Focal ratio relationship to image quality

[luca_m.]

<p>Sorry if this has been asked before. I grew up thinking that using higher f values whenever possible (for a given lens) meant ending up with better pic quality. Is this still true for modern equipment? Are there specific advantages or disadvantages in selecting a lower f number? Is there a rule of thumb for selecting the 'ideal' f value for a given lens (for instance my Sigma 150mm macro lens). Do zoom lenses behave like primes in this context? <br>

Luca</p>

 

[ShunCheung]

<p>On the D800, for example, diffraction begins to affect sharpness around f8. If you get to f16 or so, it gets rather poor.</p>

[craigd]

<p>It isn't true. It never was true. Most lenses do their best work (in terms of corner to corner sharpness, anyway) somewhere around f/5.6 to f/8, but often other considerations such as depth of field are more important than maximizing sharpness. There are also diffraction effects that can reduce sharpness at smaller apertures; in my experience, I have found that up to about f/16 diffraction is not particularly noticeable, but beyond that it can be a significant issue.</p>

<p>The best way to find what settings produce the best result with the lenses you actually use (as opposed to looking for a general rule of thumb) is to do your own tests on your own camera. Put the camera on a decent tripod, use a remote shutter release (or your camera's self-timer if you don't have a remote release) and take pictures of the same subject at a variety of settings. Compare the images on your computer and judge for yourself what range of apertures, shutter speeds, ISO values, etc. produce results that satisfy you.</p>

[SCL]

<p>Depending on the lens design and manufacturer, lenses are optimized to meet a wide range of criteria. Some lenses achieve their best resolution and contrast at fairly wide open apertures (these are quite expensive), most, as mentioned above, achieve this when stopped down 1 1/2-2 1/2 stops, others may need much more help before they get to their sweet spot. I always test my lenses within weeks of acquiring them to determine just where softness disappears, diffraction appears, and where their sweet spot is for resolution and contrast. Knowing this information allows me to take full advantage of the lens' expected results for each image I am shooting.</p>

[bruce_brown]

<p>Luca: Every lens I have used for the last 50 years, almost without exception, has generally been best about two to three stops down from wide open ... that said, sometimes you<em> need</em> wide open, and sometimes you <em>need</em> to stop down much more. Perhaps that's why so many shoot aperture priority ... a/for the quality, and b/for the dof when you need it...it sure is the reason I do. </p>

[JDMvW]

<p>A lot more than depth of field is involved, as already indicated. Increased depth of field does not necessarily mean a sharper image. Look a the results from a pin-hole camera with virtually infinite depth of field, for example.<br /> There is a lengthy tutorial on the various components of the problem at http://www.normankoren.com/Tutorials/MTF.html - a quick glance at it looked like it was ok.</p>

[rodeo_joe1]

<p>Not sure about the Sigma, but my Tamron 90mm f/2.8 macro is pretty much diffraction-limited at f/4 - meaning that in terms of absolute resolution it actually gives its best at f/4 and gets progressively worse at smaller stops. Of course f/4 is really only useful for flat copying, and d-o-f considerations mean that a working aperture of smaller than f/8 is usually needed for close-up work.</p>

<p>"Is there a rule of thumb..." - Use whatever aperture gives you the visual effect, resolution or d-o-f needed.</p>

[allan_jamieson2]

<p>The f8 rule of thumb is pretty much true with the D800 but for some types of photography like macro, f8 just won't give you that much depth of field for a non flat subject and you have the choice with lenses like the Sigma 150mm (I use the OS version) of sticking to f8 and not getting much in focus or doing focus stacking if you have a subject which is completely motionless. I took a lot of floral pictures this morning on my D800 with the Sigma 150mm macro at apertures from f14 to f18; not ideal for ultimate sharpness but anything shallower than that leaves too much just not anywhere near sharp and even in calm conditions the flowers are moving slightly all the time. The detail that I'm getting from that lens is incredible, sometimes you have to compromise a bit just to get the image in one take without focus stacking techniques. In a more controlled environment, focus stacking would certainly have a large quality edge as you would be using the lens at its optimal aperture of say f5.6 or f8.</p>

[luca_m.]

<p>Thanks everybody for weighing in. I always thought the sigma was at its best around f 4 / 5.6, which makes sense. I recently switched to a D7100, after many years of shooting a D40. Is it fair to say that the sensor was the limiting factor for sharpness with the D40? Also, is the same true for the D7100?</p>

<p>Luca</p>

[mike_halliwell]

<p>A lot of older lenses used to have a IQ > Aperture graph that resembled a molehill. Poor wide open, excellent 2 stops down, and poor again by f11/16. </p>

<p>More expensive, modern lenses, especially fast glass, are better wide open, even better 1 stop down, and gradually fall off with a flatter tail until diffraction cuts in f8></p>

<p>I'm obviously generalising some-what, but the principle's true.</p>

 

[jose_angel]

<p>In my experience, I have to check if the image quality is right enough for me. Not all the images are the same, nor are equally sized, lenses are not all the same... many people look allergic to shoot closer than recommended in web forums, whatever they need.<br /> <br /> Check the image below, plenty of diffraction at f32 with a 105VR (notice that the coin is tilted, I think it was about 30º, and 1:1 magnification):</p><div></div>

[jose_angel]

<p>Yes, I know, the pic above is so small; please check a bigger one:</p><div></div>

[mike_halliwell]

<p>Jose, it's time to clean that sensor.:-)</p>

<p>This lens review site shows <strong><em>real</em> </strong>serious diffraction happening by then..!</p>

<p><a href="http://www.lenstip.com/24.4-Lens_review-Nikon_Nikkor_AF-S_Micro_105_mm_f_2.8G_IF-ED_VR_Image_resolution.html">http://www.lenstip.com/24.4-Lens_review-Nikon_Nikkor_AF-S_Micro_105_mm_f_2.8G_IF-ED_VR_Image_resolution.html</a></p>

[jose_angel]

<p>LOL, you`re right. And this was taken time ago... now it`s even worst. Thanks :)</p>

[dan_south]

I wonder what 8x10 shooters think of image quality at f/16+. I doubt that they spend too much time worrying about it. :-)