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 Post subject: aperture and focus
PostPosted: Mon Sep 10, 2007 1:15 pm 
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A basic question about aperture and focus: If I make focus my priority, is there an optimum aperture setting? I am not referring to DOF, but rather, is there an aperture sweet spot at which my lens will focus best, at which I will get my sharpest pictures? If so, can it be calculated or determined by approximation? I know this may be a stupid question; that there may not be an optimum aperture setting for focus – just had to ask. Of course, this is in disregard to exposure.


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 Post subject:
PostPosted: Mon Sep 10, 2007 3:18 pm 
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I'm not 100% sure on this but I think, as a general rule, most lenses aren't as sharp at wider aperture settings but once you get to around f8 they enter their optimum sharpness.

I'm sure someone more technical will be able to confirm/deny that...?

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 Post subject: f8 rule
PostPosted: Mon Sep 10, 2007 3:51 pm 
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that sounds right -- I read somewhere once about an f8 rule, that suggests the most consistant quality can be gotten at f8. I still wonder if it holds up with all lenses, however. Also, I wonder why focus would be better at narrow settings. Thanks Telexstar


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 Post subject:
PostPosted: Mon Sep 10, 2007 5:31 pm 
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Brian, you have to differentiate between sharpness because of correct focus and sharpness of th elens in general:
For sharpness depending on correct focus the aperture that you set your lens to is irrelevant, as the focus mechanism does it's work before the camera stops the lens down to do the exposure. Or to put it the other way round: All AF is done with the lens opened to it's max aperture.
Sharpness depending on lens quality (even if you or your camra has focused the lens accurately depends on the aperture you shoot with. Most lenses are optimally sharp at around f5.6/f8/f11.

Hope that helps!

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Last edited by Thomas on Mon Sep 10, 2007 6:54 pm, edited 1 time in total.

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 Post subject: sweet spot
PostPosted: Mon Sep 10, 2007 6:12 pm 
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Great tombomba2, I think you are confirming that there is a sweet spot in most lenses, that they are built to optimize that area around f8 (f5.6 - 11). That's what I really wanted to know!

As to the rest: I generally set everything else (shutter speed, f stop, ISO (exposure)) before I auto or manual focus, so considering focus first is a little backwards for me, like this. I assume that shooting in some modes does what you described, but I generally prefer manual settings. Thanks


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 Post subject:
PostPosted: Wed Sep 12, 2007 12:57 am 
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Also too, to find the presice "sweet spot" you have to put it in 2 stops more, as in if your shotting at f/2.8, the sharpest aperture is at f/5.6. I read this in a book written by Scott Kelby here in the U.S.

Hope this helps a bit

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 Post subject: Sweet spot
PostPosted: Wed Sep 12, 2007 2:12 am 
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I guess I should read that book. I assume by f/2.8 you are refering to the rating of the lens, and then add 2 stops?

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 Post subject:
PostPosted: Wed Sep 12, 2007 5:23 am 
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Well, the "plus 2 stops" rule may be a little over-simplifying!
Look here (copyright Klaus Schroiff):
Image
With this Nikkor 50mm/f1.4 lens the "sweet-spot" is at around f5.6, thus 4 stops down from the maximal (open) aperture!

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Last edited by Thomas on Wed Sep 12, 2007 7:41 pm, edited 1 time in total.

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 Post subject: Sweet spot
PostPosted: Wed Sep 12, 2007 2:44 pm 
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Interesting... I think I will try to find info on my specific lenses. It would be worthwhile to know. Of course zoom lenses make this whole thing more complicated, but a general knowledge of your lens' character would be helpful. Thanks

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 Post subject:
PostPosted: Wed Sep 12, 2007 3:54 pm 
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Olympus didn't offer the chart, but PhotoZone did (thank you PhotoZone), however it’s not possible to copy it (understandable). My 40-150mm f/3.5-4.5 was surprisingly flat (just over 1400, f3.5-8.0) at 40mm, and had minimal variance (from 1350 to just over 1400, f4.5-8.0) at 150mm, with the sweet area appearing at f/8.0 (The chart did not go above f/8 ). So for this lens, the f/8 and 2 stop rules seems to hold.

I had been assuming that maximum light (low f) would give me the clearest shots (all other adjustments considered). However, this conversation has shown that to be incorrect; when it comes to focus. Lenses are indeed made to operate at intermediate aperture settings (usually f/8.0). That “sweet spot” can also be found by adding 2 stops to the lenses f/ value. However, to truly know your lens, it seems you should check its chart, since some lenses have very unique characters.

Thanks everyone for this very eye-opening conversation

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 Post subject:
PostPosted: Wed Sep 12, 2007 7:52 pm 
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BrainS, the reason for the maximum at a middle aperture is that on one side lens-errors are reduced by closing the apertur, but on the other side a general problem raises it's ugly head: diffraction. This increases with smaller apertures (=higher f-stops) and is a law of optics, i.e. cannot be corrected by the manufacturer.
You can see one absolutely crass example of this IQ-deteriorating effect on my own Nikon 105mm macro lens, which is otherwise fine glass, here (coyright Klaus Schroiff):
Image
Look at the incredible reduction in resolution from f16 to f32 :cry:
And that with a lens where you could need every cm of dof, when you shoot macro-photos :!: :(

Btw: This is one of the most relevant "off-topic" threads! Thank you, BrianS :)

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 Post subject: Thanks
PostPosted: Thu Sep 13, 2007 1:43 pm 
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Thank you tombomba2, and everyone, I am surprised how much I learned from this.

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 Post subject:
PostPosted: Thu Sep 13, 2007 2:06 pm 
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Eye opener for me too ! :shock:

Thanks everyone :D :wink:

Very informative indeed

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 Post subject:
PostPosted: Thu Sep 13, 2007 10:35 pm 
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Yes, this is very important when it comes to getting the best from your lenses. Due to diffraction, all lenses will resolve less detail when they're fully stopped-down, so avoid using above f16 unless you NEED the large depth of field.

The sweetspot is normally around f8, but be aware for some lenses it may be at a lower setting still, like f5.6 or even f4. It may also vary at different focaing distances (and for a zoom also at different focal lengths).

So if you want to know the sweetpsot for your lens, pop the camera on a tripod, set it to aperture priority, focus on the subject, then put it to manual focus to prevent any variations. Then simply take the same photo at every aperture setting.

Next open them all up on your computer, zoom into an area with lots of detail at 100% and compare them. Which looks best to you?

Also check the edges. Most lenses perform worse at the edges when the aperture is wider open, but then annoyingly this may be when it's sharpest in the middle!

Gordon


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 Post subject:
PostPosted: Thu Sep 13, 2007 11:05 pm 
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I must say, I even think the 50mm F/1.8D performs sweet at F1.8

http://flickr.com/photos/antmanisfubar/1178848532/

I thought that photo came out pretty sharp for open apature.

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