A difficult question - shooting square

[jaapv]

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An interesting article:

 

https://luminous-landscape.com/dof2/

 

As for your question:

 

The DOF will remain approximately the same as, if you change focal length but keep the subject the same size -i.e change the subject distance-, the total magnification of the subject will remain the same and the area close to your subject will have only a small change in magnification due to the perspective change.

 

But: The magnification of background and foreground will change significantly by the different perspective, so the amount of blur in fore- and background will be considerably enhanced or reduced.

This effect will give a completely different impression of DOF between the two.

See the tower in the background of the first two images in MR's article.

 

In other words, the cone formed by the light rays will be more blunt or pointed.

 

Don't forget that DOF is not a reality but an optical illusion dictated by the limited resolving power of our eyes. DOF  is in the eye of the beholder.

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[jaapv]

Additionally: modern Leica lenses are designed specifically to give a high differentiation between background blur and plane of focus resulting in a more precise DOF behaviour - and resulting in complaints about "harsh" bokeh .

[wolfloid]

But this is exactly what I am saying is not happening:

 

"if you change focal length but keep the subject the same size -i.e change the subject distance-,"

 

The issue is this: if I use a 35mm leica lens cropped to square (24x24) I can shoot the same subject size (same perspective) from the SAME distance, as I would shoot using the 80mm lens on MF. That was the very first thing I said as the very ground of the discussion.

 

I fear that some contributors are not recognising this.

 

When calculating depth of field four main conditions need to be taken into account: the aperture, the distance, the sensor size, and the lens size. Typically, it is accepted, that, all else being equal, a larger mf sensor narrows depth of field (roughly two stops for the same perspective and field of view compared to 35mm). Cut to square, the 35mm leica and the 80 mm mf give almost the same field of view.

 

I am not considering using different distances to the subject, since that would give a different perspective, and anyway, as said, the field of view matches, in that sense it is a good match.

Edited June 3, 2017 by wolfloid

[Michael Geschlecht]

Hello Wolfloid,

 

In order to take a subject from the 24mm X 24mm image & make it the same size image in millimeters in a final print as the image size in millimeters of a print made from the 56mm X 56mm image: Given that the F stop & the distance from the film plane to the subject are the same.

 

You have to blow up that 24mm X 24mm image 2&1/2 times.

 

This means that what appears as a point source because of the acceptable circle of confusion of the 56mm X 56mm image: 1/30mm, still appears as a point measuring 1/30mm:

 

While what used to appear as a point of 1/30mm with the 24mm X 24mm image is now viewed as a 1/12mm disc.

 

2&1/2 times the diameter of the other.

 

Therefore the EFFECTIVE depth of field with the 24mm X 24mm image is 40% of what is marked on the lens barrel & in the tables in the above described set of circumstances.

 

Depth of field as marked on lens barrels & as printed in tables is only a guide for use within certain parameters.

 

Best Regards,

 

Michael

Edited June 3, 2017 by Michael Geschlecht

[jaapv]

Yes, but you are introducing another variable. Sensor/film size. A larger format will always produce a more narrow DOF. Compare a small-sensor P&S. Sharp from your toes to the horizon, whatever you do.

[Michael Geschlecht]

Yes, but you are introducing another variable. Sensor/film size. A larger format will always produce a more narrow DOF. Compare a small-sensor P&S. Sharp from your toes to the horizon, whatever you do.

 

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

Edited June 3, 2017 by Michael Geschlecht

[wolfloid]

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Hello Michael,

 

Are you really saying that: within the image, perceived sharpness relationships between objects in the image will change as the image is magnified? Please think about what you are saying, since this is so counter-intuitive and goes against my 40 years of working with images.

 

My experience tells me that the perceived sharpness relationships between say a person's eyes and their ears, or between their ears and the wooden shutter a metre behind them never changes. It stays exactly the same. Of course, as you blow the image up, the overall sharpness of the image will diminish, but only if you remain at the same viewing distance. This is where coc comes in. If, however, I keep moving back from the enlarging image in a controlled way, the apparent sharpness will remain. Coc has nothing to do with perceived sharpness relationships within the image, and exactly that is what this question is about.

Edited June 3, 2017 by wolfloid

[Michael Geschlecht]

Hello Wolfloid,

 

The question I am writing about is the question that you asked originally which was essentially:

If you were to photograph a subject with a 35mm lens & then use the central 24mm X 24mm  section of the image capture surface of a 24mm X 36mm image capture surface:

 

And you would print that photo with that subject a certain number of millimeters tall (ie:50mm).

 

What would the depth of field be relative to that object (ie: A coffee cup.) in that photo when compared to:

That same subject printed the same 50mm tall:

Taken with an 80mm lens which is recording the image on a 56mm X 56mm image capture surface:

 

Taken from the same spot, of the same subject at the same spot:

Taken at the same aperture. 

 

The answer is : Reasonably the same. For the reasons I wrote in the preceding Posts.

 

Precisely the same would require a relationship between lenses of 35mm to 87.5mm, all other parameters the same.

 

If this is still not clear: Please read thru this Thread again & do the math. Mathematics are important here.

 

If there are still questions: Please ask.

 

Best Regards,

 

Michael

[pico]

Are you really saying that: within the image, perceived sharpness relationships between objects in the image will change as the image is magnified? Please think about what you are saying, since this is so counter-intuitive and goes against my 40 years of working with images.

 

[...] Coc has nothing to do with perceived sharpness relationships within the image, and exactly that is what this question is about.

 

I think wolfoid got you, folks. He is right. Call it a misunderstanding of language.

Edited June 3, 2017 by pico

[Michael Geschlecht]

I think wolfoid got you, folks. He is right. Call it a misunderstanding of language.

 

Hello Pico,

 

I don't think so.

 

Do the math please.

 

Best Regards,

 

Michael

[luigi bertolotti]

Hello Michael,

 

Are you really saying that: within the image, perceived sharpness relationships between objects in the image will change as the image is magnified? Please think about what you are saying, since this is so counter-intuitive and goes against my 40 years of working with images.

 

Frankly... I think YES... BUT only when I LOOK at the magnified image from the SAME distance from which I look at the "less magnified image".

I mean (and am convinced of) that if I make a 10x15 print and a 30x45 print from the same neg, and look at them from DIFFERENT distances (bigger for the 30x45, of course) , I can perceive the same sharpness relationship between focus planes... bit if I look at them from the SAME distance, I get a different perception : an area/subject (ideally plane) which is perfectly focused will look like this in both the prints  (supposing film grain doesn't enter in the game...), the out of focus areas will be seen with a different "degree" of Out-Of-Focus  (the "degree" term is unscientifically definible... see post #21 "DOF is in the  eye of the beholder"...  ) 

Edited June 3, 2017 by luigi bertolotti

[luigi bertolotti]

All in all, if I had to think of what happens in the situation you describe, to say :

 

- Rolleiflex TLR 80mm at f 2,8, 3,5 meter

- Leica 35mm at f 2,8, 3,5 meter

 

I agree that the 24x24 central crop of the Leica neg is next to identical to the full 56x56.

 

Now, I make two 30x30 prints from the two negs , and look at them from the same distance

 

My "istinctive" feeling is that the Rolleiflex one will show LESS DOF, but with a NOT SO GREAT difference from the Leica's

 

And... many years ago, when I used regularly my Rolleiflex 3,5f (which of course hadn't 2,8... ) and my M4 as well, I'd swear to have experienced this feeling, though by sure nver made any specific test about.

[Michael Geschlecht]

Hello Luigi,

 

What we are talking about is NOT in the "eye of the beholder".

 

What we are looking at is something concrete & measurable.

 

IF we are considering depth of field as defined by "circles of confusion", then:

what I wrote above is accurate. Because:

The original question was:

If a person stays at 1 spot & takes 2 pictures of the same quetzal with 2 lenses of 2 different focal lengths which are both set at the same aperture:

 

And then the image taken with the shorter lens is enlarged so that the 2 images are congruent when 1 image is put on top of the other:

 

Then the depth of field will be the same in both images.

 

This is math.

 

NOT the eye of the beholder.

 

Because: When the smaller image with more depth of field is enlarged to the size of the larger image:

What were previously point images or images of 1/30mm or less become larger discs & are no longer seen as being in focus.

 

All aspects of all things above being equal: The only difference between the 2 images will be that the photo taken with the longer lens will be of a better quality because it will contain more pieces of information per square millimeter in the finished print.

 

Best Regards,

 

Michael

[Michael Geschlecht]

All in all, if I had to think of what happens in the situation you describe, to say :

 

- Rolleiflex TLR 80mm at f 2,8, 3,5 meter

- Leica 35mm at f 2,8, 3,5 meter

 

I agree that the 24x24 central crop of the Leica neg is next to identical to the full 56x56.

 

Now, I make two 30x30 prints from the two negs , and look at them from the same distance

 

My "istinctive" feeling is that the Rolleiflex one will show LESS DOF, but with a NOT SO GREAT difference from the Leica's

 

And... many years ago, when I used regularly my Rolleiflex 3,5f (which of course hadn't 2,8... ) and my M4 as well, I'd swear to have experienced this feeling, though by sure nver made any specific test about.

 

Hello Luigi,

 

Part of the apparent difference MIGHT have been that both photos were taken with lenses set at their largest aperture.

 

It might be interesting to repeat the same test at F8 focused on a small something (ie: A pencil.) at a distance of 3.5 meters. With some other small things in front of it & behind it & then enlarge this central section.

 

Or perhaps just a ruler pointing away at a 45 degree angle to the lens axis.

 

Focusing on the middle number.

 

Best Regards,

 

Michael

[pico]

Michael - Consider viewing distance.

[luigi bertolotti]

Hello Luigi,

 

What we are talking about is NOT in the "eye of the beholder".

 

What we are looking at is something concrete & measurable.

 

IF we are considering depth of field as defined by "circles of confusion", then:

 

what I wrote above is accurate. Because:

 

The original question was:

 

If a person stays at 1 spot & takes 2 pictures of the same quetzal with 2 lenses of 2 different focal lengths which are both set at the same aperture:

 

And then the image taken with the shorter lens is enlarged so that the 2 images are congruent when 1 image is put on top of the other:

 

Then the depth of field will be the same in both images.

 

This is math.

 

NOT the eye of the beholder.

 

Because: When the smaller image with more depth of field is enlarged to the size of the larger image:

 

What were previously point images or images of 1/30mm or less become larger discs & are no longer seen as being in focus.

 

All aspects of all things above being equal: The only difference between the 2 images will be that the photo taken with the longer lens will be of a better quality because it will contain more pieces of information per square millimeter in the finished print.

 

Best Regards,

 

Michael

Michael...  I tend to agree NEXT to your position... but can we REALLY be confident on math on this ? I mean... the only "math" i have access to is the perennial DOF calculator on web... and seems to me that even if applying the factor you applied (correctly, imho... 1/30 vs. 1/12) to the CoC radius and even taking into account the not exact "80mm 56x56 = 35mm 24x24" formula (*), the DOF does not results the same according to DOF calc...

 

And... fact is that large formats DID need adjusting focus depth with tilt...

 

(btw... how did you compute 35mm eq. 87,5mm ? I arrived to 82 around...)

Edited June 3, 2017 by luigi bertolotti

[Michael Geschlecht]

Hello Luigi,

 

Sorry, math error.

 

56/24 = 2.33 approximately

 

35mm X 2.33 = 81.67mm approximately

 

Which means a corrected factor of 1/13 approximately.

 

As opposed to the standard of 1/30.

 

Best Regards,

 

Michael

Edited June 4, 2017 by Michael Geschlecht

[harmen]

As someone else brought up... how would that work for a small sensor point and shoot then? If I were to take a shot at f/2 with a 35mm lens with a full frame 35mm camera and also f/2 with a 7mm lens on something like a Leica C camera, with the same distance to the subject, and printing both photos at the same size. That follows the same logic of 2 lenses, 2 focal lengths, 1 spot, same aperture, and then enlarging to similar subject size.

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Hallo,

Look here: Leica M

[pico]

Sorry, math error.

56/24 = 2.33 approximately

35mm X 2.33 = 81.67mm approximately

Which means a corrected factor of 1/13 approximately.

As opposed to the standard of 1/30.

 

Let's go back and settle some terms and specifics.

 

Calculate the diagonal for each format (nice, neat squares ) 55 √(2) and 24 √(2)

or:

 

sqrt(55^2 + 55^2) == ~78mm

sqrt(24^2 + 24^2) == ~34mm

 

So we know that the the lenses for this case are equivalent, FAPP. But we knew that.

 

When calculating the CoC, the final presentation size must be considered, IOW, the degree of enlargement. Greater enlargements require smaller CoC.  Using the Zeiss method: diagonal/1730 (For comparison here we can also use d/1500 if you prefer. Outcomes will relative.)

 

CoC 78mm == ~0.045mm

CoC 34mm == ~0.019mm

 

Are we accepting 25 cm viewing distance, or are we going to be realistic and strictly propose a viewing distance of focal length * degree of enlargement? (By the way, this works only with high definition prints, not with the typical computer screen.)

 

When this is all wound down I think we will find that comparable DoF will be all of 1 stop difference. Can you see 1 stop difference?

Edited June 4, 2017 by pico

[Ko.Fe.]

Why buy Leica if you want it to be Rolleiflex? I never understand this obsession wth square, either. All your have to do with square, is take it in the middle and decide later how to crop it on print. Portrait or Landscape .