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DPChallenge Forums >> Photography Discussion >> aperture of F 20 and smaller redundant??
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 11/30/2012 04:52:05 PM · #1
Hi

I know there are some VERY technical people on DPC, so I thought, if I throw this out, there may be some interesting discussion.

I attended a very good lecture on macro photography today. One of the things that were brought up was the problem with DOF in macro - OK, so far I could follow and know from my own limited experience.
Then the guy says that "some people" (count me into those :-(), think by reducing the f stop to very small, they would increase their DOF (correct), but in actual fact, because of .... .. #$%@ forgot the term, anyway, the light rays bend before they hit the sensor and the image will be "unsharp". Apparently, this is so bad, that you can't fix it in pp either (I actually asked that). Then he continued to say that in fact those small apertures are going to be obsolete in future, as even for landscapes he never goes above 11, perhaps 13, and nobody should.

So, I would like to ask you, firstly, those of you who shoot the amazing landscapes, do you agree with that and do you use apertures around 11? Secondly how can you get enough DOF in a landscape when you shoot on 11? and lastly, do you agree in the "obsoleteness" of the small aperture?

Very curious

11/30/2012 05:00:01 PM · #2
What happens is that as the aperture gets smaller, there is an increase in diffraction from the diaphragm that decreases the resolving power. It's referred to as being "Diffraction Limited"

//www.cambridgeincolour.com/tutorials/diffraction-photography.htm

If you play with the little interactive gizmo on the linked page, you'll see that the aperture at which is due to many factors, which will vary based on pixel count, sensor size and other factors that will vary widely. So, while a given lens may be diffraction limited at f11 on one camera, it might not be diffraction limited until f13 on another camera.



Message edited by author 2012-11-30 17:07:21.
 11/30/2012 05:01:11 PM · #3
Originally posted by kasaba:
. .. #$%@ forgot the term, anyway, the light rays bend before they hit the sensor and the image will be "unsharp".

"Diffraction" (or maybe "edge diffraction") ... I think it also depends on the size of the sensor/film and the physical size of the lens and aperture (remember the f/-value is a ratio) -- certainly Ansel Adams and the f/64 Group were able to achieve sharp images, but usually shooting to an 8x10 inch piece of film.
 11/30/2012 05:01:36 PM · #4
The term is diffraction, and there is a good article on it at one of my all time favorite photography explanation sites: //www.cambridgeincolour.com/tutorials/diffraction-photography.htm

I don't know that I can comment on the future obsolescence of apertures smaller than f/11. But I can say that most of my landscapes are f/7.1 to f/11. Only times I really go smaller is when I am trying to get a longer exposure, but neutral density filters are better for that.

Edit to add: 2 posts in under 2 minutes pointing to the same link. nice! :)

Message edited by author 2012-11-30 17:02:05.
 11/30/2012 05:05:07 PM · #5
diffraction

It depends on the lens. My Canon 24-105mm f/4L IS USM has a optimum aperture at f/16. But, I do use DOFmaster to see if I can focus at the hyperfocal distance and achieve my intended Depth of field with a bigger aperture size (smaller number). It's true that stopping down excessively will reduce sharpness of detail in a landscape image.
 11/30/2012 05:10:42 PM · #6
Holy Crap! You tech guru's were on this!!
 11/30/2012 05:57:55 PM · #7
Remember that, as pointed out earlier, f/stop is a RATIO: it's the ratio of the diameter of the aperture and the focal length of the lens. So a 25mm aperture on a 50mm lens is f/2 and the same aperture on a 100mm lens is f/4, whilst on a 400mm lens a 25mm aperture would be f/16.

For all practical purposes, the amount of diffraction you get is dependent on the physical size of the aperture. So, hypothetically, if your 100mm lens does not begin to show the effects of diffraction until after f/10 (a 10mm aperture), then you're actually going to be OK at f/40 on the 400mm lens (assuming you had it available), as that's a 10mm aperture as well!

There ARE other factors to take into account system-to-system (the effect changes with different film,sensor sizes, as mentioned earlier, for example) but within a given system, and given lens quality, it's the physical diameter that counts, not the f/stop.

Regarding the "F/64 Group", bear in mind that they were shooting the 8x10 format, where a "normal" lens is around 300mm in length; so f/64 on a 300mm would be close to a 5mm aperture, not especially small by modern standards. The same aperture on a 50mm lens would be around f/10...

Message edited by author 2012-11-30 17:59:20.
 11/30/2012 06:30:50 PM · #8
Even if a lens is diffraction limited I don't think they will obsolete the small apertures. It can still be useful to use those tiny apertures to reduce the amount of light into the camera.

My top scoring image was shot at f/32 because I still had too much light after stacking three ND filters. The diffraction made it fairly soft at 100% zoom, but for an 800px DPC entry it worked just fine.
 12/01/2012 03:49:16 AM · #9
Originally posted by bhuge:
Even if a lens is diffraction limited I don't think they will obsolete the small apertures. It can still be useful to use those tiny apertures to reduce the amount of light into the camera.

My top scoring image was shot at f/32 because I still had too much light after stacking three ND filters. The diffraction made it fairly soft at 100% zoom, but for an 800px DPC entry it worked just fine.


Just looked at that image - Lovely :-).

That is an excellent point you make, and of course there may be pictures that should be soft in the first place.

Also, from a technical side, how much "more work" (i.e more cost) is involved to keep the extra f stops, even if they are used infrequently? If I look at the stuff my camera can do, I wonder how much of it "people" really use and it is still there as an option.
 12/01/2012 03:58:25 AM · #10
Originally posted by Bear_Music:
Remember that, as pointed out earlier, f/stop is a RATIO: it's the ratio of the diameter of the aperture and the focal length of the lens. So a 25mm aperture on a 50mm lens is f/2 and the same aperture on a 100mm lens is f/4, whilst on a 400mm lens a 25mm aperture would be f/16.

For all practical purposes, the amount of diffraction you get is dependent on the physical size of the aperture. So, hypothetically, if your 100mm lens does not begin to show the effects of diffraction until after f/10 (a 10mm aperture), then you're actually going to be OK at f/40 on the 400mm lens (assuming you had it available), as that's a 10mm aperture as well!

There ARE other factors to take into account system-to-system (the effect changes with different film,sensor sizes, as mentioned earlier, for example) but within a given system, and given lens quality, it's the physical diameter that counts, not the f/stop.



This is what always worries me about photography, my Physics is really poor, always has been, and just when I think I "have" another concept, there comes another whole chapter to point out my lack of understanding :-(.
Guess I will just have to chew off little bit by little bit and try to get a better base. I find it hard and infuriating not having the whole background info at my fingertips. Thanks to all of you for presenting different aspects of this "problem" in different ways, yes it showed up a big hole in my understanding, but better to know that there is a hole and try to shine some light into it, than glibbly cruise through this photographic journey not knowing why I sometimes fall flat on my face:-)

12/01/2012 04:03:43 AM · #11
Originally posted by kasaba:
Originally posted by Bear_Music:
Remember that, as pointed out earlier, f/stop is a RATIO: it's the ratio of the diameter of the aperture and the focal length of the lens. So a 25mm aperture on a 50mm lens is f/2 and the same aperture on a 100mm lens is f/4, whilst on a 400mm lens a 25mm aperture would be f/16.

For all practical purposes, the amount of diffraction you get is dependent on the physical size of the aperture. So, hypothetically, if your 100mm lens does not begin to show the effects of diffraction until after f/10 (a 10mm aperture), then you're actually going to be OK at f/40 on the 400mm lens (assuming you had it available), as that's a 10mm aperture as well!

There ARE other factors to take into account system-to-system (the effect changes with different film,sensor sizes, as mentioned earlier, for example) but within a given system, and given lens quality, it's the physical diameter that counts, not the f/stop.



This is what always worries me about photography, my Physics is really poor, always has been, and just when I think I "have" another concept, there comes another whole chapter to point out my lack of understanding :-(.
Guess I will just have to chew off little bit by little bit and try to get a better base. I find it hard and infuriating not having the whole background info at my fingertips. Thanks to all of you for presenting different aspects of this "problem" in different ways, yes it showed up a big hole in my understanding, but better to know that there is a hole and try to shine some light into it, than glibbly cruise through this photographic journey not knowing why I sometimes fall flat on my face:-)


Trust me you are not alone, most questions and the following answers that appear here are very helpful for me and I bet many more others because the combined knowledge on DPC is enormous, we can all learn here... so keep asking ;)
 12/01/2012 12:29:56 PM · #12
From a practical perspective, it's not really necessary to understand the physics... it helps, don't get me wrong, but not absolutely necessary. All you really need to remember is that for very small apertures (high f numbers) diffraction effects increase, and that the point at which they become noticeable is a smaller f number for smaller optical systems. For very small formats, like P&S cams, you rarely see anything beyond f/8 or f/11 even included as an available setting, because diffraction would be a real problem and DoF is already huge. For very large formats, different story entirely.
The moral of the story is that you just need to figure out where you begin to notice it, where it becomes objectionable to you, and don't go there.
Discussion of the physics involved would fill pages and really doesn't give all that much more insight into what we have to concern ourselves with in practical shooting scenarios. Macro is perhaps one exception to this, but again from a practical standpoint we can just follow our basic rule. For my Canon 5D, I usually don't go past f/13, occasionally f/16. That gives me results I'm happy with.
 12/01/2012 12:32:16 PM · #13
I don't see lens makers removing smaller apertures because going past the diffraction limit isn't necessarily something to avoid entirely, it's not like things get completely soft and mushy once you go past it. The effect is gradual. You might choose to use a smaller, diffraction limited aperture to gain sharpness outside your DOF at the expense of sharpness within the DOF or to intentionally force a long exposure.
 12/01/2012 04:11:37 PM · #14
and to introduce another FUN topic, things get especially tough once you get into the macro zone, for 2 reasons:

1) the more you magnify, the shallower your DOF, the more you want to stop down your lens, the quicker you get bit in the ass by diffraction.
2) past 1:1 magnification, your aperture is effectively reduced even more. a good approximation is the following:
effective aperture = physical aperture x (M+1)
where M is magnification. So let's say you are at a 2 x magnification shooting at f8, you are effectively at f24... and you get bit in the ass again by diffraction.

solution? focus-stacking. look it up, it'll blow your mind
 12/03/2012 09:03:38 AM · #15
Originally posted by Bear_Music:
Remember that, as pointed out earlier, f/stop is a RATIO: it's the ratio of the diameter of the aperture and the focal length of the lens. So a 25mm aperture on a 50mm lens is f/2 and the same aperture on a 100mm lens is f/4, whilst on a 400mm lens a 25mm aperture would be f/16.

For all practical purposes, the amount of diffraction you get is dependent on the physical size of the aperture. So, hypothetically, if your 100mm lens does not begin to show the effects of diffraction until after f/10 (a 10mm aperture), then you're actually going to be OK at f/40 on the 400mm lens (assuming you had it available), as that's a 10mm aperture as well!



Actually, this is incorrect. While your statement that the absolute size of the aperture differs for different lenses is correct, the light has farther to travel inside a longer focal length lens and thus the airy disk has a greater distance over which to diverge. In the end, the two effects cancel each other out and it's the f ratio that determines diffraction limiting, all other factors (sensor size etc.) being equal.

ETA: I don't mean to be nitpicky, but it's my nature as an engineer.

Message edited by author 2012-12-03 09:04:35.
 12/03/2012 10:00:36 AM · #16
They have to put those numbers like f22 on modern lenses so that you can see the dust bunnies.
 12/03/2012 11:17:29 AM · #17
Originally posted by MelonMusketeer:
They have to put those numbers like f22 on modern lenses so that you can see the dust bunnies.


+1 :-P
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