, does EVF technology provide an exact replica of the sensor or is there the possibility of misalignment, as found in SLRs?
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Playdo

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Re: Mirrorless/EVF/Focus?
« Reply #2 on: January 20, 2011, 10:49:45 am »

Thanks Eric. Is there anything I've missed out in terms of the AF benefits of a mirrorless system?

At the moment, the only downfall is the AF speed and tracking ability of a contrast detection system?
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Gary Brown

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Re: Mirrorless/EVF/Focus?
« Reply #3 on: January 20, 2011, 12:09:59 pm »

At the moment, the only downfall is the AF speed and tracking ability of a contrast detection system?

You might want to take a look at reviews such as the GH2 “first look” on this site. An excerpt from the section on autofocus:

“Readers will likely know that, like all non-reflex cameras, Panasonic's G series uses contrast detection autofocus, not phase detection the way that DSLRs do. …

“With the GH2 Panasonic has pulled a rabbit out of its hat. The new sensor's faster read-out allows the autofocus of the GH2 to apparently equal that of mid-level DSLRs. While I haven't done any lab tests…, my sense is that Panasonic has licked the AF speed issue on non-DSLRs.”
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Robert Roaldi

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Re: Mirrorless/EVF/Focus?
« Reply #4 on: January 20, 2011, 12:18:37 pm »

I assumed that an EVL or LCD would always show 100% field of view that is captured by the sensor. I guess I assumed this because I couldn't conceive why you'd build one that didn't. But I recently discovered that my Canon HF200 consumer camcorder's LCD only shows part of what is recorded in still jpg capture (guessing it's like 85% or so, but have not done measurements). I asked about it over on the vimeo forums, and one fellow replied that that held true in video mode too. I thought this was odd, but haven't had the time to look at it more.

Why would you design a LCD (or EVL) that didn't show 100% of what the sensor captures?  I guess it has something to do with the fact that the HF 200 is a camcorder and so the final image it generates is never actually the contents of the full sensor, pixel by pixel.

I think I will test this on my other still cameras too. I tend not to use the LCD for live view much and so would not have noticed it casually. Neither of my D-SLRs do video. I am pretty sure that the EVL on my Sony R1 shows the same thing as its LCD, but I'll check that too now.

It would be interesting to see if the current crop of D-SLRs show accurate framing on their LCDs during video capture. They must, or we'd have heard complaints about it by now.

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Playdo

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Re: Mirrorless/EVF/Focus?
« Reply #5 on: January 20, 2011, 05:37:41 pm »

Would a mirrorless system eliminate issues with miscalibrated body/lens combination or would adjustments still need to be made?

Also, if/when the likes of Nikon/Canon bring out pro mirrorless systems will a new lens mount (ie new lenses) definitely be needed?
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feppe

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Re: Mirrorless/EVF/Focus?
« Reply #6 on: January 20, 2011, 08:01:17 pm »

Also, if/when the likes of Nikon/Canon bring out pro mirrorless systems will a new lens mount (ie new lenses) definitely be needed?

Not necessarily, but using legacy lenses exclusively wouldn't make much sense.

Current 35mm format lenses would work as long as the lens backfocus distance is the same - probably some other factors involved as well to reach optimal IQ, but that's the main point. Going mirrorless allows for a smaller (shallower) body due to lack of mirror assembly and smaller backfocus distance. Building a body to only use legacy lenses would force them to make a bigger body thus largely negating the size advantage of mirrorless cameras.

My bet is that they'll go for a compact body and new lens system, and enable legacy lenses to be used with an adaptor. This is what Micro Four Thirds is doing already - a 135mm L-series lens looks pretty hilarious on my E-PL1 with an adapter :) A new lens system makes business sense as they'll get to re-sell lenses to their customers; and it might mean that the lens line is upgraded to meet the requirements next few generations of sensors put on optics.

There are some pretty strong rumors about Nikon bringing out a mirrorless (semi-)pro-level system this year. Olympus and Panasonic are also supposed to announce new (semi-)pro-level MFT cameras in the coming months with a new sensor, so these are exciting times for mirrorless cameras.

Playdo

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Re: Mirrorless/EVF/Focus?
« Reply #7 on: January 21, 2011, 06:28:46 am »

Hi Feppe. Yes, it makes very good business sense to be able to start selling lenses from scratch again. I definitely see the future going in the direction of mirrorless pro cameras too. So, do you know if it eliminates any AF issues due to lens/body miscalibration? I'm sure some of the big companies wouldn't want to eliminate this, being such a large source of income for them.
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telyt

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Re: Mirrorless/EVF/Focus?
« Reply #8 on: January 21, 2011, 07:45:11 am »

...  do you know if it eliminates any AF issues due to lens/body miscalibration?

Calibration issues would be gone and in theory you would be able to use AF anywhere in the picture area, not just at focus "points", and the focus would be equally accurate everywhere.

I'm sure some of the big companies wouldn't want to eliminate this, being such a large source of income for them.

Why are you sure of this?  As a consumer I'd stay far far away from any camera that is known to have calibration problems.  No camera maker is getting any of my money if they see mis-calibration as a source of income.
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Playdo

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Re: Mirrorless/EVF/Focus?
« Reply #9 on: January 21, 2011, 08:01:29 am »

That's what I initially thought, but someone brought up the point about liveview, saying that when testing for front/back focusing you use liveview. This would clearly imply that you still need to adjust lenses to the body for contrast detection AF. Or am I missing something?

Why are you sure of this?
Not 'sure', but if you look at it logically from a business point of view, it's profitable. There a lot of people sending their equipment in at high cost for alignment/calibration issues due to quality control. But I'd rather keep on topic :)
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telyt

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Re: Mirrorless/EVF/Focus?
« Reply #10 on: January 21, 2011, 12:10:19 pm »

That's what I initially thought, but someone brought up the point about liveview, saying that when testing for front/back focusing you use liveview. This would clearly imply that you still need to adjust lenses to the body for contrast detection AF. Or am I missing something?

No you don't need to adjust the lenses to the body for contrast detection.  The adjustment is to make phase-detect AF match actual focus.  Contrast-detection AF algorithms don't know which way to adjust focus or how far to adjust focus, that's why they're slower.  They detect maximum contrast which by definition is correct focus.

... if you look at it logically from a business point of view, it's profitable.

Unless you have access to the company's internal cost structure you have no idea if it's profitable.  Logically, intentionally selling poorly-calibrated equipment is not a sustainable business model because the customers will run away as soon as another manufacturer sells a better product.
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Playdo

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Re: Mirrorless/EVF/Focus?
« Reply #11 on: January 21, 2011, 01:29:43 pm »

Hi Telyt. That's great news; being able to accurately focus anywhere on the sensor in pro cameras in the future would be amazing, especially at wide apertures. I look forward to it.

Re:profits - It's simple. I mean, either:

1. a company spends extra costs on QC, eliminating calibration issues, and receives no extra income from calibration repair.
2. a company spends less on QC, allows a percentage of miscalibrated products through, then receives extra income from calibration repair. As long as it isn't too big of a percentage, the user base stays. In many cases warranty would have expired when matching old with new products.

Certain brands are known to have lower QC, and that's their business plan.
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stever

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Re: Mirrorless/EVF/Focus?
« Reply #12 on: January 21, 2011, 01:34:32 pm »

it's not obvious from the manuals, but Canon 5D2 and later slrs use CD AF in liveview.  it decent light, is fairly slow, and the focus area is larger than desired (although it can be moved about) but in situations where these limitations are acceptable it provides accurate autofocus

it's also useful for checking against your micro-adjust

and has the benefit of working with any lens and extender combination regardless of maximum aperture - e.g. a 400 f5.6 with 2x extender (so it's autofocusing at f11) - i find that with long lenses it's at least as accurate, more consistent, and quicker than focusing with magnified liveview

with Panasonic's example, all manufacturers should be challenged to offer faster and more sensitive CD autofocus
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telyt

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Re: Mirrorless/EVF/Focus?
« Reply #13 on: January 21, 2011, 03:57:52 pm »

Hi Telyt. That's great news; being able to accurately focus anywhere on the sensor in pro cameras in the future would be amazing, especially at wide apertures. I look forward to it.

Yup.  As good as manual focus with a pre-AF viewfinder.


Re:profits - It's simple.

Do you know what the cost of after-sale calibration is?  Is it less than Canon charges?
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AFairley

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Re: Mirrorless/EVF/Focus?
« Reply #14 on: January 21, 2011, 05:43:56 pm »

it's not obvious from the manuals, but Canon 5D2 and later slrs use CD AF in liveview.  it decent light, is fairly slow, and the focus area is larger than desired (although it can be moved about) but in situations where these limitations are acceptable it provides accurate autofocus

it's also useful for checking against your micro-adjust

and has the benefit of working with any lens and extender combination regardless of maximum aperture - e.g. a 400 f5.6 with 2x extender (so it's autofocusing at f11) - i find that with long lenses it's at least as accurate, more consistent, and quicker than focusing with magnified liveview

with Panasonic's example, all manufacturers should be challenged to offer faster and more sensitive CD autofocus

Although the Panny is supposed to be snappy with the tiny micro 4/3 lenses, I have read that that speed disappears if you use larger lenses where you have to move heavier glass around to focus.  As I understand it, you have to move the lens elements back and forth much more with CDAF than with PDAF, (CDAF is iterative to get to hightest contrast, PDAF is predictive so only moves the glass once or twice to hit focus if I understand correctly).  Still, I believe that ultimately CDAF will be the standard for most SLRs.
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ErikKaffehr

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Re: Mirrorless/EVF/Focus?
« Reply #15 on: January 22, 2011, 02:38:02 am »

Hi,

Contrast detection AF has some issues. It works by moving the lens back and forth and measure contrast. It can work very well, but needs lenses built for purpose. For instance Sony's NEX cameras work with Sony SSM (ultra sonic motor) lenses but are said to be terribly slow.

I'm much in favor of EVFs, the Sony A55 I have has an EVF, the viewfinder image can be enlarged 15X which is very useful for exact focusing, but the focusing rings on AF lenses are not really meant for critical AF, to short stroke and to much play. So I would say that MF with an EVF is exacting but slow. With fast lenses there is also an issue with focus shift, the position of correct focus shifts when stopping down.

Even if the EVF actually shows the sensor image, the sensor may be misaligned with the lens. In that case it's not possible to achieve correct focus across the image.

I'd be pretty certain that EVFs are taking over in a few years, but I don't think we are there yet.

Best regards
Erik


Thanks Eric. Is there anything I've missed out in terms of the AF benefits of a mirrorless system?

At the moment, the only downfall is the AF speed and tracking ability of a contrast detection system?
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NikoJorj

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Re: Mirrorless/EVF/Focus?
« Reply #16 on: January 22, 2011, 04:18:50 am »

Yup.  As good as manual focus with a pre-AF viewfinder.
And maybe better, because that viewfinder needs a correct calibration of the mirror and groundglass position to be accurate, whereas such problems are gone away with live-view AF.

Speaking of that, Fuji promoted a phase detection AF on sensor not that long ago... Could bring best of both worlds if it works proprely.
Some news?
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feppe

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Re: Mirrorless/EVF/Focus?
« Reply #17 on: January 22, 2011, 06:48:01 am »

I'm much in favor of EVFs, the Sony A55 I have has an EVF, the viewfinder image can be enlarged 15X which is very useful for exact focusing, but the focusing rings on AF lenses are not really meant for critical AF, to short stroke and to much play.

That's not a feature unique to EVFs - my E-PL1 without an EVF can zoom up to 14x for manual focus.

ErikKaffehr

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Re: Mirrorless/EVF/Focus?
« Reply #18 on: January 22, 2011, 07:45:05 am »

Hi,

Sorry, nerve meant that the feature is specific to Sony, I presume it's present on all but the simplest cameras using "live view", but my experience is with the Alpha 55.

Best regards
Erik


That's not a feature unique to EVFs - my E-PL1 without an EVF can zoom up to 14x for manual focus.
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Erik Kaffehr
 

telyt

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Re: Mirrorless/EVF/Focus?
« Reply #19 on: January 22, 2011, 02:39:04 pm »

... Fuji promoted a phase detection AF on sensor not that long ago... Could bring best of both worlds if it works proprely.

I agree, this could be an excellent solution: phase detection for rough focus and contrast detection for fine focus.  Speed + accuracy.
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