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### AuthorTopic: Medium format redefined  (Read 65607 times)

#### Petrus

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##### Re: Medium format redefined
« Reply #80 on: September 12, 2015, 01:35:05 pm »

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!

How come DOF calculation formulas do not take this into account at all? They only need focal length, relative aperture, distance and film/sensor format to calculate DOF.
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#### razrblck

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##### Re: Medium format redefined
« Reply #81 on: September 12, 2015, 02:16:33 pm »

How come DOF calculation formulas do not take this into account at all? They only need focal length, relative aperture, distance and film/sensor format to calculate DOF.

They do.

f = Focal length
s = Subject distance (or focus distance)
N = Lens f-number
c = COC

You put the numbers in and get a pretty accurate result. It's not going to be perfect because there are always design, engineering and operating errors at play in the real world, but this is the complete formula to calculate how big will your depth of field be at a given focusing distance.

The formula changes a bit for very close distances (macro).

P.S. COC is calculated separately, and as stated above by others it depends on various factors. It's not directly related to sensor size, but some systems may use that as a quick way to calculate COC based on standard values, thus having a single factor for the main sensor formats for quick selection and calculations. It's not going to be accurate for all cameras with a certain sensor size, but it's a simplification that is usually close enough. Maybe that is why you think they don't take other factors into account.
« Last Edit: September 12, 2015, 02:19:58 pm by razrblck »
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#### Theodoros

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##### Re: Medium format redefined
« Reply #82 on: September 12, 2015, 03:06:38 pm »

How come DOF calculation formulas do not take this into account at all? They only need focal length, relative aperture, distance and film/sensor format to calculate DOF.
They do take COC into account, in fact the existance of COC is why DOF exists, so it couldn't be otherwise... But I thought you have jumped out of the conversation, which is a very good thing to do instead of you (and some others) misinforming people...

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#### Petrus

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##### Re: Medium format redefined
« Reply #83 on: September 12, 2015, 03:33:15 pm »

I still do not see the mount distance in the calculations. This is getting weird. Focal length: yes. Image circle size: never.

If I put a lens shade which is too long on a lens, it will make the image circle smaller. Is the DOF going to change?
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#### Theodoros

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##### Re: Medium format redefined
« Reply #84 on: September 12, 2015, 04:21:48 pm »

I still do not see the mount distance in the calculations. This is getting weird. Focal length: yes. Image circle size: never.

It's because you don't want to see it... It has been explained to you (and others) at elementary school levels many times above... Once more then... To have larger COC value (which directly affects DOF) the lens must be able to project a larger image area... But on DSLRs (of what ever format) for this to happen the mounting distance increases respectively... I hope now that it has gone below elementary  school level you (and some "usual" others) get it...

EDIT: It's like using a 24x36mm old MFDB on an MF camera, if you use a (say) 180/2.8 lens on it at full aperture, you have "thinner" DOF than using a same focal length and speed lens on a Nikon or a Canon.... Why? ...Because COC value has changed! ...simple isn't it? (real elementary school level)
« Last Edit: September 12, 2015, 04:38:21 pm by Theodoros »
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#### Petrus

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##### Re: Medium format redefined
« Reply #85 on: September 12, 2015, 04:35:59 pm »

To have larger COC value (which directly affects DOF) the lens must be able to project a larger image area.

Lenses do not have COC values.
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#### Theodoros

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##### Re: Medium format redefined
« Reply #86 on: September 12, 2015, 04:44:28 pm »

Lenses do not have COC values.
Size has a value (so that it can be measured)... but I think this is assumed even at elementary schools and thus it is not taught...
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#### ErikKaffehr

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##### Re: Medium format redefined
« Reply #87 on: September 12, 2015, 04:59:51 pm »

Hi,

CoC, as used in DoF calculations depends only on viewing distance and magnification. It is the largest disk of unsharpness that seems acceptable to the eye at a given viewing distance.

But, the sensor image is normally much smaller than any image viewed at a realistic distance, so the CoC value needs to be multiplied the image magnification.

Say we want to have an 7x5" print viewed at 10" and let's assume that it needs a CoC of 0.15 mm to appea sharp. Now, a 24x36 image would need to be enlarged around 5 times for 5"x7", so we would need a CoC of 0.15 / 5 = 0.03 mm for a "sharp image".

Now, assume that we use a P45+ sensor instead which is 49x37 mm. Looking at the 37 mm dimension we would need around 3.5x magnification, so the needed CoC on the P45+ would be 0.15/3.5 around 0.43 mm.

This has nothing to do with flange distance, just image scaling and human vision.

Now, with a larger sensor we need a longer lens for same field of view. The longer lens will give a shorter DoF at similar aperture.

On the other hand, short flange mounting distances allow for more symmetric lenses, which have less aperture enlargement. But aperture enlargement plays a small role in DoF calculations.

So neither flange distance or CoC are really related to lenses and both are red herrings in this context.

Best regards
Erik

Lenses do not have COC values.
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#### Bart_van_der_Wolf

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##### Re: Medium format redefined
« Reply #88 on: September 12, 2015, 05:16:14 pm »

They do.

f = Focal length
s = Subject distance (or focus distance)
N = Lens f-number
c = COC

You put the numbers in and get a pretty accurate result. It's not going to be perfect because there are always design, engineering and operating errors at play in the real world, but this is the complete formula to calculate how big will your depth of field be at a given focusing distance.

The formula changes a bit for very close distances (macro).

P.S. COC is calculated separately, and as stated above by others it depends on various factors. It's not directly related to sensor size, but some systems may use that as a quick way to calculate COC based on standard values, thus having a single factor for the main sensor formats for quick selection and calculations. It's not going to be accurate for all cameras with a certain sensor size, but it's a simplification that is usually close enough. Maybe that is why you think they don't take other factors into account.

That's essentially correct. It's only about focal length, (effective) aperture, subject distance (or magnification factor), and COC (fixed value determined by viewing constraints). Pupil factor can be added to the equation for increased accuracy with asymmetrical lens designs. Image circle doesn't play a role as long as it's larger than the sensor.

Cheers,
Bart
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#### Bart_van_der_Wolf

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##### Re: Medium format redefined
« Reply #89 on: September 12, 2015, 05:20:06 pm »

Lenses do not have COC values.

Correct. COC is determined by viewing distance and acceptable amount of magnified blur at that distance.

Cheers,
Bart
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#### fdisilvestro

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##### Re: Medium format redefined
« Reply #90 on: September 12, 2015, 05:38:32 pm »

Please, CoC is not a structural characteristic of a lens, it is the maximum amount of defocusing or unsharpness that will be indistinguishable from a perfectly focused object, based on OUTPUT (will I be able to stress this enough?) size and viewing distance. It is a concept based on the angular resolving power of the human eye. IT HAS NOTING TO DO WITH IMAGE CIRCLE.

Now, in relation to mounting distance, I think there is an indirect relation between it and DOF, but the real factor is pupil magnification. I will try to explain

Pupil is referred to the apparent size of the aperture. Entrance pupil is viewed from the front, exit is viewed from the back of the lens
Pupil magnification is the ratio between exit pupil and entrance pupil

Think of three common lens designs:
- Symmetrical - used typically in normal lenses and most large format lenses, the pupil magnification is 1
- Retro-focus - Typical of wide angle lenses, to allow greater distance from the sensor/film plane and clear the mirror box in DSLR/SLR , pupil magnification is greater than 1
- Tele - Allow for a lens that is shorter than its focal length and so it can be mounted closed to the sensor plane and pupil magnification is less than 1

What is the effect of this?

The Pupil magnification is a factor that does affect DOF and it is included in the complete equations, but it is usually discarded because its effects are only relevant at close focusing distances

The effect of pupil magnification is Inverse to DOF, which means, the greater the pupil magnification (as in retrofocus designs) the smaller the DOF. Since the retrofocus lens will have a greater mounting distance than a symmetrical lens, you can erroneously assume that the change in DOF is due to mounting distance, but it is really due to pupil magnification.

Another effect at close focusing distances: Lens focal length, it turns out that for a given field of view, the longer the lens, the shorter the DOF for the same entrance pupil diameter.

What to consider as a close focusing distance? A rough approximation is 10 times the focal length.

#### Theodoros

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##### Re: Medium format redefined
« Reply #91 on: September 12, 2015, 05:39:55 pm »

Lenses do not have COC values.
Correct... none ever said that they do! The COC has a (measurable) size though, depending on the viewing distance... Still the size is determined with respect to the whole projected image area... "value" is a number used in the equation that razrbick posted... Now we can all play with words to "cover" Petrus trolling all this time...
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#### Theodoros

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##### Re: Medium format redefined
« Reply #92 on: September 12, 2015, 05:48:39 pm »

Please, CoC is not a structural characteristic of a lens, it is the maximum amount of defocusing or unsharpness that will be indistinguishable from a perfectly focused object, based on OUTPUT (will I be able to stress this enough?) size and viewing distance. It is a concept based on the angular resolving power of the human eye. IT HAS NOTING TO DO WITH IMAGE CIRCLE.

Now, in relation to mounting distance, I think there is an indirect relation between it and DOF, but the real factor is pupil magnification. I will try to explain

Pupil is referred to the apparent size of the aperture. Entrance pupil is viewed from the front, exit is viewed from the back of the lens
Pupil magnification is the ratio between exit pupil and entrance pupil

Think of three common lens designs:
- Symmetrical - used typically in normal lenses and most large format lenses, the pupil magnification is 1
- Retro-focus - Typical of wide angle lenses, to allow greater distance from the sensor/film plane and clear the mirror box in DSLR/SLR , pupil magnification is greater than 1
- Tele - Allow for a lens that is shorter than its focal length and so it can be mounted closed to the sensor plane and pupil magnification is less than 1

What is the effect of this?

The Pupil magnification is a factor that does affect DOF and it is included in the complete equations, but it is usually discarded because its effects are only relevant at close focusing distances

The effect of pupil magnification is Inverse to DOF, which means, the greater the pupil magnification (as in retrofocus designs) the smaller the DOF. Since the retrofocus lens will have a greater mounting distance than a symmetrical lens, you can erroneously assume that the change in DOF is due to mounting distance, but it is really due to pupil magnification.

Another effect at close focusing distances: Lens focal length, it turns out that for a given field of view, the longer the lens, the shorter the DOF for the same entrance pupil diameter.

What to consider as a close focusing distance? A rough approximation is 10 times the focal length.

Does this (irrelevant) mean that your calculation that you posted earlier is correct? ...In other words, can DOF differ more than two stops between a FF DSLR and an MF camera for the same AOV or not? ....Can the difference be up to three stops (as Eric Weiss posted earlier) or not? ...by the way... wasn't you (as well) leaving the conversation earlier?
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#### Theodoros

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##### Re: Medium format redefined
« Reply #93 on: September 12, 2015, 05:52:51 pm »

I propose to some people, to do some more (homework) reading and come back next week to play the "Einstein"...
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#### fdisilvestro

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##### Re: Medium format redefined
« Reply #94 on: September 12, 2015, 06:04:35 pm »

Does this (irrelevant) mean that your calculation that you posted earlier is correct? ...In other words, can DOF differ more than two stops between a FF DSLR and an MF camera for the same AOV or not? ....Can the difference be up to three stops (as Eric Weiss posted earlier) or not? ...by the way... wasn't you (as well) leaving the conversation earlier?

Yes, it can be more than two stops at close focusing distances. The equation I posted earlier is the simplified version that works for distances greater than 10 X focal length (which is correct). I have no issue in reviewing and correcting myself if proven wrong.

I really don't care what you (Theodoros) think, my concern is with the vast audience that uses LuLa as reference, that might get the wrong concepts and ideas about this subject

#### Theodoros

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##### Re: Medium format redefined
« Reply #95 on: September 12, 2015, 06:14:18 pm »

Yes, it can be more than two stops at close focusing distances. The equation I posted earlier is the simplified version that works for distances greater than 10 X focal length (which is correct). I have no issue in reviewing and correcting myself if proven wrong.

I really don't care what you (Theodoros) think, my concern is with the vast audience that uses LuLa as reference, that might get the wrong concepts and ideas about this subject
This (your later comment above) is certainly better than supporting any troll earlier with a "+1" comment...

EDIT: By the way... can you define "close focus distances"? I ask this because if I'm not mistaken, where shallow DOF maters, it's always "close focus distances"... I mean, who cares if DOF covers infinity at a long distance landscape anyway?
« Last Edit: September 12, 2015, 06:26:57 pm by Theodoros »
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#### ErikKaffehr

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##### Re: Medium format redefined
« Reply #96 on: September 13, 2015, 12:05:01 am »

Hi,

Petrus has indicated that pupil magnification stated to be significant at around 10x focal length.

But, that is not really relevant as macro lenses are normally used at closes distances and those used to be quite symmetric designs. As macro lenses are often on the longer side, there is little need to inverted telephoto designs. Indeed, most of the classic macro lenses are symmetric double gauss designs, perhaps combined with a field flattening group and variable airspace.

Modern 135 macro lenses can be quite complex designs, however, so they may be a bit different.

When Eric Hiss talks about 3 stops of difference in DoF he compares an 80 mp back to a 36 MP 135 DSLR. Not clear from his posting if he compares prints of similar size or actual pixels on screen. This is an example of publishing the findings but not disclosing the methods can give confusing results.

Best regards
Erik

PS

Just to say, some one sharing insights and knowledge isn't exactly what I would call trolling.

This (your later comment above) is certainly better than supporting any troll earlier with a "+1" comment...

EDIT: By the way... can you define "close focus distances"? I ask this because if I'm not mistaken, where shallow DOF maters, it's always "close focus distances"... I mean, who cares if DOF covers infinity at a long distance landscape anyway?
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#### Petrus

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##### Re: Medium format redefined
« Reply #97 on: September 13, 2015, 12:12:56 am »

It is all in the equations. Einstein, who was mentioned earlier, played around with his equations, and to his surprise, E=mc2 popped out. As the calculations were correct, this result had to be taken at the face value, and it has held ever since.

There is no way to squeeze the size of the lens image circle (or mounting/flange distance, which just a mechanical engineering decision, not optical) out of any DOF equation known to man.

That is all there is to it.
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#### synn

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##### Re: Medium format redefined
« Reply #98 on: September 13, 2015, 01:20:56 am »

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#### EricWHiss

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##### Re: Medium format redefined
« Reply #99 on: September 13, 2015, 03:08:49 am »

When Eric Hiss talks about 3 stops of difference in DoF he compares an 80 mp back to a 36 MP 135 DSLR. Not clear from his posting if he compares prints of similar size or actual pixels on screen. This is an example of publishing the findings but not disclosing the methods can give confusing results.

Yes quite right. When Eric Hiss posts about what he found by actually testing in real life with real cameras and real models in the studio,  he knows that people who are doing same will find the information useful and carry on,  and conversely those who enjoy to spend time in forum discussion will continue on in an endless circle endlessly anyhow.  Eric Hiss also understands the difference between publishing a paper in a respected academic environment with peer review  and posting a few empirical observations to a forum full of old men who have nothing better to do than pick at each other.
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