Soft Focus Issues??

[nathan_rigg]

<p>Hi<br>

I posted about this subject last week and was given some great advice about tests to undertake.<br>

I have now taken a number of photos using a tripod on a bright sunny day.<br>

The results (on my Sigma) seems to be showing what I think is acceptable focus somewhere in the middle of the range of f stops i.e. at 2.8 (widest) its not great and at 32 its also, not great. 9 to 16 seem to be acceptable?<br>

On the Canon, again wide open is not great and improves as you increase the f stop.<br>

I have seen on other threads that the lens wont be as sharp at either end of the spectrum, however does my Sigma have an issue, is just not a great lens or is this normal?<br>

This is quite important, as this is going to influence the purchase of a new lens if there are problems.<br>

My test pics: -<br>

http://www.flickr.com/photos/nathanrigg/<br>

I have focused on the number 15, using a tripod.<br>

Thanks<br>

Nathan</p>

[Matt Laur]

<p>At f/32, you're guaranteeing yourself softness because of diffraction. That will start to show up past f/11 or so, depending on what you're using and how you're looking at things. But when you really crank that aperture down to f/22, f/32 etc - you're fighting with the laws of physics (and the laws of physics <em>always</em> win!). That's going to happen no matter which lens you use.</p>

[wogears]

<p>In order to test a lens, if you must, you have to shoot a FLAT object which is perpendicular to the lens axis. This is not an opinion, it is how all serious testing is done. Autofocus points are too large to focus with absolute precision on a slanted scale. Manual focus with high magnification in Live View is better, but still can be affected by user error, especially at wide apertures.</p>

<p>More to the point, pixel-peeping is deadly. Don't do it. Take pictures. If your pictures are unsharp, then you can start to eliminate the causes. Use a tripod, and stop down about two stops from wide open. If you still have problems, then you can set up a rigorous test.</p>

[bob_sunley]

<p>Well your tests prove you are doing it wrong. :)</p>

<p>Plus you could crop those images so all you are looking at is the scale itself, not a whole lot of floor.<br>

As Les said, you need to do it right, and a curved surface scale just doesn't work at all. It has to be flat, supported at an angle to the camera and not tipped to either side, so a line drawn perpendicular to the edge of the scale is parallel to the image plane of the camera.</p>

[nathan_rigg]

<p>Thanks for the direction.<br>

However, now I may have identified the problem I have been through many old photos and most, unless in amazing light seem to have poorer focus at either end of the scale. Is this a normal feature of lenses? i.e. will I see the same results on a new lens, would a more expensive lens (been looking at Canon 15-85) be sharper at lower and higher f stops rather than just in the middle range.<br>

I would still be interested to know if its 'normal' for a lens not be sharp outside of the middle range.<br>

Thanks</p>

[jcuknz]

<p>Nathan ...I have always assumed that to be a fact of life except for when a fast lens is designed to be used wide open which I believe happened back in film days.<br>

Les makes a good point in his second -para but after using the Panasonic G3 for awhile I was delighted with its optional very small single spot focusing area ...a real advance for those who want to use AF .... I think photographers should press every maker to include this option .... it can be a trap for some shots, the problem with a fast moving object and getting it behind the trget spot .. but really good in most situations where you want a small area to be picked up as the point of focus. </p>

[Wouter Willemse]

<blockquote>

<p>would a more expensive lens (been looking at Canon 15-85) be sharper at lower and higher f stops rather than just in the middle range</p>

</blockquote>

<p>-with a word of caution that I do not shoot Canon, so I won't comment on that specific lens-<br>

With lenses, look up tests; price isn't going to tell the whole story, so it's impossible to say that a more expensive lens will by definition give you better performance over a wider range of f-stops. More generic, yes, it is normal for a lens to be weak at the widest aperture, and for the smallest aperture, read what Matt Laur already responded - diffraction is a problem with any lens. Some lenses are by design more optimised towards wide-open performance, but those lenses usually come with a price tag to match. For normal money, you should expect to stop down 1 stop on a modern lens (older designs, 2 stops), or suffer "problems" such as slight softness, lowered contrast etc. Genericly speaking, f/8 is a good place to be.<br>

Another option of course, is to learn to know your lens, and use its weaker points creatively. Super sharpness and high contrast is not always the proper choice, nor is it going to make or break a photo. Learn to use a lens to its benefits. If you want to a do-it-all lens that is sharp end-to-end, you're dreaming, sorry.</p>

<p>And re-read what Les said. Pixel peeping serves little, there will always be some niggles. Its the entire image that counts, and more important: how that image works as a composition, its visual appeal, language, its message. Absolutely epic photos were taken with lenses that have a much poorer design than what you're using.</p>

[nathan_rigg]

Thanks for comments, really useful.