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Ian
27th January 2011, 09:41 AM
Lovely subject title! :)

But in all seriousness, the fact is that if you use too small an aperture you will suffer from diffraction softening. I have written an article on DPNow.com that explains the phenomenon and the example shots were all taken using an Olympus E-5 and 35-100 ZD lens. I hope you will find the article illuminating:

http://dpnow.com/7644.html

Ian

sapper
27th January 2011, 10:41 AM
Very helpful Ian.

Ian
27th January 2011, 10:42 AM
Thanks all - the sample shots below f/8 are a bit scary don't you think!?

Ian

OlyPaul
27th January 2011, 11:45 AM
Ian its only what I would expect, f22 on 4/3 is equivalent to f64 on 35mm and I would rarely go above f16 on 35mm because of diffraction.

At f5.6 on 4/3 you are getting the same dof as f11 on a full frame so there is rarely any reason to go above f8 unles it is close-up/macro work.:)

Radar
27th January 2011, 11:47 AM
Great article! I tend to do my indoor work with flash on f8 and f11. f8 is fully usable and I don't find the softening on f11 disturbing. But I never go smaller than f11 as the diffraction is visible.

Ian
27th January 2011, 12:01 PM
Ian its only what I would expect, f22 on 4/3 is equivalent to f64 on 35mm and I would rarely go above f16 on 35mm because of diffraction.

At f5.6 on 4/3 you are getting the same dof as f11 on a full frame so there is rarely any reason to go above f8 unles it is close-up/macro work.:)

There is another reason - to reduce brightness if you don't have a ND filter to hand.

I tend to aim for f/6.3 or f/7.1 for ideal lens resolution on Four Thirds.

Ian

snaarman
27th January 2011, 12:09 PM
Yep. Its usually f8 and be there for me as well :-)

Pete

Ian
27th January 2011, 12:31 PM
Another thing - the 35-100 is impressively sharp even at f/2 - but we all knew that already didn't we? :D

Ian

photo_owl
27th January 2011, 12:49 PM
Another thing - the 35-100 is impressively sharp even at f/2 - but we all knew that already didn't we? :D

Ian

yep.......*chr

Radar
27th January 2011, 06:22 PM
I want one :D

David Morison
28th January 2011, 12:08 AM
Very interesting article, I learn something new every day. But a question comes to mind - with my Leica 14-150mm and EC20 (which I rarely use) I am starting at a base f stop of 11 @ 150mm, so according to your tables I can usefully go no further. However when the EC20 is used with this lens my minimum f no. registers on the camera as f45! So where do I go from here?

Thanks

David

Ian
28th January 2011, 11:01 AM
Very interesting article, I learn somrthing new every day. But a question comes to mind - with my Leica 14-150mm and EC20 (which I rarely use) I am starting at a base f stop of 11 @ 150mm, so according to your tables I can usefully go no further. However when the EC20 is used with this lens my minimum f no. registers on the camera as f42! So where do I go from here?

Thanks

David

Yup, with the EC-20 and the widest aperture set to f/5.6 (tele end of the zoom), the effective aperture is f/11, so the lens is already diffraction affected without stopping down. Moral of the story - don't use the EC-20, the EC-14 would be better (f/8 effective aperture). Having used that lens for a few months - it's a glorious performer. I really wish I still had the use of one. But I wouldn't use a converter with it.

Ian

theMusicMan
28th January 2011, 11:44 AM
Great article there Ian, very informative thanks.

Makonde
28th January 2011, 12:03 PM
Good images, well illustrating the effect.

Another reason to go for small aperture on specific occasions is to get good starburst effects - having said that, I rarely manage to get good starbursts myself whatever I do! Any tips on that?

While you are playing with that lens and E5, Ian, how about some tripod shots with/without IS on to see just how much difference IS makes to the clarity when using a tripod (my own tests with the 520 a couple of years ago suggested: 'not much' but I didn't have a long lens of that quality)

photonutter
28th January 2011, 05:18 PM
Great piece of info, just wonder how the 8meg E500 fairs and is the focal distance a contributing factor?

stevednp3
28th January 2011, 05:47 PM
Wow that is scary, I knew quality degraded when using a smaller aperture, but not like that wow :eek:

Cheers Ian

David Morison
28th January 2011, 05:52 PM
Ian,

Your excellent articles on diffraction softening and hyperfocal distance refer to the problems of light rays suffering diffraction when passing through a small diaphragm aperture. I am interested in how this affects the image when using a lens whose "effective" aperture is reduced by a teleconverter which in the case of EC20 is a 2 stop loss. As I mentioned in an earlier post the minimum effective f stop with the EC20 on my Leica 14-150mm is f45 whereas without it is f22. The actual aperture in the diaphragm hasn't actually changed but the amount of light hitting the sensor has, so does this have the same diffraction effect? I am afraid my physics is not quite up to spec. on all this so please excuse me if I don't appear to be "seeing the wood for the trees".

I am very much in the early learning phase as far as photography goes so I can't promise not to continously ask less than intelligent questions!

Kind Regards

David

P.S. Anyone want to swap an EC14 for my mint EC20?

StephenL
28th January 2011, 05:55 PM
This article has certainly opened my eyes. I'd heard about the diffraction effect, but thought it was too little to have much real-world effect. Looks like I was wrong (again!).

Garrie
28th January 2011, 06:58 PM
Thanks for putting this together. Its totally opened my eyes to diffraction, its a bit scary really, I had no idea how bad F22 could be :eek:

stevednp3
28th January 2011, 07:18 PM
Going back a few months ago I did some long exposure shots at a beach at f22 to get the light down (I did use a ND grad as well) got back home looked at the shots and the quality was awful and binned them all, I thought I had done something wrong or the wind blew the camera, but this answers it now.

Least I know in the future that I have to let the light die enough to get up to f11 or f8 for doing shots like that again :cool:

Wee man
28th January 2011, 07:24 PM
Very useful post I never considered that the F stop would be changed with the sensor. I am another who has just found why some shots were only fit for the bin.

Homer Simpson
28th January 2011, 08:40 PM
Interesting*chr

I sometimes have trouble as my brain/memory reverts to 35mm days
Its easy to forget that F8 is really f16 in film terms

Now what about ISO ?
My default setting on my E520 is 400 - is this general concensus as being "A good thing?"

If I want to set my max aperture as being F8 but want to slow things down a bit, is chosing ISO 100 good or bad
Actually, what are you really altering when you alter ISO on a DSLR ?

photo_owl
28th January 2011, 08:58 PM
Interesting*chr

I sometimes have trouble as my brain/memory reverts to 35mm days
Its easy to forget that F8 is really f16 in film terms

Now what about ISO ?
My default setting on my E520 is 400 - is this general concensus as being "A good thing?"

If I want to set my max aperture as being F8 but want to slow things down a bit, is chosing ISO 100 good or bad
Actually, what are you really altering when you alter ISO on a DSLR ?

if you are asking those questions I would (tactfully) suggest a new thread HS

Homer Simpson
28th January 2011, 09:33 PM
PO

You might be right but I was really asking if they were related issues, then I got a bit carried away.:rolleyes:

David M
28th January 2011, 11:18 PM
I tend to aim for f/6.3 or f/7.1 for ideal lens resolution on Four Thirds.

Ian

I just checked my aperture values in Lightroom. About 97% of my shots are f/5.6 to f/8.

I've never shot 4/3 at f/22.

Nick Temple-Fry
29th January 2011, 12:00 AM
Ian,

As I mentioned in an earlier post the minimum effective f stop with the EC20 on my Leica 14-150mm is f45 whereas without it is f22. The actual aperture in the diaphragm hasn't actually changed but the amount of light hitting the sensor has, so does this have the same diffraction effect?



The actual way you have phrased it makes this a difficult question to answer, so forgive me if I wander around the topic a little.

Ian states, as an assumption to avoid technical geekery that

The main limiting factor for digital cameras is when Airy discs exceed the size of individual pixels on the sensor. At this stage an individual pixel is no longer able to record a single Airy disc or minimum point of detail from the subject, and so resolution progressively falls off. You may also notice a reduction in contrast

accepting that (and it is a good way to express it without adding in quite a lot more physics), then the key point is the size of the airy disc that is produced on the sensor.

Before we get on to tele converters lets consider the case of two lenses one 50mm focal length and one 100mm focal length, we set both lenses to F5. Can we find out the aperture, well yes because we know that

F = Focal length/Aperture

so for the 50mm lens

5= 50/Aperture

Aperture = 50/5 = 10

And for the 100mm lens

Aperture = 100/5 = 20

So we can see that as you get longer focal lengths then the size of the aperture increases for the same f number. But does this alter the size of airy disc at the sensor. The answer is no because the diameter of the airy disc is contolled by this formula

D = 2.44 * λ * Fn

where

D= diameter of the airy disc
2.44 = one of those geometric constants derived from the maths of a circle
λ = wavelength of light
Fn = Focal Ratio

So for both our lenses the size of the airy disc is = 2.44 *λ * 5
= 12.2λ

(if you ask why, well I'll refer you to Mr Airy and Mr Raleigh (Mr Raleigh was the chap who first explained how diffraction limits lens resolution))

(You can think of the airy disc starting infinitely small at the aperture and growing the further it travels to the sensor)

Might be worth restating it this way

Since Fn = Focal length/Aperture

then for our 50mm lens

D = 2.44 * λ * 50/10 = 12.2λ

and for our 100mm lens

D = 2.44* λ * 100/20 = 12.2λ

Hopefully this will have demonstrated that the fall off in resolution is altered in proportion to the fnumber. It might even have answered your question.

Nick

Zuiko
29th January 2011, 12:44 AM
It's very true that diffraction with small apertures does degrade the image but that doesn't mean they are unusable. Generally small apertures should be avoided but what if there is a specific reason to use them?

In this shot I wanted to blur through motion the incomming tide as it swirled around the old sea defences. I did not have an ND filter and even at 100 ISO I needed a small aperture to get a slow enough shutter speed. This was taken at 1/20th sec @ f16. Sure, I could have got sharper at f8 but wouldn't have got the effect I wanted.

http://e-group.uk.net/gallery/data/500/SPURN_POINT_forum.jpg


Here's a 100% crop

http://e-group.uk.net/gallery/data/500/SPURN_POINT_100_CROP.jpg