Focus, what or who is to blame? - DPChallenge Forums

Sigma 28-80mm f/3.5-5.6 Aspherical HF for Pentax

12/14/2007 06:31:12 AM · #1

I'm using my 70-200 2.8 lens, and noticing a lot of focus problems. This lens has been to the shop, and it's a lot better than before, but still has intermittent problems. Here's what I'm seeing...

2 photos, shot basically identical situation. 77mm zoom, ISO 200, f/2.8, shutter around 1/500s. This OOF photo is one of the worst ones, but roughly half of them come out front-focused, nearly half are good, and the occasional shot is backfocussed.

So, what's responsible for this bad focussing? The lens, or the camera, or what? I don't really understand how the focus system works, so I don't know what to do.

Do I need a better camera to focus accurately at f2.8? My kit lens seems to be OK at focussing, but with it's much smaller aperture and focal length, it's not going to show the problem anywhere near as much. Plenty of people have the 50mm 1.8. Does that focus just as badly on the 400D?

Obviously I can get around it by closing the aperture - it certainly improves with f5.6, but I didn't buy a 2.8 lens to shoot closed up. The occasional shot in focus proves that it can be plenty sharp at 2.8. I can even live with the odd missed shot from focus problems, but not 50%!

I've tried center spot only focussing, and strangely enough, this actually seems to make it worse. It gets more accurate with all the points selected.

So, anyone with a long 2.8 or bigger on a 400D want to comment? Maybe I'll have to hunt around for another camera and another lens to test stuff side-by side in the real world and find out what the missing link is.

Message edited by author 2007-12-14 06:33:26.

Pug-H

Pentax K10D

12/14/2007 07:10:16 AM · #2

Try using manual focus and see how that works out.

justamistere

Nikon D3s

Nikon AF-S DX Zoom Nikkor 17-55mm f/2.8G IF-ED

12/14/2007 09:02:43 AM · #3

For the best results, I think auto focus needs to be some "minimum", direct light, such as a modeling light.

Even if using a flash/strobe. I have heard that Nikon uses infra red light for it's auto focusing system.

I know the D200 has a menu choice that requires the auto focus system to focus on a subject before allowing you to click the shutter. That can be turned On/Off.

Leica 18-56mm f/3.5-5.6 Vario Elmar-T ASPH

12/14/2007 09:33:45 AM · #4

Tried using manual focus on another shoot. Over a decent distance, focussing plus or minus one foot was something like half a mm movement of the focus ring! Add that to the fact that with my eyes, I can't even see these focus problems in the little viewfinder, and manual focus is definitely going to be worse. :) I will have to give this a good go sometime though, just to get a feel for it.

We actually had pretty good light for this shoot. We were outdoors, with the models in diffused or dappled light, with handheld reflectors reflecting direct sunlight onto the models from a few metres away. This particular angle is side-lit, but it was the same on other more front-lit shots.

I've had the same sort of problems shooting ISO 1600, f2.8, 1/30s, and there I can understand low light being an issue - although focus doesn't seem to be noticeably worse, even at this low light level. Anyway, at ISO 200, 1/500s, that's about 7 stops more light, and should be no problem.

I still don't know if it's the lens or the camera that does the focussing. I've heard both. It has to be the camera that does most of the work, I would think, because different cameras have different systems, but lenses are always blamed for back or front focussing. I don't really understand how the lens comes into it, or why the camera isn't responsible for adjusting the lens until it is right!

MAK

Leica CL

12/14/2007 09:42:05 AM · #5

My wife has the 350D and we had focusing issues with her lenses that are sigma.. now we know that there are softness probs using some sigis on the canons but this was a bit too soft, we sent it back to the shop and there is a problem with the 350D. the 400D is very similar so it might be an issue with the actual camera itself. You may want to ask them to test it out for you and even let you put the 70-200 on another body in the shop to check it is not the lens that needs setting up.

Good luck

dwterry

Canon EOS-1D Mark III

Canon EF 24-70mm f/2.8L USM

12/14/2007 10:44:57 AM · #6

See if this helps: Testing auto focus accuracy

Ann

Olympus OM-D E-M1

Olympus 12-40mm f/2.8 Pro

12/14/2007 10:55:00 AM · #7

I'm convinced that focusing for close up portraits is art as much as science. I'm pretty sure I haven't fully mastered it yet. I'm assuming the lens actually focuses correctly. You *have* tried sticking it on a tripod and focusing on things that don't move, right?.

I don't have a 350D, but with the dynamic auto focus on the D300, I can see in the viewfinder what the camera is trying to focus on, and half the time it's not what I want (cheek, ear, eyebrow, etc). I've only been working with it for a week, so it may be that I haven't learned how to use it yet. At f/5.6, it doesn't matter that I've just focused on the cheek instead of the eye, but at f/2.8, of course, it does matter.

In single point focus, it's really easy to get sharp focus on whatever you want, but then in that second when you recompose before shooting, either you or the model can move, which will affect the focus.

Solution? I have no idea. But one of these things is probably your problem. I leave the solution to the big kids here.

Prof_Fate

12/14/2007 11:15:31 AM · #8

How are you focusing? One point, all points, one shot, AI servo?? There are different modes and methods for a reason.

What mode is the camera in? Auto modes will screw you up. You know what you want, so use a 'creative' mode to get it.

Try setting to AF point to one point, the center point only.
Set it to one shot focus.
go to AV mode and dial up F 2.8. The camera will pick a shutter speed - get 1/500 or better(for now).
STAND STILL. Have the subject SIT STILL. At 2.8 the DOF is shallow and in one shot focus mode the camera/lens will not adjust once it's focused, it's focused. Movement from you or the subject will cause problems.

The camera focuses using CONTRAST. Look at the focus points in the viewfinder. The center one is a cross point - the others see 'lines of contrast' that are 90 degrees to their orientation (the top point sees vertical lines - it won't focus on a horizontal line) So now focus on an eye - there are lines there for the camera to see.

If you are using the shutter button only (there are other buttons that can be set to do the focus) HALF PRESS and the point that has focus (the center one if you've set it so) will light up. If the camera does not have focus you should get a blinking (one blink or constant on...depends on teh camera model) light on the line with the shutter speed/ap in the viewfinder. You can also turn a beep on or off I believe - when it gets a focus lock will will BEEP. Then push the shutter the rest of the way and take the picture.

Face parts with low contrast (a cheek for example) will not allow the camera to focus. Using all the points (in theory) means the camera should pick the closest/largest/most centered item to focus on, BUT in reality it will pick the highest contrast item, often the background.

Also watch for the minimum focus distance - it looks like on your lens it can't focus on subjects closer than 5 to 6 FEET.

Message edited by author 2007-12-14 11:18:15.

12/14/2007 07:05:24 PM · #9

Hi Prof, I've used a number of different modes, but my normal method for shooting is exactly what you say. AV mode, one-shot focus. I would normally use centre point only for these type of shots, but I've found it to be worse than using all points, so I force myself to stick to all points. Everyone is standing still, and so am I. I can recognise camera shake in the final shot compared to out of focus, and this is definitely out of focus.

The camera indicates that it has got successful auto focus in every shot - it's just not always right. I've turned off the beep, but it does flash the appropriate focus points, and I keep an eye on this to make sure it's right.

dwterry, I have done focus tests before, but that one is a little more detailed than most, so I will give this one a whirl with both my lenses. I will also test it out under stome different lighting conditions to see. I like the testing of from front end or from back end.

Nikon AF-S Nikkor 70-200mm f/2.8G ED VR II

12/16/2007 09:16:48 AM · #10

OK, I ran some more serious focus tests, using the procedure in

dwterry's link. Anyone know what it means if the manual focus is out? Sure some of it is my eye, but some of the final shots came out so badly out of manual focus, I can't believe that it was that bad when I shot it, and for one particular set, the manual focus was out very much the same as the grossly out of focus auto focus.

Would problems with manual focus maybe indicate a problem with the mirror positioning? Or focus screen, or prism, or something like that?

I'm beginning to suspect a camera problem, because the kit-lens was shown wildly varying focus results too, with roughly half the shots having the focus point noticeably outside the DOF. It's just all a lot more noticeable on the 2.8 long lens. However, when it was good, it was very very good, with some amazingly accurate runs of focus, followed by some amazing runs of out of focus shots, including several runs where it would say in focus, no matter where I turned the focus ring!

MattO

Nikon D4

12/16/2007 12:36:29 PM · #11

Any chance there is a local DPC member that you can put a Canon 70-200 on and take some test shots. Or perhaps put your lens on their camera to test it. ALso do you have the diopter set the way the manual tells you to have it set?

MattO

skylercall

Canon EOS-7D

Canon EF 70-200mm f/2.8L IS USM

12/16/2007 12:46:13 PM · #12

Originally posted by surfdabbler:

So, anyone with a long 2.8 or bigger on a 400D want to comment?

I have shot literally thousands of pictures using my 400D and the Canon EF 70-200mm f/2.8L IS USM with no problem such as you describe. Sorry!

dwterry

Canon EOS-1D Mark III

Canon EF 24-70mm f/2.8L USM

12/16/2007 12:48:39 PM · #13

So... using the test procedure I gave you, you did not find a consistent "front focus" or "back focus" issue? I would expect one or the other. And if that happens, you can send your camera and lens in to Canon to have them calibrated together.

As for not getting the focus in manual mode, two thoughts there:

1) Without one of those high precision focusing screens, I would think it would be hard to tell, accurately, how well you've focused in manual mode.

2) As you said that, it just dawned on me ... if there is a camera/lens adjustment required, chances are the same issue will be present whether using AF or manual because in manual you'll be "seeing" the image front or back focused as well.

FocusPoint

XIAOMI Redmi Note 8 Pro

12/16/2007 12:50:49 PM · #14

Originally posted by surfdabbler:

I'm using my 70-200 2.8 lens, and noticing a lot of focus problems...

Hey... don't blame me :/

Prof_Fate

12/16/2007 01:00:06 PM · #15

manual focus can't be out.
how is the Diopter adjusment on the viewfinder set? Mine gets bumped every once in a while and that is the ONLY way manual focus can be off - you focus to what you see in the viewfinder of course and if it's not focused you're going to focus incorrectly.