spencer
Topic Author
Posts: 1518
Joined: Sun Apr 25, 2004 8:30 pm

### Sweet Spots And Diffraction DLA

I've been reading a lot of the technical literature about a camera I have (Canon 7D) and there's a point I've got to where I'm confused. I wonder if anyone else could help me out?
I really don't want someone's interpretation of a Google search, I can do that myself. If someone could shed some light on the subject then please could you help me with this:
IF a certain lens, (any lens, say), is sharpest at (say) f/8, and Canon say the camera's DLA is at f/6.8, then what aperture should one be aiming at, given the ideal conditions to go for the sharpest shot?
Simply put, if I dial in f/8 in Av, (aiming for the sharp aperture), but the DLA says fall-off starts around f/6.8, then is f/8 still accepted as the sharpest aperture?
Another camera I own is the 1D4 and DLA fall-off is at f/9.1! Same question applies....
Footnote; I knew nothing of DLA until I read this literature, so I'm quite interested in knowing more.

From the website's definition:

DLA (Diffraction Limited Aperture) is the result of a mathematical formula that approximates the aperture where diffraction begins to visibly affect image sharpness at the pixel level. Diffraction at the DLA is only barely visible when viewed at full-size (100%, 1 pixel = 1 pixel) on a display or output to a very large print. As sensor pixel density increases, the narrowest aperture we can use to get perfectly pixel sharp images gets wider.

DLA does not mean that narrower apertures should not be used - it is simply the point where image sharpness begins to be compromised for increased DOF and longer exposures. And, higher resolution sensors generally continue to deliver more detail well beyond the DLA than lower resolution sensors - until the "Diffraction Cutoff Frequency" is reached (a much narrower aperture). The progression from sharp the soft is not an abrupt one - and the change from immediately prior models to new models is usually not dramatic. Check out this specific diffraction comparison example using the ISO 12233 chart comparison tool. The mouseover feature will show you the degradation at f/11 compared to f/5.6.

Thanks,
Spence.

[Edited 2011-02-06 08:10:16]
EOS1D4, 7D, 30D, 100-400/4.5-5.6 L IS USM, 70-200/2.8 L IS2 USM, 17-40 f4 L USM, 24-105 f4 L IS USM, 85 f1.8 USM

ckw
Posts: 4586
Joined: Fri Aug 27, 2010 12:26 am

### Sweet Spots And Diffraction DLA

This is tricky, partly because in a technical sense there is no single quantity called "sharpness" which can be measured on its own. Perceived sharpness is a combination of two distinct properties - resolution and acutance.

Resolution is quite easily measured by line pairs per millimetre. Acutance can be thought of as constrast between brightness levels. When you "sharpen" an image, obviously you cannot improve resolution, what you do is improve the acutance.

I think, simply because we have some ability to enhance acutance, then the key factor here must be resolution.

Of course any two lenses could have very different resolutions. A cheap lens at its optimum aperture may not match a quality lens wide open. Similarly with sensors, different cameras will have varying abilities to record resolution in the sense of LPPM. Pixel count is not a reliable determination of resolution, as how the pixel sites work (micro lenses etc) and of course the Bayer filter will impact on this measure.

So to answer the question you would need to know both the resolving power of the lens and sensor for a given aperture. Until fairly recently, any reasonable lens would out resolve the sensor (ie produce more LLPM than the sensor could record). But now this is changing.

I think that IF the lens can out resolve the sensor, then you need to shoot at the DLA to capture as much of that resolution as possible. If the sensor out resolves the lens, then probably shooting at the lens's best aperture is the best solution. In another words, you have to work out which component is the limiting factor, then get the best you can from that component.

Unfortunately this info is not readily available - and indeed I've seen heated debates over the validity of measuring methods.

However, I think it is all a bit theoretical - in practice unless working in ideal test conditions I would expect that the tiniest of vibrations, misalignment of lens elements, inaccuracy of focus, heat haze etc. would all have a far more significant impact than moving away from the DLA by a couple of stops.

What we do need to realize though is that a) stopping down excessively can cause loss in image quality, and crop cameras are more prone to this than full frame b) lenses may perform slightly differently on different bodies.

Cheers,

Colin
Colin K. Work, Pixstel

cpd
Posts: 5169
Joined: Sat Jun 28, 2008 4:46 am

### Sweet Spots And Diffraction DLA

 Quoting ckw (Reply 1):However, I think it is all a bit theoretical

I think so too - because out in the open, there are so many other variables that you've got no control over that can affect the quality of your image.

I don't look at it as a technical thing - and to be perfectly honest - I let the camera do its own thing now - within the limits that I set. With the lens I often use, I haven't noticed much/any quality differences between F/8.0 or F/9.0 (or even smaller apertures) or wide-open at F/4.0 at 400mm. Maybe my eyes aren't trained to look for the differences (which is possible). Sometimes the camera will select F/13 or smaller in bright light.

What I have noticed however is that since moving over to a D3S from a D700 - using the same lenses, techniques and methods, the sharpness of very fine details in the image is remarkably better. It was the single biggest difference I noticed.

spencer
Topic Author
Posts: 1518
Joined: Sun Apr 25, 2004 8:30 pm

### Sweet Spots And Diffraction DLA

Thanks guys! Colin, again your input is invaluable!

 Quoting ckw (Reply 1):I think that IF the lens can out resolve the sensor, then you need to shoot at the DLA to capture as much of that resolution as possible.

I wondered about this but wasn't sure how to find out whether my lenses did in fact out-resolve my sensors!   It was only after reading about said DLA that it got me thinking about what aperture should be used, for the so-called sharpest point. It kind of makes me wonder now when the tests are done, whether the results would be any different if taken by different bodies...?

 Quoting ckw (Reply 1):If the sensor out resolves the lens, then probably shooting at the lens's best aperture is the best solution.

This also makes sense, but then one could argue(?) whether the lens' tests are valid at all? But I suppose your next paragraph just about sums it up pretty well:

 Quoting ckw (Reply 1):However, I think it is all a bit theoretical - in practice unless working in ideal test conditions I would expect that the tiniest of vibrations, misalignment of lens elements, inaccuracy of focus, heat haze etc. would all have a far more significant impact than moving away from the DLA by a couple of stops.

I mean, who really expects to get such results that these test show? I don't, not exactly anyway. It's so hit and miss, after all, what with all the points you meantioned above.

 Quoting cpd (Reply 2):I don't look at it as a technical thing - and to be perfectly honest - I let the camera do its own thing now - within the limits that I set.

I totally respect that and for the most part I tend to do something like that myself. However I also like to learn everything there is to learn about ANYTHING I own. (Not saying you don't)!

 Quoting cpd (Reply 2):What I have noticed however is that since moving over to a D3S from a D700 - using the same lenses, techniques and methods, the sharpness of very fine details in the image is remarkably better. It was the single biggest difference I noticed.

Couldn't agree more! I was disappointed with upgrading originally from the 10D (years ago, of course), to the 20D, then to the 30D. I notice this a lot when I look back on unedited files from my 10D and see an improvement over 20/30D files. So, personally, those upgrades were pretty poor.
From the 30D I went to the 7D. WOW. That WAS an upgrade, to say the least! But it wasn't the camera I had really wanted and it was more of a gap filler before I got the 1D4.
With the 1D4 I'm seeing things I really didn't think I would ever see. Such clarity in the details, contrast and colours. It's like the camera is 10 steps up from anything I've ever seen or used. And that includes the 1D2 and 1D3! It's just a monster.
Spence
EOS1D4, 7D, 30D, 100-400/4.5-5.6 L IS USM, 70-200/2.8 L IS2 USM, 17-40 f4 L USM, 24-105 f4 L IS USM, 85 f1.8 USM

masi1157
Posts: 174
Joined: Sun Feb 06, 2011 11:56 am

### RE: Sweet Spots And Diffraction DLA

 Quoting spencer (Thread starter):IF a certain lens, (any lens, say), is sharpest at (say) f/8, and Canon say the camera's DLA is at f/6.8, then what aperture should one be aiming at, given the ideal conditions to go for the sharpest shot? Simply put, if I dial in f/8 in Av, (aiming for the sharp aperture), but the DLA says fall-off starts around f/6.8, then is f/8 still accepted as the sharpest aperture? Another camera I own is the 1D4 and DLA fall-off is at f/9.1! Same question applies.... Footnote; I knew nothing of DLA until I read this literature, so I'm quite interested in knowing more.

You should be aware that diffraction might decrease the resolution of your lens if you go above the DLA, so you wouldn't get the absolute optimum in "sharpness". But depending on the size of your print and/or how much you crop it, you might not even see any unsharpness due to diffraction up to f/16, f/22 or even beyond. As long as your can't resolve it, it appears sharp and everything is fine.

Regards, masi1157
485 different segments on 95 airlines to 200 airports in 54 countries

spencer
Topic Author
Posts: 1518
Joined: Sun Apr 25, 2004 8:30 pm

### RE: Sweet Spots And Diffraction DLA

 Quoting masi1157 (Reply 4):You should be aware that diffraction might decrease the resolution of your lens if you go above the DLA, so you wouldn't get the absolute optimum in "sharpness".

Yes, I'm aware of that, but (even though we're talking minute detail now, possibly even undetected detail), would diffraction start closer to the lens' (let's call it diffraction point) or the camera's DLA diffraction point, if there are such things? I'm just interested to know if I should go by the one or the other if I needed to....
Spence
EOS1D4, 7D, 30D, 100-400/4.5-5.6 L IS USM, 70-200/2.8 L IS2 USM, 17-40 f4 L USM, 24-105 f4 L IS USM, 85 f1.8 USM

cpd
Posts: 5169
Joined: Sat Jun 28, 2008 4:46 am

### RE: Sweet Spots And Diffraction DLA

 Quoting spencer (Reply 3):With the 1D4 I'm seeing things I really didn't think I would ever see. Such clarity in the details, contrast and colours. It's like the camera is 10 steps up from anything I've ever seen or used. And that includes the 1D2 and 1D3! It's just a monster.

I never thought that at ISO200 I'd see the benefits as well - and I'd always subscribed to the theory that at those ISO levels, on two 36x24mm cameras, any differences were academic - but after using it for a while, that's not correct - and the newer camera has more clearly defined small detail in the images. I never expected that to be so much better.

Your message now has me looking into this DLA stuff as well.   The technical guide for the D3S makes no mention of this, but does give lots of hints on how I should take photos of figure-skaters and others at the Winter Olympics...   Thom Hogan suggests on the older D3 the DLA is F/16.

I'll admit, I've noticed no difference in sharpness between F/4.0 and even F/13, which probably is in keeping with what was above.

[Edited 2011-02-09 15:06:39]

[Edited 2011-02-09 15:07:08]

masi1157
Posts: 174
Joined: Sun Feb 06, 2011 11:56 am

### RE: Sweet Spots And Diffraction DLA

 Quoting spencer (Reply 5):Yes, I'm aware of that, but (even though we're talking minute detail now, possibly even undetected detail), would diffraction start closer to the lens' (let's call it diffraction point) or the camera's DLA diffraction point, if there are such things? I'm just interested to know if I should go by the one or the other if I needed to....

The DLA defines one exact optimum. Above that the diffraction blur becomes larger than a pixel size. But that is only relevant, if you actually use the camera's resolution to its maximum (i.e. 100% on your monitor, very large prints, very small crops etc.). If you do larger crops or use the entire sensor, and if you make smaller prints, this maximum is not needed, and you won't see any diffraction blur even at smaller apertures. For a normal web image with 1000px you can probably use f/16 or smaller without any problem.

Regards, masi1157
485 different segments on 95 airlines to 200 airports in 54 countries

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