sharpest focal lenght and f stop for sharpest large images

<p>I have been using f8 as the sharpest f stop on my canon 12-135 lens on a canon xti 10mp<br>

I am wondering if the there is an optimum focal lenght to select (when all other things ale not rele)v<br>

Can i set my manual at f8 and focal lenght at 55m or whatever?<br>

ie is there an optium sharpnes ( focal lenght and f stop )for a digital blowup in a canon 18 -135 zoom lens ?<br>

I have always believed f8 was the sharpest on any camera but I would lovwe to hear if this is not true</p>

 

<p>The only way for you to find this out for your equipment is to systematically test it yourself. </p>

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<p>I have always believed f8 was the sharpest on any camera but I would lovwe to hear if this is not true</p>

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<p>It is not true.<br>

There are many factors that go into the sharpest aperture for a particular lens.<br>

OTOH, f/8 isn't a bad approximation, but it's just that.</p>

<p>I'd suggest looking at some of the more technical lens-review sites (ex. photozone.de). The resolution or MTF curves they measure are good data for identifying the sharpest settings for a particular lens.</p>

<p>Not true. Sharpness can be limited by f/number, diffraction, lens aberrations, and focussing errors. The aperture that gives maximum sharpness in the center of the image might not be the aperture that is sharpest at the edges. The aperture for maximum sharpness varies widely from lens to lens, and perhaps widely from wide angle to tele settings of your zoom. f/64 was long considered as the best compromise for sharpness with normal lenses on an 8x10 view camera. With a normal lens on a 35mm camera that aperture would yield images unsharp due to diffraction while f/5.6 or f/8 should be much better. While serious photographers may find a study of optics beneficial, it's the photograph that is most important. What gives you the best photographs is best for you.</p>

<p>With crop sensors try opening up a bit. I find f5.6 and f6.7 to be optimal for my lenses, but if you need more depth of field, by all means pick a narrower aperture.</p>

<p>A widely accepted rule of thumb is to stop down a lens by 2–3 stops for best sharpness. In general that holds true, but there are types of lenses, such as the very fast and expensive telephotos, that perform admirably even when used wide open and show little improvement stopped down.<br>

Regarding focal length, zooms tend to be weakest at their extremes (shortest/longest FL).</p>

<p>I wouldn't lose sleep over these issues though: if a certain focal lenght-/aperture combination gives you the desired result, then you should use that for your image. Say if you wanted to isolate your subject from a busy background, you'd choose the longest zoom setting and use the lens wide open. Stopping down just wouldn't make any sense here.<br>

You can always improve sharpness in post processing.</p>

<p>BTW, based on the photozone.de Imatest measurements, the optimum aperture for the Canon EF-S 18-135mm IS is about f/8, pretty much across the focal-length range (the longest focal-lengths are the least sharp - a common characteristic in zooms). There are trade-offs in the center vs. edge sharpness at other apertures that you might find worthwhile (i.e. slightly less center sharpness for an improvement at the edges):<br /> http://www.photozone.de/canon-eos/462-canon_18135_3556is?start=1</p>

<p>You are looking for a" formula" to help you take the best photographs. Sadly there is no such thing . There is simply no substitute for studying your scene and deciding what is the best way to treat that specific photograph to meet your objectives. There is no aperture that you should use every time or even most times- thats why they make cameras with a wide range of aperture options.</p>

<p>Lets assume that for your lens, f8 gives you the sharpest image at the centre and the edge ( this also could be the subject of debate). Now lets look at two types of photograph. The first is of a painting. Everything is on one plane. Materially there is no depth of field requirement because everything important to you is at the point of focus. Here's a case where it might be good to use the sharpest overall aperture.</p>

<p>Now imagine a different scene , one in which you have a house 100 feet away and a mountain range at five miles away, which we'll treat as infinity. You want to take the shot at 100mm. If you use f8 and focus on the house, the mountains will be somewhat unsharp. If you focus on the mountains the house will be somewhat unsharp. If you focus between the two you'll find there is a point ( the hyperfocal point) where the mountains and the house are just about in acceptable focus- though of course as they are both right on the edge of your depth of field neither will be as sharp as you could get them individually if you focused directly on one of the other. You have a number of choices to make- </p>

 

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<li>You can choose which of the objects- house or mountain- you want to be sharpest and focus on that, maintaining your f8</li>

<li>You can focus between the two objects in the belief that they will both be somewhat sharp, maintaining your f8 but accepting that your point of sharpest focus is somewhere in the inconsequential ether between the two. Within depth of field does not mean as sharp as you can get it.</li>

<li>You can reduce the aperture to say f16 to get dof up and position both objects more clearly within it. You'll get closer to the optimum sharpness available at this aperture, but of course that optimum might well be worse than you'd get at f8. Is that trade off worth it? Sometimes it will be. In fact you could focus on the house- so achieving maximum sharpness on that- whilst still having the mountain within your depth of field.</li>

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<p>There are other things you can do too to produce a different apparition of sharpness, but this is probably already complicated enough. There are even times when your image may be enhanced by having some or all of it out of sharp focus, and times when you have to open the lens wide to freeze subject motion in a breeze.. Fact is that there is no formula, and no effective substitute for thinking how best to treat a subject in the prevailing conditions. </p>

<p>My rule of thumb for cases when sharpness is my primary consideration is that I avoid the focal length extremes (if using a zoom), and choose an aperture between f/4 and f/8 (unless I'm using an f/4 zoom, in which case I stop down to f/5.6 or so).</p>

<p>If you want sharp get a prime. The 50mm f/1.8 is a great place to start, especially since its less than $100. As for your 18-135mm, since its such a broad range, its probably not especially sharp at any FL. But for f-stop, the general rule is a couple stops down from wide open, but its always better to get the DOF needed first, then choose the optimum f-stop, if there's any choosing to do.</p>

<p>i always shoot at iso 100, 1/250sec, and f8. anything else and i wont get a photo thats sharp enough to print big.</p>

<p>ron interesting comment<br>

1. I wonder if you feel , if using a zoom u have a prefernce for best point on the 18-135m zoom<br>

2. At iso 100 and f8 at 1/250th what light conditions are yu working with and if outdoors does that mean that you sacrice exposire for sharpness and correct in PS for exposure?</p>

 

<p>my point exactly. Dont get caught up with this crap. Set the camera up for the photo you're taking. Very rarely will i revert to the sharpest focal length and Fstop (which actually is 35mm and F8 for my kit lens, the rest was a put-on).</p>