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04-13-2018, 08:40 AM   #11761
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QuoteOriginally posted by IgorZ Quote
what I'd like to know is how do you guys know all this stuff? Books on optics? I had some basic optics in my high school physics class some 23 years ago, but can't remember much of it.
I learned this from designing high-resolution, large-format film scanners back in the late 1980s. Most of this stuff is just turning the crank on some geometry and algebra.

These days, wikipedia is a rich resource on this stuff. But it still requires some mental noodling to see what a given math formula means to a photographer who sees a cool picture taken with one format of camera and wishes to replicate that image or that look with their own different format camera. And it does not help that the ways that lenses are speced in mm of focal length and relative aperture can make some photographic calculations easy but make other ones hard.

04-13-2018, 10:12 AM - 2 Likes   #11762
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QuoteOriginally posted by photoptimist Quote
This is exactly true. The optical refraction of light, the convergence of the rays behind the lens, and any blur if the film or sensor aren't at the exact point of focus is only a function of the focal length and the various distances from subjects to lens to focal plane. The width and height of the film or sensor have no influence on the size of the circle of confusion. The lens does not know how big the film or sensor is.



But that has nothing to do with equivalence (except to create a lot of arguments about using the same focal length lens on different formats zooming with the feet etc.).



The effects of equivalence arise when one wants to take truly equivalent images in two different formats.



Imagine I see some beautiful 16x20 image taken on a 6x7 with a wide-open 105/2.4 and I want to make the equivalent 16x20 image (with identical foreground-subject-background perspective, identical framing, and identical blurring of out-of-focus regions) but using a K-3.



1) To get the identical foreground-subject-background perspective, I need to put the K-3 in the same location as the 6x7 was in with all the various lens-to-subject distances being the same.



2) To get the identical framing, I can't use a 105 on the K-3 because it's too narrow. I need to use a 30 mm lens on the K-3 (30 = 105 * 16mm APSC_frame_height / 56mm 6x7_frame_height ) to get the equivalent angle of view for getting the equivalent framing.



3) To get the identical blurring of out-of-focus regions with a 30 mm lens on APS-C as a 105/2.4 produces on 6x7, a messy bit of math reveals that I need a 30 f/0.7. (The reason for this is due to the smaller optical magnification of a 30 mm lens on APS-C and the effects of having a smaller circle of confusion on APS-C. That forces me to need a physical aperture on the 30 mm lens as big as the physical aperture on the 105/2.4. That's a 44 mm diameter physical aperture which then is equal to a f/0.7 relative aperture on a 30 mm lens. The overall result is that the ratio of the format sizes determines the ratio of relative apertures needed and that 105/2.4 on 6x7 is actually a very shallow DoF lens compared to a 30/2.4 on APS-C.)





Equivalence is about attempting to make equivalent images on different formats and has nothing to do with the same lens or focal length looks on different formats.


I think if you’ll read my later post, you’ll see that I added the effect of enlargement as an offsetting effect. And you’ll also note my care in distinguishing between “aperture” and “f/stop” or focal ratio.

But that does not mean the optical magnification is unimportant. Blur is controlled by absolute aperture and magnification. The magnification may be in the lens or in the “enlarger”. When print size is controlled, enlargement magnification required by a smaller format undoes some of the increased depth of field resulting from less lens magnification (resulting from a shorter focal length used with a smaller format).

Of course, wanting large prints is one reason people use larger formats, so the assumption of equal print size is also a model rather than a reality.

But, as you say, the first step to mathematical understanding is the distinction between aperture and focal ratio. That’s why large format often uses very long shutter speeds—the absolute aperture needed for acceptable depth of field with the longer lenses results in a much higher focal ratio—f/22 instead of f/8, etc. But the focal ratio is what controls the intensity of light and therefore the exposure.

The reason I know this stuff? 1.) I work in multiple formats from APS-C (and smaller) to 4x5, often adapting lenses and cameras to work across formats. 2.) As an engineer, I’m not afraid of math. 3.) As a teacher, I relish opportunities to explain things, but only things I actually know, which requires a willingness to learn stuff that works from first principles. 4.) And I’ve been involved in various photographic forums since the late 90’s, where these topics come up routinely.

I learned about lens calculations from optical texts, mostly. Kingslake, etc.

And one learns a lot enlarging various formats using various lens focal lengths and a good grain focuser.

Rick “who has done the math—years ago” Denney
04-13-2018, 10:28 AM   #11763
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QuoteOriginally posted by IgorZ Quote
what I'd like to know is how do you guys know all this stuff? Books on optics? I had some basic optics in my high school physics class some 23 years ago, but can't remember much of it.
50 years in electro-optics systems design, analysis, and requirements generation.

I hope I can further clarify this subject with the following gedanken experiment.

Consider a camera-lens assembly where the focal plane width is 35mm, the lens focal length is 100mm, and the aperture is 10mm (f/10). Ignoring all aberrations, the angle of the ray bundle to a point on the focal plane will be twice the arctangent of half the reciprocal of the f/no. This is roughly 100 mrad. From focus, one can displace the focal plane by 1 mm and get a growth in the ray bundle size (in this model also the circle of confusion) of 100 microns.

Now, if one dimensionally scales the entire camera and lens by a factor of 2 to provide a 70-mm focal plane width (but don't double the glass indices of refraction), one will get twice the focal length, twice the aperture, twice the lens group spacings, twice the lens curvatures, twice the barrel size, etc., but the same f/no. Rotating the focus ring will move the back focus distance by twice as much as it did with the 35-mm assembly. Moving focus from the minimum focal distance to infinity will require the same angular rotation of the focus ring because the gearing will have also doubled. Moving the lens 1 mm toward the focal plane will again yield a blur growth of 100 microns.

From the point of view of focal plane motion to achieve a given blur, the depth of field is the same for both cameras. However, photographers' minds are looking outward, not inward, and depth of field is meaningful when applied to the field. If a focus ring rotation moves the focus distance by twice as much with the 70-mm camera, then it follows that 1-mm of motion at the focal plane of the 70-mm camera-lens assembly will change the far field focus by half as much as it would for the 35-mm camera-lens assembly. Thus, the depth of field, measured in the field, is half a much for the doubled focal length lens.

I hope this makes the last few comments made here on this subject consistent.
04-13-2018, 01:17 PM   #11764
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Thanks to everyone

I sure appreciate all the in-depth and knowledgeable responses to what I originally thought was a pretty simple question. I will re-read the responses in an effort to understand, but may need an optics text to do so! Again, thanks to all of you for sharing your knowledge and perspectives.

04-13-2018, 05:59 PM - 5 Likes   #11765
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Pentax 645N II, Pentax smc 645 FA 200mm f/4.0 (IF), Fujicolor NPS 160
f/2.8 1/250 ISO 160 Epson Perfection V550 Photo
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04-14-2018, 12:35 PM - 6 Likes   #11766
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---------- Post added 04-14-18 at 12:50 PM ----------



04-14-2018, 02:08 PM - 6 Likes   #11767
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My first 645 film photos.

Thanks to Ivan
I am now also into 645 film.
Below is a couple of the photos and linked to my photo album with the cherry picked others.
Pentax 645 with A120 Macro
The detail in the film is truly different than 35mm.
I think this is going to be an interesting and fun journey.

Clarence



04-14-2018, 04:37 PM   #11768
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QuoteOriginally posted by c.perkins Quote
Thanks to Ivan
I am now also into 645 film.
Below is a couple of the photos and linked to my photo album with the cherry picked others.
Pentax 645 with A120 Macro
The detail in the film is truly different than 35mm.
I think this is going to be an interesting and fun journey.

Clarence

Nice shots, what film did you use?
04-14-2018, 04:48 PM   #11769
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QuoteOriginally posted by c.perkins Quote
Thanks to Ivan
I am now also into 645 film.
Below is a couple of the photos and linked to my photo album with the cherry picked others.
Pentax 645 with A120 Macro
The detail in the film is truly different than 35mm.
I think this is going to be an interesting and fun journey.

Clarence

very nice/glad to be a small part of it!
04-14-2018, 04:53 PM   #11770
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QuoteOriginally posted by Jacquot Quote
Nice shots, what film did you use?
It was a roll of Ektar 100 that just expired which came with the camera.
Av mode set to f8

Clarence

---------- Post added 04-14-18 at 06:55 PM ----------

QuoteOriginally posted by ivanvernon Quote
very nice/glad to be a small part of it!


Clarence
04-14-2018, 11:22 PM - 6 Likes   #11771
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from this morning up in Mendocino County


Milky Way over Bowling Ball Beach
645Z & DFA 25/4
04-15-2018, 12:14 AM - 9 Likes   #11772
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Perhaps it's not too noticeable but a guy is standing waist-deep in the water just get front of everyone for his picture.

From some Acros development testing back in 2013 at Cannon Beach, Oregon.
P67, 67 55/4, Orange Filter, 100 Acros


04-15-2018, 02:27 AM - 2 Likes   #11773
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tuco, love this picture
04-15-2018, 03:58 AM   #11774
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QuoteOriginally posted by tuco Quote
Perhaps it's not too noticeable but a guy is standing waist-deep in the water just get front of everyone for his picture.

From some Acros development testing back in 2013 at Cannon Beach, Oregon.
P67, 67 55/4, Orange Filter, 100 Acros


I think perhaps he enhanced the photo by providing a measure of perspective . . . great photo, though, either way -- with or without!

---------- Post added 04-15-18 at 07:05 AM ----------

Here is Adorama's listing of the top five medium format lenses. I was pleased to see the Pentax-D FA 90mm f 2.8 listed, though a little piqued that it is listed as the best choice for the budget-conscious photographer . . . especially since it is the most expensive lens I own!

5 Best Medium-Format Camera Lenses - ALC

---------- Post added 04-15-18 at 07:06 AM ----------

Here is Adorama's listing of the top five medium format lenses. I was pleased to see the Pentax-D FA 90mm f 2.8 listed, though a little piqued that it is listed as the best choice for the budget-conscious photographer . . . especially since it is the most expensive lens I own!

5 Best Medium-Format Camera Lenses - ALC
04-15-2018, 05:05 AM - 3 Likes   #11775
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