Aperture & Depth of Field

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Chris S.
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Re: Aperture & Depth of Field

Post by Chris S. »

Barrelcactusaddict wrote:
Tue Dec 28, 2021 12:15 pm
Now I'm a little confused, as to how shortening a focal length when focusing on a macro lens increases magnification. . . .
Kaegen,

You seem to have paused posting, perhaps while contemplating Rik's question about optical formulae. If these are throwing you at all, I'll point out a simplified way of looking at this. Here goes:

Yes, most of the general use "macro" lenses for DSLRS do shorten focal length as they are focused closer. This is because the lens designers took a mechanical shortcut. Let me emphasize that point: This shortcut is more about mechanics, cost, and bulk than it is about optics.

Imagine you are using a simple lens on a view camera. The camera is a shallow, light-tight box with film at the back; at front, there is a bellows full of empty air between the camera and the lens. To focus on a subject at infinity, you adjust the length of your bellows so that the amount of space between the lens and the film (or later, sensor) is equal to the focal length of the lens. So if you have a 100mm lens, you have 100mm of space between lens and film when focused at infinity. (This is close to the definitions of "infinity focus" and "focal length.")

Now, imagine that you want to focus on a closer subject. To do so, you lengthen your bellows. For a subject at 1:1 magnification (1x), you need a bellows length that creates lens-to-film distance of twice the focal length of the lens. So for a 100mm lens, you have 200mm of space between lens and film at 1:1 magnification.

This is true for any simple lens shooting at 1:1: The distance between film and lens will be twice the focal length of the lens. So for a 200mm lens to focus at 1:1, it must be 400mm from the film.

In the days of view cameras, photographers adjusted focus by changing the length of their bellows ("bellows draw"). By the time of SLRs, most lenses included a focusing helicoid that replaced the bellows. A focusing helicoid is a tube that changes length as a portion of it is turned. Helicoid focus is not optically different from bellows focus--it's still a tube of empty air, simply in a more convenient and portable form.

For SLR photographers who were still willing to fuss with a bellows in the field, there were some good, inexpensive, and simple macro lenses made for bellows use. They lacked a helicoid. But these lenses--when combined with the required bellows--were bulky, a bit heavy, and quite fiddly to use in the field. (I can't begin to say how many shots I missed while wielding such things--while you are adjusting the bellows draw, the bug flies off.)

So camera companies came up with "macro lenses." (In quotes because most lenses of this type focus from infinity to 1:1, and many would argue that this is not truly macro.) These lenses are comparatively small, compact, and used helicoids, so no bellows to mess with. The problem is that helicoids are either limited in their range of length, or expensive and bulky. For a 100mm macro lens to focus the full range from infinity to 1:1--which most users expect--requires 100mm of length adjustment. This is difficult to do with a helicoid.

So here is where the design engineers accepted a tradeoff. The designed most macro lenses to shorten their focal length as they are focused down from infinity toward 1:1. As the focal length decreased, so decreased the need for a long helicoid. For example, if you decrease a nominally 100mm macro lens to 60mm as it approaches, 1:1, you only need a 60mm helicoid.

Does this make any sense?

--Chris S.

Barrelcactusaddict
Posts: 72
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Location: Rexburg, Idaho

Re: Aperture & Depth of Field

Post by Barrelcactusaddict »

Scarodactyl wrote:
Fri Dec 31, 2021 12:26 am
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.
Well, I do have just one image I saved from using strictly the EOS 500D and Canon 60mm f/2.8 lens (some others were taken with Hoya magnification add-on lenses to the 60mm, but those don't really count); this one is the only image I'm happy with and have published: it is of 10 faceted 20mm cubes of Cordierite var. Iolite, which I had posted on Mindat.org; the field of view is 134mm, ISO 400, f/3.2, 1/125 sec.. It is not as high a magnification as I usually use for specimens, so I found its quality acceptable; the file's too large, and a resize is too small, so I have a link to the image:https://www.mindat.org/photo-1139989.html

Two other pics I had saved using the DSLR and 60mm f/2.8 lens involved the stacked addition of a 2+ and 4+ Hoya circular magnification lens; the first is of the whole specimen (37x15x11mm), and I am not sure of the details as I took the image back in June or July of this year, but I do believe I had the lens set to infinity, so it was roughly 6x magnification: http://www.thefossilforum.com/index.php ... fm-~30-ma/ Another is of the same specimen, but of an inclusion within it measuring about 3mm in length: I believe the lens was set at full magnification to 1:1, while the two lens filters were attached as well. This resulted in terrible quality, but it was the best I could manage:http://www.thefossilforum.com/index.php ... fm-~30-ma/

This last is of a photo taken with my Samsung WB35F with a BelOMO 10x loupe positioned between the lens and the specimen. I only include this image because it is actually much higher quality than the same photos I rejected, when using the exact same setup and conditions, but with the DSLR and 60mm lens without Hoya add-ons. I had used a tripod, as usual, with the DSLR, and tried for a couple hours to get a single decent photo, until I gave up and went with the mirrorless. I had taken turns slowly adjusting the ISO, aperture, and shutter speed, adjusting only one setting while keeping the others constant, from one extreme to the next, but never obtained a good picture that would even rival the image I took with the mirrorless soon after. I took around 100 or more images, all useless. Please note that I did adjust the position of the BelOMO loupe so that the 60mm lens was in as good focus as possible; this was the case for both the mirrorless and the DSLR. http://www.thefossilforum.com/index.php ... 3-1365-ma/

I use the macro lens by manually setting the focus ring to the desired *magnification (*edit: not "aspect") ratio beforehand, then I physically move the camera and lens toward/away from the subject. I didn't do this with the BelOMO loupe setup, though; with that, I tried moving either the loupe or the lens, as well as both lenses together. I managed to take the mirrorless photo successfully within 10 minutes.

Looking ahead to anyone's response, I'm certain there will be said there is a great deal that is wrong with my methods. I do think the first two photos (iolite and blue amber specimen) that I did save turned out okay. I truly understand the relationship between aperture, ISO, and shutter speed, but I have trouble reliably putting that into practice, even after more than a year. I don't know, my work and methods are probably so shoddy and incorrect it'll make anyone who reads this laugh and/or shake their heads in disbelief. I want to think that I'm trying everything I can to get a good result, but it seems nearly anything I understand or do is incorrect. I guess it makes sense, since in HS Chemistry and Physics I somehow managed to get good grades on assignments, but miserably failed every single chapter's test: I just cannot extrapolate concepts and put things into practice in a distinct situation. Even when I try to replicate my literally few successes, I run into countless issues. I can't explain it, it is ridiculously pitiful.
Last edited by Barrelcactusaddict on Mon Jan 03, 2022 4:46 pm, edited 1 time in total.

Barrelcactusaddict
Posts: 72
Joined: Wed Sep 29, 2021 10:04 am
Location: Rexburg, Idaho

Re: Aperture & Depth of Field

Post by Barrelcactusaddict »

rjlittlefield wrote:
Fri Dec 31, 2021 3:01 pm
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
Chris S. wrote:
Sun Jan 02, 2022 12:23 am
Barrelcactusaddict wrote:
Tue Dec 28, 2021 12:15 pm
Now I'm a little confused, as to how shortening a focal length when focusing on a macro lens increases magnification. . . .
Kaegen,

You seem to have paused posting, perhaps while contemplating Rik's question about optical formulae. If these are throwing you at all, I'll point out a simplified way of looking at this. Here goes:

Yes, most of the general use "macro" lenses for DSLRS do shorten focal length as they are focused closer. This is because the lens designers took a mechanical shortcut. Let me emphasize that point: This shortcut is more about mechanics, cost, and bulk than it is about optics.

Imagine you are using a simple lens on a view camera. The camera is a shallow, light-tight box with film at the back; at front, there is a bellows full of empty air between the camera and the lens. To focus on a subject at infinity, you adjust the length of your bellows so that the amount of space between the lens and the film (or later, sensor) is equal to the focal length of the lens. So if you have a 100mm lens, you have 100mm of space between lens and film when focused at infinity. (This is close to the definitions of "infinity focus" and "focal length.")

Now, imagine that you want to focus on a closer subject. To do so, you lengthen your bellows. For a subject at 1:1 magnification (1x), you need a bellows length that creates lens-to-film distance of twice the focal length of the lens. So for a 100mm lens, you have 200mm of space between lens and film at 1:1 magnification.

This is true for any simple lens shooting at 1:1: The distance between film and lens will be twice the focal length of the lens. So for a 200mm lens to focus at 1:1, it must be 400mm from the film.

In the days of view cameras, photographers adjusted focus by changing the length of their bellows ("bellows draw"). By the time of SLRs, most lenses included a focusing helicoid that replaced the bellows. A focusing helicoid is a tube that changes length as a portion of it is turned. Helicoid focus is not optically different from bellows focus--it's still a tube of empty air, simply in a more convenient and portable form.

For SLR photographers who were still willing to fuss with a bellows in the field, there were some good, inexpensive, and simple macro lenses made for bellows use. They lacked a helicoid. But these lenses--when combined with the required bellows--were bulky, a bit heavy, and quite fiddly to use in the field. (I can't begin to say how many shots I missed while wielding such things--while you are adjusting the bellows draw, the bug flies off.)

So camera companies came up with "macro lenses." (In quotes because most lenses of this type focus from infinity to 1:1, and many would argue that this is not truly macro.) These lenses are comparatively small, compact, and used helicoids, so no bellows to mess with. The problem is that helicoids are either limited in their range of length, or expensive and bulky. For a 100mm macro lens to focus the full range from infinity to 1:1--which most users expect--requires 100mm of length adjustment. This is difficult to do with a helicoid.

So here is where the design engineers accepted a tradeoff. The designed most macro lenses to shorten their focal length as they are focused down from infinity toward 1:1. As the focal length decreased, so decreased the need for a long helicoid. For example, if you decrease a nominally 100mm macro lens to 60mm as it approaches, 1:1, you only need a 60mm helicoid.

Does this make any sense?

--Chris S.
I'm sick to my stomach to say this, but after trying to comprehend those equations and also the explanation on helicoids and macro lenses, I just cannot visualize it.

The second equation makes some sense to me and I can visualize it a bit, but I just don't know what to think about the first one; I still don't know how either would actually work. I don't know how the bellows would work by extending from 100mm to 200mm or 400mm or any length; I can only see that by doing that, it would just make the image out of focus (I keep visualizing light rays focusing to a point and then when the bellows are extended, diverging not to reach the film or sensor). I don't know how to put it into words or how to visualize it.

This has to be equally maddening for everyone else: Scarodactyl, Rik, and Chris, you have no idea how sorry I am for the frustrations my dumb questions are causing. I just don't know what to think anymore, I can't understand. I don't think anything can be done, I'm just humiliating myself, I shouldn't even be trying to learn more about these things. I thought I was beginning to understand, and it's just falling apart in my mind; I don't think I understand at all. :(

I'm so rattled, it's affecting my ability to even construct a decent paragraph; that's why I've edited it multiple times now. I just can't do it, it's a total disaster. I'm sorry. It just occurred to me that my confusion might be partly due to my developing health condition (I had mentioned this back in October on a different topic). Maybe it's best if I forget about all of this, it's hopeless. :cry: I'll take some time, and check back in a couple days.

-Kaegen

Barrelcactusaddict
Posts: 72
Joined: Wed Sep 29, 2021 10:04 am
Location: Rexburg, Idaho

Re: Aperture & Depth of Field

Post by Barrelcactusaddict »

Okay, I think it would be best if I give up trying to understand anything more about how light and lenses work or how lenses focus. I still cannot comprehend or visualize much of what has been explained. I'm sorry for wasting everyone's time, please know that was not my intent.

Things just seem to be getting worse; I had contacted Allan Walls with questions about an adapter I own (Pentax 645 to EF mount), and he had provided some encouraging insight I had overlooked. However, I mentioned my setup and intended use, and that led to discouraging news: he told me that using a 5x Plan Apo Mitutoyo objective on a 100mm tube lens might not be a very good idea (that objective's ideal performance is met when used with 200mm lens), and that the Laowa 25mm 2.5-5x Ultra Macro lens has diffraction issues at high magnification. He had recommended I use an El Nikkor 50mm F/2.8N enlarging lens (reversed) with a PB-6 Nikon bellows, and that this would be a better option. I don't know how bad the image quality would be with using a 100mm tube lens with the 5x Mitutoyo, or if this would even work well with my camera's APS-C sensor; I also don't know if a bellows would work well if mounted vertically. I'm even more confused at this point, and I feel I'm back to square one; I've poured hundreds of dollars and many months in pursuit of a vertical setup. I just don't know what to do.

I'm sorry, I'll just give up asking any more questions about how light and lenses work, or any kind of abstract theoretical principles whatsoever. I seem to have too much trouble as it is with getting a setup completed. Maybe I wasn't meant to get involved with this kind of photography, and it might be best if I just give up pursuing the whole idea. I just don't know what to think anymore. I'm sorry. :cry:

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