Stopping down.

[rantasam]

I've heard a few people mention that stopping down at the extremes of a zoom lens' focal length improves the picture quality.. and can improve the performance of a lens. I just can't get my head around why this would be the case. Surely with a smaller aperture at maximum zoom, stopping down further would reduce the available light even further and yeild worse results in average light conditions?

I'm sure there's a simple explanation, but I just can't seem to find it. Would anyone be kind enough to enlighten me?

 

[Alison]

Me too!

 

[Canon Bob]

Lens design revolves around a series of compromises to allow the flexibility that we all demand.
A simple lens would be a fixed aperture and fixed focal length....very simple to manufacture and we'd all be walking backwards and forwards and adjusting ISO to get the correct exposure.

Bring in the zoom portion and then we need a compromise....a lens that works sufficiently well at varying focal lengths. The solution is to move some internal groups to keep things acceptable but there is always a benchmark point from where quality will degrade as the focal length changes in either direction.

Add a variable aperture and another compromise is built in. We now have lens that needs to work in light and dark situations so the designer will pick a useful aperture and then begin to see how far the aperture can be varied whilst keeping image quality within bounds.

Some lenses push these compromises a little too far others have been treated conservatively.

Your zoom at the end of its travel and wide open is pushing the two worst case scenarios. Backing off the focal length or stopping down a bit will bring benefits.....you're moving closer to optimum performance settings of the lens.

HTH

Bob

 

[who?]

My understanding was always that stepping down helps with issues around the lens design - flare and such like (can you tell yet that I'm talking ballcocks?) - and by reducing the aperture reduces the amount of light going through the edges of some of the internal lenses where image quality is degraded.

But I'm looking at that and thinking .... hmmm....


edit: that was posted without seeing Bob's post.

Question - what is it that the reduced aperture does (in terms of light rays) that makes a difference?

 

[Jamougha]

If you're light limited then stopping down will produce more blur from camera and subject motion. Obviously this will decrease image quality.

If you can stop down and keep a fast enough shutter speed then you will get better images doing so. The larger the aperture the more different directions light is entering the lens from, and so you need more glass with more complex shapes to focus accurately. The extreme here is a pinhole camera, which requires no lens at all. Therefore it's easier to make a lens that is sharp and has low CA's, low vignetting and good contrast, when it's at a small aperture than at a large one.

After about f/11 diffraction causes the lens to loose sharpness, so f/8 or f/11 is the point of maximum IQ for many lenses.

 

[Canon Bob]

Question - what is it that the reduced aperture does (in terms of light rays) that makes a difference?

Spherical abberations in lens manufacture cause the light from the outer edges to be mis-focussed slightly (actually, they will focus on a slightly different plane to light travelling through the centre of the lens).

By stopping down a little, the rays from edges will be focussed closer to the true focal plane.

Bob

PS...hope that didn't sound like gobbledegook

 

[Canon Bob]

....
After about f/11 diffraction causes the lens to loose sharpness, so f/8 or f/11 is the point of maximum IQ for many lenses.

It's also worth noting that the design of the pixel lenses on the sensor can make them more or less receptive to the diffracted light. Some sensors with poor lenses will show diffraction at wider apertures...f/4 or f/5.6 whilst a sensor with a different front surface could well be immune upto f/22 or beyond.

Bob

 

[rantasam]

Ta very much for the explanations.. very well-worded all of them. Thanks for taking the time to explain.

 

[who?]

Spherical abberations in lens manufacture cause the light from the outer edges to be mis-focussed slightly (actually, they will focus on a slightly different plane to light travelling through the centre of the lens).

By stopping down a little, the rays from edges will be focussed closer to the true focal plane.

Bob

PS...hope that didn't sound like gobbledegook

thanks Bob - that sounds clear, but does it imply that you might need to refocus if changing the aperture ?

 

[Canon Bob]

thanks Bob - that sounds clear, but does it imply that you might need to refocus if changing the aperture ?

I hadn't considered that point.......

Most bodies will AF with the max aperture and stop down for the shot...so unless you're using MF and focussing whilst holding the DOF preview button then you're not in control of this situation.

Some higher spec lenses have a floating rear element to cater for any focus shift when the aperture closes down. The infamous 50L has a focus shift problem when shooting at f/1.4 to f/2 at short range and this has been attributed to it not having a floating rear element.

There's probably more guesses that facts in the above :shrug:

Bob

 

[Wail]

I must say, this has been a very informative thread .. thanks to all who contributed