Your lens sweet spot...explained technically

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empoy78th:
Hello po!

I just want to share something to my fellow newbies out there. I am just happy about a good read that I had yesterday.

Read on:
http://3mp0y.multiply.com/journal/item/26/Your_lens_sweet_spot...explained_technically.

Cheers!

jdee:
Hmmmm. I've heard the term lens sweet spot before and I also read somewhere and it says "f8 and stay there!". The excerpt talked about the details of a film SLR, is it the same with DSLR? DSLR (except full frame) has smaller sensor than a full frame 35mm.

I'm not an expert so let's see what the experts will have to say.

empoy78th:

--- Quote from: jdee on January 13, 2008, 11:20:37 PM ---Hmmmm. I've heard the term lens sweet spot before and I also read somewhere and it says "f8 and stay there!". The excerpt talked about the details of a film SLR, is it the same with DSLR? DSLR (except full frame) has smaller sensor than a full frame 35mm.

I'm not an expert so let's see what the experts will have to say.

--- End quote ---

I think you're in the same position where I was a year back hehehe. Sticking to f/8 won't bring me creative dof so I won't stay there definitely ;D... I would want to experiment different shots using those f stops though.

I'm no expert too, but the book has been revised to cater digital SLRs. I think the point there is how those apertures make images more appealing. The concentration is within light, blades, glasses and how they work together before it reaches the medium that records the images, may it be a film or a digital sensor. That's just how I interpret it though... ;D

Any master care to share their insights?

caterpillar:

--- Quote from: empoy78th on January 13, 2008, 11:03:04 PM ---Hello po!

I just want to share something to my fellow newbies out there. I am just happy about a good read that I had yesterday.

Read on:
http://3mp0y.multiply.com/journal/item/26/Your_lens_sweet_spot...explained_technically.

Cheers!

--- End quote ---

Empoy,

The explanation is incomplete. That's not the entire thing.

The "sweet spot" is supposed to be the aperture by which a lens performs at its best in terms of resolution and likely in contrast too. And hopefully also from corner-to-corner. In general, most lenses once you get to f8, is already very good. Even those cheap consumer lens perform well at f8 or no more than f11.

Peterson's explanation is not complete because he does not expound further as to the effect of focal length (FL) on sharpness (resolution & contrast). Wide angle lenses are tougher to get right because if you have a FOV of 130 degrees, then you are bending light to go parallel as it exists the end of the lens onto the sensor. That takes lots of glass and technological gymnastics to get those corners sharp as well. That means a very good design, good use of modern optics, etc. And in the manufacturing process, the tight tolerances to assemble the lens so the corners will also stand up. Otherwise the center is the only one sharp or one corner is sharp while the other is not. This is the problem commonly encountered by most Sigma users (poor QC) especially on wide angle lenses. There are 10-20 sigma's that have decentering problems that I have mentioned. So with the 35FF 12-24. If you get a good copy, then you are in heaven. If not, then it's hell. Problem is half of the lenses seem off spec and choosing a good one is a clay pidgeon shoot.

Of course, if the design is also good, then your sweet spot is better than another lens. Witness the better all around performance of the new nikon 12-24 f2.8 vs a prime of Canon, the new 14mm f2.8L. The former trounces the latter even to the corners, considering it is a zoom. Zooms aren't supposed to be as sharp in the corners compared to primes. But this shows you how design can affect the sweet spot. The Canon need to be stopped down past f8, maybe f11 to get it right at the corners, whereas the Nikon is just fine at f5.6. Going past that won't improve it much.

One will also notice that longer lenses tend to be sharp even wide open. The sweet spot is very close to wide open, if not wide open. Hardly any need to stop down to get it sharper, except maybe just to increase DOF. The 400 f5.6L for example is of this. Stopping this lens down to f8 or f11 hardly makes a difference in resolution. The 300 f2.8L IS is another example. At f2.8, the 300mm L is already at its sweet spot. Going to f8 is just going to increase the DOF. You can always check their MTFs to show you that I am not kidding. And the field experience verifies this. MTF charts are very good at identifying the sweet spot. But there are caveats.

There are other lenses that behave like this, but its usually the longer FL. What this shows is that longer FLs are easier to get their sweet spot because the FOV in degrees are narrower and there is not much need to bend light and refocus them involving more glass w/c adds to the problems of going to the sweet spot.

But what the author also forgot to mention another very important factor affecting the sweet spot. A pity because his book is about digital cameras. He forgot to mention that the sensor size or the size of the capture medium and the no of pixels for or size of the pixel and it's distance from each other (pixel pitch) also changes the sweet spot. Yes, it's not the glass that is the limiting factor but also the sensor. Why you ask?

Well, if you use a crop camera like the D80 or 40D, the smaller sensor with more pixels makes the lens reach the diffraction limits earlier. It means, that the image will be blurry even if your lenses have higher potential resolution at smaller apertures. In plain words, it means that when the sensor is smaller but with more pixels, the lens true potential is blunted so that it the sweet spot is lowered. Diffraction lowers the resolving ability of the lens and is caused by the light scattering (diffraction) as it hits the corners of the pixel. This scattering is a normal beahviour of light as a wave and causes the image that is to be formed to be of lower quality (less sharp). If the pixel is smaller and the distances of each on are further apart, the effect is to lower the resolution projected and thus the sweet spot.

So, assuming a sensor or a film that is not limited, and a lens that really has the highest MTF score at f16, your sweet spot will be f16 --- the limit of the lens. But if you put that lens on a 30d, even if the MTF shows it is at its best at f16, due to the smaller pixel pitch and size, you can be limited to f8. Past that, your higher MTF would be blunted. In fact, if you insist on shooting at f16, you might find the image softer compared to shooting at f8.

For 1.5x or 1.6x crop cameras, with today's 8-10mp count, the sweet spot for most lenses is f8. I tested my 10-22 ef-s and it is at its' best at f7.1, not f8. But that is at 10mm. You must also factor the FL in a zoom. My 24-105L is very sharp from 40mm-70mm at f4 that stopping it down hardly improves it except the DOF. But I would say, it's sweet spot at 40-70mm is f5.6. But it's overall sweet spot is f6.3 from 24-105mm.

So, your sweet spot varies with 1-aperture, 2-type of lens 3-sensor size and pixel pitch.

Why is knowing your sweet spot important, even if you know you won't need it? Because if one time you find you need a job for a big poster as big as the side of a building, you know what aperture, camera or lens you need to make it so. Or when you are shooting landscape and you will enlarge to 20x30, you know which aperture combination for your body is the best. When you are shooting macro, you know how to compromise DOF vs hitting your diffraction limits and thus softening the image.

But you don't have to be a slave to the sweet spot. Yes, you don't have to use your sweet spot all the time. I have, for example, complete confidence using my 100 f2 usm at f2 or f2.2. Yet I know that my lens is at its best at around f5.6-6.3. Same with my 50mm f1.4 or f1.8. I use them at f2.2 or f2.5 even if know they are at its best at f6.3.

But one should not hesitate using apertures past their sweet spot if need be. At times, with landscape, or macro, you sacrifice resolution for DOF. If it requires f11 or f16 to get that deep DOF, then you must be willing to trade that softness for it. Besides, even if your sweet spot is f8, going to f9, or f10 isn't such a big loss in sharpness. Maybe you'll notice fully at f14 and surely at f16. But then again, if you need to stop down, stop down. If you need to open up way below the sweet spot, open up. Just use PS to overcome the softness of using past or below the sweet spot.

The sweet spot, though giving you the best image, may not be the most appropriate aperture for you to use. Do not be a slave to it. But neither should you be ignorant of it.

I hope this has helped understanding what the "sweet spot" is.