Hunger for quick focus

[Doctor Andrei]

If the auto-focus is too slow, don't use it !

Here is a simple way to have your camera always ready for that shot.

First, you need to determine a minimum close "working" distance that you'll use for your camera to subject distance. From this you need to do a bit of math, the goal is to determine the camera settings to keep everything from this minimum working distance to infinity in an acceptable focus.

If you are wondering how a camera could be set to keep everything in focus with auto-focus turned off, check out the concept of Hyperfocal Distance. The trick is quite simple: set the auto-focus off, set the camera to aperture priority, select an aperture that yields a Hyperfocal distance which is twice as far as your minimum working distance, Set the lens to focus at the Hyperfocal Distance, and adjust the ISO to bring your shutter speed where you want it. ( if the camera meters a long shutter, raise the ISO to shorten it ) Now, every time you snap a photo, everything, from your minimum working distance to infinity, will all be equally focused.

Normally, the Hyperfocal Distance is calculated from the focal length of the lens ( The Real Focal length, not factored by the imager size ), aperture, and a parameter called the "Circle of Confusion".

The best way to think of the Circle of Confusion is the diameter projected by a point in the scene onto the imager that is considered acceptably focused. Often, the Circle of Confusion uses standard values based on the size of the imager or film, but I have done the calculations for digital by the photosite spacing. If the largest acceptable blur a point in a scene makes on an imager is no greater than the spacing between photosites, then the image focal clarity is only limited by the photosite pitch of the imager. ( You can't get a better focus )

For this example I looked up the specifications of the Canon 550D which states that the Max resolution is 5184 x 3456 across a Sensor size being APS-C ( 22.3 x 14.9 mm )

* DANGER * Formulas to follow *

So, to calculate a Circle of Confusion yielding an optimal focus:



Since we have already determined the minimum working distance, we need to calculate the necessary aperture by:



or, in terms of the Minimum Working distance:



For example, you are using a 24 mm lens and you want the minimum working distance to be 12 feet, 3.7 m. Well, if you are one of the few in the world who measures in feet, you first need to turn feet into mm by:



and in this example:



The rest of us know to multiply meters by 1000 to get mm. Now on to calculating aperture:






So, if you set the aperture to f/18 on a 24mm lens with the focus to a distance of 24 feet, 7 m, everything within 12 feet, 3.6 m, and infinity will be focused with the maximum blur not exceeding one pixel. A trade off here is the small circle of confusion means that you need to use a small aperture and long shutter times ( even at high ISO settings )

A more traditional Circle of Confusion for Canon cameras is 0.019 mm ( 4.4 times greater than the photosite pitch ), lets do the calculation again:






Obviously, now having a f/4 is a lot easier to manage at the expense of image clarity, but play with the numbers and see how you go with your lens, a minimum working distance that you need, and post your results. Keep in mind that if you are using a Zoom lens, the calculation needs to be with the focal length that your lens is set to.

Oh my God((((((((

Why does photography require being a professional?????????

Why dont they just make cameras with one button, seriously? We don't need to be Formula One drivers to drive our cars perfectly, and our wives are no Jamie Olivers and they still cook good. So, I'm sober, got two hands and two eyes, got me a nice camera and still have to take a day-off to read and figure out all the above to make nice pictures. Damn.

Stephany thanks a whole lot, typing work like yours deserves praise! Seriously. And please don't tell me about manual focus. To me it's like killing a cow instead of buying a piece of meat in a grocery.

[Doctor Andrei]

Looks to me like just some practice and attention may fix your concerns.

Definitely! But what IF I take out my camera and start figuring out which shutter speed and ISO should I use and WB and all that on a cloudy winter day with snow all around (btw anyone tell me how to make that m.f. look white like SNOW not grey dirt!) with my kid runnin around, and before I cope with all that the kid just runs to the next street?
Does anyone know how to take shots IMMEDIATELY?

[Doctor Andrei]

Andrei:

Further to Dave's comments, my first additional comment is that I can't recall when I used an ISO setting as low as 100. Now this may shock some purists, but on my ancient 30D, I commonly use a value of 640, and although noise can be seen by peeping, 11" x 14" prints don't show any noise.

Looking at the individual images:

1) two boys in the snow: ISO 100, 1/100th second @ f/4. At f/4 there isn't much depth of field to work with, but still I can't find anything that is in focus - even the leaf Dave mentioned is not great. At ISO 400, the f/stop would have been f/8 which may have salvaged something.

2) Group of people: ISO 100, 1/100th second @ f/4. Again, nothing much in focus - this is strange.

3) Your son in blue T-shirt: ISO 100, 1/30th second @ f/2.8. Because your son's right arm and the background on the left look reasonably sharp, the slow shutter speed can't be to blame. This must be partially due to missed focus. The wide open f/stop (at f/2.8) gives so little depth of field that the focus must be accurate (and preferably on the eyes). It looks as though focus was somewhere between his right arm and the wall. As above, ISO 400 would have used f/5.6 which might have pulled his face into focus.

I am certain that this camera will produce very good images (it's a better camera than my 30D which I still use with great results). But - it may be that the lens is letting you down.

I don't know if you have access to a camera shop where you live, but if so, I would suggest trying one of their better Canon lenses on your 550D body, taking a few shots - take your son along - he will probably move around enough.

But first set the camera up as I suggested. One central focus point. Use the "star" button the back for focus, and use AI Servo (which is for moving subjects).

I didn't mention it earlier, but the advantage of this focusing method is that you can keep the "start" button pressed while you follow your subject, then press the shutter with your finger when you're ready to take the image.

Waiting to hear how things are going.

Glenn

Just borrowed a Canon glass from my cousin (Tamron was also her leftover). It's EFS 18-55 58mm. Just took a few shots of stable objects indoors and finally figured out how Aperture effects DOF, and how to play with S Speed without turning everything black. Next winter walk with my Son, maybe there will be a result. And BTW the Canon lens focuses 2 times quicker.

[Doctor Andrei]

If the auto-focus is too slow, don't use it !

Here is a simple way to have your camera always ready for that shot.

First, you need to determine a minimum close "working" distance that you'll use for your camera to subject distance. From this you need to do a bit of math, the goal is to determine the camera settings to keep everything from this minimum working distance to infinity in an acceptable focus.

If you are wondering how a camera could be set to keep everything in focus with auto-focus turned off, check out the concept of Hyperfocal Distance. The trick is quite simple: set the auto-focus off, set the camera to aperture priority, select an aperture that yields a Hyperfocal distance which is twice as far as your minimum working distance, Set the lens to focus at the Hyperfocal Distance, and adjust the ISO to bring your shutter speed where you want it. ( if the camera meters a long shutter, raise the ISO to shorten it ) Now, every time you snap a photo, everything, from your minimum working distance to infinity, will all be equally focused.

Normally, the Hyperfocal Distance is calculated from the focal length of the lens ( The Real Focal length, not factored by the imager size ), aperture, and a parameter called the "Circle of Confusion".

The best way to think of the Circle of Confusion is the diameter projected by a point in the scene onto the imager that is considered acceptably focused. Often, the Circle of Confusion uses standard values based on the size of the imager or film, but I have done the calculations for digital by the photosite spacing. If the largest acceptable blur a point in a scene makes on an imager is no greater than the spacing between photosites, then the image focal clarity is only limited by the photosite pitch of the imager. ( You can't get a better focus )

For this example I looked up the specifications of the Canon 550D which states that the Max resolution is 5184 x 3456 across a Sensor size being APS-C ( 22.3 x 14.9 mm )

* DANGER * Formulas to follow *

So, to calculate a Circle of Confusion yielding an optimal focus:



Since we have already determined the minimum working distance, we need to calculate the necessary aperture by:



or, in terms of the Minimum Working distance:



For example, you are using a 24 mm lens and you want the minimum working distance to be 12 feet, 3.7 m. Well, if you are one of the few in the world who measures in feet, you first need to turn feet into mm by:



and in this example:



The rest of us know to multiply meters by 1000 to get mm. Now on to calculating aperture:






So, if you set the aperture to f/18 on a 24mm lens with the focus to a distance of 24 feet, 7 m, everything within 12 feet, 3.6 m, and infinity will be focused with the maximum blur not exceeding one pixel. A trade off here is the small circle of confusion means that you need to use a small aperture and long shutter times ( even at high ISO settings )

A more traditional Circle of Confusion for Canon cameras is 0.019 mm ( 4.4 times greater than the photosite pitch ), lets do the calculation again:






Obviously, now having a f/4 is a lot easier to manage at the expense of image clarity, but play with the numbers and see how you go with your lens, a minimum working distance that you need, and post your results. Keep in mind that if you are using a Zoom lens, the calculation needs to be with the focal length that your lens is set to.

Alright, I'm using a Canon 18-55 58mm, and taking a walk with my Son on a cloudy day with snow all around, and he's moving all around. What EXACTLY should I do to spend all the time after that just pressing release button, not scratching my head searching thru the menu and endlessly turning manual focus?

[rpcrowe]

Dr. Andrei,

I can relate to your frustration. Let me answer a simple query of yours: (btw anyone tell me how to make that m.f. look white like SNOW not grey dirt!)

Your camera's meter is designed to expose for an 18% reflectance. That is because a lot of normal scenes contain much area that is 18% reflectant, green grass is an example of a subject with that reflectance. When the meter senses a very bright area such as snow, it tells the camera "Oh! Oh! You had better cut down on the exposure so that the scene will be registered at 18% gray. Therefore it underexposes the snow. The way to counteract this very normal problem is to over expose what the meter is telling your camera to shoot at. The way to do that with a Canon DSLR is to dial in exposure compensation of +1 or +2 stops. Your manual will tell you how to add this exposure compensation. If you cannot figure it out, let me know, I will download a copy of your camera user manual and perhaps be able to point you to the explanation how to do it.

BTW: The same thing happens in reverse when shooting a very dark subject, the proverbial black cat in a coal bin. The meter tells the camera, Oh oh! That subject is too dark. You had better add exposure to get it to look gray. The camera adds exposure and the black cat shows up as gray. To prevent this, you need to subtract exposure from your image by selecting -1 or -2 stops of exposure compensation....

[Doctor Andrei]

Dr. Andrei,

I can relate to your frustration. Let me answer a simple query of yours: (btw anyone tell me how to make that m.f. look white like SNOW not grey dirt!)

Your camera's meter is designed to expose for an 18% reflectance. That is because a lot of normal scenes contain much area that is 18% reflectant, green grass is an example of a subject with that reflectance. When the meter senses a very bright area such as snow, it tells the camera "Oh! Oh! You had better cut down on the exposure so that the scene will be registered at 18% gray. Therefore it underexposes the snow. The way to counteract this very normal problem is to over expose what the meter is telling your camera to shoot at. The way to do that with a Canon DSLR is to dial in exposure compensation of +1 or +2 stops. Your manual will tell you how to add this exposure compensation. If you cannot figure it out, let me know, I will download a copy of your camera user manual and perhaps be able to point you to the explanation how to do it.

BTW: The same thing happens in reverse when shooting a very dark subject, the proverbial black cat in a coal bin. The meter tells the camera, Oh oh! That subject is too dark. You had better add exposure to get it to look gray. The camera adds exposure and the black cat shows up as gray. To prevent this, you need to subtract exposure from your image by selecting -1 or -2 stops of exposure compensation....

Thanx a lot...
I just thought that there's a trick I didn't know... Overexposure yes, I was using it that day. Without that, the daylight looked like evening dawn.

[Glenn NK]

Richard:

Thanks for stepping in. Of course the snow is one of the problems - can't believe I forgot about that.

The T2i is a pretty sophisticated piece of equipment - for a new photographer it is a bit like an F-1 car and I can appreciate the frustration Andrei is going through. However his main concern does seem to be achieving good focus.

The AI Servo is what I use myself with reasonable success (along with the single centre focus point and the back button focus). However my lenses are pretty quick when it comes to focus, and I still think the Tamron is at least part of the problem if not all of it.

Here's hoping that I'm not offending anyone with Tamron lenses (I think I already upset some with Olympus cameras).

The manual for the 550D/T2i:

http://software.canon-europe.com/sof...014.asp?model=


Glenn

[Doctor Andrei]

Richard:

Thanks for stepping in. Of course the snow is one of the problems - can't believe I forgot about that.

The T2i is a pretty sophisticated piece of equipment - for a new photographer it is a bit like an F-1 car and I can appreciate the frustration Andrei is going through. However his main concern does seem to be achieving good focus.

The AI Servo is what I use myself with reasonable success (along with the single centre focus point and the back button focus). However my lenses are pretty quick when it comes to focus, and I still think the Tamron is at least part of the problem if not all of it.

Here's hoping that I'm not offending anyone with Tamron lenses (I think I already upset some with Olympus cameras).

The manual for the 550D/T2i:

http://software.canon-europe.com/sof...014.asp?model=


Glenn

The link goes to a Czech language manual))))))) Alright I'll read thru my Russian manual 1 more time))) What's T2i?

[rpcrowe]

Hi again,

I experienced vey slow focusing and a long lag between pressing the shutter button and acquiring the image when I shot with an Olumpus 5500Z P&S camera. But, I have not experienced slow focusing with any of my Canon lenses even when I used them on a 350D Rebel. Most of my Canon lenses are equipped with the Canon USM system which speeds up auto focus greatly. I have a Tamron 90mm f/2.8 AF SP macro lens which auto focuses rather slowly. However, once it locks on, it is quite accurate. Since I normally shoot macros using manual focusing, this slow AF doesn't propose a problem for me.

I was doing some research on the 550D and or the Tamron 17-50mm f/2.8 slow focusing that you are experiencing. This what I have found

Camera:

When you are shooting your image, are you using liveview on the LCD monitor or are you viewing through the eye-level viewfinder? Apparently the liveview on this camera is not terribly speedy. See this link which mentions the following about the 550D "The live view mode is all right for tripod-based shots with relatively static subjects that allow plenty of time for the Canon to find the right focus, but it’s not very practical for day-to-day shooting."

http://photo.tutsplus.com/articles/r...specs-deliver/

Lens

Additionally, here is a review from a person who is apparently exeriencing slow/eratic focus when using the 17-50mm f/2.8 Tamron lens. However, I don't know which camera he used it with. I also noticed in other reviews that the Tammy focused rather slowly in low light situations...

http://www.bhphotovideo.com/c/produc...2_8_XR_Di.html

[revi]

I'd also suggest to do any tests at rather high shutter speeds, to minimise the effects of camera shake (clearly visible in the 2nd posted image: horizontal lines are sharper than verticals), even if this means high ISO (we're testing things here, not producing high art ). And indeed make sure the selected focus spot is on the target you want (that one bit me often enough).

Also, with respect to the Tamron lens used, which version is it exactly? There seem to be several versions around with different AF motorisations, the older ones might be slower... And, any AF system needs a bit of contrast to work on, they won't be able to focus on a flat texture-less surface. If the contrast at the focus spot is low, focusing will take longer. So the black coat in the first image might not give enough contrast, the T-shirt in the 3rd image should be fine.

As for the complexity of an SLR: of course they are more complex than a point-and-shoot, that's the whole point of them...
However, if you set an SLR to P-mode, you should get pretty close to point-and-shoot mode, with easier ways to apply needed corrections (and getting the better quality inherent from a larger sensor)

[Splatsworth]

I have the 550D and haven't had any issues with focusing or the speed of focussing. It could be that you have a either a faulty camera or a faulty Lens.

[Glenn NK]

The link goes to a Czech language manual))))))) Alright I'll read thru my Russian manual 1 more time))) What's T2i?

Sorry Andrei, I selected the wrong language. The manual is available in many languages; this should be the English one:

http://software.canon-europe.com/sof...013.asp?model=

The Canon 550D is also called the T2i by Canon.

http://www.dpreview.com/reviews/canoneos550d/

Were you able to set up the focusing system using AI Servo, a single focus point, and the back button (star button) for focusing?

Glenn

[Glenn NK]

I have the 550D and haven't had any issues with focusing or the speed of focussing. It could be that you have a either a faulty camera or a faulty Lens.

Trev:

What lens are you using?

Glenn

[vladimir]

Hi Andrei,
I often have similar problems, and they have nothing to do with Canon, or Tamron, or equipment in general, or with lack of theoretical background. They simply have to do with my SHAKY HANDS!
That's the reason I got myself Pentax, which has in-camera image stabilization (I knew I'd need that with every lens), and even with that, and even in the range below 50mm, I very often get blurred images at shutter slower than 1/100 (and no flash).
Same goes for focusing speed. If the camera moves/shakes [just a little] when it tries to focus, then it can't focus.
So - just an idea - it may be the stability of the camera in your hands that's the source of all these problems. If (hopefully) you have steady hands (unlike mine), then just pay attention to holding it steady as you aim it and shoot. If you have unsteady hands (like mine) - know the limit of the shutter speed that you can use, and try to stabilize the camera by other means for faster focusing.
Hope it helps.

[Dave Humphries]

Reading all the intermediate replies above, I think Richard maybe on to something - if Andrei is using LiveView on the LCD (because he's used to a P&S), that would definitely explain the slow focus speed, my D5000 is MUCH slower using that method.

[MrB]

Reading all the intermediate replies above, I think Richard maybe on to something - if Andrei is using LiveView on the LCD (because he's used to a P&S), that would definitely explain the slow focus speed, my D5000 is MUCH slower using that method.

I am not familiar with Canons but I agree with Dave that Richard might have identified the problem, and there is also a paragraph in the review article to which Richard refers that might be useful to Andrei -

"The 550D retains an exposure mode seen on earlier EOS models that’s so useful it’s surprising other makers don’t do something similar. In the Canon’s A-DEP mode, it checks all 9 focus points for the nearest and farthest object in the scene, then automatically selects a lens aperture which will extend the depth of field to cover both of them."

Philip

[rpcrowe]

I have not been able to get an objective focus speed measurement between of liveview focus on the 550D but, here is a chart with the live view focus speed for a 60D. There is no reason for me to believe that the 550D would be any more efficient than the 60D in this respect. As you will see in the referenced website, the eye-level speed is but a fraction of the live view speed.

Canon 60D Live View Focus Speed http://www.imaging-resource.com/PRODS/E60D/E60DA6.HTM

Full Autofocus Live View “Quick Mode” Phase detect: 0.999 second
Time from fully pressing shutter button to image capture. This is phase-detect autofocus, the camera drops the mirror to focus, then raises it to grab the shot.

Full Autofocus Live View "Live Mode" (Contrast Detect) 1.100 seconds
Time from fully pressing shutter button to image capture. This is contrast-detect autofocus, the camera reads Live View data from the image sensor to determine focus.

Prefocused Live View 0.122 second
Time to capture, after half-pressing and holding shutter button.

This puts the 60D using liveview in the area of a P&S camera for focus speed and lag. As most of you probably realize , I am not a great fan of live view to start off with but a full second for focus on a moving subject is ludicrous and IMO, negates the value of the greatly touted articulating LCD...

[Glenn NK]

Richard:

Seems that you may have nailed it, however I wish Andrei would try out the back button focusing method and set the camera to AI Servo. I have an 8 year old whirling dervish granddaughter - with out AI Servo, I'd be lost. The nice thing about the back button focus is that you can hold it in which operates focus continuously, follow your subject as they bounce around and when satisfied with the composition, fire the shutter.

As for LV focusing, I suspect it's slower than other modes on other bodies too. My 5Dii with LV is really only useful for tripod work, and even then I find it frustrating.

Glenn

[rpcrowe]

One final posting on this subject and then I will let the thread die unless we hear from Dr. Andrei

http://www.ephotozine.com/article/ca...r-review-12948

"During LiveView, two focusing options are available – Contrast Detection and Quick-mode. The contrast detection is improved over previous Canon models, taking around a second to lock onto a static subject in good light. In poor light the system struggles and can take four of five seconds to lock on, if it manages to do so at all. The Quick-mode option is certainly quicker and more reliable, but the screen is blank while the camera attains focus, which is not ideal, especially if your subject is in motion."

[Splatsworth]

Trev:

What lens are you using?

Glenn

I use either the 18-55mm kit Lens a Canon EF 100-400mm f/4.5-5.6 L IS USM or a Tamron SP AF 90mm f/2.8 Di Macro Lens. I guess his problem could be that its set to one shot AF, which locks focus once you have half pressed the button, mine is set on AI Servo which solves all my problems with focusing other than my own shaky hands :-)