What's So Special About f/8?

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Jayceyned

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Hey guys,

I recently read that shooting @ half open aperture will always provide the sharpest image?

Does this sound familiar to anyone and is it correct?

I'd appreciate any info anyone is willing to offer up, thanks very much in advance,

Jay.
 
No doubt those far more learned than I will be along to correct me shortly, but there is no 'holy grail' of sharp images. Each and every lens will have its own 'sweet spot', where it performs as well as it possibly can. That varies from type to type though, and often even from lens to lens.

Bear in mind it will also vary from subject to subject. Shooting a moving train at f8 for example, will in many cases not allow you a fast enough shutter to stop the motion, resulting in blur. Likewise, shooting a live band on stage..f8 just isnt going to cut it.
 
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afaik that is correct! Anything below around half open is softer, and anything above around half the maximum aperture will degrade sharpness due to diffraction.

My 50mm 1.8 is pretty soft wide open, it starts getting nice and sharp at about f/4 and it's amazing at f/8.

If I'm wrong someone please correct me too! :p
 
If you want to shoot a fast moving train at F8 you can,you just use manual & set the shutter speed you want.:)
 
F/8 is about as small as you can go on most APS-C cameras before diffraction starts to kick in and soften images.

Lenses tend to get sharper as you stop down and on a lot of bodies f/8 is as far as you can go before diffraction starts to soften images again.
 
A lens is always sharpest at the point where the aberrations reduced by raising the f/number are overtaken by diffraction, which is increased by raising the f/number. The crossover diffraction point is also related to format (to magnification actually) and in practise what this means is that on full frame most lenses peak around f/8, and the same lens will peak at f/5.6 on crop format.

The very best lenses will beat that by maybe a stop (ie f/5.6 and f/4 respectively) and they will peak at a higher level but that's the best I've ever seen, ie Canon 100L macro, 300L 2.8 and a couple of others get close to that.

Poorer lenses don't peak until maybe f/11, and sometimes you get the centre and edges of the frame peaking at slightly different f/numbers.

You can see the same lens on the two different formats here, Canon 50 1.8 http://www.dpreview.com/lensreviews/canon_50_1p8_ii_c16/page4.asp
 
Depends on what it is you photographing as if you have subjects in the foreground close up and subjects in the background far back (very technical lingo) and focus in the center the image OVERALL is going to be sharper at f22 than f8. But if photographing where the distance is of subjects is fairly relative and small aperture is going to be sharper OVERALL.
 
Done it many a time at motorsport.:)
 
I dont doubt it, but then I also said 'in many cases...'.
It was intended as an example, not a hard and fast rule.
 
:shrug:Nobody is arguing the fact.
 
So each lens will have a sweet spot but this is generally going to be under f8. Didnt know that, thanks!
 
retrogamer1990 said:
afaik that is correct! Anything below around half open is softer, and anything above around half the maximum aperture will degrade sharpness due to diffraction.

My 50mm 1.8 is pretty soft wide open, it starts getting nice and sharp at about f/4 and it's amazing at f/8.

If I'm wrong someone please correct me too! :p
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Actually the 50mm f/1.8 is about the sharpest it will ever get at f/4-f/5.6, and is still very sharp by f/2.8. If you are having softness issues below f/4 it's probably because you're missing the focus.

Other lenses don't sharpen up so much when stopped down, the 17-40 being a prime example. On full frame I find it perfectly sharp at f/4, and only slightly better at f/5.6.
 
A[L]C;3493882 said:
So each lens will have a sweet spot but this is generally going to be under f8. Didnt know that, thanks!
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Yes, any lens worthy of the name will generally peak at f/8 or lower f/number.

However, the very sharpest lenses, and we have a few of them about now that have been designed post very high-res digital, must peak at lower f/numbers than that, or you will never see the advantage due to the the diffraction limit which is on optical chacteristic defined by the physical size of the aperure, regardless of lens quality.

There was a guy posted some comparison pictures on here a year or so ago, to illsutrate his disappointment at hiring a £1200 Canon 16-35L II for a weekend of landscaping. He found it was no sharper than his kit lens, but that's because he was shooting at f/11 to get good depth of field and both lenses were equally limited by diffraction.

trencheel303 said:
Actually the 50mm f/1.8 is about the sharpest it will ever get at f/4-f/5.6, and is still very sharp by f/2.8. If you are having softness issues below f/4 it's probably because you're missing the focus.

Other lenses don't sharpen up so much when stopped down, the 17-40 being a prime example. On full frame I find it perfectly sharp at f/4, and only slightly better at f/5.6.
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That's true. Most lenses are pretty sharp in the centre right from the off. It's the edges and corners that mostly get better as the f/number is raised. It's well illustrated by the 50 1.8 test in the link post #6. That lens is also interesting in that the centre peaks early at a pretty high level and then starts to decline as diffraction weakens the middle, but the improvement in other aberrations is still making the edges better.

So what's the peak aperture there? Depends which part of the frame you're looking at, and whether the centre is all important, or overall edge to edge sharpness is what you're after.
 
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Just wanted to thank everyone for their contributions. Is all making sense.

Appreciate all of your help, hope I can do the same for someone else in the future.
 
Hers a question then, for you technical types.

What's the score if you use a lens that has a maximum aperture of say 6.3, as some of the larger Sigma zooms do ? Should you still be looking at f8, or maybe something like f10 ?

Steve

Sent from my iPad using TP Forums
 
Jelster said:
Hers a question then, for you technical types.

What's the score if you use a lens that has a maximum aperture of say 6.3, as some of the larger Sigma zooms do ? Should you still be looking at f8, or maybe something like f10 ?

Steve

Sent from my iPad using TP Forums
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F/8-11 for the Sigma 18-250 at the long end where it starts at 6.3. And at a pretty low level - those lenses tend to just get worse as focal length increases.

Review here http://www.dpreview.com/lensreviews/sigma_18-250_3p5-6p3_os_c16/page3.asp
 
This site is good for lens reviews where they go into a bit of detail measuring the sharpness in the centre, border and extreme parts of the lens during the review. They also measure at different focal lengths for zoom lenses. which may change the sharpest f stop. If will give you an idea where your lens is sharpest. ;)
 
redhed17 said:
This site is good for lens reviews where they go into a bit of detail measuring the sharpness in the centre, border and extreme parts of the lens during the review. They also measure at different focal lengths for zoom lenses. which may change the sharpest f stop. If will give you an idea where your lens is sharpest. ;)
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Yup, there is no magic formula. Even different lenses of the same type are likely to differ slightly in their behaviour.

If you're really, really worried about it then the best thing is to do some tests with your lens. Personally, I've got better things to do than measure imperceptible differences between f 5.6 and f8.
 
I've read all about diffraction and understand the theory - well, on a simplistic basis I suppose.

It all makes sense to me, but can anyone explain why in all the photo mags when you read about landscapes, they advise f16 or f22? At what stage do you trade off diffraction for depth of field?
 
It's true that most lenses have a sweet spot but it's not compulsory to always shoot at it. Doesn't matter what aperture you use as long as it's giving you the DOF you want and the quality is good enough for the size you want to print at. Best way is to do a test shoot of the same subject with your camera mounted on a tripod using every aperture, then compare to see which apertures are acceptable and which are not at your normal maximum print size.
 
parkmoy said:
I've read all about diffraction and understand the theory - well, on a simplistic basis I suppose.

It all makes sense to me, but can anyone explain why in all the photo mags when you read about landscapes, they advise f16 or f22? At what stage do you trade off diffraction for depth of field?
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...the simplistic answer is that diffraction isn't as bad as it sounds, unless you have a fetish for shooting brick walls.

If you are shooting landscapes, the greater DoF is more significant in ensuring that distant trees/rocks are in focus than the slight increase in blurring due to diffraction. Its even more critical if you are shooting macro (...unless you want to focus-stacking...)


Getting back to the original question, for most lenses, the highest and lowest f-stop they are rated at will be noticably a bit softer than the rest of the range...but the difference in sharpness over that range will be marginal (obviously consumer lenses will be worse than pro ones at this...you get what you pay for...). The result of this is that a F1.7 lens at f2.8 will be sharper than a f2.8 lens at f2.8, but both will be very good at f4 and virtually indistinguisable at f8!
 
parkmoy said:
I've read all about diffraction and understand the theory - well, on a simplistic basis I suppose.

It all makes sense to me, but can anyone explain why in all the photo mags when you read about landscapes, they advise f16 or f22? At what stage do you trade off diffraction for depth of field?
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Such narrow apertures are rarely needed for landscape photography. One tends to use short focal lengths which give a wide depth of field at wide apertures anyway.

It's the macro shooters who need f22 or worse. But, as Richard says, diffraction isn't as bad as some people make out. Try it yourself. I did and found that no real problems happened until around f22, rather than f8 as the theorists would have you believe.
 
I wouldn't worry too much about diffraction either, if only because there is very rarely any need to get too far into it. But if you do shoot at f/22 on a crop format camera, the losses are quite serious. Just take a couple of identical shots, at say f/8 and f/22, and zoom in on the LCD - you'll notice it for sure if your camera has a decent LCD screen.

However, the OP was about the sweet spot, where a lens is at its absolute sharpnest. You may have to look closely, but diffraction at least starts to take the edge off very fine detail at a lot lower down the scale than that. For folks that spend £1k on a L glass and are a bit obsessive about peeping at everyone of their 18m pixels or whatever, that's what they need to know.

It's not good for your picture-taking health though ;)
 
Ahhhhhhhh...f8

f8 for me goes way back to the days of black and white.

f8 at 1/125 was the universal setting for sunlit days.
Fast'ish shutter speed and a decent depth of field to cater for any focus errors.

If you needed greater accuracy then one used ma light meter.

D in W
 
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