Sensor size and diffraction.

Erik Ohlson wrote:

"Hmm, I wonder if my better pics were taken using A mode...

Now I need to experiment. Thanks for the comment (should have read all the posts earlier!).

It is very likely that this is, indeed, the case.

"A" mode is the best for any of the pocket zooms, by any manufacturer.

It's technical - physics, actually. The sensor is quite small, therefor the lens must have a very short focal length - usually about 4.3mm (that is approximately 1/3inch).

The 'aperture' is the "hole" the light comes into the camera. The aperture can be changed, and the numbers used to keep track of this are the RATIO of focal length to the size of the opening. (Divide the size of the opening in millimeters into the focal length in millimeters = the "ƒ/number" value.

There is nothing magical about this - but where the "Magic" comes in is that - due to physics again (sorry!) when light passes through a hole, it "scrapes" on the edges. All waves do this, including light and water. Here is a picture of water waves passing through an aperture, notice how the wave pattern changes and smooths (blurs) out.

The waves are coming toward you in this photo:

View: gallery page

When light does this, it blurs the image. It's called "diffraction". It can NOT be avoided.

The wider the hole, the less blur.

There is a critical size "hole" for visible light - approximately 3mm (1/8 inch).

This 3mm hole happens to be about ƒ/3.3, and going smaller than that (larger number - yes, its confusing) the blurring effect gets much worse.

So setting the Aperture to ƒ/3.3 and LEAVING IT THERE will ensure the sharpest image the lens is capable of. Period.

There is no "downside" to this: the camera will then choose a shutter speed (from 1/2000th second right down to 1 full second). This "wide open" aperture is best for "low light" too - it lets in more light.

When you zoom out, you will see larger ƒ/ numbers although the actual "hole" remains the same, the changing numbers are just how many times the diameter is divided into the focal length - which is changed by zooming.

Forget all you have ever heard about "stopping down for depth of field" - with this extremely short focal length you have extremely deep depth of field (Physics again, sorry).

Once you become more familiar with photography, you can use different apertures at the long zoom focal lengths where the diffraction effects do not apply. (Yep, physics, again.)

Thank you, Erik, for letting us know this fact. It is important for me. I do a lot of macro work and a couple of days ago I found a video at youtube where a photographer told, that he prefers macro photos with a long focal length and extended DOF as he does not like to do image stacking with short focal length and the time consuming process at the computer, afterwards. I did not know why this should work - but now I know it.

I could also imagine that these macro photos done with e.g. Lumix cameras which are great most of the times, use the same effect. If the sensor size is very small the light will hit the sensor with a sharp angle and thus less diffraction at the aperture.

I could also imagine that it is best to have the aperture in the middle of the optical construction we use e.g. a distance ring, a zoom lens with a nice (many blades) and very small closing aperture and a lens in reversed position in front of it with aperture wide open.

I plan to do some testing the next days and I also have an extremely long lens (an old Novoflex 600 mm) which could be part of strange constructions ...

Best regards

Holger

So, set it on "A" mode - about -.66 EV is a good idea to control the brightest parts of the picture and improve the general appearance of your images, and the camera takes totally automatic pictures by changing the shutter speed.

That's all you really need to know for using this type of camera.

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4 mm is much closer to 1/6 of an inch.

Sorry, that just popped right out at me.

Erik Ohlson wrote:

"A" mode is the best for any of the pocket zooms, by any manufacturer.

It's technical - physics, actually. The sensor is quite small, therefor the lens must have a very short focal length - usually about 4.3mm (that is approximately 1/3inch).

Actually, most small-sensor cameras simply don't have an aperture to adjust when the sensor is already at the diffraction limit -- this is why so many Canon PowerShots have a built-in ND filter (which is motor driven to go in/out of the optical path).

The ones that have an adjustable aperture iris generally have a long enough zoom range so that the smaller apertures are usable at least at the longer focal lengths. In fact, the fully automatic program modes will generally do the right thing in terms of avoiding the potentially worthless combinations of focal length and aperture.

However, the principle is correct, and especially when using fully manual lenses adapted to a smaller-than-expected-film-size sensor camera (Pentax Q, Nikon 1, etc.), the smaller apertures (higher f/numbers) should be avoided.

1/3" is a camera sensor size but it's not related to any dimension of the sensor!

Erik Ohlson wrote:

drh681 wrote:

4 mm is much closer to 1/6 of an inch.

Sorry, that just popped right out at me.

And "well-popped" it is.

I was not looking for mathematical accuracy - the salient point being that, in round figures, the "breaking point" is around 3mm, 1/3" depending on which physicist one consults - there was quite a "dust-up" some time ago between followers of two different [real: Ph.D] Physicists on another forum.

Thanks for your input.

Erik Ohlson wrote:

So setting the Aperture to ƒ/3.3 and LEAVING IT THERE will ensure the sharpest image the lens is capable of. Period.

Erik Ohlson wrote:

Cute cartoon, but you are obviously not considering the SMALL sensor, diffraction-limited cameras under discussion.

Your f/3.3 rule is bunk as it does not take into account the size of the sensels, which is about the only thing you don't mention, and the only thing that matters other than f-stop.

Tom Axford wrote:

BobORama wrote:

Erik Ohlson wrote:

Cute cartoon, but you are obviously not considering the SMALL sensor, diffraction-limited cameras under discussion.

Your f/3.3 rule is bunk as it does not take into account the size of the sensels, which is about the only thing you don't mention, and the only thing that matters other than f-stop.

Your rather blunt response suggests that you are one of the small minority who believe that depth of field should be defined in terms of the number of pixels in the image rather than just the overall size of the image in the camera (i.e. the sensor size).

If the conventional definition of depth of field is accepted, then Erik is entirely correct. By conventional definition, I mean that used for the past hundred years; e.g. the definition used when depth of field scales are engraved on manual focus lenses and also the definition used by popular online DoF calculators.

If you wish to start yet another argument about the "correct" way to define depth of field, it might be better done by starting a new thread on that topic.

No, Bob's right: you need a well-specified resolution reference to give a CoC (circle of confusion), and that can be taken either by pixel dimensions or by approximating human visual acuity for a specific print size and viewing distance. The old depth of field scales from different manufacturers rarely used to agree, because there was not a universal agreement about the desired CoC with film.

In fact, this is part of the problem with infinity focus of some old lenses. It turns out that it was not uncommon for the infinity hard stop to be placed not at infinity, but at the point where infinity is covered by the DoF as they computed it. Unfortunately, for film most folks used a larger CoC, so using these lenses on high-resolution digitals reveals that they don't quite reach infinity. This is one of several reasons that adapters are often built just a little short....

Tom Axford wrote:

Your rather blunt response suggests that you are one of the small minority who believe that depth of field should be defined in terms of the number of pixels in the image rather than just the overall size of the image in the camera (i.e. the sensor size).

My rather blunt response suggests I know optics, physics, math, and causality.

If the conventional definition of depth of field is accepted, then Erik is entirely correct. By conventional definition, I mean that used for the past hundred years; e.g. the definition used when depth of field scales are engraved on manual focus lenses and also the definition used by popular online DoF calculators.

What y'all seem to be taking about is matching the resolving power of the lens with the resolution of the sensor to maximize the overall performance of the camera as a system.   However this ignores several contributing factors to overall IQ.  ISO, NR, and the desire to manage DoF, managing shutter speed.  So of course picking an particular f-stop as "ideal" is ludicrous.

If you wish to start yet another argument about the "correct" way to define depth of field, it might be better done by starting a new thread on that topic.

I am not sure I ever mentioned DoF in my original post. Can you please argue with something I actually wrote? 'Cuz that would be great.

Tom Axford wrote:

ProfHankD wrote:

No, Bob's right: you need a well-specified resolution reference to give a CoC (circle of confusion), and that can be taken either by pixel dimensions or by approximating human visual acuity for a specific print size and viewing distance.

Well, you seem to want to re-open that argument. I'm not biting, I stick by what I said. We have been through it all before!

What? Yeah, Bob's post was a bit inflamatory, but I'm not opening an argument, I'm ending one with the formal definition (and even gave some useful implications which you handily edited out in your response).

DoF is computed based on an assumed allowable CoC. That's all there is to it; even wikipedia gets it right. You can argue that a particular sensor format should be associated with a particular CoC based on the magnification needed to create a specific print size to be viewed from a particular distance, and the wikipedia article even quotes some common CoC assumptions for various film formats and explains some of the subtleties of CoC choice, but go back to the OP:

"There is a critical size "hole" for visible light - approximately 3mm (1/8 inch)."

That's completely wrong. There is only a particular diffraction limit once you pick an allowable CoC. The exception to that rule would be an aperture close to the wavelength of the light, which would be a lot smaller than 3mm. 3mm wavelength would be well beyond what cameras can see (deeply into the IR band)... and yes, CoC does vary with wavelength -- diffraction comes earlier for reds than for blues. (Actually, there's also the recently discovered extraordinary optical transmission (EOT)  that sort-of breaks all the rules and allows apertures smaller than the wavelength of the light, but that doesn't really apply to refractive lenses.)

Anyway, as I posted earlier, most engineers designing cameras are not complete idiots and hence don't bother putting an aperture iris in that can go way beyond any reasonable diffraction limit -- this is why so many Canon PowerShots have an ND filter insertion mechanism rather than an iris. On properly-designed cameras with permanently attached lenses, the only thing that can get you in trouble is wide-angle use of small apertures (large f/numbers) that were intended to accommodate the long end of the zoom range. That is not what the OP said, but what he said is useful in that it roughly corresponds to application of this fact.

Tom Axford wrote:

ProfHankD wrote:

No, Bob's right: you need a well-specified resolution reference to give a CoC (circle of confusion), and that can be taken either by pixel dimensions or by approximating human visual acuity for a specific print size and viewing distance.

Well, you seem to want to re-open that argument. I'm not biting, I stick by what I said. We have been through it all before!

While we are posting things not in any way salient to the discussion...   I'm going to let my photography do the talking.   With micro-photography, managing the overall resolution is pretty much the only consideration.   You guys posting cut and past from Google I'm Feeling Lucky searches is rather boring, especially when you get it wrong.  Post something that demonstrates you know what you are talking about.

The FoV is about 3mm wide.

FoV is about 3mm wide.

In terms of diffraction, the following was created by placing two of these tiny, fine patterned  honeycomb metal screens about 3mm apart.  The FoV is about 4mm wide.   The image below is the screen BEHIND the front most screen.  So we are seeing through the front most screen.  The image of the rear screen is rather intact, however the diffraction caused by the front most screen creates the fine honeycomb pattern.

FoV about 4mm wide.

Yeah, so why view diffraction limited performance from a technical perspective when you can cut and paste stuff from the Internet?

At least if I am going to post stuff that has nothing to do with the discussion, its my own work.  Well, except for the Sponge Bob thing.

ProfHankD wrote:

What? Yeah, Bob's post was a bit inflamatory, but I'm not opening an argument, I'm ending one with the formal definition (and even gave some useful implications which you handily edited out in your response).

Just remember, inflammation is the first phase in wound healing.

I view the OP as the optical equivalent of "everyone should drink only orange juice, its the best juice."   Really?   Why?   Based on what?   How about diabetics?   People with ulcers?  Do you even know what juice is?   Or what an orange is?

Erik Ohlson wrote:

I created this thread to help relatively new photographers to understand what's going on with small-sensor cameras as opposed to large sensor &/or larger film.

Other than posting inaccurate information in the wrong forum, thereby failing in that mission....

I have had many thanks over the years during which this and preceding threads have been posted.

Stockholm syndrome, I suspect.

But of course it attracted some ditz with a forehead inflated by ego who just wants to argue how many angels can dance on the head of a pin.

Actually I didn't argue that at all.   But while we are talking pins...   salt and pepper on the head of a pin...  made it with a $2 lens.

No Angels, just seasoning.

Don't bother replying, as the "Ignore" button, so thoughtfully supplied by DP, is invoked.

Much like infants do when they want to make the world disappear.   Good news, only you have your eyes closed.