Depth of Field and F-Stop Test

A forum to ask questions, post setups, and generally discuss anything having to do with photomacrography and photomicroscopy.

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Cyclops
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Post by Cyclops »

Bill D wrote:I took a 100% crops out of the lower right corner of the original pictures.
Wow thats one sharp lens youre using to be able to blow up that large !
Is the Rebel XT a digital? I think its the digi version of my film camera.
Canon 5D and 30D | Canon IXUS 265HS | Cosina 100mm f3.5 macro | EF 75-300 f4.5-5.6 USM III | EF 50 f1.8 II | Slik 88 tripod | Apex Practicioner monocular microscope

Charles Krebs
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Post by Charles Krebs »

The lens I am using is a f2.8 to f32. Can I conclude, if I had a lens that went from, say, f.2 to f128, there would be two or three aperatures, evenly spaced, where the pixels are sharp?
I'm sure Rik will answer in detail... but here's one "take" on this...

In theory, the larger the lens aperture, the greater the potential resolution. In reality, certain aberrations will impact the image, and to overcome them most lenses will need to be "stopped down" to some degree. So typically you will find 2 or three apertures where the lens is "best". But you really need to test to find where this is. Although rare, some very highly corrected lenses might actually provide the sharpest image "wide-open". Depth of field will always increase as you use smaller apertures. So in reality it becomes a trade off... you need to find the aperture that will provide the best combination of resolution and depth of field to portray the subject. In some close-up cases, stacking software such as Helicon Focus adds a new wrinkle. With stationary subjects it now becomes possible to use an aperture that might provide much higher resolution, but would otherwise provide very inadequate DOF.

Rik has done really nice work with the "math" on this subject. I have always been impressed how on an empirical level, you always seem to hit a "wall" at an effective aperture of f32. No matter how people's opinion differ on the visual "compromise" between DOF and resolution, once the effective aperture hits about f32 nearly everyone is forced to admit that the resolution really suffers.

(I use "effective" aperture deliberately, because with most close-up situations there is a considerable difference between "marked" and effective aperture. It is the effective aperture that need be considered when discussing the effects of diffraction and resulting loss of resolution.)

.... wow... just think...
an f.2 50mm lens would need a front lens diameter of 250mm. I'd need one of my sons to carry that baby! :wink:

Bill D
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Post by Bill D »

Cyclops- Yes, the Rebel XT is a digital camera. The Canon f2.8 EF 100mm Macro USM is a very sharp lens. It is a true flat field macro. It weighs 20.5 ounces, and is 4.5 inches long. It's a little pricey, but I think worth the weight and cost.

Charles-250mm front element! You will need your other son to carry the filters!!
Bill

Cyclops
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Post by Cyclops »

Thats a 10 inch front element!! Imagine the cost of making it! The biggest lens Ive seen for a telescope is 6 inch,they dont usually make them bigger than that,probably couldnt find anyone strong enough to lift it ;)
Canon 5D and 30D | Canon IXUS 265HS | Cosina 100mm f3.5 macro | EF 75-300 f4.5-5.6 USM III | EF 50 f1.8 II | Slik 88 tripod | Apex Practicioner monocular microscope

rjlittlefield
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Post by rjlittlefield »

This has been another one of those really interesting threads. Good discussion, experiments, and url's, folks!
Bill D wrote:Can I just pretend that pixels and RGB are Silver Halide Crystals and Dye Layers? LOL
Wait...you mean they're not?! :shock: :? :lol:

Those latest crops tell the story, don't they? From wide open down to around f/11, the diffraction blur is smaller than a pixel, so you gain DOF without losing sharpness. Around your f/22 setting, the blur goes to a couple of pixels wide, and you start to lose sharpness. At f/32, it's several pixels wide, and the blur really gets noticeable.

Because of differences in how the lenses focus, at 1:1 your Canon's "f/22" setting is probably about like my Sigma 105's "f/16" setting. As shown in my article, the Sigma is noticeably fuzzy at 1:1 and f/22.

That Canon lens looks very nice, by the way. Not only is it pretty sharp all the way out to f/2.8, but I don't see nearly as much focus shift as the Sigma has. With the Sigma, its wide-open focus point is really pretty close to the front, not the center, of its f/11 focus range. I have to take this into account when choosing focus point for a single shot.
DaveW wrote:However this does not explain why it happens more at smaller effective apertures that at their actual physical size.
There are a couple of easy ways to think about it. The more accurate one is to think that diffraction depends on the angular diameter of the aperture. Suppose you fix the aperture at so many mm in diameter, focus at infinity, and measure the size of the Airy disk. Then add extension so that the lens focuses at 1:1. How big is the disk now? Well, the distance from sensor to lens has doubled. So, the fixed size aperture now subtends half the angle that it used to, and diffraction is twice as bad -- the Airy disk doubles in size. The other way is to just imagine that the diffraction blur scales linearly with distance to the lens. Doubling the distance doubles the size of the blur.
Charles Krebs wrote:.... wow... just think...
an f.2 50mm lens would need a front lens diameter of 250mm. I'd need one of my sons to carry that baby!
Not only that, but it wouldn't help much with the diffraction problem, even ignoring the, um, rather formidable problem of aberrations.

What diffraction depends on is actually the angle subtended by the aperture. (More precisely, it depends on "numerical aperture", symbol NA, which is the sine of half the angle.) Make the lens as wide as you like, it's not going to subtend more than 180 degrees! 8)

I like Charlie's description of how resolution varies with aperture. If you think of a graph, resolution (height) versus aperture (width), it usually looks like a hill with its highest point someplace in the middle. Like most hills made of dirt, you can move a fair distance laterally from the high spot before you lose much elevation. I usually see one click-stop that's measurably best, sometimes two about the same (with the best aperture between clicks), but going one stop on either side of best is seldom enough to attract attention.

However, the preceding paragraph only applies if you're looking at just resolution, ignoring DOF.

If you're trying to optimize aperture to get maximum DOF at a specified resolution, then the story is different. In that case, what you end up doing is stopping down until diffraction by itself consumes most of your tolerance. From that optimum point, making the aperture one f/stop smaller is like falling off a cliff -- your useful DOF ("depth of detail") drops to zero because the image is too fuzzy everywhere! (Think of going from f/22 to f/32 in Bill's examples.)

On the other hand, opening up one f/stop from the point of optimum DOF doesn't make much difference. I forget the exact number, and I'm away from my references at the moment, but as I recall, the DOF at one f/stop wider than optimum is still like 80-90% of DOF at optimum.

--Rik

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