A difficult question - shooting square

[mikemgb]

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Reading through this thread one thing that strikes me is that a better comparison may be made with a 50mm lens on 35mm. I have always considered 50mm on a 35mm camera to be equivalent to 80mm on a 6x6. This is because I look at the horizontal aspect, rather than the vertical.

 

If you used a 50mm lens on the 35mm, then cropped the 6x6 to 4x6 landscape, then enlarged both images so the quetzal was the same size on both prints, would the depth of field then be the same?

 

Or, if you insisted on using a 35mm lens on 35mm film wouldn't you be better comparing it to a 6x9 so no crop was necessary?

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[Michael Geschlecht]

Hello Mike,

 

Reasonably close.

 

But: Both lenses must be set at the same F stop & both lenses must be focused on the same eye of the same quetzal.

 

The angle of coverage of an 80mm lens on a format of 56mm X 36mm is about the same as the angle of coverage of a 50mm lens on a 36mm X 24mm image capture surface.

 

You could also use a 80mm lens that covers 56mm X 72mm. All else as above.

 

Because: Just as 6 X 6 is 56mm X 56mm.

 

6 X 7 is 56mm X 72mm.

 

Best Regards,

 

Michael

[01af]

I always appreciate your insight & knowledge.

Obviously, you don't.

 

 

But: Both lenses must be set at the same f-stop ...

 

Sheesh ... how hard can it be!?

 

At the same f-stop (and equivalent focal lengths), the smaller-format camera will yield deeper depth-of-field. Understand? For equal depth-of-fields, the smaller-format camera's relative aperture, or f-stop, must be wider (same absolute aperture) ... as explained above several times by now.

[Michael Geschlecht]

Hello 01af,

 

I actually do appreciate your insight & knowledge. I think it adds a lot to this Forum.

 

Best Regards,

 

Michael

[Michael Geschlecht]

Hello 01af,

 

If we are maintaining the acceptable circles of confusion to be at 1/30mm on each of the 2 finished photos, regardless of the original lens or image capture surface format, etc, of the 2 photos, both focused on the same eye, of the same quetzal:

Which is part of the parameters at the beginning of this Thread:

 

And if we have 1 image that appears as a point because that image is 1/30mm in diameter when when it is enlarged 10X:

 

And if we have a second image where there is something which also appears as a point at a magnification of 10X, BUT when further enlarged to 23.3 X appears as a disc/blur because what had appeared as a point because it was 1/30mm in diameter is now 1/13mm (approximately) in diameter & it appears as a disc/blur.

 

The greater enlargement has effectively lessened the depth of field.

 

Best Regards,

 

Michael

 

[luigi bertolotti]

At the same f-stop (and equivalent focal lengths), the smaller-format camera will yield deeper depth-of-field. Understand? For equal depth-of-fields, the smaller-format camera's relative aperture, or f-stop, must be wider (same absolute aperture) ... as explained above several times by now.

+ 1... (btw... "horses for courses"...the OP entitled this thread as "a DIFFICULT question"... )

[harmen]

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If we are maintaining the acceptable circles of confusion to be at 1/30mm on each of the 2 finished photos, regardless of the original lens or image capture surface format, etc, of the 2 photos, both focused on the same eye, of the same quetzal.

 

Some of us focus on a different aspect in comparing the two cameras/lenses. Instead of asking what is acceptably sharp, the other question is how not sharp do out of focus areas become. When I calculate from scratch (see drawings earlier), I get the same results as others are mentioning, which is that larger formats make for stronger blur.

[jankap]

Reading through this thread one thing that strikes me is that a better comparison may be made with a 50mm lens on 35mm. I have always considered 50mm on a 35mm camera to be equivalent to 80mm on a 6x6. This is because I look at the horizontal aspect, rather than the vertical.

 

 

The thread is called shooting square.

56mm divided by 80mm is equal to 24mm divided by 35mm. More or less!

Jan

[lct]

Cropping 24 x 36mm to 24 x 24mm determines the CoC value i.e. 0.023mm as suggested above. Changing aperture and/or focal length will change DoF though necessarily. To mimic that of the Rolleiflex 80/2.8 (0.055mm CoC), a 50mm lens should be open at f/2.8 then instead of f/1.4 for a 35mm lens.

 

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[01af]

If we are maintaining the acceptable circles of confusion to be at 1/30 mm on each of the two finished photos ...

... then the depth-of-field wouldn't be the same. Equal DOF naturally means smaller COC in the smaller-format capture. So please stop fantasizing about "maintaining the COC the same" already!

 

 

...the OP entitled this thread as "a DIFFICULT question" ...

No idea as to why. As a matter of fact, it's a totally simple question.

 

 

... I get the same results as others are mentioning, which is that larger formats make for stronger blur.

Exactly! And that's why the larger-format camera needs to be stopped down more if you want the same depth-of-field, or the same degree of out-of-focus blur beyond the in-focus depth of the field.

[luigi bertolotti]

No idea as to why. As a matter of fact, it's a totally simple question.

 

 

Don't overevaluate the mean "man of the road" familiarity with math...   

[adan]

Luigi - better idomatic English would be "Don't overestimate the average "man in the street's" familiarity with math." But we know what you meant.

___________________

 

I went through the whole "crop my M9 to simulate 6x6" thing - even sort of works, with a "sharp everywhere" lens and aperture (15mm Voigtlander vs. Hassy Superwide).

 

Ultimately, it was more satisfying and authentic to just get the real thing and shoot film with it. Plus I like seeing the complete finished picture as I shoot it, rather than taking a hatchet to it to get a particular shape later.

 

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[harmen]

I'm finding this topic very interesting. I had another go at comparing the two seemingly opposite features that a change in focal length and film format creates more blurry background for the same perspective, while there is no change in depth of field when changing focal length and object distance while keeping film format the same. Hope this makes it easy to see what happens.

 

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Main principle to base all calculations on: 1/f = 1/o + 1/i. This simply gives the distances before and after a lens to get an object in focus.

 

d is usually given as f/x, so we'll do the same.

b is the size of the circle that a dot at infinity becomes when focusing closer than infinity. It's also useful to express it as a percentage of the negative size: b/n. Let's call that b%. For large o, b%=f^2/(x*o*n)

 

From this formula, you can see that b% goes down with film format (n), but increases quadratically with focal length (f) because we scale up the diaphragm together with focal length. Not unexpected is that stopping down (x) reduces blur.

Let's look at some scenarios

* How does the blur percentage change when using different formats of film (n1 vs n2)? If you change the focal length of the lens, so you retain the same perspective from the same distance, you will find that b%1/b%2 = n1/n2 * x1/x2. In other words, for the same f-stop, a larger format film has a linearly larger percentage of blur for distant subjects. ( take f2=f1*n2/n1 and o2=o1 and the above formula for b% will result.)

 

 

* How does b% change when using different focal length lenses, while keeping the format (n) and f-stop (x) the same?

- First case, keeping distance constant. b%1/b%2 = (f1/f2)^2. However your main subject will increase within the frame with larger focal lengths, unless you step back...

- Second case, changing distance (o) to maintain the size of the subject in focus. b%1/b%2 = f1/f2. This means similar depth of field, since the increase in blur of distant subjects with larger focal lengths scales with the increase in size of distant objects while keeping the main subject similar in size in the frame.

(No drawing to illustrate, but the link someone posted earlier to the luminous landscape photo comparison demonstrates this very nicely.)

[luigi bertolotti]

I'm finding this topic very interesting...

* How does b% change when using   different focal length lenses, while keeping the format (n) and f-stop (x) the same?

...

- Second case, changing distance (o) to maintain the size of the subject in focus. b%1/b%2 = f1/f2. This means similar depth of field, since the increase in blur of distant subjects with larger focal lengths scales with the increase in size of distant objects while keeping the main subject similar in size in the frame....

 

And THIS is a situation I personally verified many times when taking pictures of roses of our garden... (here posted ad nauseam...  ) : Leica M (now 240.. but is a longtime story started by my wife's passion...  ) with bellows-tripod... and ONE of my lenses/lensheads : be it head of TE135, head of Elmar 90 3 el., head of Cron 50 DR... same image size-same f/stop--->same DOF in the main subject, no doubt even going a bit extreme (last year took some with Sonnar 250 for Contarex, on bellows... funny setup...)

[Tailwagger]

I am.

 

As am I, but with one important exception. If this really were the 60's, my brain would have sufficient empty space as well as resolving power to allow me to muse thoughtfully about the arguments presented.  These days my personal CoC is far too deep to get involved in this sort of exercise.  As such, I simply enjoy threads like this one as a motivating force for spending more time contemplating the spiritual aspects. Thats all that's left which I can pretend to understand. 

[jmahto]

Hello Jaap,

 

That is the case IF both images are enlarged the same number of times.

 

That is not the situation being discussed here:

 

What we are writing about here is a smaller image being enlarged 2&1/2 time as much as the larger image.

 

That is when the effective depth of field becomes the same.

 

We are producing a same sized photo in millimeters with the smaller image: As the photo from the larger sized image in millimeters.

 

Both subjects (ie: The coffee cup.) being the same size in the finished photo.

 

If both lenses cover the same angle of view & the F stop is the same for both lenses, etc: As I described previously:

 

Then, regardless of what depth of field scales or tables might say:

 

If the finished subject (ie: Coffee cup.) is the same height in millimeters in both finished photos (ie: 50mm.):

 

Then the depth of field in the finished photos will be the same.

 

Best Regards,

 

Michael

Michael I had done this test with APS-C and FF before. I used APS-C+28mm and FF+40mm (they have same FOV).

 

Same FOV

Subject (bucket) is the same size in finished picture.

Same f stop f/2

Same finished output size (see the crop as well)

 

I found that (with all the assumptions in your mail quoted above):

APS-C+28mm @f2 *DOF is not equal to* FF+40mm @f2  (in fact bigger sensor, FF, has more blur)

 

(Left: APS-C, Right: FF)

 

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Crop

[jmahto]

If I do the same test with 

APS-C+28mm @f2  and FF+40mm @f2.8 

 

Then both background blur are similar. Bigger format, FF in this case, needs a smaller aperture to do the same blur.

Crop:

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