Lots of shots out of focus at wide apertures

[peter_viitasaari]

I got a DSLR less than a year ago and have started noticing that lots of shots are out of focus when running at f/1.4

 

I am running a Rebel XT with a 50-f/1.4 lens.

 

I thought the problem was caused by the AF not being able to resolve some subjects properly which is probably

true however, it's actually worse when I try manually focusing.

 

I am thinking that the design of the focusing screen does not allow for precision focusing but I am not sure.

 

Any info on running DSLRs with wide apertures would be great.

 

Best Regards, Hedghog

[john_g10]

Post some samples and provide some info like how far are you from the subject, etc.

[g dan mitchell]

You 50mm f/1.4 lens will, indeed, be less sharp at f/1.4 - that is a characteristic of this lens.

 

In addition, the very narrow DOF at f/1.4 makes accurate focus more critical. If you want to test focus, you can put the

camera on a tripod, AF a very easy to focus subject, and make a photo. Now switch AF off and manually focus just a

tiny bit closer, take a shot, focus a tiny bit further away, and take another shot. Compare the results.

 

(Not the most sophisticated test, but simple and it works.)

 

Dan

[john_hanlon3]

Hello Peter,

 

As G Dan rightly points out, the depth of field at f/1.4 is very limited and there's nothing wrong with your lens or camera. For a fuller understanding of depth of field go here: http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

[michael.gregory]

Peter,

Stop running.

[john_hanlon3]

Nice one Michael, LOL!

[bob_osullivan]

Peter:

 

Usually we can chalk this up to operator error. Using F1.4 is very difficult for some people even F2.8 can present problems do to the very limited DOF. Realize that at 5' subject distance you only have less than 2" DOF.

 

Anyway it is possible there is an equipment problem. So, to rule that out do this test. Lay a ruler flat on a table. Hold the camera such that you are shooting the center of the ruler at a 45% angle. Step into it such that the ruler fills the frame. Try with both AF and manaual. Focus on the 6" mark. This will help indicate if you have a front/back focus problem or if the lens cannot resolve any of the numbers sharply. Either way the lens needs to go back to Canon for calibration. If you can get the 6 to register sharply under controlled circumstances, then you need to work on your technique.

 

Never use multi AF points only use center one, and use one shot only. That's a good place to start when encountering AF problems. The manual focus issue you describe is probably just a case of the AF being better at it than your eye. The stock screen sucks for MF. There are options avialable like the Hoada screen, or a Canon Ee-S but I'd hold off on that till you do this test.

 

Good Luck.

[peter_viitasaari]

Thanks for the info everyone.

 

John G. - I am working on getting some pictures up.

 

Bob O'Sullivan - The focus test here http://www.photo.net/learn/focustest/ is what got me started on this. It seemed

that the AF shots seemed to consistently be a little fore-focused which is fine. As long as something is

consistent I can deal with it. The manual focused shots however where all over the map...some as bad as 2-3 cm

fore-focused to some as bad as 2-3cm back-focused and everything in between. This was at f/1.4 with the scale

about 2 ft. from the film plane.

 

I am used to shooting FM cameras which never have focus problems unless shooting in low light. I recently shot 6

rolls of film with not a single picture out of focus. The Canon rig I am running now just does not seem to

provide focused shots consistently enough to what I am used to.

 

Best Regards, Hedghog<div></div>

[peter_viitasaari]

Worst auto focus shot out of 15.

[peter_viitasaari]

Best man focus shot out of 15

[peter_viitasaari]

Worst man focus shot out of 15

[mark u]

The standard focus screen on the Rebel XT is quite hopeless for manual focus at f/1.4. Try the following simple test: set the aperture at f/2.8, and press the DoF preview button and see if you can notice any difference in viewfinder brightness (in theory, stopping down by 2 stops should be very noticeable). Try again at f/4. The viewfinder is effectively acting as an additional aperture stop, so the image you see shows the same depth of field and brightness as the reduced aperture. If you want to be able to focus manually at f/1.4 you will need a replacement focus screen from Haoda or Katzeye with a split prism field.

[beauh44]

On the "Worst auto focus shot" the coffe-pot is sharp. The cereal box, not so much. Looks like your autofocus point wasn't over where you thought and/or you focused/recomposed perhaps?

[peter_viitasaari]

All AF shots where only allowed to focus on the middle of the corn flakes box after turning the focus ring to one extreme or the other (infinity or 1.5 feet).

 

All manual focus shots where brought into focus in the same manner.

 

Every shot looked like the corn flakes box was in focus before firing the shutter.

 

Hedghog

[michael.gregory]

Peter: Were all of these shots done on a tripod preferably with a cable/remote release or by using the auto timer? I ask

because In low light situations where the camera will automatically open your aperture all the way to 1.4, It will probably

also have slowed down the shutter to the point where holding the camera still by hand is very difficult. So if you are

going to do test shots you need to either put the camera on a tripod, or set it on a flat table where it can sit solidly. You

also need to provide the basic metadata of ISO, f-number and shutter speed. These are important to understanding if the

"out of focus" issue is a lens issue or camera shake. even mirror slap can cause shake. So a sandbag or bean bag or

pillow or folded up dish towel under the lens to support it so that the camera does not move is critical if a tripod is not

available. Lots of people who are new at this stuff don't realize how hard it is to hold still for 1/30th or a second. Make

sure the camera is absolutely rock solid still when the shots are taken. Use the self timer so you are not touching the

camera when the shutter goes off. Also if you have a flash, try using that. Flash speeds are so fast they freeze motion.

If you don't ever get out of focus shots with a flash, either the flash has an autofocus assist beam to help the camera

see in the dark via IR, or it is freezing the motion. Either way it will tell you that the auto focus either is or is not working,

and that the problem may be that the photographer is unaware of how shaky he is.

[peter_viitasaari]

I have the ISO up at 1600 which gives a shutter speed of around 1/500s.

 

Hedgehog

[peter_viitasaari]

Does anyone else have a fast lens they can try this with?

 

Is it possible the focusing screen on the Rebel XT just has too much focus latitude for large apertures?

 

Is it possible the 50/f1.4 was never designed to work through it's full range of apertures on the Rebel XT?

 

Another thing I should mention is that this camera had a cooked shutter strait from the factory. I had to send it in for warranty the day I bought it.

 

I am starting to really wonder about digital photography...it is getting to the point where I am afraid to use my Rebel XT because when I get the pictures home a lot of them won't have turned out.

 

Hedghog

[stanleys]

I would wait before blaming the camera. Try a tripod, single point, single servo AF, and self timer. If the focus point continues to move.Then, you might have a problem.

[peter_viitasaari]

1/500s should not need a tripod.

 

All the AF shots are single centre focus point, one shot AF mode.

 

Unless someone has a good reason not to, I am gonna call up Canon tomorrow and get the paperwork started for another warranty return.

 

The only thing I am unsure about is whether its the camera or the lens. With the cameras history I bet its the camera and not the lens.

 

Thanks for the help guys.

 

Best Regards, Hedghog

[stanleys]

Shutter speed is not one of the variables that needs to be eliminated to isolate an AF problem. Moving the camera a couple inches forwards or backwards at f1.4 is.

[peter_viitasaari]

Stanley - If AF requires me to jump through this many hoops to focus then I would rather just focus manually...its faster.

 

Besides, if you look at the pictures above, my manual focus shot is a much bigger problem than the AF one.

 

So...I am just trying to get this thing to focus manually.

 

Maybe I am just asking too much out of digital camera to be able to set the exposure, focus and press the shutter release like I do with a manual camera and expect the picture to be in focus.

 

If don't think this basic concept has changed.

 

Best Regards, Hedghog

[willhl]

What Stanley said, it is very easy to move a couple of cm back or forward after focusing when you take a photos, also using a lower ISO will make it more obvious what is sharp and what isn't.

[roger_smith4]

"Maybe I am just asking too much out of digital camera to be able to set the exposure, focus and press the shutter release like I do with a manual camera and expect the picture to be in focus.

If don't think this basic concept has changed. "

 

I use a 20D which is similar to the XT. I also have a Pentax 50mm 1.4 lens.

 

You haven't isolated the problem here so running to Canon to exchange your camera or lens may not solve anything.

 

If the camera is consistently front or backfocusing (put camera on tripod, print test chart, shoot at 45 degree angle) then that's one thing and you can get the body fixed. I use the center point on an edge with contrast and find my camera does a good job reasonably quickly. It also focuses fine on a test chart.

 

Whether you can manually focus on the supplied focus screen is another issue entirely- there are aftermarket replacement screens (Katz, Haoda) which are more similar to what we used to use with manual focus cameras. I find the little viewfinder challenging to use for focus even under good light and my vision is excellent and focus isn't a problem with my bright big manual focus camera screens.

 

If you are determined to use manual focus with a camera that really wasn't meant for it- fine. Consider using the AF-confirm button for the camera to tell you when you hit focus or an aftermarket screen. Also try to stay in the f4 range to cover up your errors.

 

Myself- I stick to autofocus 90% of the time and live with the knowledge of likely user errors when I use manual focus lenses.

[craig_hall4]

Such a huge difference in the 2 manual focus shots leaves nothing to my mind but forwards/backwards movement after focusing. The time lag between achieving manual focus and shutter release is longer than that of AF therefore you'll get worse results.

Without using a tripod you're never going to get consistent and usable test results at f1.4

[philk]

In manual focus, perhaps your diopter adjustment is out of whack. Not sure whether this camera has one or not, but that could be the issue. As for AF, you need to do the ruler test using a tripod (handhold adds another variable even if you use a fast shutter speed, so why not eliminate it altogether) in good light with a high contrast target. A ruler may not be a big enough target for the AF points. Lay the ruler at a 45deg to the camera, place a large box at the 6" position behind the ruler and focus on the box. Look at the ruler to see what is in and not in focus. This will tell you whether the camera (and lens) is front or back focusing. Do all of this at the f1.4 end, centre spot selected, one-shot AF.