Aperture & Depth of Field

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Barrelcactusaddict
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Aperture & Depth of Field

Post by Barrelcactusaddict »

Hi,

Lately, I've had a few questions that have been severely troubling me; I can't get them out of my mind, and it's actually causing me to lose sleep (not just from staying up late, it's the stress as well). Yesterday I was reading numerous articles that supposedly addressed my questions, from about 5:30 PM to 11:00 PM: needless to say, I don't have any clearer an understanding. I'll just visit two of the questions in this topic; the third is probably unrelated, and I'll create that as a separate topic.

1. Why doesn't changing the aperture on a lens at a fixed focal length affect the field of view (i.e., why isn't an image "cropped" when stopping down)?

2. Why does depth of field increase/decrease with change in aperture?

I must say that diagrams make much more sense to me than conceptualizing an explanation; this is an extremely inconvenient limitation, especially for a photographer. After all my searching, I cannot seem to find any visual explanation that makes sense to me. For the most part, I can comprehend how diffraction works with aperture, but I don't have any idea as to the basics of aperture as outlined in my two questions.

Thank you.

-Kaegen

Scarodactyl
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Re: Aperture & Depth of Field

Post by Scarodactyl »

As a maybe dumb model, to sharply cut off the field of view you'd need the iris to be in a plane of sharp focus. Where the iris sits it is, instead, so defocused that it fuzzes darkness evenly across the image instead.
In microscopy you have two irises, the field iris which is in sharp focus and cuts off excess field of view and the condenser aperture which affects the resolution and brightness.

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Re: Aperture & Depth of Field

Post by Barrelcactusaddict »

Scarodactyl wrote:
Mon Dec 27, 2021 12:43 pm
As a maybe dumb model, to sharply cut off the field of view you'd need the iris to be in a plane of sharp focus. Where the iris sits it is, instead, so defocused that it fuzzes darkness evenly across the image instead.
In microscopy you have two irises, the field iris which is in sharp focus and cuts off excess field of view and the condenser aperture which affects the resolution and brightness.
Yeah, I really don't understand what that all means; unfortunately, I'm not familiar with using microscopes or how some of their components (i.e., eyepiece) coordinate to produce an image. I see the field iris and condenser iris in the diagram, but how is the field iris in sharp focus, as I don't see any lenses to focus that light, or do you mean it is at the focal plane? Could your description be applied to a camera lens? So basically, after light comes through the front focusing element on a lens, the point of convergence within the lens is the "plane of sharp focus" you mentioned? And "iris" is just the aperture-adjusting component made up of blades in a camera lens? If the iris/aperture is placed on the point of convergence's plane, shouldn't light that is possible to control with the aperture be absent (effectively no aperture at all)?

I'm probably not understanding your response correctly, but I don't think it answers either of my initial questions. This gives me more questions, this time about how microscopes work: I don't really want to do that. Could you please correct me if my interpretation is wrong, which it likely is? I'm extremely confused now. :(
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Copyright© Kohler Illumination

Scarodactyl
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Re: Aperture & Depth of Field

Post by Scarodactyl »

I should probably let someone who is more qualified answer--my only real education in optics was a two week internship in high school. I'll have another stab but I may not be entirely right here.

On the microscope you have two metal-bladed iris diaphragms which are mechanically the same and only act differently because of where they sit vertically. There are a few focal planes within a microscope just like there are in any complex lens system. The field aperture sits in one of these, so when you stop it down you are essentially superimposing two images, one of the subject in focus and one of the iris in focus. That gives you a nice cutoff of the light and reduces flare when you set it to be just outside the field of view. If the iris was moved a bit out of that focal plane the edges of the iris would blur, and the more out of focus it got the less distinct the boundary would be. Soon, instead of a sharp edge, you have a fuzzy black blur over the entire field of view. It's not a secret to photomacrographers that you can see right through out of focus parts of an image. That's one reason why stacking hairy bugs with overlapping legs is such a pain. That's the aperture iris, a dark ring put into a plane so out of focus that you can see right through it when you're using the lens to spec, even as it affects brightness and resolution. Of course it isn't actually seeing 'through' it, since the light doesn't actually pass through the metal blades--more like seeing around it by light rays going through at an angle and then being refracted back onto your sensor by another lens element (and the more you close it the narrower the set of angles that can make it through and onto your sensor).
Normal lenses don't have field irises because they wouldn't serve a practical purpose, they only have the aperture iris.

Uhhh please someone tell me if I'm going wrong here.

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Re: Aperture & Depth of Field

Post by rjlittlefield »

Kaegen, can you clarify what sort of optics you're thinking about?

For example, are you thinking about say an ordinary macro lens, or are you thinking about microscope objectives?

The basic issues are similar, but the details of one can be distracting if you're thinking about the other.

--Rik

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Re: Aperture & Depth of Field

Post by Barrelcactusaddict »

Thank you Scarodactyl, that detailed explanation does make sense to me. I guess I have always had a false understanding of how images are formed on a camera sensor by a lens; I was confusing single point source diagrams with double point source diagrams (this was compounded when I began comparing 3d models): I've mistakenly thought that an image was formed after the point of convergence within a lens, and that the larger the base of a cone of light that reaches the sensor, the larger the image would be (I thought a perfect image was formed after convergence, and just increased in size).

Thank you Rik. Actually, within the last 30 minutes, I think I found a video that answered the two initial questions I had regarding aperture and depth of field; it also was the only explanation I've ever read, heard, or seen that helped me understand what the Circle of Confusion is. I'm still a bit confused about one aspect with the video's presentation, but it really isn't as significant as the overall concept. https://www.youtube.com/watch?v=W34sLbA ... dex=1&t=2s

One thing I still am confused about is how focusing in a prime lens works, i.e., my Canon 60mm f/2.8 macro; I've read that a prime lens' rated focal length never changes, while others say it does change and that the former were describing a "fixed focus lens". I'm not sure what to think here, as maybe either could be true depending on how focal length is measured (distance between front element, vs. rear focusing element, vs. optical center of lens, vs. point of convergence and the focal plane/sensor). I've only seen conflicting definitions for a photography definition of "focal length"; I also can't visualize how changing focal length affects angle/field of view, and I've never seen a decent diagram with accurate angles clearly depicting this.

It seems for every concept I come to somewhat understand, several more take its place. I still can never seem to find the right combination/balance of aperture, shutter speed, and ISO when I go to photograph anything under 50mm long, using the Canon 60mm f/2.8 with my EOS 500D: everything turns out fuzzy and is under- or overexposed, no matter how much I adjust any single or combination of those settings. I know how they work together, but I never get it right in practice when photographing very small subjects. I take better pictures using a Samsung WB35F than with my DSLR, which I find infuriating and discouraging.

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Re: Aperture & Depth of Field

Post by rjlittlefield »

Barrelcactusaddict wrote:
Mon Dec 27, 2021 9:56 pm
I'm still a bit confused about one aspect with the video's presentation, but it really isn't as significant as the overall concept. https://www.youtube.com/watch?v=W34sLbA ... dex=1&t=2s
I assume the aspect that you're confused about, is the one the you asked in the YouTube comments:
think I can finally understand how the Circle of Confusion forms, for which I am most grateful! Still, I can't understand this one aspect in the animation: why doesn't the focal point from the bottle opener form at a different distance once the aperture is changed to F/22, when the focal point from the bottle at F/22 forms earlier than it did at F/1.4 (the same can be said of the cork)? Does stopping down:
A) decrease ONLY the diameter of the cone of light that forms the focus point (seems to be the case with the corkscrew), or
B) does it decrease both diameter AND length (seems to be the case with the bottle and cork). Please, can you help me understand this anomaly?
If it is depicted properly, why is there a change in distance of the focal points (of out-of-focus objects) when changing aperture, while no change with the in-focus object? Please, can you help me understand this? Thank you.
The aperture does not change the path of any light ray as it passes through the lens. Therefore the points of focus do not move. Stopping down changes only the widths of the cones, not their lengths. If the points of focus appear to move in the video, then this is either an illusion or the video is not accurate.

That's a very good video, by the way. Congratulations on finding it.

I've read that a prime lens' rated focal length never changes, while others say it does change and that the former were describing a "fixed focus lens".
The term "prime lens" means different things to different people. Most commonly it means just "not a zoom lens".

If you dig deeper into what that means, a zoom lens will be able to adjust its angle of view at a fixed focal distance, and a prime lens will not.

But note, that definition is silent regarding exactly how focus gets adjusted when using a prime lens. In the old days, prime lenses had fixed spacing between their elements. Lenses like that have a fixed focal length, and focus is adjusted solely by changing the distance from lens to sensor. But in modern designs, many prime lenses have movable elements, that is, the spacing between elements changes as focus is changed. This change of spacing also changes the focal length of the lens. Sometimes the change in focal length is small, but sometimes it is large. A typical macro lens that focuses from infinity to 1:1 will shorten its focal length to around 70% of nominal, when focusing at the closest distances. For example a lens that is rated at 100 mm focal length will usually measure quite close to that value at infinity focus, but when focused down to 1:1, it will measure only around 70 mm focal length. (The exact values vary significantly between lens models.)

The term "focal length" describes how much the lens deflects the path of a light ray as it goes through the lens. For lenses that focus at infinity, a conceptually simple method of measuring focal length is to set infinity focus, take a picture of stars in the night sky, and look at linear separation of stars in the image, versus angular separation of those same stars in the sky. Then focal length can be accurately computed as
FL = (d/2) / (tan(A/2)), where
FL is the lens focal length,
A = angular separation between two stars whose midpoint is on the optical axis, and
d = linear separation between the images of those two stars on the sensor.

Note that for fixed angular separation, larger FL gives larger separation in the image. That is, larger FL makes things "look bigger".

The formula can also be run backwards, as
A = 2*arctan(d/(2*FL))

Then if we fix d to be the sensor size, A will be the angle of view. In this case, with FL in the denominator, larger FL gives a smaller angle of view. That is, "short" lenses have a wide angle of view, and "long" lenses have a narrow angle of view.

A more detailed discussion can be found at https://en.wikipedia.org/wiki/Angle_of_view .

Another way to measure focal length is to measure magnification at two different extensions (taking care to not turn the lens focus ring!), then calculate as
FL = (ext2-ext1) / (mag2-mag1)

This formula can also be run backwards, in a couple of useful ways:
mag2 = mag1 + (ext2-ext1)/FL
ext2 = ext1 + FL*(mag2-mag1)

The first of these tells you how much magnification you get by changing the amount of extension (ext2-ext1). The second tells you how much extension you need, to get a specified change in magnification.

Note that when an image is focused at infinity its magnification is zero, so these formulas correspond to the usual simplification that:
extension added from infinity focus = FL*magnification

Notice that there is nothing in these formulas that relates FL to any particular physical point on the lens: no "front element", no "rear element", etc. Focal length has nothing to do with those things, only with how the light bends.

--Rik

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Re: Aperture & Depth of Field

Post by Barrelcactusaddict »

Thank you for clearing that up; I was confused by that apparent change in focal point distance with aperture. I'm also glad I found that video, otherwise I don't think I'd ever understand the relationship between depth of field and aperture.

Now I'm a little confused, as to how shortening a focal length when focusing on a macro lens increases magnification (i.e., going from ∞ (nominal) to 1:1 (70% of nominal)); I always thought increasing focal length meant higher magnification. So when I rotate my Canon 60mm Macro's focusing ring counterclockwise, away from ∞, the element(s) move backwards toward the sensor decreasing the focal length (assuming focal length is measured from focal point to sensor)? Shouldn't it be the other way around? Unless the cone of light (starting from the front element) is being extended in length toward the sensor by the moving element(s), then the angle of view would be decreased : if I'm not visualizing that correctly, then I don't understand why shortening the focal length would increase magnification. Is there some kind of diagram that shows why this would be the case? I don't think I'm understanding this correctly.

Sadly, I can't comprehend those equations or what fixed and linear separation are; my knowledge of Algebra is extremely limited, only to solving for "x" in fairly simple equations. I'd have to figure out what the magnification value of the lens would be in those instances, and that makes it more confusing to me. The equations look simple, but I cannot for the life of me conceptualize in the least what they mean. However, I do see how the last two equations were derived from FL = (ext2-ext1) / (mag2-mag1); beyond that, I have no clue.

Sorry, I don't think I'm understanding anything here: not about focal length and magnification in a macro lens, and calculating focal length. I have a lousy mind, and if it's not one concept I have trouble with, it's another. :(

Barrelcactusaddict
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Re: Aperture & Depth of Field

Post by Barrelcactusaddict »

I really appreciate everyone's help, time, and effort: I'm just sorry I can't understand it.

I really hope I didn't and don't come across as ungrateful or narrow-minded. I apologize if I did. Thank you.

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Re: Aperture & Depth of Field

Post by rjlittlefield »

No worries. I'm just taking some time to figure out what to try next, as a way of helping you to understand.

--Rik

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Re: Aperture & Depth of Field

Post by rjlittlefield »

Barrelcactusaddict wrote:
Mon Dec 27, 2021 9:56 pm
It seems for every concept I come to somewhat understand, several more take its place. I still can never seem to find the right combination/balance of aperture, shutter speed, and ISO when I go to photograph anything under 50mm long, using the Canon 60mm f/2.8 with my EOS 500D: everything turns out fuzzy and is under- or overexposed, no matter how much I adjust any single or combination of those settings. I know how they work together, but I never get it right in practice when photographing very small subjects. I take better pictures using a Samsung WB35F than with my DSLR, which I find infuriating and discouraging.
Kaegen,

In reviewing everything that you have written in this thread, I am struck that
  • (1) you say you "never get it right in practice", being unable to take a suitable picture with your Canon 60mm f/2.8 and EOS 500D
and at the same time
  • (2) you are asking tough theoretical questions about properties of lenses and how they work with light.

In my opinion it is pointless to worry about (2) until you get past (1).

In fact it's probably worse than pointless, it is actively distracting.

I suggest to concentrate first on learning how to use your camera and lens.

After you can reliably take pictures that are properly exposed and are sharp somewhere, then we might productively talk about theory.

I suggest to practice with the following setup:
  • choose a subject that does not move
  • mount the camera on a rigid tripod
  • use continuous illumination
  • set the camera and lens on manual exposure and manual focus
Start at ISO 400, f/11, and 1/30 second. Focus, frame, and shoot. If the image is too bright, use a shorter exposure time; if it is too dark, use a longer exposure time. If your image is fuzzy everywhere, then most likely either you did not set focus correctly or your setup is vibrating. Whatever happens, if you need help, then please start a new thread in either Beginners Macro or Macro and Micro Technique and Technical Discussions .

--Rik

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Re: Aperture & Depth of Field

Post by Barrelcactusaddict »

Thank you Rik, I appreciate it! That is excellent advice, and I will continue to try to take suitable photos with my DSLR.

I suppose I've become somewhat disenchanted with the DSLR, but mostly with my lack of success despite my effort: so, I've lately been more interested in finding out how light and lenses work. And you are right, it's certainly not the proper way to go about it and is counterintuitive, yet I'm trying to work at it from a different angle, so to speak.

Actually, I pretty much never set my lens to AF, as I greatly prefer manual focus (despite the Canon 60mm f/2.8 being great at AF); I only use it to photograph specimens of amber or minerals, usually at just 1:1 scale and I try to avoid cropping in post-production. I sometimes use either Tv or Av, but mostly Manual Mode; I've experimented in the past with Program AE and A-Dep, but I didn't care for them. I currently only use constant LED illumination (transmitted, indirect, or both) at about 70-90 lumens (although I do have a cold illumination halogen microscope illuminator, I will only use it when photographing inclusions) and try to avoid using flash settings or direct light, as it doesn't usually end favorably with highly polished amber specimens or crystal faces.

I do think I try to avoid extremes in settings, and to keep it as balanced as possible (comparing similar settings used by my WB35F mirrorless), and I really should be taking good photos with the DSLR. Even with adjusting, the brightness may turn out all right, but the sharpness never really works out. I'm at a loss, as the 60mm lens is practically brand new, and the DSLR sensor is clean. I made a point of taking a week or more to study the 500D's manual cover-to-cover when I first purchased it over a year ago, but I do still refer to it when I need to. I'm trying not to make excuses, I'm just listing what I do and have done so far.

So yes, this might not be the proper place to discuss basic user techniques. I almost always have a difficult time in being successful in practice, and often even more-so in comprehending abstract principles, regardless of what the subject may be. I was just hoping to find some answers, which seem to elude me. I can honestly say I have been unsuccessful in life in most regards, but I want to at least achieve greater success with the few hobbies and time I do have. It's a struggle.

-Kaegen

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Re: Aperture & Depth of Field

Post by Barrelcactusaddict »

I would like to know how a macro lens focuses, how a shorter focal length equates to a larger image; however, it seems that this is not deemed necessary. Hopefully the YouTube channel "Animations Xplaned" will produce a video detailing how macro lenses focus: maybe I'll reach out to them, since I'm not as yet seeing diagrams or videos detailing this online. I'll just keep looking: something will turn up eventually.

I know that my lack of understanding is an inconvenience, and my way of working at this particular issue is unconventional, perhaps even foolish. It certainly is frustrating, and I can tell it is not to myself only. Other users' time could certainly be of more benefit elsewhere than with my questions, I see that.

I appreciate the effort and assistance everyone has provided, regarding my questions within this particular post, and in other past topics.

-Kaegen

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Re: Aperture & Depth of Field

Post by Scarodactyl »

Barrelcactusaddict wrote:
Thu Dec 30, 2021 11:19 pm
Other users' time could certainly be of more benefit elsewhere than with my questions, I see that.
This is not a good way to think about this. The questions you're asking about lens design are interesting but I believe they fall pretty solidly outside the normal realm of photomacro knowledge. I personally don't know and don't want to blindly speculate.

If you post some of the pictures you were unsatisfied with and specific info about how they were taken and I imagine we can be a lot more helpful on that front.

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Re: Aperture & Depth of Field

Post by rjlittlefield »

Kaegen, a quick question: are you familiar and comfortable with these basic lens formulas?
  • 1/f = 1/o + 1/i
  • magnification = i/o
where
  • f = focal length,
  • o = distance from object to lens, and
  • i = distance from lens to image
--Rik

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