Correct focus in viefinder but far from it in picture, with DSLR.

Hi :)

 

Can I first off say that my newly aquired 350D is my first ever AF camera.

Focusing...well I'm very well used to it and over recent years it has been

primarily on my 5x4 MPP and my Nikon 35mm kit (secondarily M645 and 124G).

Suffice to say I prefer to focus manually and know how to do it.

 

So I have got a 350D (second hand but new to me) and a Canon EOS 50mm 1.8 MkII

(Had it a ling time for use with EOS bodies never noticed anything wrong with

it but never used it AF). I was trying out the combo in the garden - the lens

was wide open and set to manual focus. When I finally remembered there was a

preview screen available (!) I saw the actual flower I focussed on was well

out of focus and the plane of sharp focus was considerably nearer. I went and

investigated further by photographing a ruler.

 

Picture Link :

http://i271.photobucket.com/albums/jj139/BillsBucket/focusjump.jpg

 

I manually focused on the 1 foot mark (red dot with a little bump of blu-tack

behind it) and withoubt a doubt the focus on the picture was no where near

where it should have been (half way to the 2 foot mark). I am totally

perplexed as to how this is happening. I though it impossible that the

viewfinder image could be out of alignment with that of the 'film' (sensor)

plane. And the focus does stay where I put it from frame to frame.

 

Anybody any ideas what is happening here? Somebody said in the camera settings

there will be something stopping it focusing the way I want it but I have just

re-read the 350D Users Manual and the section on manually focussing reads just

how I have been doing it.

 

PS after the above manual focus shot I allowed the camera to AF at the same

point...and it focussed no problem : :-\

The diopter? adjustment is probably off. There should be a small wheel next to the

viewfinder that allows you to adjust the viewfinder.

This does NOT sound like the diopter adjustment is the cause. In that case the view through the viewfinder would never be

sharp, but the photograph might well be. In other words, the symptoms would be the opposite of what we see here.

 

Sometimes when a problem like this is reported it is due to an autofocus problem - often one of two. First, the camera/lens

could be out of adjustment causing the autofocus system to set the lens incorrectly and giving front or back-focus. A second

type of autofocus problem can be user error - the photographer is not using AF correctly and perhaps the system is not using

the focus point that it on the subject or the photog is using the focus and recompose method at a wide open aperture (narrow

DOF).

 

Not sure that this is what is happening here.

 

For one thing, don't trust the distance scale on the lens barrel. There are reasons that it might not register accurately, even if

the lens is capable of good manual focus. Try to focus manually using the (somewhat small and dim) viewfinder. Perhaps take

a few photographs with the focus displaced slightly in front of and behind the "best" manual focus setting determined this

way. This should rule in or out a problem where the lens simply is incapable of focusing correctly.

 

Dan

Even assuming the camera is OK, the diopter is correct, and your eyes are good, you're still likely to have problems focusing fast lenses with the screen in an entry-level body. An explanation for why is <a href="http://www.jayandwanda.com/photography/dslr_man_focus/man_focus.html">here</a> (that's not using a 350D, but the same ideas appy). You might want to investigate an after-market focusing screen if you're going to use MF a lot.

Once I received my camera back from Canon repair, and either the mirror or the pentaprism became misaligned. Basically, looking through the viewfinder, when the camera achieved focus, everything looked out of focus. However, the image came out fine.

 

Look for a similar problem. Set the center AF point only and let it focus on point on your measuring tape. If the AF points aren't lighting up (or the lights are showing up in the wrong place), you have your telltale sign.

If the AF is focusing properly, but the manual focusing isn't, then the problem would seem to be a misaligned focusing screen. Has the camera been fitted with an aftermarket screen - like a split image one? These are often not the same thickness as the original and so won't be aligned properly unless shimmed to be exactly the same. Some are even too thick so shimming worsens the problem. Alternatively, maybe the original screen has been refitted by the previous owner but the shim missed out.

did you get a focus confirmation when you were manually focusing? i.e. a beep or the green light in the viewfinder?

Hi thanks for the responses so far.

 

 

Adam, as Dan says the Diopter, seems to just pull the whole picture in or out of focus. I have just been playing with it to see if I could squeeze anything unexpected out of it.

 

 

Dan, I may be missing your point but I'm not usung AF and I'm not using barrel markings (the lens has none).

 

 

And to Dan and David I am using the 'normal slr' viefinder. The 350D only shows pictures on the LCD screen once taken. And to me with the 1.8 the viefinder seems fine to use.

 

I have just replicated the 'ruler' shots. Infact I even tried assisted manual focus - where the camera bleeps when you get focus confirmed over one of the focus spots. In each case Manual fell short of the mark. If I flick to AF (using the centre marker) focus is near enough spot on. And when set to AF, even though the camera had just assisted to confirm focus in the centre spot, the lens barrel still moved. It moved away and back, but not exactly to where it had been. I wonder, eventhough the lens was wide open and accurately focused through the viefinder and set to Manual focus, does the camera body materially actuate anything in the lens when shooting?

 

I have an early start tomorrow so better get some kip but for now all shots will have to be AF till I get to the bottom of this. Thanks again all.

Sorry..slow typer.

 

Pete... AFAIK the screen is standard but I do not know its history. I may try to get in touch with its previous owner if all else fails.

 

Mark... Yes just replicated the ruler shots with shutter button half depressed to get assisted manual focusing. But Manual Focusing - full or assisted gets the errant focus on the picture. AF..where I suppose a mechanical action is applied internally to the lens in a different manner - spot on.

 

I do wonder if somehow a lens element (or something?) is held taught (or something? -yep clutching at straws) if AF is being used vs MF? Humm.

 

Catch up soon!

I use manual focus zeiss lenses on my rebel xt. At first I had a simple adapter and had to rely on my eyes to focus. When I used the nice bright viewfinder from the contax slr's I had very little problems with focusing. However with the small dim viewfinder on the rebel getting focus was almost impossible. I then swithced to a AF confirm adapter and have since been successful with manual focus. I am surprised that your photos are out of focus when using the manual focus assist. When I go home tonight I will try some experiments and see if I can get a better idea. Take care.

>>>>"And when set to AF, even though the camera had just assisted to confirm focus in the centre spot, the lens barrel still moved. It moved away and back, but not exactly to where it had been."<<<<

 

Normal for THAT lens.

 

Jason

Remember it's not just the screen position that affects focus in an SLR, it's the mirror position too. It could be that the mirror isn't seating properly, or has been knocked or adjusted out of whack. If the mirror rest is broken, that would explain why focus varies from frame to frame.

Further to the above. The Canon AF system uses a semi-transparent patch in the middle of the mirror to focus through, so AF operation wouldn't be overly affected by the mirror position. Whereas with MF, you're focusing using the reflected image on the viewfinder screen.

I think David Long has it pegged best... the AF screen simply cannot focus at f/1.8. Either stop down to f/4 when focusing manually, or install a manual focus screen if you want to use fast manual lenses.

Hi

 

I have been lucky enough to get my hands on two other lenses today. I tested a lot and have just been through lots of comparisons.

 

I think my conclusion is the 50/1.8 seems to have an intermittent problem. It can seem focused in the viewfinder and can confirm it is focused during autofocus but sometimes it does and sometimes it doesn't produce a focused image. This is not restricted to a wide aperture nor focus distance. I am hoping it is the lens and not the camera body thats faulty. The details and test shots are...

 

I have UL some new ruler shots I did with these lenses. Some people might be interested in how a manual focus (MF) stacks up aginst auto focus (AF) over a range.

 

I got a Canon 35-80 and a Cosina 28-80. There are a few 'new ruler' shots here. The file names hold the info eg 3580-50 is the 35-80 at focal length 50mm with MF, AF and AFASS (the latter I'm terming assisted autofocus meaning I slowly focus manually till I get the focus confirmation beep). Each is a screen grab of organised files in photoshop. Sound scarey but isn't really, and I'm only showing 3 groups. I don't want to bore anybody badly!

 

http://i271.photobucket.com/albums/jj139/BillsBucket/3580-50newruler.jpg

 

http://i271.photobucket.com/albums/jj139/BillsBucket/3580-80newruler.jpg

 

http://i271.photobucket.com/albums/jj139/BillsBucket/2880-50newrulerII.jpg

 

 

The take home message for me is that MF is every bit as good as, if not better than AF and PLUS in the case of this original query, both the borrowed lenses I used focused on the money.

 

AS a departure from near shots I decided to shoot the metal grid in safety glass about 50 to 60 yards away (roughly estimated) and try a few other things...namely to shoot not at 1.8 but at 5.6 with the Canon prime 50mm lens I was having trouble with in isolation. I also threw mirror-lock into the mix. And now we have the resurgence of a problem with the 50/1.8 which now is focusing with MF (not that surprising given the distance and f-stop) but isn't focusing the picture with AF (btw in AF mode the camera refuses to shoot if it can't focus...so it obviously thinks it is focussing...which is almost contrary to what it was doing to prompt me to start this thread. Anyway here is the prime 50mm at 5.6 MF vs AF both with mirror-lock and 'normal'.

 

http://i271.photobucket.com/albums/jj139/BillsBucket/1850atf56MFvsAF.jpg

 

Amazing eh? That the camera can think that mess is in focus! I then threw in the 35-80 (at 80) simply to compare MF vs AF and both were on the money even if the lens was a bit bleary and low in contrast (lower file is AF).

 

http://i271.photobucket.com/albums/jj139/BillsBucket/3580-80MFvsAF.jpg

 

I had not seen at this point the AF shots had been totally messed up and was playing around with the free play of the barrel and had moved to f8 for a comparison to the 5.6. And now would you believe the prime 50mm is working again.

 

http://i271.photobucket.com/albums/jj139/BillsBucket/1850atf8MFvsAF.jpg

 

 

 

Summary.

 

As I pointed out at the start AF/DSLR is new to me. I have concluded that the lens has an intermittent fault. Although its nice when it works ( ! ) it is only that lens that fails to focus either MF or AF and at apertures of 1.8, 5.6 and 8. If anyone is still there ad knows something I have missed please pipe up. (Don't worry the details done with now I think).

blimey...I have never given up and have come across back focus and now front focus, and now front focus specifically with this lens...and for the brave how to fix it...

 

http://forums.dpreview.com/forums/read.asp?forum=1029&message=21351476

Coming back to this thread I learned something new - how the venerable 50mm f/1.8 is adjusted for focus during service, which gives a further insight into Canon's AF system. And David Long's link to Jay Turberville's excellent description of manual focus through a viewfinder complete with example images to illustrate is really worthwhile.