Will the collective DGrin Borg educate the Beginners on "Stopping Down"?

frnofrno

:scratch Beginner Grinner seeks advice. I Understand Aperture, ISO and Shutter, but can't grasp the concept of smaller aperture giving better photographic results. I know it gives greater depth-of-field but aside from a desired DOF what other reason or benefit is a smaller aperture? (The explanation is probably embarrassingly obvious, but I sacrifice myself on the altar of vulnerability and public ridicule for all of my Beginner Grinner Comrades.)

pathfinder

Actually, other than increasing depth of field, it is better to avoid the smaller apertures [ f16, f22, f27 on 35mm film size cameras ( DSLRs), if you can].

As the lens aperture gets smaller, diffraction of light becomes a limiting factor in resolving power of the lens, so that although the dof IS greater with the smaller apertures, larger apertures may give sharper images overall, but with less dof.

Lenses, typically, are their sharpest about 2-3 stops down from their maximum aperture. I try to shoot around f4-f5.6 or f8, unless I have a specific reason to choose a wider or smaller aperture, assuming the lens has a maximum aperture of f2.8. An f1.4 lens will be sharpest around f2.8 to f4.

These statements are mild recommendations, not commandments written in stone I shoot from f1.4 to f29 depending on a long list of considerations, but what I said above, is usually what I do shoot.

angevin1

but can't grasp the concept of smaller aperture giving better photographic results.

Hi there,

This statement from your post is a very relative term. Perhaps visiting a site that has blur charts such as: http://www.slrgear.com would help. When you read their text and study the blur charts, hopefully it'll become a bit more clear. Just choose the brand and the lens you are wondering about and see if they tested it. If not choose another and look at the tests for it.

cheers, tom

Robinivich

frnofrno wrote:

Beginner Grinner seeks advice. I Understand Aperture, ISO and Shutter, but can't grasp the concept of smaller aperture giving better photographic results. I know it gives greater depth-of-field but aside from a desired DOF what other reason or benefit is a smaller aperture? (The explanation is probably embarrassingly obvious, but I sacrifice myself on the altar of vulnerability and public ridicule for all of my Beginner Grinner Comrades.)

Well, hopefully without going too too far into the physics of it, a lens is made of of a bunch of glass elements lined up, ie:



As far as design and manufacture go, the central parts of each element are the most perfect, and they have to bend light the least. The farther you get from the centre of each chunk of glass (the left-right line in the diagram), the more problems creep in, both due to design and optical laws.

When the lens is "wide open", all, or almost all of each element is being used to project the image onto the sensor/film. Even though you have very well corrected rays of light coming through the centre of each lens element, you also have less well corrected rays coming in from the outside edges. These all add up to an image that might not be as sharp, or as evenly lit, and some of the rays might not even refocus correctly (Chromatic aberrations, etc). However, since the lens does use all available rays of light, your picture happens faster...

When stopping down, you narrow the aperture, or diaphragm inside the lens, and this progressively blocks more and more of the possibly less well corrected rays from the edges of each element. You have less light getting through, but generally speaking, the light will be more coherent, having come from the central, better parts of each element exclusively, and having been less bent and mucked around with.

The problem is that as the aperture blades close tighter and tighter, they start to bend the light themselves, introducing an effect called diffraction, where light waves/rays are changing course as they pass through, which starts reducing sharpness.

Stopping down to the sweet spot of the lens is a matter of finding the highest quality balance between the lens imperfections that can be fixed by stopping down, and the slow onset of diffraction at the far end. On the vast majority of lenses on typical digital SLRs, this is somewhere around f8.

Following Angevin's link will give a very good idea of where the best spot is on each lens.

Also check out www.photozone.de, a favourite of mine.

frnofrno

"Lenses, typically, are their sharpest about 2-3 stops down from their maximum aperture. I try to shoot around f4-f5.6 or f8, unless I have a specific reason to choose a wider or smaller aperture, assuming the lens has a maximum aperture of f2.8. An f1.4 lens will be sharpest around f2.8 to f4."

"Stopping down to the sweet spot of the lens is a matter of finding the highest quality balance between the lens imperfections that can be fixed by stopping down, and the slow onset of diffraction at the far end. On the vast majority of lenses on typical digital SLRs, this is somewhere around f8."


That is exactly what I was looking for - thanks! I heard so many comments about "stopping down", and now I see that it is because that is where they are "typically" sharpest.

divamum

frnofrno wrote:

Beginner Grinner seeks advice. I Understand Aperture, ISO and Shutter, but can't grasp the concept of smaller aperture giving better photographic results. I know it gives greater depth-of-field but aside from a desired DOF what other reason or benefit is a smaller aperture? (The explanation is probably embarrassingly obvious, but I sacrifice myself on the altar of vulnerability and public ridicule for all of my Beginner Grinner Comrades.)

I am definitely NOT one of the forum experts (just getting that part out of the way first!). But I have also been bowing to the Collective Borg with extreme gratitude and ever-increasing understanding for the last few months, so just one point to add...

Many lenses have a "sweet spot" which is often, as pathefinder says, a couple of stops smaller (higher number) than the maximum possible.

I have only just - as in TODAY - truly realised the importance of this. I have a "nifty 50" Canon 50mm 1.8 which I usually use because of its low light and background-blurring capabilities, thus I tend to use it a lot around 2.2 or so. It's fine there - nicely sharp.

HOWEVER... today I was using it at f6.4-f8 because I needed greater dof.
It may be the sharpest photo I have EVER taken, so finally understand what folks mean about a lens's "sweet spot" and I suspect I will be trying to stop it down whenever I can... I'm bowled over by it, actually - with the pixel density of the xsi that lens seriously impressed me today

Tee Why

Other than creating a deeper depth of field, stopping down on the aperture (if from wide open aperture) may do a few things to improve image quality.

It can generally decrease vignetting. Stopping down about two stops from wide open generally gives the sharpest image from the lens. Generally, I've found stopping down a touch reduces chromatic aberration as well.


So there are some good reasons to stop down a touch if the increase in the dof does not distract from the image.