sharper images when stopping down

[Tickhiller]

I have been reading some lens reviews and a regular comment is that sharpness is improved by stopping down. How does this improve the sharpness of the image?
(Am I being dim today?)

[Malcolm_Stewart]

It might be worth checking out this article and then clicking on the word "aberrations" for more info as to what's going on. Very simply, the more that light has to be bent, the more compromises which have to be fixed. So wide aperture lenses bend light more than a lens with a stopped down aperture. And that causes aberrations.

It's a big subject... and if you stop down too far, you run into the problems of diffraction.

Hope this gets you started!

[TimF]

Most lenses will have some residual aberrations (the very finest, such as super-telephotos by Nikon and Canon, and most modern Leica glass, will have reduced these so they are pretty well un-noticeable even at maximum aperture), which are reduced or eliminated by stopping down. See here for example.

Sharpness is a factor of resolution and contrast or acutance - see here. Lens design has always been a matter of compromising between various factors. In the 30s, Leica lenses were optimised for sharpness; Zeiss for contrast. If you look at the three comparison photos on the link, you can see that the high acutance/low resolution picture appears sharper than the low acutance/high resolution one (until you study them critically at least). That made many people think the Zeiss lenses were superior. Some at least may have been, but the higher contrast they had was a useful trick in that regard.

That said, you shouldn't stop down too much otherwise you will begin to suffer from diffraction limitation

[Tickhiller]

Thanks Malcolm, I will read that article tomorrow, I concentrate better in the mornings, just glancing at the article I think I will need all my faculties.

[TimF]

So wide aperture lenses bend light more than a lens with a stopped down aperture.

Really? As we've both noted, stop down too much and you get diffraction problems which is caused by squeezing the light through a narrow opening after which it has to spread out again.

It's true to say that faster lenses are more prone to certain problems than slower ones (unless they're very expensive). Fast wide angles even more so. However, they're usually things like flare, distortion (which also affects most zooms of course) and chromatic aberration.

[Malcolm_Stewart]

So wide aperture lenses bend light more than a lens with a stopped down aperture.

Really?

Of course it does, at the outer edge of the lens.

[nimbus]

All lenses have an aperture where sharpness is at an optimum, further stopping down only serves to greaten depth of field, whilst definition is reduced, usually down to diffraction. Most primes will hit their optimum around f5.6, zooms will be generally around f8-11.

[Monobod]

Is it not also true that prime lenses are sharper, buck for buck, than zooms because having one focal length they can be designed more closely to the requirements of that focal length. Combine this with choosing an optimum aperture, gives the best result of all.

I was not certain that with modern zoom design this was still so important, but having gone back to using my two Pentax prime lenses, the 100mm f/2.8 macro and the 50mm f/1.7 I am now seeing far better resolution and sharpness than with, for instance, my 17-70 Siggy.

This has started me thinking more about getting prime lenses in the future.

[daft_biker]

Indeed David. My worst experiences with diffraction softening has been whilst using a zoom even although macros should suffer more. In practice I'll stop my prime lenses down further than I'd be happy to stop my zooms down...I can live with a little diffraction softening on a properly sharp image better than I can live with a little diffraction softening on an image that was only sort of sharp to begin with.

Am tempted to blow the dust off my 28-105mm and see if it really is as bad as I remember it being at f/22.

[Tickhiller]

Thanks for all your contributions, I have studied the linked articles and think I follow what you are saying.
I am concerned that stopping down in other than very good light will cause images to be dark or, if I slow the shutter to compensate, blurred due to movement/camera shake.
The opinion seems to be that the more expensive the lens the better. My Bigma was expensive for me but is considered a cheaper lens by the 'experts'
I would like a wide lens 10-20 or 12-24 as I photograph landscape regularly but the reviews are very mixed and they all seem the require stopping down to improve sharpness. I do not want to pay £400/500 and find I have wasted my money.

[Malcolm_Stewart]

Thanks for all your contributions, I have studied the linked articles and think I follow what you are saying.
I am concerned that stopping down in other than very good light will cause images to be dark or, if I slow the shutter to compensate, blurred due to movement/camera shake.

You can sometimes cure that by raising the ISO. Depends on the camera, but if you normally shoot at around an ISO of 200, raising the ISO to 800 will allow shorter exposure times, and shouldn't increase noise too much with a fairly recent body.

The opinion seems to be that the more expensive the lens the better. My Bigma was expensive for me but is considered a cheaper lens by the 'experts'

I expect I fall into your category of 'experts'! To get here, I initially travelled blissfully thinking I could buy 300mm lenses which I could easily afford and which would give me pleasing results. No way!
Somewhere along the line, I compared my Soligor 300 f5.6 against a Canon 300 f5.6 FD. The negatives from the Canon were so much better, I got rid of the Soligor more or less immediately, and it helped pave the way eventually for my purchase of a Canon EF 300 f2.8L. All I had to do then was to find out where the focus errors were! Thank goodness for the AF MicroAdjust feature on the latest bodies.

I would like a wide lens 10-20 or 12-24 as I photograph landscape regularly but the reviews are very mixed and they all seem the require stopping down to improve sharpness. I do not want to pay £400/500 and find I have wasted my money.

I bought a used 12-24 Sigma about a year ago, at a very good price, and I've posted some examples here. Check the Willen church example (shot at f11) at the bottom of the thread. Unfortunately the RRP, and the price on the used market has risen substantially since I paid £300 for my mint copy.

The other problem with sharpness which some of us encounter is a poor example of our chosen lens. It hasn't happened to me that often, but I'd advise checking out this site and reading the reviews which give low marks to a lens. If poor quality control seems to be an issue, ask yourself why a lens is on offer used, and consider instead buying new from a dealer with a reputation for no-quibble replacement. (Warehouse Express are good on this.)

[beejaybee]

My worst experiences with diffraction softening has been whilst using a zoom even although macros should suffer more.

?

Diffraction doesn't care whether a lens is a zoom or prime, the number of elements or the optical design. All that matters is the apparent diameter and shape (round is best) of the iris.

The only reason why "macros should suffer more" is that you're working at a smaller focal ratio than indicated because of the extension of the lens. i.e. the diameter of the iris is the same but the diffraction pattern is magnified by the extra distance between the lens's nodal point and the image plane.

[TimF]

My Bigma was expensive for me but is considered a cheaper lens by the 'experts'

Yes, leaving aside the 'expert' bit, my experience has been similar to Malcolm's. I could tell the difference between the Sigma 28-200mm I started with and the similar Nikon it was replaced by after a year. When I then began using prime lenses; firstly Nikon and then Leica the effect on image quality was similar except magnified greatly.

Now it is true to say that Nikon's pro-spec zooms are generally as good if not better than many of their prime lenses; however it's equally true that many of those prime lens designs are pretty old now - some date back to the beginning of the autofocus era (if not before!)

[beejaybee]

however it's equally true that many of those prime lens designs are pretty old now - some date back to the beginning of the autofocus era (if not before!)

The fact that the designs are old doesn't mean they are bad.

The essential point here is that, when most of these lenses were designed, film was king and film was (and remains) slow - so wide apertures were necessary - consequently there was pressure on camera manufacturers to make lenses a stop or two faster than the design was optimised for. That's why they're often a bit wooly when used at full aperture.

The same pressures are still there, though they're transformed by digital into wanting to market a lens which is lighter, cheaper and yet still faster than your competitor. Use less, smaller elements than are required to get maximum performance at full aperture and you achieve these design goals, at the expense of resolution.

Then along come ultra high density sensors & your design criteria are no longer adequate for optical performance to match the capability of the sensor....

[El_Sid]

The fact that the designs are old doesn't mean they are bad.

Our old friend the 50mm f1.8 being a classic example...

I do agree with Andy though. I don't find zooms quite as sharp when heavily stopped down as primes. Personally I put this down to the compromises that any zoom requires in order to function across a broad range of focal lengths - primes after all can be much more effectively optimised as it only needs to cover the far more restricted focal length variations required to cover focusing. I also suspect that standard zooms which have to cross over between retrofocus and telephoto requirements may be particularly prone to quality issues.

[TimF]

The fact that the designs are old doesn't mean they are bad.

Our old friend the 50mm f1.8 being a classic example...

Well, as our old chum Mr Hicks would say - you have to be pretty dumb to make a bad 50mm (though doubtless there have been some!)

In response to BJB, I don't think I said that just because the lenses were old it means they are bad - though that said, some are decidedly mediocre; the 28mm f/2.8 springs to mind. Many long time users say that one is inferior to its manual focus predecessor.

[nspur]

Am tempted to blow the dust off my 28-105mm and see if it really is as bad as I remember it being at f/22.

If it's the Canon one, my memory is of it being pretty awful at any aperture.

[Benchista]

Am tempted to blow the dust off my 28-105mm and see if it really is as bad as I remember it being at f/22.

If it's the Canon one, my memory is of it being pretty awful at any aperture.

Depends which one it is - the f3.5 versions are OK< the f4 is very poor.