Stopping the uwa down

Quick question. I know you could probably do tests to figure this out, but what aperture is the lowest to stop down without decreasing image quality. In particular I'm using a Tokina 12-24mm. If I want maximum sharpness with no degradation, what aperture would be best? Is f/11 too far stopped down for a lens like this? I remember reading Bryan Petersen's book years agonamd he would stop down to f/22 which seems overkill....

Frodge wrote in post #17760468
Quick question. I know you could probably do tests to figure this out, but what aperture is the lowest to stop down without decreasing image quality. In particular I'm using a Tokina 12-24mm. If I want maximum sharpness with no degradation, what aperture would be best? Is f/11 too far stopped down for a lens like this? I remember reading Bryan Petersen's book years agonamd he would stop down to f/22 which seems overkill....

Depends on your purpose.

An ultrawide really likely doesn't need to be stopped down more than F8, if even that, to achieve a sharp complete depth of field full image (12mm~16mm on full frame). The reason some folk, myself included, stop down to F22 or more, is for the specific qualities you get from the blades, such as the kinds of star spikes you can get from certain lens's qualities. Sometimes you stop down as far as you can go because you want longer exposure time for the purpose of long exposure effects (blurring water, sky, etc).

Every lens has a specific "sweet spot" so to speak in terms of sharpness (corner to corner). You could search for that sort of terminology regarding your specific lens.

Very best,

It also depends partially on which camera you're using. IIRC, high-resolution, high-pixel-density cameras are more prone to diffraction image degradation at larger apertures (lower f-stop numbers). As MalVeauX says above, you usually should not need a small enough aperture with an UWA lens to have diffraction issues.

Frodge, I think the question you are asking is, "When stopping down, at what aperture do I begin to lose image quality?"

There are really two answers to that question. One is lens related, the other sensor related.

Lens first. There is some aperture, specific to each lens, where you will get the highest possible resolution and contrast. Both of these, taken together, produce what is called by most of us, sharpness. This is specific to each lens, and is what MTF charts attempt to show graphically. Generally, but not always, that point comes when you are stopped down 1 or 2 stops from wide open. Most lenses have a range of about 3 stops where they perform best. Fast L-primes like the 135 f/2L are at their best stopped down an bit. It is excellent (PZ.de's rating) from f/2.8 through f/8. Slow consumer zooms like the efs 10-18mm STM are at their best near wide open. (You can see all of this graphically depicted in lens reviews at photozone.de.) Not because they are really good, but because efs lenses mounted on apsc cameras run into the other aspect of aperture affecting image quality: the sensor.

The sensor. Each sensor has a specific size to its individual light buckets. Apsc sensors crammed with 20 MP have relatively smaller sensors. A full frame sensor with the same 20 MP will have larger light buckets. A full frame sensor with only 12 MP will have comparatively huge light buckets. The size of the light bucket matters because of the lens. As light passes through the lens, it does a lot of bending and bouncing: it gets diffracted, or spread out. Stopping down and forcing the light through a smaller hole makes for more bending and bouncing. When the light from a particular point spreads out too much, it spills over the edge of its particular light bucket. For each specific sensor, the aperture that causes this to degrade your image is known as the diffraction limited aperture, or DLA.

Michael at TDP (http://www.the-digital-picture.com …D-DSLR-Camera-Review.aspx) usually includes DLA data in his camera reviews. Go to the site just quoted and look at the DLA table. The DLA of the 60D is f/6.9. Beyond that, you begin to lose IQ due to the sensor. The 135L on full frame is excellent at f/8. If it were tested on a 60D, f/8 would be detrimentally affected by DLA. Compare the 5Dc to the 5D Mk III. DLA's are f/13.2 & f/10.1, primarily because the Mk III has more MP, which translates into smaller light buckets, which means DLA sets in at a lower aperture.

I don't have the best eyes, but I have LR and a 22" monitor. I can't see any negative effects of DLA until well past what TDP lists as DLA. It is my opinion that you can go about 2 stops past what he lists. Diffraction may be there, but it is not visible or detrimental to IQ. I have never had a problem using my crop cameras at f/11-16. I tend to use my 6D with fast primes wide open, so I will never run into DLA. I use UWA lenses (<20mm) a lot on full frame. At 15mm I get all the DOF I need at f/5.6-f/8, so again, I'll never run into DLA on my full frame.

I'd basically agree with Bassat except with a few quibbles. Theappearance of diffraction softening isn't really a function of the number of pixels, but it is a function of the degree to which you are enlarging the image (from sensor size to print or display size). Pixel count comes in to this because if you have very few pixels, the detail lost to diffraction wasn't going to be resolved anyway.

But to a more practical bent.....I've found that for 35mm sensors and pixel numbers up to ~ 20 MP, anything up to f/16 looks fine. Some people say f/11 is about the maximum, but I've not seen softening from f/11 to f/16. I have used f/22 at times (usually trying to blur a waterfall without having a ND filter on me) and I do indeed see diffraction softening at that aperture.

If you are using a smaller 1.6X sensor, you will need to shift these numbers about one stop larger in aperture because you will be enlarging more for any given output size.

Thanks for all the replies. I know you can stop down for exposure length purposes. I was just wondering about diffraction. I generally will stop down to f/8-f/11 for some landscape shots. I've done this just to keep super sharp images.