Focus points, back focus, and cult of sharpness issues

Blurmore

I'm a single point multi controller direct focus point kinda guy on my 20D and 30D and although my new 17-55 f2.8 EF-S focusses superbly at telephoto or normal angle of view, the wide anlge focussing leave something to be desired.
It seems that if I use a single point for a bridal party or wedding group my focus is almost ALWAYS behind the party. Here comes the anal part. Not bad enough that clients notice on 8x10's but bad enough that I notice when viewing the image full. I started a'thinkin that this might not be a endemic backfocus problem with lens/camera but a over extension of the limits of the focussing system in general. Kinda like one point in a wide angle is just not enough information for the camera to accurately judge where I want the focus to be. After my wedding tonight I set up a little test with clothes pins and other doo dads like a group picture, and replicated the results when using a single focus point. BUT Eureka when I put the camera in 'uncle harry' all point mode it more accurately determined where I wanted focus seemingly by anchoring more points of focus in a plane and guessing more acutely.


So how do you use YOUR points?? From now on I'm still going to use single point for everything BUT groups, but if I can break the habit I'm going to start using 'stupid' focus for groups.

Red Bull

I've always used single point focus with all of my lenses and I haven't had any problem before. I always use the center focus point, lock focus, and recompose.

colourbox

On my Rebel XT I use single point which I move around with the controller, because I read on threads here and in a Canon tech note that focus-and-recompose using center point only can lead to parallax-related focus errors when depth of field is narrow. Changing your angle to the subject during recomposition can rotate the narrow focal plane enough so that the subject is no longer within it after you recompose. To minimize recomposition, I use the controller to pick the single point closest to the subject.

I can see how using auto and letting the camera pick multiple points might be more accurate due to oversampling, but I haven't gotten around to trusting it. I worry it might pick the wrong point and not shift to the right point before I need to press the shutter the rest of the way, so I pick my own point. I seem to be satisfied with the focus point in most frames. Some are not right, but I blamed myself. I'll have to take a closer look next time.

pathfinder

Steven,
I am sure you are aware that focus, recompose guaranties that your focus will be incorrect at distances less than 10 feet or so!? Canon even had a link on its website NOT recommending focus, recompose. ( I can't seem to google it here this mornming!)

There have been several discussions here on DGrin about this issue also.

Mind you, I'm not saying you're wrong, or that I don't do it from time to time. But I think you must know what you are doing, and why, and that it WILL give poor focus at 5 feet at f2.8 or f2 for certain.

The best links I have found re Focus Recompose are

http://visual-vacations.com/Photography/focus-recompose_sucks.htm

http://www.outbackphoto.com/workshop/phototechnique/essay06/essay.html

http://www.dgrin.com/showthread.php?t=5642

Blurmore,

As you stated, the AF sensors on the 10D, 20D, 30D, tend to see and react to areas larger than the points that display in the viewfinder, and there is a strong software driven drive to find bright, close, highlighted areas to AF on. That is why eyelashes and eyebrows tend to be good areas to focus on, particuarly at portrait distances of less than 6 feet and with wide apertures.

I tend to agree with you that wide angle lenses need a little different technique and that using several AF points may work better.

If you are going to use a single AF point with a WA lens, you must be MORE careful not to rotate the camera as you recompose even slightly - it roatates the plane of focus much more dramatically than with a longer telephoto lens, and will build in the errror of focus recompose.

As I think about this subject, Focus -Recompose has been widely discussed on the web, but no mention has really been made on the effect of focal length on focus recompose - usually the discussion centers around portraits and moderate length teles.

But Focus -Recompose with wide angles will have an even greater effect since the angle of view is so much wider for the shorter focal lengths, the plane of focus can be rotated much more than with a telephoto lens. And since the FOV is so much wider, there will probably be lots more straight, bright contrasty lines for the AF to be confused about as well. Does this make sense re focusing with WAs,, Blurmore??

I lost a snapshot of a cousin because my 10D focused on the strong pattern in the denim immediately behind her head - I do not use focus recompose indoors or at near distances. I do use it shooting birds at telephoto distances.

A craftsman needs to know the limits of his tools I think, but should not blame them his usage of them ..

In my google to respond to the posted questions, I came across a technique to help focus in dimly lit areas that I have not tried, but that I believe I will use a lot more. That is to use the AF assist light from the 430ex or 580ex without firing the flash, or even better just using the ST-E2 for its AF assist light.

Blurmore

pathfinder wrote:

Steven,


Blurmore,

As you stated, the AF sensors on the 10D, 20D, 30D, tend to see and react to areas larger than the points that display in the viewfinder, and there is a strong software driven drive to find bright, close, highlighted areas to AF on. That is why eyelashes and eyebrows tend to be good areas to focus on, particuarly at portrait distances of less than 6 feet and with wide apertures.

I tend to agree with you that wide angle lenses need a little different technique and that using several AF points may work better.

If you are going to use a single AF point with a WA lens, you must be MORE careful not to rotate the camera as you recompose even slightly - it roatates the plane of focus much more dramatically than with a longer telephoto lens, and will build in the errror of focus recompose.

As I think about this subject, Focus -Recompose has been widely discussed on the web, but no mention has really been made on the effect of focal length on focus recompose - usually the discussion centers around portraits and moderate length teles.

But Focus -Recompose with wide angles will have an even greater effect since the angle of view is so much wider for the shorter focal lengths, the plane of focus can be rotated much more than with a telephoto lens. And since the FOV is so much wider, there will probably be lots more straight, bright contrasty lines for the AF to be confused about as well. Does this make sense re focusing with WAs,, Blurmore??

Let me first start by saying that I don't use focus recompose, ever. I've broken myself of that habit. I have on occasion zoomed in focussed and zoomed out, which seems to work especially at f8-f11 where dof is huge.
When I shoot groups I feel like I need a split, plain ad simple, as the off center focus points only respond to horizontal or vertical lines and that just doesn't really work for groups of people. Wedding dresses and tuxes are notorious for fouling up contrast based focus in low light. In good light no problem. I'm also going to experiment with focussing then using a bump of the focus ring right to get into the hyper focal range, kinda like focus bracketing. I would chalk it up to backfocus issues and send the camera and lens in (if I had the time) but this only occurs with groups at wide angle by the time the shot is compressed even to 35-50 mm focus is fine.

pathfinder

Jason,
I really wasn't insinuating that you are using Focus Recompose. You were very clear in your original post and I am sure you were correct in that regard. My post should not be construed as critical in any way, that was not my intention whatsoever.

I was more in the mode of thinking out loud about WA lenses, and it occurred to me that wide angle lenses will have the focal plane shift much more when shot hand held ( as opposed to using a tripod - you did not specify and some wedding shooters use tripods, some don't) if the camera is rotated at all, and that this resembles the way focusing gets done erroneously in focus recompose. So even without recomposing, unless the camera and its associated plane of focus is rock stable, a wide angle lens is going to have more error than a longer focal length - With a long tele you cannot rotate the focal plane very much or your subject disappears, but with a wide angle, shifting the subject from central to the edge of the frame will rotate the plane of focus significantly.

I know I find myself less fastidious about focus with a wide angle lens sometimes because I know DOF will save my buttt. Something I never do with long glass, because I know there is no margin. Just human laziness sometimes, I guess, on my part.

And your comments about the general tendency of the central AF point to being so much more sensitive than the peripheral points, which is why people tend to prefer to use the central AF point, even if it is poorer technique to do so, are so pertinenet. This is more true for the APS sensor based camera than the 1 series. But it is something we all have to deal with from time to time.

Blurmore

Path- No worries I was more stating that I used to focus recompose and I dropped it like a bad habit after I got the 85 f1.8. I do use a tripod but only when absolutely neccessary to burn in the background. I won't go over 400 iso for my groups so a dimly lit church will often mean I am on the tripod. The tilt aspect of focus recompose is something I've pondered trying to use to my advantage for hyperfocal distance. Like focussing on the feet or legs then recomposing to the whole group. Unfortunately weddings aren't the place/time to expirement, and inviting 15 of my closest friends to a church for group pictures well... But each week I try something new on few shots to see how it works, and foot focussing is one I am definately going to try. Ultimately I think what I need is a katzeye split, after all I do have a 20D as a backup to the 30, I could use the 20 for WA groups and the 30 for everything else.

ziggy53

Blurmore wrote:

... It seems that if I use a single point for a bridal party or wedding group my focus is almost ALWAYS behind the party. ... BUT Eureka when I put the camera in 'uncle harry' all point mode it more accurately determined where I wanted focus seemingly by anchoring more points of focus in a plane and guessing more acutely. ...

Blurmore,

This is "exactly" what I noticed with my XT body and Sigma 18-50mm, f2.8 lens. When I used single center-point focus with the wider part of the zoom, the camera was consistantly focussing to the background. When I use all focussing points, I get much more consistant group shots. (I have two bodies and it is seemingly the same with both.)

Since you and I are getting similar results with different bodies and lenses, but the lenses are similar in focal length, I would suggetst this is more than coincidence.

I have already adopted using all focus points as the standard for that lens as it doesn't seem to impact the longer focal lengths negatively.

Thanks for pointing this out! (I thought it was just me.)

ziggy53

Red Bull

pathfinder wrote:

Steven,
I am sure you are aware that focus, recompose guaranties that your focus will be incorrect at distances less than 10 feet or so!? Canon even had a link on its website NOT recommending focus, recompose. ( I can't seem to google it here this mornming!)

Eeek! Looks like I need to break a bad habit.:uhoh I've never realized any problem when I focus and recompose. Maybe I'm just blind. Anyway, thank you for telling me.

THE TOUCH

Red Bull wrote:

Eeek! Looks like I need to break a bad habit.:uhoh I've never realized any problem when I focus and recompose. Maybe I'm just blind. Anyway, thank you for telling me.

ME TOO! I would have never saw this one!

Thanks guys! Great article on the first link.

gtc

also
if you are using a tripod then stopping down should give you more DOF so that the af works correctly-also you can switch to manual and get it right that way.

3rdPlanetPhotography

Now that I've focused, I guess I need to recompose and change my setting from single focus point to all-of-em.

thebigsky

Wow, I'm glad I found this thread, guilty as charged, no more focus recompose for me, the article in that first link makes it very clear.

erich6

Could it be that when you use multiple points the camera is able to find the most optimal focus plane to make sure all of the subjects in the field are in focus? If you use only one focus point then the camera will optimize for that point only. I usually try to put the focus point at least 1/3 of the depth of field that I want to capture since the depth is usually split 1/3 toward you and 2/3 away from you....

Erich

Scott_Quier

erich6 wrote:

Could it be that when you use multiple points the camera is able to find the most optimal focus plane to make sure all of the subjects in the field are in focus? If you use only one focus point then the camera will optimize for that point only. I usually try to put the focus point at least 1/3 of the depth of field that I want to capture since the depth is usually split 1/3 toward you and 2/3 away from you....

Erich

I don't know about optimal, but the dRebel and the 20D (the only cameras with which I have direct experience), when using multiple focus points, will using the focus point that registers on the closest object. That is, all the focus points will determine the distance to focus and the camera will choose the closest focus distance so registered and indicate that by flashing the focus point(s) that provided that focus distance.

Blurmore

erich6 wrote:

Could it be that when you use multiple points the camera is able to find the most optimal focus plane to make sure all of the subjects in the field are in focus? If you use only one focus point then the camera will optimize for that point only. I usually try to put the focus point at least 1/3 of the depth of field that I want to capture since the depth is usually split 1/3 toward you and 2/3 away from you....

Erich

This is sort of like A-dep mode that selects the right aperture for subjects in different planes. I'm not sure that I've read about this in the manual, a hyperfocal compensation using multiple points, but it makes sense. My testing in the last week has brought me to the conclusion Ziggy came up with, all are better than one for a stand focal length lens, unless focussing on something way off access. My tests with all points have yielded much less focus error at wide angle, and no detrimental effect when shooting at 55mm telephoto and large fstops.

gtc

dof preview and stop down metering
i tend to only use the single focus point for macro,choosing to focus on the mid-point of the area you want in focus.as the efs-60mm macro is my only af lens thats the way it has to be for the time being.

the dof preview button is then used to determine how much wider your dof has become,either side of the mid point you selected.if you dont want to do this or if its too dark then the auto depth of field feature is a good one-they should have kept it in my opinion.

if your group or subject is too deep to achieve the focus you want, then you have to either stop down even further until it becomes sub -optimal

eg diffraction starts to affect sharpness

or shutter speed is so low that the subjects have to freeze and stare, a la 19th century

or you have to bump up iso to lumpy 800 or higher

or you need to use a flash (flash and tripod can't be beat for a group shot)

or you have to recompose the group(can be good fun)

the good old manual lens ,using stop down metering ,does this DOF preview for you when you stop down.

nature shooters use single point AF to focus on the neck of ,say, a bird in flight flying towards you, so that they get the eye in focus.

or predictive AF (havent used this with a tele yet so can't comment)

or with a manual focus lens, focus manually ,wide open ,then stop down with the auto- diaphragm off in aperture priority mode until you get the shutter speed and dof that you want.

Dante

Great discussion
Great post, information and discussion!

Thanks all.

-Dante