Aperture confusion

Hi chaps .... again a basic amateurs problem but please advise, I always thought that a small aperture would generally give a sharper result ... trying out my 28-135 at full zoom (135) in low light with 100 ISO I was surprised to see that it was softer closed down .... quite a lot .... is this diffraction or a problem ?

Many thanks Roger

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Most lenses are at their sharpest when closed down a couple of stops from their widest aperture, so I would expect the result you have shown, especially the f/5.6 image.

What the smaller apertures (F/11 onwards) give is a greater range of acceptable focus, ie greater depth of field. That doesn't mean the sharpest images are at the smaller apertures.

That's how I understand it anyway. I'm sure someone with more in-depth knowledge will correct me.

When you get past f16 or so, you start losing sharpness due to diffraction. Most lenses are sharpest around f5.6-f8.

Almost all lenses are best across a certain range but then diffraction sets in so theres a trade off, wide open the light passing through the entire surface of the lens is being used (eg f3.5 on yours), once stopped down a few f stops you will mostly use the centre of the lens and that gives better results (f5.6-8), once you go past that diffraction becomes more and more of a problem (f18-36) so they are softer.

ps hadnt seen the other replies but you get the idea

What was your SS at f36?

Hi windpig ... 1/6 second ... on a tripod with a 2 second delay on shutter speed to help steady it ... Roger

I of course meant "shutter release" not speed ...

http://www.photozone.d​e …anon_100_28is_5​0d?start=1

Look about halfway down the page and you can see what diffraction does at higher apertures. It sets in on full frame much higher than most crop cameras due to pixel density. For larger aperture lenses it starts to limit a lenses performance usually around F7.1-F8. On Full frame this is typically pushed out to F11 or so. Smaller aperture lenses still typically benefit from being stopped down a stop or so for an F5.6 lens might still improve at F7.1 unlike larger aperture lenses.

Sdiver2489 wrote in post #12388416
http://www.photozone.d​e …anon_100_28is_5​0d?start=1

Look about halfway down the page and you can see what diffraction does at higher apertures. It sets in on full frame much higher than most crop cameras due to pixel density. For larger aperture lenses it starts to limit a lenses performance usually around F7.1-F8. On Full frame this is typically pushed out to F11 or so. Smaller aperture lenses still typically benefit from being stopped down a stop or so for an F5.6 lens might still improve at F7.1 unlike larger aperture lenses.

Lens' maximum aperture has nothing to do with diffraction, it appears depending on the aperture used for the given shot.

Diffraction appears when the size of the Airy disk becomes larger than the pixel size of the sensor.
Pixel size is set by the sensor (camera) manufacturer: there is nothing to be changed there.
Airy disk size depends on wave length of the light and the aperture size - once again - used for the particular shot.

http://www.cambridgein​colour.com …ffraction-photography.htm

rogertb wrote in post #12388396
Hi windpig ... 1/6 second ... on a tripod with a 2 second delay on shutter speed to help steady it ... Roger

Were you using a truly solid tripod or a cheap tripod? That could make a huge difference in whether or not the camera was truly steady for the long shutter speeds.

Yes it was a flimsy travel one - the nearest I could find ... I'll find my heavier one and try the test again ... thanks Skip ... Roger

Hermeto wrote in post #12388533
Lens' maximum aperture has nothing to do with diffraction, it appears depending on the aperture used for the given shot.

Diffraction appears when the size of the Airy disk becomes larger than the pixel size of the sensor.
Pixel size is set by the sensor (camera) manufacturer: there is nothing to be changed there.
Airy disk size depends on wave length of the light and the aperture size - once again - used for the particular shot.

http://www.cambridgein​colour.com …ffraction-photography.htm

I never said it did. Large aperture lenses max out their own resolving properties earlier than small aperture lenses. That stems from the commonly held belief that most lenses are at their best 1-2 stops from their max aperture.

Therefore, small aperture lenses can benefit from stopping down a stop or two even though diffraction has begun to have an effect because the increase in the lenses resolving ability outweighs the diffraction effect. Meanwhile, a large aperture lens has typically already maxed out its resolving ability by the time you get to the diffraction limit and therefore softens earlier.

compare the earlier posted results of the 100mm F2.8L IS to this:

http://www.photozone.d​e …0300f456vceosap​sc?start=1

And you will see this effect in action.

rogertb wrote in post #12388396
Hi windpig ... 1/6 second ... on a tripod with a 2 second delay on shutter speed to help steady it ... Roger

Kinda slow, even on a tripod when trying to do focus critical stuff. Also, mirror lock up should be engaged along with using the shutter release timer. I think diffraction is a part of it, but you'd need to eliminate camera shake out of the test.

Ahhhhhhh windpig ... mirror lock up - haven't get to that bit in the manual yet - you have just prompted me to dig in thank you .... I'll try the test again with all the recommendations applied thank you (it is out of my office window so pretty simple ... and the sun has come out just to confuse matters) ... Roger

SkipD wrote in post #12388542
Were you using a truly solid tripod or a cheap tripod? That could make a huge difference in whether or not the camera was truly steady for the long shutter speeds.

You can tell from the images that it is not camera shake. That's what a 100% crop of f36 diffraction should look like.