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Ollie
04-03-2006, 10:10 AM
Say, I was hoping somebody here might be able to give me a good, quick answer. In case it's important, I'm using a Canon 350D and a Canon 50mm f/1.8 lens, the one that's just under $100 USD.

At Tekko this weekend, I had the first chance to use the f/1.8 lens. When I did closeups (say, bust and up), it worked great. Even at 2.0 or 1.8 I could get the people in focus and the background out of focus, using the autofocus function. However, when I tried to get fullbody shots, the focus was usually off from mildly to horrible unless I went to 4.5 or 5.6.

At about 4 to 5 meters away, about the distance I needed to get a full body shot, the DOF would be about 30-some centimeters (about a foot for you imperialists). I figure that should be enough, but I tended to keep getting blurry photos. In fact, in some cases, the background was more in focus than the subject. :lost:

So, either I still can't use the darn autofocus (grrr) or maybe the lens just won't do autofocus at that distance (doh). I was only using one autofocus point. My guess is that since it uses a cheaper motor, it can't adjust the focus to within the 12 inches or so needed to get the right focus on the subject, and uses a "best guess." Or maybe that at a high aperture the extra light decreases the SNR for the autofocus system.

Anybody able to give me a hint at what either I'm doing wrong or if I'm expecting too much of the camera?

jtnishi
04-03-2006, 10:22 AM
You're certainly not expecting too much of the camera. That's very strange behavior, admittedly. There might be something with your technique or the equipment. How are you dealing with your focusing? Are you putting the single focusing point over the right area (eyes)? Are you using a focus and recompose method, in which case are you considering the possibility that the lens might be changing it's focus (one shot mode works best then, if the subject isn't moving)?

You might also want to take a very close look at your closeups again. Think about where you put the focus point (presumably the eyes), and check visually and see that the focus is on the eyes. For example, I noticed that with my 50mm lens on my XT, the focus is indeed off, though I never noticed it until I actually looked at my closeups on screen and looked where the focal plane was.

You can hunt down on dpreview about all sorts of tests you can do to check the focus of your lens. However, the simplest and most pragmatic ones are probably just to try a few subjects at about 5 feet or so from a somewhat oblique angle, put the focus point over a point that you can remember, focus, shoot, repeat a few times, and then check on a monitor whether things are in focus or not.

phaedrus
04-03-2006, 10:30 AM
Depth of field issue. At F1.8 it's going to be very shallow, so any errors in focusing (manually or autofocus) will push the field past where you want it to be. Might be your autofocus is a little off: Not so bad that a smaller aperature of 4.5 or 5.6 won't compensate, but enough that F1.8 will show it.

Re-reading your post you already posited that possibility so...'yes, that's probably it'. :p

Ollie
04-03-2006, 10:42 AM
Thanks, I'll have to try that out.

I am focusing and recomposing, but it's always first on the eyes and then only a bit of movement. Unfortunately, the pictures are obviously blurry. I'm not talking about a slight front or rear focus bias of a few centimeters, but like a foot or two at least. Nothing is in focus, which is what aggrivates me. If it were a small error I'd just count it up to poor eyesight and the shallow DOF.

The autofocus is in one-shot mode, but even if it were on servo or such I'd expect it to have at least part of the subject in focus. Again, same thing with the shallow DOF; at least part of the image should be in focus. It's not slightly off, it's way the heck off.

Here's an example: http://images.cosplay.com/showphoto.php?photo=646398
and another http://images.cosplay.com/showphoto.php?photo=646399

And here's a perfect example, where you can tell the focal plane is several feet too far back.

http://images.cosplay.com/showphoto.php?photo=646399

I'm going to hazard another guess that since a larger focal distance means a larger DOF (and hence a better chance of getting a non-blurry subject), the AF system biases towards longer distances when it's not sure. The lighting is not particularly good here, so I figure the AF system couldn't resolve contrast very well. On top of me not focusing well in the first place. I'll just have to try to learn to use the manual focus better.

jtnishi
04-03-2006, 12:02 PM
Okay, now that's definitely a problem. It almost looks like the system is completely changing it's focus.

This is where focus and recompose might indeed be the problem. There is a slight limitation of the camera that you might've noticed: the half hold position on the camera isn't exactly all that steady. In easier words, it's easy for your finger to slip, come back on the button, and then for the camera to change it's focus, only to throw everything off. This looks especially like it might be a possibility. I've sometimes had issues where I focused, recomposed, and my finger slightly slipped and changed focus.

What I'm about to note is very specific for your camera, since it's also the same body that I use. This is somewhat advanced from basic point-and-shoot style operation with the camera, but if you're willing and understand what I'm about to say, try this: camera in one shot autofocus, single focus point (I like center myself), and change custom function 4 from 0 (default - exposure & autofocus on half press / exposure lock on *) to 1 (auto exposure & exposure lock on half press / auto focus on *). To shoot with this style: you point your focus point on the subject focus point, you press * until it gets your focus right, you half hold the shutter to get a proper exposure, recompose, then shoot. The big disadvantage of this style of shooting is that it does not work very well with moving subjects at all, and it's a bit cumbersome to quickly get a shot off. The big advantage is that focus never changes once you've set it - it's locked on. For cosplay portraits in a steady pose, this method works VERY well for me, and is my default mode on the camera, much to the chagrin of friends who try to borrow my camera and use it expecting it to behave like theirs. ^^;; Try it perhaps, and see if it helps.

Ollie
04-03-2006, 12:40 PM
Okay, now that's definitely a problem. It almost looks like the system is completely changing it's focus. This is where focus and recompose might indeed be the problem. There is a slight limitation of the camera that you might've noticed: the half hold position on the camera isn't exactly all that steady. In easier words, it's easy for your finger to slip, come back on the button, and then for the camera to change it's focus, only to throw everything off.

As for just recomposing, that would change it by a few inches. This is a few feet.

Son of a gun, I've noticed that happen a few times, and wondered why it was all of a sudden refocusing. Might be happening here, too, and I just didn't hear the focusing beep. I'll certainly have to try that out with the separate focus lock button. Does one have to hold *, or does just hitting it once lock it?

AdamN
04-03-2006, 01:23 PM
I have the same lens and camera. With an F-1.4 to 2.8 range lens focusing is critical.

*** YOU MUST 1st point your focus on the target (face), then HOLD / HALF-PRESSED the shutter while you reposition view, then CLICK. Do NOT let go of the shutter while repositioning ***

For closeup headshots / portraits it is easier to pick a focal point. Incidentally, it is better to focus between the eyes near the nose because that will keep the overall face in focus.

For full length body shots you want to set your In-Camera focal point (red dot in viewfinder) to the center (or wherever you want if you are comfortable with quickly adjusting it per shot). Set your F-Stop to whatever you want and aim the red dot in the viewfinder to the focal point (face), half press the shutter, reposition view, and click.

NOTE: If you want more blurred out backgrounds behind the person at a full body shot then you need to get a longer / zoom lens.

Program to see different Aperature / Zoom effects: http://www.tamroneurope.com/dof.htm

If you are still having problems after testing out your lens, then you may have an issue called BackFocus.

http://homepage.ntlworld.com/dltucker/nikond70/faqs_backfocus.html

Some lenses that fall through the quality control testing can focus properly with closeup objects but for larger scenes pick the background as the focal point even after repositioning (your pictures for examples).

Do some test shots. Set up a test of 5 cans spaced about 1 foot apart and behind each other and change the red focal dot from left to right in your camera and take shots.

Other focus testing methods: http://www.leongoodman.com/d70focusnew.html

If you think your lens has BackFocus problems then you should get your lens replaced from the store you got it from or send it to a Canon lens repair (on warrenty).

jtnishi
04-03-2006, 02:52 PM
As for just recomposing, that would change it by a few inches. This is a few feet.

Son of a gun, I've noticed that happen a few times, and wondered why it was all of a sudden refocusing. Might be happening here, too, and I just didn't hear the focusing beep. I'll certainly have to try that out with the separate focus lock button. Does one have to hold *, or does just hitting it once lock it?

It's hit once. The idea is that the focus can only change while the * button is depressed when CF4 is set to 1 (or 3). Usually, you hold down the * button until it gets focus lock (in most cases, very fast). Then you let it go. It's pretty much like a button press that you just hesistate on letting go for a moment. From that point on, the focus is fixed. Focus won't change, even if the subject moves (ergo, why it isn't good for moving subjects - to adjust the focus, you need to hit * again).

Sometimes, it's hard to notice that the finger slightly slips off the half-hold position, but I have noticed it. There have been times before I changed the custom function where I'd have the button half-held, but then start hearing these beeps come off after I have the button held down. It seems that there isn't a lot of error room for "half held". That is, even loosening up ever so slightly on the half hold on the button can cause it to register as the button being let go.

With the custom function set this way, not only does focus only change when you want it to (godsend for focus/recompose), but if you use the partial metering mode, it makes it relatively easy to make sure you meter the right part of the picture before you recompose and shoot, so that the exposure comes out the way you want. Again, this method of shooting requires a bit of practice. But once you get used to having to use * to set your focus, it becomes second nature, and I sometimes wonder how I used to live without setting my camera this way. I only switch back when I know that things are going to move a lot (like in a concert or a masquerade, where people are moving a lot).

skypirate
04-03-2006, 05:49 PM
It might be the the combination of the 50mm 1.8 and the 350D.

I had a 350D last year from March thru May, and seem recall a LOT of people having trouble with back focusing. Check out the dpreview forum on the 350D.

Ollie
04-03-2006, 05:50 PM
In case anybody is interested, I did a test with the camera and a chart. Here are the results, all shot at ISO100 and f/1.8:

http://orobouros.cjb.net:81/~ollie/test/best_auto.png
http://orobouros.cjb.net:81/~ollie/test/worst_auto.png
http://orobouros.cjb.net:81/~ollie/test/best_man.png
http://orobouros.cjb.net:81/~ollie/test/worst_man.png

Overall, using manual focus got 7 out of 10 acceptable images, whereas the autofocus got only 3 out of 11 acceptable results.

I still can't tell why close shots are easy to get in focus but distance (> 2 m) are not. But overall, I guess I just need to figure out how to focus correctly.

SolarTempest
04-03-2006, 07:10 PM
First three images looks like you slipped your finger while recomposing. Focus lock button is a very good solution.
http://images.cosplay.com/showphoto.php?photo=646398

I had a similar discussion @ Flickr.com (Nikon D70/s group) a few months ago and we all kind of concluded on a few things:

- I own a Nikkor 50mm F1.4 and manual focus is practically impossible to do well
- DOF is only an inch or so (just breathing throws focus off)
- Lots of shooters (pro and amateur) use AF for very fast lenses because it's usually more accurate
- Split prism viewfinder is a good solution, but nobody wants to modify their $$$ DSLRs
- Recomposition results in focus errors, due to the hypotenuse effect
- Focusing on someone's face usually gets the nose, resulting in a softness in their eyes from F1.4-F2.
- Even if you manual focus, the camera's focus indicator is wrong almost all the time when it indicates your manual focus is correct (D70/s, but likely extends to many others).

Solutions:
- Use a tripod
- Use central AF sensor on less contrasty subjects because it's likely to be more accurate (both horiz and vert contrast sensors are there)
- For portraits, focus on the nose, lock focus, move camera in depth of nose, then shoot
- Continuous servo is good for non-fixed objects


I'd like to note the recomposition hypotenuse effect is compensated at further distances because DOF drops slower as you focus at further distances. (If I remember correctly).

Most people don't care so much about small errors in focus, but I like my shots tack sharp, and the F1.4 and F1.8 lenses are both capable of stunning quality and shots :eek:

staereo
04-03-2006, 07:12 PM
your camera is choosing the wrong autofocus point.

check your autofocus mode. im not sure if the 350d has selectable point af. if so, its selected on an outside point in the viewfinder, not center.

I have this lens. its impeccable, I assure you. Its your af point.

prime lenses are inherently fast, even if a slow prime lens.

jtnishi is unquestionably 100% correct here.

His advice will fix it.

Use manual point selection on your center point, use one shot mode, and try this out. It *IS* the issue. Textbook case.

Ensure that your subject is 1.5ft (.45meters) or more away from you. That lens wont focus closer.

Then follow jtnishi's advice. This is the issue, without a doubt.

Bruce

Joshuaaaaaa
04-03-2006, 09:10 PM
Others have kind of implied this, and if you have spot focus, you may want to use it. At closer ranges, or in some lighting situations, autofocus may get confused. Spot focus will let you focus on the center of the image, even if you have to focus on an object and then swing the body.

Ollie
04-03-2006, 09:21 PM
Use manual point selection on your center point, use one shot mode, and try this out. It *IS* the issue. Textbook case.
Bruce

Thanks for the tips. I tried it out and it's still not focusing correctly. In fact, it's got a strong back focus from what I can tell. I set up a row of action figures and shot them, using the custom setting, with the center point, and on one shot AF. Each time, I can tell where the focus really lies, and it's not quite in the right place.

I'm strongly suspecting that this is related to the fact that I have to wiggle the manual ring a bit on occasion to get the AF drive motor to move it. At this point, I'm too paranoid not to take it somewhere to get looked at. But that being said, I quite appriciate everybody's tips and will not be surprised if the lens (bought new two months ago) turns out to be A-ok.

jtnishi
04-03-2006, 09:34 PM
Thanks for the tips. I tried it out and it's still not focusing correctly. In fact, it's got a strong back focus from what I can tell. I set up a row of action figures and shot them, using the custom setting, with the center point, and on one shot AF. Each time, I can tell where the focus really lies, and it's not quite in the right place.

I'm strongly suspecting that this is related to the fact that I have to wiggle the manual ring a bit on occasion to get the AF drive motor to move it. At this point, I'm too paranoid not to take it somewhere to get looked at. But that being said, I quite appriciate everybody's tips and will not be surprised if the lens (bought new two months ago) turns out to be A-ok.
Wiggle the manual ring? Hrm, okay, that does obviously suggest something physically wrong, like a broken lens. Wouldn't be a big surprise, admittedly, since the 50/1.8 is pretty much the paragon of cheap plastic lenses, even if it does carry a lot of punch for it's price. And despite steraeo's point, these lenses don't always carry absolute accuracy, since it's the combination of body/lens (I know for a fact that my 50 does front focus on my XT by about 2" at a distance of 5' for autofocus - more than enough to be annoying when I want pinpoint sharpness on the eyes). That said, it would've been a bit helpful if you had mentioned the wiggle thing earlier, since that suggests that the AF motor isn't quite completely engaging the focus system, and that does mean that it might just be a mechanical problem with the lens causing misfocus. You shouldn't be too worried about taking it in. It is, after all, a cheap lens.

It does pose a more interesting question though: not to insult your technique, but are you looking carefully enough at the images through the viewfinder when you shoot? My eyesight's not that good myself, and I wear glasses when I shoot with my camera. That said, with manual or automatic focus, unless the lighting was really bad, my eyes could probably tell that the subject was out of focus if they're off by that much as long as I hesistate long enough to actually look at my viewfinder. My presumption was that the image at least looked in focus when you half held the shutter, which led to me presuming at issue was the AF system, which can do strange things like throw images off focus at times (I've had it happen often to me before). If the camera isn't completely engaging the autofocus, however, and in fact the image was incorrectly out-of-focus anyway, it would seem like it should be apparent in the viewfinder once you half hold the shutter. One of the first few rules I learned when handling the modern SLRs was to never completely trust autofocus.

As an aside, I do miss the days when split finder prisms were installed in SLRs. And I can't justify the cash to go up to the $3k+ Canons that can use the alternate finders.

Ollie
04-03-2006, 09:52 PM
I never really thought about the ring until I started worrying about the AF being correct. It's odd, seeing as it works wonders when the subject fills at least 2/3s of the frame, but after that, kinda goes to poo.

Here's one of the test shots, scaled down but you can still clearly see the effect.

http://orobouros.cjb.net:81/~ollie/test/figures.png

It's cropped, but I'm focusing right at Sagat (guy with the eyepathch) who was right in the smack dab middle of the frame. I didn't move the camera at all after getting an AF lock on him, and using only the center. If there is some other focal point being chosen, it's some sort of very odd camera problem. it seems pretty obvious the real focus is on THawk (guy with the torn shirt), who is a good six to eight inches behind Sagat. I'd even say the bishop and queen are in decent focus, too, and they're another few inches farther back. Even the boxer dude seems more in focus than the actual focus point.

My eyesight is a bit weak, but through the viewfinder things looked pretty good. Off by maybe a little, but not bad. I have a number of closeup shots where the focus is a bit off due to DOF, but otherwise fairly good. But, I think this last experiment shows that regardless of how it looks, the focus is shifted back. I may not have picked it up through the viewfinder, but even so, the images shouldn't have ended up that far off.

skypirate
04-03-2006, 11:10 PM
Have you got another lens to compare it to? Repeating the tests with different lenses, using comparable apertures could help you determine if it's the lens or the camera.

staereo
04-04-2006, 03:48 AM
I also agree, if you have to wiggle the manual ring to get it to work, somethings wrong.

I will say that I must have lucked out, because my 50 1.8 II has spot on focus. Even more odd, I can shoot soccer or even indoor volleyball and get a fast enough autofocus to grab sharp pictures. And they are spot on.

http://www.staereo.com/knc3.jpg

Isnt it funny how the same lens can be so different case by case. Definately an arguement for trying your lenses before you buy them.

After this was posted I dug it out and lined up some shots, and my manual focus only served to soften and remove focus from an already autofocused shot. Even being as careful as I possibly can.

Bruce

SolarTempest
04-04-2006, 07:16 AM
As an aside, I do miss the days when split finder prisms were installed in SLRs. And I can't justify the cash to go up to the $3k+ Canons that can use the alternate finders.
I was thinking about buying a split prism focusing screen (http://www.katzeyeoptics.com/page--Custom-Focusing-Screens--store.html) for my camera last year. Would be so nice! Still not willing to void my extended warranty though... :untrust:


I like Skypirate's idea.
Even if if you've got bad eyesight, picking a focus sensor and putting it on an object is pretty foolproof :toothy:

If you want to test your autofocus, maybe it's time to bust out the measuring tape. This idea seems a bit approximate, but might be worth a shot:

Set up a single object (something biggish might be good) a specific distance from the sensor in your camera (on a tripod would be best). My lens has focus markings at 1m, 3ft, and 4ft - hopefully yours is similar. Pick one of those distances to use. Then see if your camera focuses to the right distance on autofocus.

Best of luck on figuring this out!

Ollie
04-04-2006, 09:09 AM
Isnt it funny how the same lens can be so different case by case. Definately an arguement for trying your lenses before you buy them.


I guess that's a risk with cheap lenses. They mostly work as they should, but not quite. Luckily, it works great for anything within about six or seven feet.

http://images.cosplay.com/showphoto.php?photo=647079

Granted, it's sized down, but it's an order of magnitude better in focus. It does work for what I really wanted (closeups), so I guess I get what I pay for. Until I ask for work done under warrenty. Seeing as I've always had focusing issues, though, I'm going to try to use some other lenses and see what I can cook up. It could be that the autofocus finds a point at the very front end of the DOF. If that's the case, I hear that can be adjusted by screws inside the camera (which I'd leave up to a professional).

staereo
04-04-2006, 10:42 AM
All canon lenses are warranteed. I strongly suggest you return that lens for another.

I've had some nice canon lenses that were crappy, but a replacement of hte same lens yielded a much higher quality lens, even when they were identical models.


Return that lens and get another one. The majority of the people that have this lens are very happy with it. Here is fredmiranda.com's review of it. If you read peoples ratings on there, and compare them to other lenses that you know are high quality, like L lenses, the scores for this lens is SUPER high. These people are a good cross section of the photography field.
http://www.fredmiranda.com/reviews/showproduct.php?product=30&sort=7&cat=2&page=1

Get a new one. For sure. The lens is good stuff. To shrug it off would be a pity.

gmontem
04-04-2006, 12:17 PM
Here are a couple of links to focus test charts that some may find useful during those boring and lonely rainy days. :p

http://www.focustestchart.com/chart.html
http://photo.net/learn/focustest/index

Ollie
04-04-2006, 12:27 PM
Here are a couple of links to focus test charts that some may find useful during those boring and lonely rainy days. :p

http://www.focustestchart.com/chart.html
http://photo.net/learn/focustest/index

Yup, I acutally already used that when I did my first set of tests. I got the center just barely in focus (at about two or three feet distance) with the area in front of the center out of focus, but the DOF going back noticably. The real point of focus was actually several centimeters behind where it should have been. This was my first clue to something being off. I checked my "good" pictures, and at full rez can see how the focal point is actually a few inches behind the person, despite my focusing on their eyes. At that scale, though, the fuzziness is negligible once you scale the image down.