For those good at calculations

Asha

Asha

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Asha
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What would be the approximate amount of dof ( ie front to back acceptable sharp focus) using an F/3 aperture, 250mm focal length and 10x8 inch sheet film ?

Édit, at à subject - lens distance of 2.5m
 
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Online Depth of Field Calculator

Depth of field and hyperfocal distance calculator
www.dofmaster.com

Is probably your best friend! Seems to suggest about 0.055m in front and behind the point of focus based on half way between f/2.8 and f/3,2, given your other figures. Might need a little adjustment because they might calculate everything on a film plane datum rather than a front element one.
 
Or, alternatively, cut out the middleman (i.e. the calculation) and knowing how big a print you're going to make use a loupe of that magnification to view the ground glass. This method does have the advantage that it will automatically take into account any lack of parallelism of the standards (either by accident or design).

Of course, being you, my assumption that the maximum size of print you're contemplating is no more than five feet by four feet is probably far too conservative. [Based on the 6x loupe being one in your possession.]
 
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StephenM said:
Or, alternatively, cut out the middleman (i.e. the calculation) and knowing how big a print you're going to make use a loupe of that magnification to view the ground glass. This method does have to advantage that it will automatically take into account any lack of parallelism of the standards (either by accident or design).
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That is such a useful insight, thank you
 
AndrewFlannigan said:
Total0' 3.06"77.8 mm
Near Limit6' 5.20"1.96 m
Far Limit6' 8.30"2.04 m
Infront Subj.0' 1.50"38.1 mm
Behind Subj.0' 1.56"39.7 mm
1/8th0' 0.38"9.73 mm
Hyperfocal Dist296'90.3 m
Circ. of Conf.0.217 mm [reset to def.]

From here: https://www.pointsinfocus.com/tools/depth-of-field-and-equivalent-lens-calculator/#{%22c%22:[{%22f%22:21,%22av%22:%223.2%22,%22fl%22:250,%22d%22:2000,%22cm%22:%220%22}],%22m%22:1}
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Thanks for that. I’ve never heard of points in focus.
A quick look around the website shows some pretty nifty tools / calculators .
The best thing being that they actually accommodate formats upto 10x8 in LF too.

Doesn’t appear at first glance that there is an app available ….. ill look closer tomorrow.
 
Had a go using the F-stop app on my iPhone. Added 250mm to the subject-lens distance as a rough guess for the focal plane distance. At f/2.8 it gives a near limit of 2.69 and a far limit of 2.81m. Changing to f/3.2 gives 2.68 and 2.82m.

EDIT, subtract 0.25 from those numbers for the lens distance, obvs!
 
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Looking back to your first post, the answer to your actual question seems to be a depth of field of around 8-12 cm. So pretty close to what you'd need to get from the tip of the nose to the ears in focus for a forward-facing portrait (if that's what you're thinking of)?

Thinking of my answer above, I realise that adding 250 mm to the lens-subject distance would be wrong. Presumably the film plane is 250mm behind the lens (nodal point, wherever that is) when focused at infinity. 2.5 mm seems really quite close for such a setup, so I'm guessing the lens to film plane distance would be significantly greater than 250mm. So not only does the film-subject distance increase, but the effective aperture decreases; both of these would have the effect of increasing the depth of field. But since I've no idea of the amount of extension at 2.5m, I can't comment much further!
 
Calculations are a wonderful thing, and I've many times intended to take the complete (in all its excruciating precision with its myriad variables) and apply partial differentiation to give a set of equations that allow the variation of DOF with the various parameters to be seen. That I've never done so is down not only to my inherent laziness, but also my inherent lack of confidence in the amount of effort people are really prepared to put in to get an answer that is both exact and of no (or very limited) practical use.

In the present case, we've seen that the actual amount of DOF is quite small. So, we're going to have to use a tape measure to see where the limits lie in our subject. Hang on, though - where exactly are we measuring from? The front of the lens? The film plane? A lens nodal plane (which one, and how on earth do we know whereabouts in the lens mount we should be measuring from)? Is the size of the circle of confusion appropriate for our intended degree of enlargement and viewing distance?

Hence, I tend to be very skeptical about the practical use of this type of calculation, except insofar as it gives a feel for approximate distances and quantities. The only exception I might make personally would be cases where an exact reproductiion ratio was required in technical work (which I don't do).
 
wornish said:
Way too much maths for me.
I prefer to look through the viewfinder and press the shutter button.
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Quantum mechanics can appear to be fearsomely mathematical. Probably because it is...

Back in the 1960s when I first started to study the subject, the textbooks had lots of equations on every page. Well, all except one. And that single solitary exception was written by a professor of mathematics. I've always suspected that because he actually really did understand the mathematics, he was happy to drop mathematics to give simple qualitative explanations that actually made it clear to the reader exactly what was really going on under all the equations that everyone else bandied around. He did use maths, and there were equations - but rather simpler than everyone else's.

I generally prefer plain English to equations where and when I can get away with it.
 
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Asha (like one or two odd people here) uses a view camera, which has a ground glass at the rear of the camera where the image projected by the lens can be viewed. No viewfinder in the conventions sense.

A ground glass is of course a glass used to store your teeth in overnight after you have ground them in frustration at the difficulties using a view camera has caused you...:)
 
StephenM said:
Asha (like one or two odd people here) uses a view camera, which has a ground glass at the rear of the camera where the image projected by the lens can be viewed. No viewfinder in the conventions sense.

A ground glass is of course a glass used to store your teeth in overnight after you have ground them in frustration at the difficulties using a view camera has caused you...:)
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Thanks for the explanation. Lesson learnt.:exit:
 
StephenM said:
Quantum mechanics can appear to be fearsomely mathematical. Probably because it is...
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One thing that I wonder about is: just how useful are the theories of quantum physics?

If something can only be proven using a system of mathematics so esoteric that few people can follow the argument and fewer still agree on the results, is it any more relevant than angels dancing on the head of a pin? :naughty:
 
I'll be brief as this is completely non-photographic.

The justification for quantum mechanical calculations lie in their giving the correct (i.e. in accord with experiment) results. Like any theory, the value lies in being able to make predictions, which if correct, tend to add weight to the theory. But in the end, it's all down to making a coherent linking of apparently unrelated facts and suggesting new experiments to expand our knowledge. If you can track down a copy, I recommend a later edition of Cotton and Wilkinson's Advanced Inorganic Chemistry for a reading of the preface as to why they removed much of the theory from later editions...

On the question of how many angels can fit on the head of a pin, despite the very bad press this question has received (I think mainly from people who didn't understand what the point (pin point? :)) of it was), it is actually a question which is totally reasonable once the actual basis of asking it has been understood. And I'm NOT going to get into a question of philosophy or scholastic theology. My own viewpoint is firmly that of Reformed theology and far removed from Roman Catholic views, but I can still appreciate some of the scholastic theology.
 
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StephenM said:
...it is actually a question which is totally reasonable once the actual basis of asking it has been understood.
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...which implies that you accept the existence of angels.
 
AndrewFlannigan said:
...which implies that you accept the existence of angels.
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My last word on this - actually, it doesn't. Not per se, anyway. That wasn't the actual point (no pun intended) of the question.

As far as this thread goes, I'm now going to stay on topic.
 
StephenM said:
As far as this thread goes, I'm now going to stay on topic.
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Always a good plan! At least it prevents the making of silly claims... :naughty:
 
AndrewFlannigan said:
....and which of those technologies cannot be described using basic binary arithmetic?
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Perhaps they could im unsure what you mean by binary arithmetic

However its probable we wouldn’t have the technology at all without the more exoctic maths that allowed us to discover and test so much.

id also argue everybody agrees on quantum mechanics. Where people disagree is how to bring newton and quantum together
 
If you've used a computer in the past 60 years, you're benefiting from quantum physics, AFAICR. All transistors and integrated circuits depend on quantum effects.
 
StephenM said:
In the present case, we've seen that the actual amount of DOF is quite small. So, we're going to have to use a tape measure to see where the limits lie in our subject. Hang on, though - where exactly are we measuring from? The front of the lens? The film plane? A lens nodal plane (which one, and how on earth do we know whereabouts in the lens mount we should be measuring from)? Is the size of the circle of confusion appropriate for our intended degree of enlargement and viewing distance?

Hence, I tend to be very skeptical about the practical use of this type of calculation, except insofar as it gives a feel for approximate distances and quantities. The only exception I might make personally would be cases where an exact reproduction ratio was required in technical work (which I don't do).
Click to expand...
You may be right in your exception, Stephen. The question Asha asked was about depth of field. I think the examples discussed above show that with various assumptions, Asha should have a pretty good idea of the depth of field. We don't know the actual application, so we don't know whether these approximations mean his application is feasible, marginal or infeasible. But I suspect he's better informed to make that decision now!
 
Thanks for all the feedback guys.

The calculations and methods of obtaining them are very helpful.(y)

:)
 
You know you can always count on F&C. Better than an abacus.
 
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AndrewFlannigan said:
...which implies that you accept the existence of angels.
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I've met a few and been to a couple of their parties. Not winged, other than their colours...
 
Nod said:
I've met a few and been to a couple of their parties. Not winged, other than their colours...
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Are you sure you're not confusing them with right little devils? :naughty:
 
Not many of them are little!
 
Depth of field is best estimated on the ground glass in large format.
A major problem, is that almost no field cameras are exactly in parallel.
And I have never found a monorail that was set up perfectly.
But surprisingly it does not seem to matter.
Even in medium format like a Rolleiflex, they soon get slightly out of parallel. One of the few that stays exact is a Minolta Autocord as the lens is mounted on a large helical threated focus tube mounted to the body.

With movements you can adjust the plane of focus very easily, and to distribute the depth of field as needed.
So the zero point of the set up is not critical.
 
Terrywoodenpic said:
One of the few that stays exact is a Minolta Autocord as the lens is mounted on a large helical threated focus tube mounted to the body.
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I've used Flexarette, Mamiya, Microcord, Rollei, Semflex and Yashica TLRs but I've never used a Minolta. I doubt I ever will, now, but I think I would have enjoyed it if I had.
 
AndrewFlannigan said:
I've used Flexarette, Mamiya, Microcord, Rollei, Semflex and Yashica TLRs but I've never used a Minolta. I doubt I ever will, now, but I think I would have enjoyed it if I had.
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The Auto cord was my favourite wedding camera. Mainly because the lever focussing was so much quicker than the knob on my Rollie's.
But it had another great advantage in the way the film tracked so as the right angle of the film was after not before the next fresh exposure. If a Rollie is left overnight with a film in. the second shot always had a slight kink in the middle of the frame. This was not always noticeable, but best practice was to take one and waste one.
As they say, not everyone knows that.
 
Terrywoodenpic said:
If a Rollie is left overnight with a film in. the second shot always had a slight kink in the middle of the frame.
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I left films in my Rolleis for several days if I wasn't busy and never noticed kinking.

The same applied to all the TLRs I used. Like the saying goes: your mileage may vary. :naughty:
 
AndrewFlannigan said:
I left films in my Rolleis for several days if I wasn't busy and never noticed kinking.

The same applied to all the TLRs I used. Like the saying goes: your mileage may vary. :naughty:
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Very few noticed it as it was usually within the Back depth of focus. But it was certainly measurable.
We once did a few tests on an undeveloped film and the kink was visible on the unrolled film after two days.
The pressure plate would have removed it at the edges of the frame but not in the middle.

I shot literally thousands of rolls on Rolleiflex cameras,, and as far as I am aware I never had a noticeable problem.
However I have no way of knowing which of my less than perfectly focussed shots may have been affected by this problem. What is certain is that is a real possibility, that is not so on Autocords.
 
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