Fumbling Around With Focus Issues

[erainn]

Hi, I am having a challenging time getting a subject in sharp focus when taking close-up photographs with my Canon EOS 450d/Sigma DC 17-50 mm 2.8 HSM

I usually have AF active and have selected a single-point selection, the resulting images though do not have a consistent sharpness and I am wondering if I am missing something very obvious that maybe causing this.

Here's an example, a photo taken without a camera-stand (which I usually use, but still get the focus issue) certain parts seem sharper, while others are much softer.



http://www.anony.ws/image/Dzge

As relative newcomer to photography I am happy to consider that I may well be making some very basic error or lacking technical awareness that can explain this, if that is so I welcome any advice or experience which can help me better understand how to ensure a much sharper images when taking close-up shots. For example is there a setting AF points selection which would be more likely to generate the image being more sharply focused? Alternatively is it just my ineptitude on physically focusing upon a subject?

Thanks for taking time to consider my request. Regards, Erainn

[JohnRostron]

I am not an expert on these matters, but my first question would be: Can you get the image acceptably sharp using manual focus? For an image such as you have I would recommend doing it manually. I would guess that your camera will have a magnifier so that you can see what is on your screen or viewfinder more precisely. Have you tried that?

It would also help if you gave the exif data: f-stop, shutter speed, ISO, etc.

John

[george013]

Did you take care of the shortest distance of the lens?

George

[erainn]

Will defo give those suggestions a try, thanks.

[erainn]

Hi, I take it you mean that shortest distance which the lens operates at? I think so, as at a certain closeness the lens has difficulty to focus, so I moved back to take the shot.

[erainn]

John, details were: 1/250 f3.2

[travvie]

Hi Erainn,

I'm not an expert either but I can't see anything wrong with your focusing or your camera. Your problem is the type of shot you have taken: shooting close-up, with a telephoto lens, at high magnification, i.e. 'macro' photography, results in very narrow depth of field. This is why people often use a technique called 'focus stacking' for sharp macro photography. I've not tried it myself but the results I've seen are staggering! You'll find info about it on the web - I'm sure there's plenty here, in fact.

Good luck!

Best wishes, Nick

[shreds]

At f3.2 the depth of field will be wafer thin. So either consider mounting the camera on a tripod and stopping down the aperture to something like f8 or f11, then focus on the flower again. You may need to up the ISO or bring additional lighting to the subject.

Focus Stacking kit such as

www.cognisys-inc.com

is not cheap but really is the only way to do it and achieve razor sharp results.

[erainn]

travvie, thanks for your suggestion

[erainn]

shreds, thanks for your advice on that, I'm working on the knowledge (misunderstood perhaps) when taking close-up photos such as this, I need to switch to AV mode and set to low aperture as possible to get detail along with blurred background...what puzzles me also is why when applying such settings certain elements of the image focused upon are sharp yet others less so? That's why I'm wondering if it's an AF focus points issue?

[DanK]

As Ian pointed out, the main problem is your use of a wide aperture, which will create very shallow depth of field. The closer you get, the shallower the depth of field for any given aperture and focal length. If the camera focused on the wrong point, which it seems to have done, you have very little room for error. That seems to be the case here, because you have parts in focus. You could focus manually, or you could use a single AF point and make sure it is pointed at whatever you want to be the point of focus.

I hope you won't mind, but my thought is that you are not going about this in the right way. You should not be thinking first of a mode, like Av. The choice of mode is simply a matter of whatever gets you most easily to the settings you want. The starting point should be the settings you want. Then think about which setting will get you what you need.

In this case, here is how I would go about it: First, decide what aperture will give you the depth of field you need for the flower. Second, decide whether that will sufficiently blur the background. This second you can find out by taking a test shot, or by using the DOF preview button on your camera. If the DOF you need won't blur the background enough, you either have to move to a different viewpoint or blur the background in postprocessing. Third, see if you have enough light for the aperture you need. Having a tripod will help because it will let you use slower shutter speeds. Then, chose your settings and your AF method.

Re stacking: yes, that is the main way to deal with macros that are too deep for a reasonable aperture. I stack all the time, as in this thread that I posted yesterday. However, I wouldn't think about that yet. That should come later, after you have mastered control of DOF and close focusing. Incidentally, in the first of the two images in that post, I had to blur the background in the lower left in postprocessing; the rest of the plant was simply too close.

[rpcrowe]

When one portion of the image is sharp while other portions are blurry, that usually indicates a focus problem. I don' know how far the stem was from your lens but, you can determine the depth of field by using a depth of field simulator http://dofsimulator.net/en/ and/or referring to the DOF tutorial here on CiC.

I agree that a smaller aperture will give a wider DOF. However, unless you increase the ISO, the smaller aperture will also require a longer shutter speed. Any breeze might then move the stem resulting in motion blur...

A way to experiment with the differences in shutter speed and aperture, while still retaining the correct exposure is to place the camera in "P" or programmed mode and the run through the entire gamut of f/stop and shutter speed combinations (your manual will tell you how to do this). You can then see for yourself what your images look like using the different shutter speeds and f/stops. You can also experiment using different ISO values.

The capability to do this experimentation without incurring the cost of film and processing; as well as the ability of the DSLR to retain the f/stop and shutter speed information for each shot is one of the great learning tools available to the DSLR camera user. I learn best when I see for myself what is happening...

I urge you to yake advantage of this great learning tool...

[erainn]

DanK, thanks for your helpful advice, most welcome.

[erainn]

rpcrowe, thanks very much for your suggestions, which I shall certainly try out. I noticed during the process of taking the image that my lens was having a hard time focusing, was on and off, until I moved back slightly from the object. I'm on a learning curve, so really appreciate all the advice from folks here.

[xpatUSA]

Erainn,

Your posted image has no EXIF data in it, so we have no idea of the settings you used. Doesn't stop me commenting of course



With this type of subject, I not would expect any kind of AF to work properly. The ideal method, lMHO, is using a magnified live view and manual focus - one of the best of such is found on Panasonic G series cameras in my experience.

Some camera shake or subject movement is evident, see flowers at left of stem. Some means of supporting the camera will help if it was shake. Alternatively, selection of a faster shutter speed - if you prefer hand-held - is mandatory. It is even possible in this shot that focusing is not the main problem.

There is blur caused by insufficient depth-of-field which, again, is not caused by poor focus. More depth of field is given, as we all know, by higher f-numbers. I would have thought f/11 to f/16 for this shot. Before I get jumped for that, there are sharpening techniques that almost remove diffraction blur, for example RL deconvolution. Some depth-of-field can be gained by stepping back (with the same focal length) and accepting the need for cropping in post (assuming you have the obligatory mega-mega-MP sensor).

Composition plays a part: different views of a subject can reduce the depth-of-field that it occupies - which is why we see fewer side-shots of individual flowers that we do "full-frontal".

So:

- More DOF.
- Less camera or subject movement.
- Manual focusing by best possible means.
- Better composition.

Pardon me where I have repeated any points already made by other members.

P.S. you probably know that the Sigma 17-50mm is not the finest for macro work:

http://slrgear.com/reviews/showprodu...ct/1329/cat/31

If you ever get serious, Sigma's true macro lenses are pretty good: the 50mm, 70mm and 105mm are all excellent, I've owned them all.

Here's a Sigma 50mm f/2.8 macro EX DG shot:



Sigma SD14, f/11, 1/125 sec, ISO 100.

Manually focused on forepart of the flower, DOF estimated to get the insect in, cropped from original 4.7MP. Manually exposed to kill the background on a whim (with the ISO-less SD14 you can get away with it).

[jcuknz]

The simplest answer would be to use forced flash and that will probably result in a smaller aperture giving you enough depth of field for all the flower... I assume you focused on the mid point of the flower but unfortunately you wavered a bit and only the bottom is even reasonably sharp.
I rarely manual focus because if you use single centre spot properly it will be a lot faster than messing around in manual focus.
A problem working wide open with a long lens, or any lens, is that the movement between taking focus on part of the flower and re-composing for the shot is highly likely to put it out of focus. However have said that you should be aware of the technique of setting focus and then moving camera in and out until you get correct focus which would probably be my prefered way of working for big close-ups.[ when I am not using AF and my camera permits me to place the single spot anywhere, not just centre, but again I rarely have need of this ].

finally I believe the helpful tool 'save for the web' is the biggest culpret in stripping EXIF details off photos ... black mark Adobe!