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Author Topic: Medium format redefined  (Read 65620 times)

ErikKaffehr

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Re: Medium format redefined
« Reply #60 on: September 04, 2015, 12:23:20 am »

Hi,

Is that observation based on prints or viewing on screen. If on screen at what scale?

At actual pixels the DoF will seem to be thinner with an 80 MP-sensor (as a smaller crop would be shown). But, that would normalise in a print.

Best regards
Erik


I got to the 3 stops difference in aperture by direct comparison in studio with AFi-ii 12 compared to Nikon d800e.  It's really quite amazing the difference.  If you shoot a model at f/16 with the AFi-ii 12 you can't even get their whole body from front to back within the envelop of DOF, but at f/8 you can get that and more with the same framing.  There probably is a mathematical relationship that expresses this but I'll just go by experience.  Stopping down FF DSLR 3 more aperture stops seems about right to get to the same DOF.
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Erik Kaffehr
 

Theodoros

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Re: Medium format redefined
« Reply #61 on: September 04, 2015, 08:02:03 am »

Theodoros
Is this the reason why my Schneider Digitars (35XL + 24 XL) have the longer DOF than the Rodenstocks (32 + 23)?
I never compared them directly, but I always had the impression that the Rodies have much less DOF than the Schneiders,
maybe because of the retrofocus design?
Even the 24 PC-Nikkor (with a D800) seems for me to have less DOF at the same F-Stop as the 35 XL Digitar with a 44 x 33 sensor.
Best regards,
Ben
 
Ben, I'm not familiar with modern view camera lenses as I've never owned one with a digital back... I do have access to a P3 with 75H back that a friend owns, but the wider he uses is a 47mm lens... The lenses I used to have in the past for the P2, where older lenses aimed for 4x5 sheet film and I've sold them with the camera as I now use a Fuji GX-680 with my two digital backs alongside my Contax 645 system. However, I would expect the lens that has a wider image circle, to also show a narrower DOF since the value of the circle of confusion (COC) that the lens exhibits should increase.
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Garry Sarre

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Re: Medium format redefined
« Reply #62 on: September 04, 2015, 10:28:53 pm »

Agree Synn

I appreciate your real world comparisons. Thank you.

I see this all the time with corporate portraiture skin tones. The smaller format has this flat red smear and cutting in cyan makes a different problem. In the past with only dslr's available to me, Celtic skin required matting down with green tinted foundation in the makeup room.

MF file shows lots of different colours within the skin colour. The clarity and depth of colour shows through even at a 500 pixel webpage size.

I'll put some samples up when I get back to the studio. They are chalk and cheese.

On another note, As far as DR goes,  I was surprised after switching to MF, even with my studio portraiture, I have found I am barely within the DR capabilities of the camera - H5D-50. I have to expose as accurately as I did Transparency film. The MF CMOS solution would fix that, I just don't like the smaller size of the sensor and the puny image size through the viewfinder that the Blad and Pentax CMOS offer. I like to manual focus sometimes.
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Petrus

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Re: Medium format redefined
« Reply #63 on: September 06, 2015, 03:28:36 pm »

I would expect the lens that has a wider image circle, to also show a narrower DOF since the value of the circle of confusion (COC) that the lens exhibits should increase.


Not so, why would it? Only the focal length and relative aperture matters here.

You seem to have some quite funny ideas about optics.
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Theodoros

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Re: Medium format redefined
« Reply #64 on: September 06, 2015, 06:31:43 pm »

Not so, why would it? Only the focal length and relative aperture matters here.

You seem to have some quite funny ideas about optics.

I don't like people to call "funny ideas" what is common knowledge among optics engineering... in the mean times, this is a very general approach about the relationship between COC, image format (that the lens is able to cover) and DOF...  I hope you'll find it helpful as to correct the funny ideas about optics that you have... https://en.wikipedia.org/wiki/Circle_of_confusion
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Petrus

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Re: Medium format redefined
« Reply #65 on: September 07, 2015, 01:46:06 am »

I don't like people to call "funny ideas" what is common knowledge among optics engineering... in the mean times, this is a very general approach about the relationship between COC, image format (that the lens is able to cover) and DOF...  I hope you'll find it helpful as to correct the funny ideas about optics that you have... https://en.wikipedia.org/wiki/Circle_of_confusion


I did not find any references to "image circle" in that article and it does not feature in any of the formulas for circle of confusion or DOF, so I must assume it does not factor in at all. And please enlighten me how it even could.

Until then I hold on to my viewpoint about "funny ideas" about optics...
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Theodoros

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Re: Medium format redefined
« Reply #66 on: September 07, 2015, 02:26:15 am »

I did not find any references to "image circle" in that article and it does not feature in any of the formulas for circle of confusion or DOF, so I must assume it does not factor in at all. And please enlighten me how it even could.

Until then I hold on to my viewpoint about "funny ideas" about optics...

You did find image format size didn't you?  ??? There is even a table shown that gives you the COC value increase depending on the image area that the lens is designed for  ;)... The image circle of the lens under question is supposed to exactly cover that same image area... In other words, if you use another (smaller) size image area, it doesn't change the lens characteristics ... it's quite easy, try rethink of it...  :'(
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Petrus

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Re: Medium format redefined
« Reply #67 on: September 07, 2015, 04:31:36 am »

You did find image format size didn't you?  ??? There is even a table shown that gives you the COC value increase depending on the image area that the lens is designed for  ;)... The image circle of the lens under question is supposed to exactly cover that same image area... In other words, if you use another (smaller) size image area, it doesn't change the lens characteristics ... it's quite easy, try rethink of it...  :'(

No and no. Film format affects the circle of confusion, because it dictates how much the picture is to be enlarged for the print os a given size. What kind of a lens  you use is fully up to you: if you use a lens with overly large image circle you just wasting your money on a lens designed for a bigger format, gaining nothing, or if you use a lens which has a too small image circle you vignette the corners badly. How could this affect the DOF?

That table you mention shows the effect of the image size, not the lens image circle. Does a 135 system 24 mm T&S lens have a different COC from a regular 24mm prime, because the image circle is bigger? Hah, no it does not. Your misunderstanding probably stems from the fact that you mistakenly think this table has something to do with the image circle, when it really just is the image size and the image circle has nothing to do with it (except that in practice you need to use a lens with big enough IC to cover the format).

Like I said, if the image circle had anything to do with COC or DOF, if would be part of the formulas, or it could be derived from the formulas. Please show this instead of typing funny faces on your posts.  
« Last Edit: September 07, 2015, 05:10:17 am by Petrus »
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Theodoros

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Re: Medium format redefined
« Reply #68 on: September 07, 2015, 08:07:00 am »

The T/S lens can't be used as an example because it shares the same mounting distance... if the larger format lenses would mount at the same distance as smaller format lenses the COC value wouldn't change... what increases the COC value and thus reduces DOF is precisely the mounting distance as the COC widens with the mounting distance... as I said before, think...  ??? ...or read one of the references that are on the appendix of the link that I provided...  ;)

It's all common logic really... the more the mounting distance the larger the COC... Another way to think of it is as a percentage of the image area that the lens covers... Assumming that glass is the same and because COC should be the same in analogy, the larger the image area that can be covered by the lens, you get in proportion the increased COC value... and thus DOF decreases... As simple as that...  ;D  8)  :P
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Theodoros

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Re: Medium format redefined
« Reply #69 on: September 07, 2015, 08:39:43 am »

Petrus (and others) keep forgetting that if one uses a larger format lens for a shorter format image area, what one sees is a crop of the original image area of that larger format... But, OTOH, he admits that the larger image area does have increased COC and thus reduced DOF... Then he claims that if cropping a part of that area, DOF will be increased as if a smaller format lens (of the same focal length) would be used (which shows smaller COC)... I know no magic and testing proves that it doesn't... The increased COC value, still affects the whole image area that the lens is designed to cover and thus, the cropped image area retains, (of course!!!) the DOF that was recorded on the projected image circle... (which of course is reduced because of the larger COC value)!

Obviously he confuses the fact that the result print, if being the same in size, is still a cropped proportion of the one that was projected by the lens (minus what was not recorded on the image area) and thus what one sees on a print of the same size is a result of a bigger enlargment... Simple, isn't it?
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Petrus

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Re: Medium format redefined
« Reply #70 on: September 07, 2015, 09:33:20 am »

The T/S lens can't be used as an example because it shares the same mounting distance... if the larger format lenses would mount at the same distance as smaller format lenses the COC value wouldn't change... what increases the COC value and thus reduces DOF is precisely the mounting distance as the COC widens with the mounting distance... as I said before, think...  ??? ...or read one of the references that are on the appendix of the link that I provided...  ;)


I think I understand now enough to step out of the "discussion"…

But I wonder if a, say, Sigma Art 50mm lens really has different COC and DOF when mounted on Canon, Nikon and Sigma 135 format bodies, which all have different mounting distances?

(sorry, just could not resist after all…)
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Theodoros

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Re: Medium format redefined
« Reply #71 on: September 07, 2015, 10:49:57 am »

I think I understand now enough to step out of the "discussion"…

But I wonder if a, say, Sigma Art 50mm lens really has different COC and DOF when mounted on Canon, Nikon and Sigma 135 format bodies, which all have different mounting distances?

(sorry, just could not resist after all…)
Yes... In fact I've mentioned that a bit earlier (look above)... Nikon has a (very) little narrower DOF than Canon... most people would never notice as the mounting distance differs by only 2.5%... With 6x4.5 DSLRs where the mounting distance can differ up to 30% it is quite visible though and with 6X6 even more so... Try A P-6 mount (Exakta 66) lens on a DSLR and it becomes quite obvious (the 180/2.8 is a very good example - no wonder why many still consider it as the bokeh king)... Eric Weiss mentioned that he finds a three stops difference earlier from MF to his DSLRs... This may sound larger than it should to some, but don't forget that Eric uses Rollei which is a 6X6 camera...
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ErikKaffehr

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Re: Medium format redefined
« Reply #72 on: September 07, 2015, 11:45:09 am »


Hi,

There is a factor called pupil enlargement, but that is dependent lens design and not on mounting distance, per sé. Anyway, it is clearly second order.

So, as long as the lens has an identical design it will yield identical defocus and bokeh for different focusing distances.

Best regards
Erik

I think I understand now enough to step out of the "discussion"…

But I wonder if a, say, Sigma Art 50mm lens really has different COC and DOF when mounted on Canon, Nikon and Sigma 135 format bodies, which all have different mounting distances?

(sorry, just could not resist after all…)
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Erik Kaffehr
 

Theodoros

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Re: Medium format redefined
« Reply #73 on: September 07, 2015, 01:00:00 pm »

The entrance pupil position is another aspect I also mentioned, but although it does affect DOF, it makes a small difference... The main thing that can affect DOF by a considerable value is the COC...
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fdisilvestro

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Re: Medium format redefined
« Reply #74 on: September 07, 2015, 02:21:58 pm »

I think I understand now enough to step out of the "discussion"…

+1

ErikKaffehr

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Re: Medium format redefined
« Reply #75 on: September 09, 2015, 10:12:34 am »

Yeah, good point.

I think much of the discussion is about lack of objectivity much more than any demonstrable facts.

Best regards
Erik

I think I understand now enough to step out of the "discussion"…

But I wonder if a, say, Sigma Art 50mm lens really has different COC and DOF when mounted on Canon, Nikon and Sigma 135 format bodies, which all have different mounting distances?

(sorry, just could not resist after all…)
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Erik Kaffehr
 

Theodoros

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Re: Medium format redefined
« Reply #76 on: September 12, 2015, 05:01:57 am »

Hi,

There is a factor called pupil enlargement, but that is dependent lens design and not on mounting distance, per sé. Anyway, it is clearly second order.

So, as long as the lens has an identical design it will yield identical defocus and bokeh for different focusing distances.

Best regards
Erik


Naturally the universal lens makers like Sigma or Tamron, make their lenses to mount for a certain distance from the censor, but since the mounting distance between DSLRs differs by a little, one may notice that the lens frame is made longer for Canon than for a Nikon, this obviously means that extra length has been added on the frame to act as a spacer so that the (active) design of the lens is always at constant distantce from the sensor despite the different DSLR that it is mounted on. Hence the mounting distance stays constant and so does the COC and the DOF, but one may notice that a respective Canon lens and a Sigma lens made for Canon may have a (very) small difference between them as far as DOF is concerned....

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MatthewCromer

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Re: Medium format redefined
« Reply #77 on: September 12, 2015, 09:29:33 am »

The T/S lens can't be used as an example because it shares the same mounting distance... if the larger format lenses would mount at the same distance as smaller format lenses the COC value wouldn't change...

Mounting distance is irrelevant. It doesn't show up in the optical equations at all. Lenses focus rays to a particular distance, and one designs the mount to hold the glass elements at that distance. But the actual mount distance (flange focal length) is irrelevant, except that it means you cannot mount certain lenses on certain cameras.
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MatthewCromer

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Re: Medium format redefined
« Reply #78 on: September 12, 2015, 09:32:51 am »

Yes... In fact I've mentioned that a bit earlier (look above)... Nikon has a (very) little narrower DOF than Canon...

Uh, no.

You can use Nikon lenses on Canon cameras with a mount adapter, and they will show exactly the same DOF. Or, of course, use the same Sigma lens design on both - and both will have the same DOF.
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Theodoros

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Re: Medium format redefined
« Reply #79 on: September 12, 2015, 10:52:22 am »

Mounting distance is irrelevant. It doesn't show up in the optical equations at all. Lenses focus rays to a particular distance, and one designs the mount to hold the glass elements at that distance. But the actual mount distance (flange focal length) is irrelevant, except that it means you cannot mount certain lenses on certain cameras.
Matthew, both your previous comments are irrelevant to what I say and what I say is totally different. Mounting distance would be irrelevant only if the image circle was the same, it's the larger image circle of larger format lenses that causes the larger value of the COC and DOF is directly related to the COC value...

The mounting distance is not relevant to theory... it only happens that with DSLRs the larger the image circle, the bigger the mounting distance... So, a 6x6 lens mounts further away that 6x4.5 which in return the later mounts further away from the light sensitive area than a 35mm DSLR lens... As a consequence, because larger image area lenses have increased COC and thus reduced DOF, this reduced DOF is then (of course) retained no matter what (cropped) size image area is used behind the lens, but the lens still has to be mounted where it should be for the image area it was originally designed for... and that's why and adapter is used to cover the extra distance...

So as a conclusion, the mounting distance is relevant because all lenses that can be used via an adapter for smaller image areas, have been designed for larger ones, that have increased COC value and thus reduced DOF... which has the direct conclusion that DOF decreases with the mounting distance increase... Does this means that if one uses a 6x4.5 lens on a DSLR it will present reduced DOF for the same focal length and aperture value than another lens that was designed for this DSLR? Yes it does!  All that use larger format lenses via an adapter on their DSLRs too can confirm that! I personally use all the Contax 645 series of lenses (but the 350) on my Nikons via a JAS adapter...
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