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FORUMS Canon Cameras, Lenses & Accessories Canon EOS Digital Cameras
Thread started 08 Apr 2010 (Thursday) 15:58
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STICKY: -=What to do if you suspect a focus problem with your DSLR=-

 
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lungdoc
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Apr 08, 2010 15:58 |  #1

It seems that there is often a lot of unnecessary confusion when members have concerns about the performance of a camera or lens. This can lead to long threads with often (but by no means always) inappropriate questions about possible user error.

In another thread I thought it would be helpful to have a relatively simple set of steps for members to follow when this arises to allow faster troubleshooting of their problem and to avoid either making simple mistakes or being improperly accused of making them. CyberDyneSystems asked me to write a draft sticky..so here it goes
Please consider this a draft initially and we can hopefully make it a sticky once it has been finalized. Comments and suggestions are welcomed. I don’t consider myself a focus expert by any means but do know something about taking a logical and scientific approach to diagnosing problems.

This thread and it's suggestions were written primarily about focus issues, however please note that most of the suggestions regarding how to go about framing your questions, and what information to include when posting sample images will pertain to any issues you may be having.

If you suspect a focus problem


1. Please post some sample shots that show the problem
, with Exif information intact or included. Linking to full sized images is useful

a. This post has useful information on how to do so. This site http://www.mediafire.c​om/ and others will allow free image hosting for samples.

b. You can use Zoombrowser to show the active focus points for a shot, then take a screencapture and upload that image to show others where the focus points were. You can use various tools for this including free Screenhunterexternal link to do so.


2. Verify the camera/lens can in fact focus under good conditions first.

a. Take test shots in good light, outdoor shots at decent minimum distance preferred, for most lenses it is a good idea to conduct focus testing at 50x the focal length or more. At the very least, people should try to focus test lenses at ranges that they plan to use the lens at. One of the biggest problems with the 8x10 piece of paper focus test chart is that it encourages people to do focus testing near the MFD for most lenses. Bad idea.

b. Verify adequate shutter speed – suggest a minimum of 1/(2 times focal length) for troubleshooting, to keep it simple use 1/1000 or faster, or use a tripod/stationary camera with remote release or self timer


c. Use a reasonable depth of field (online DOF calculator http://www.dofmaster.c​om/dofjs.htmlexternal link ). Really large apertures with tiny depth of field can easily lead to misfocus even with a perfect camera, though avoid testing with really small apertures as depth of field can be large enough to hide focus errors.


d. Verify the lens switch is set to AF not MF, IS is off if shooting on tripod
e. For testing use center point focus with a central object, avoid recomposing. Use One Shot focus mode at least initially to verify camera can focus and to reveal focus point selection red square.

f. If AF seems to be causing problems try manual focus, Live View is easiest for this if your camera has it. Verify you can focus sharply in manual.

g. If it's the camera that is in question verify the problem with a second lens if possible, if the lens is the question try a different camera if you can.

h. If with the above there seems to be consistent problems there may be front or back focusing, testing for this is well described here, member Teamspeed has a free lens adjustment tool to aid focus adjustment, described and available here

3. If the issue is with focus on moving targets

a. It’s helpful to link to a series of shots and to list the custom function settings for the camera. This can be seen in EXIF with Zoombrowser by right-clicking on image (Windows) and selecting “properties”

b. There’s very good information on the Canon Learning site hereexternal linkwith specific information for many models , the videos on the 7D focus system are very good (hereexternal link) as are those at BH - click http://www.video.bhpho​tovideo.com/external link and search for 7D.

c. Try to describe the problem in a way that others with the same gear can test for themselves to help.

d. Consider shooting some semi-reproducible motion targets e.g. oncoming cars, a friend or family member on a bicycle, etc. if tracking is the issue.

e. For more specific problems limited to certain scenarios it is often helpful to ask in the forum relevant to that topic e.g. Bird Talk, Macro Talk, Motorsports Talk etc.

f. As always searching the forum to see if others have had or solved similar problems can be very helpful.

I hope this is useful.Please add comments or suggestions and we can maybe have this made into a sticky.


Suggestions for replying to focus threads:

  • Please be courteous in responses and respect the rules of the forum. Direct users to this sticky.
  • Take a moment before assuming user error and be careful with your language even if you do suspect it. Have a look at the poster's other posts, website or gallery if applicable to get a sense of their expertise before making judgmental statements or asking questions that may be seen to be insulting. While anyone can be wrong it is clearly different to ask some things of a novice versus people who obviously have proven photographic skills.
  • Helpful to state if you have similar equipment and to post your settings and/or your examples with EXIF if you believe it will help.
  • Although threads may get long please make an effort to read through the prior answers so issues already answered don't need to be rehashed.


Mark
My Smugmugexternal link Eos 7D, Canon G1X II, Canon 15-85 IS, Canon 17-85 IS, Sigma 100-300 EX IF HSM, Canon 50mm 1.8, Canon 85mm 1.8, Canon 100mm 2.8 Macro, Sigma 50-150 2.8, Sigma 1.4 EX DG , Sigma 24-70 F2.8 DG Macro, Canon EF-S 10-22, Canon 430EX,

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tonylong
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Apr 08, 2010 19:12 |  #2

Static Subject Focus Test and Lens/Body Characterization

Focus issues crop up frequently, and it is good to have a standard "starting point" in judging the possible cause of these issues, and also to get an analysis of how you should expect your camera and lens combination to perform over a variety of settings. This test is critical to achieve this basic understanding. It will help you to understand the optimal settings to use and what level of perceived sharpness to expect when you are utilizing these settings and proper techniques and skills.

So, here is an outline for static focus and lens/body characterising:

This is a useful test for characterising your camera/lens combination as well as a beginning point in analysing possible focus problems. It will help you, for example, to note variations in the sharpness of a lens over a range of apertures and focal lengths as well as a look at the effect of diffraction at narrower apertures.

First, you need to have either a sturdy tripod or another means of solid support. Trying this hand-held will defeat the purpose of this test. Hand-held shooting requires a combination of skills and techniques that have an effet on image sharpness, and as a result "soft" images often are a lack of those skills and techniques ("user error") rather than a failure of the camera and lens.

Second, you want to shoot with good light at a subject or a grouping of subjects that has/have good detail with contrast over three dimensions but a broad enough center area to fill a good portion of the center "circle". This will give the camera a proper target to lock focus on. It will also enable you to get a good exposure with a low ISO so that noise in the image will not interfere with detail. The three-dimensional subject matter will allow you to asess a proper focus from the desired focal plane compared to "rear focusing" (focus being behind the desired subject) and, if you have your subject(s) with detail in the front of the focal plane, "front focusing". It will also aid you in assessing "Depth of Field" using wider apertures (see below).

If your lens has IS, turn it off. IS, if active, will turn off if you don't have the shutter button/focus button depressed after a time and will cause a "slump" as it deactivates. This test is not designed to help you with using IS>

Set the camera to use Mirror LockUp so that vibrations from mirror motion will not affect your picture. In "real world" shooting mirror vibrations only affect a range of shutter speeds but to simplify your procedure just use mirror lockup for the whold process.

For best results you should use a cable release or a wireless remote shutter release. If you don't have one, then you will need to use the self-timer. These will help to minimize camera vibration from ppushing the shutter button. Again, this will only affect a range of shutter speeds, but again, incorporate it in the whole procedure to save mix-ups.

Now, you want to take a series of shots of the same subject both over the full range of apertures and the full range of focal lengths if you have a zoom. You don't need to use all possible apertures and certainly all focal lengths, but get a good enough representation, especially at the wider apertures and the center apertures and the wider and longer focal lengths so that you can spot variations and "windows".

Things to note:

Most lenses are "soft" to one degree or another with a wide open aperture. How soft they are depends on the lens. It will be useful to you to note whether or not a particular lens is "usable" wide open for various things -- this is an important part of your "characterizing". Another thing that is important is that lenses typically what is called a "sweet spot", meaning a range of apertures that will yield the best degree of sharpness. The amount of variation varies from lens to lens, and this is why this test is important to the process of characterizing your lenses -- some lenses are quite sharp over the range of apertures until they reach "diffraction" (see below) while others are quite soft when wide open. A "safe" assumption for most lenses regarding the "sweet spot" would be a range of f/5.6 to f/11, but this can vary, hence this characterization test.

"Depth of Field" (DOF), or the part of an image that appears to be in acceptable focus, will vary in your shots with changes in the aperture and the focal length (other factors won't be a consideration since you will be shooting at a fixed distance and with a single camera/sensor). You will with this test be able to observe these changes in the DOF of your images between shooting with a wide aperture which will have a "narrow" DOF and a narrow aperture with a "wide" DOF). Depending on the maximum aperture of the lens and the focal length used, this difference may be very striking but if your subject(s) have a good 3-D depth it should be quite visible if viewed at a good size on your monitor. Please note that many concerns about focusing come from not understanding and applying proper focus techniques when shooting with a lens wide open. If you carefully follow this setup you will at least see the effects of a narrow depth of field.

When you shoot with narrow apertures you will eventually see another effect, which is diffraction. Again, using a variety of apertures up to the max f-number of your lens (the narrowest aperture) will help you to analyze this effect and judge tradeoffs. For examle, the narrowest aperture will enable you to have something in the very close foreground in decent focus as well as something in the background in decent focus but will also lead to some softness from diffraction. Your test won't "see" the whole picture of this unless you have something closer to you than the main subject, so you may want to actually put an object fairly near to you for focal lengths that are wide enough to take it in. Check these shots at a magnified view such as 100% and you will see how the closer object will come into better focus but the sharpness of the detail will suffer as you decrease the aperture opening.

It should be noted that this test does not address a range of shooting conditions and techniques. Things like hand-held shooting, use of IS, low-light shooting, and shooting of moving targets using AI Servo AF are "big" and complex topics that involve a variety of skills and sometimes specialized gear. This test is based on one assumption, and that is the assumption that your camera and lens combination is capable of taking good, sharp images of stable subjects if used correctly.

I hope this helps!


Tony
Two Canon cameras (5DC, 30D), three Canon lenses (24-105, 100-400, 100mm macro)
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Wildlife project pics hereexternal link, Biking Photog shoots hereexternal link, "Suburbia" project hereexternal link! Mount St. Helens, Mount Hood pics hereexternal link

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CyberDyneSystems
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Apr 14, 2010 16:12 |  #3

Draft and Comment thread can be found here.
http://photography-on-the.net/forum/showthre​ad.php?t=854727

If you have any input, feel free to post it in the linked thread.


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