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Barney

Barney

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I am not sure If I am confusing myself here, but here goes anyway,

Reading about compression and 35mm equivalencies it occurred to me that I may possibly be making fundamental mistakes in exposure calculations, please bear with me.

35mm format with 35mm lens at f1.4 is, by my possibly imperfect understanding, equivalent to MF 70mm f2.8, or thereabouts depending on actual film size.

MY dilemma is as follows, If my incident meter reading shows a reading of iso 100 @ f16 for 100th second which format is the reading for? Do i need to make any adjustments for MF exposure or is the relativity of the F stop purely a compression/DOF consideration?

It the often referred to "rule" to shoot a stop under box, an attempt to mitigate the deficiencies in my exposure calculations due to errors in the lenses and exposure meters calculations?

I have read that a couple of times and am not sure if my thoughts have translated adequately to the question,
 
As above, exposure is exposure, regardless of format (ignoring things like bellows compensation on LF).

The negative is simply larger, with the same amount of 'in focus' portion according to the aperture used, which is why a 6x6 medium format image will have a larger out of focus ('bokeh') area than an equivalent image shot on 35mm film, as the image circle is projected onto a larger area. The same is seen when comparing a medium format negative to a large format negative, for example.
 
A very simplistic diagram but this shows a 35mm negative next to a 6x6 negative, both with the same central area of 'in-focus' frame (in green), as dictated by aperture/focal length;

1759305593856.png

The out of focus portion of the negative is considerably larger on the 6x6 negative, meaning that our eye sees a much shallower depth of field as a whole when comparing the in-focus and out-of-focus areas, even though the actual portion of the frame in focus is the same size.
 
Barney said:
I am not sure If I am confusing myself here, but here goes anyway,

Reading about compression and 35mm equivalencies it occurred to me that I may possibly be making fundamental mistakes in exposure calculations, please bear with me.

35mm format with 35mm lens at f1.4 is, by my possibly imperfect understanding, equivalent to MF 70mm f2.8, or thereabouts depending on actual film size.

MY dilemma is as follows, If my incident meter reading shows a reading of iso 100 @ f16 for 100th second which format is the reading for? Do i need to make any adjustments for MF exposure or is the relativity of the F stop purely a compression/DOF consideration?

It the often referred to "rule" to shoot a stop under box, an attempt to mitigate the deficiencies in my exposure calculations due to errors in the lenses and exposure meters calculations?

I have read that a couple of times and am not sure if my thoughts have translated adequately to the question,
Click to expand...
f stop values take into account focal length so f/1.4 is f/1.4 whatever lens or format you are using. That's why f stops are called that - because it's a ratio of the focal length.
 
Two different topics in play here!

First, exposure. f numbers are expressed as f/2 etc for a reason - they are fractions. The diameter of the lens opening to the focal length in fact. f/2 on a 100mm lens has an opening diameter of 50mm; f/2 on a 50mm lens has an opening 25mm wide. Why? Because of two laws of physics.

The amount of light passing through a hole - like the amount of water through a pipe - becomes greater as the diameter increases. The increase depends on the area though, and the area is proportional to the square of the diameter. The second law is the inverse square law. Double the distance a torch beam travels (torch because you can perform the experiment at home) and the diameter of the beam on the wall doubles. Which means the area quadruples; and as extra light isn't attracted by gravity into the beam of the torch as it crosses the room (nobody split hair here about gravitational effects!) this means that the brightness has actually not halved but quartered. That's the inverse square law.

The second part of this is that to focus on infinity, a 100mm lens has to be 100mm away from the film plane. A 50mm lens only 50mm. So the amount of light passed for a given lens opening has quartered at each image point. So we have to let 4 times as much light in by doubling the opening. Which is why f numbers are defined in terms of fractions (or ratios if you're continental).

Depth of field is a separate topic. A perfect lens reproduces a point in the object as a point in the image. Unfortunately, there is no such things as a perfect lens. And even if there was, physics has another trick up its sleeve to prevent imaging perfection. Diffraction. Light is a wave motion, and as a result the image produced by a perfect lens isn't a point but a disc. Look up Airey disk. So we never have a point, always a (small) circle. If the eye accepts it as a point. all well and good - it looks sharp. With a perfect lens, the image rays from the focus point are brought to a point at the film. The image from closer focuses behind the film (or would, if it got through) and so hits the film while still converging, giving a circle not a point. Light from further away than the focused plane meets in front of the film, so is diverging again when it hits the film. Another circle, not point.

As it's all down to what the eye will accept, depth of field will depend on things outside the pure physics of this explanation. First, how good is your eyesight? Second, how closely are you examining the image from? And how much have you enlarged the image? All of these subjective factors are combined with the maths to produce depth of field tables.

The interesting point here is that for the same size of print a 35mm negative is enlarged more than a medium format one. A longer focal length lens will have shallower depth of field, so a medium format negative taken with a standard lens (let's say from a distance that with that lens shows a half length portrait) would exhibit shallower depth of field than a 35mm image with a standard lens taken from the same position. But the 35mm image would contain more extraneous matter, and to arrive at an identical image size in the print would need enlarging to not only use only the central part, but also to match the print size from the larger negative. What effect does this have?

Someone asked this as a question to Victor Blackman in the 1960s, and he devoted a column to answering it. The punchline was a pair of photos, one from medium format and one from 35mm taken from the same position of the same seated person. Enlarged to show the upper body at the same size, the depth of field appeared to be actually the same - the extra magnification just about canceled out the shallower depth of field.

I haven't checked the equations, and I haven't repeated the experiment, but it is worth considering...
 
Monopix said:
"The out of focus portion of the negative is considerably larger on the 6x6 negative, meaning that our eye sees a much shallower depth of field "

That is just so wrong. The DOF we see has nothing to do with that.

Try reading this:https://lenspire.zeiss.com/photo/app/uploads/2022/02/technical-article-depth-of-field-and-bokeh.pdf
Click to expand...
“A very simplistic diagram…”

Maybe you could summarise the 45 page very technical document into an explanation to help the OP with their question instead of just dropping a link?
 
stevelmx5 said:
As above, exposure is exposure, regardless of format (ignoring things like bellows compensation on LF).

The negative is simply larger, with the same amount of 'in focus' portion according to the aperture used, which is why a 6x6 medium format image will have a larger out of focus ('bokeh') area than an equivalent image shot on 35mm film, as the image circle is projected onto a larger area. The same is seen when comparing a medium format negative to a large format negative, for example.
Click to expand...
Bellows compensation is needed in all formats, but it's usually only for macro.
 
Barney said:
I am not sure If I am confusing myself here, but here goes anyway,

Reading about compression and 35mm equivalencies it occurred to me that I may possibly be making fundamental mistakes in exposure calculations, please bear with me.

35mm format with 35mm lens at f1.4 is, by my possibly imperfect understanding, equivalent to MF 70mm f2.8, or thereabouts depending on actual film size.
Click to expand...
FOV multipliers (Crop factor) for medium format depend on which medium format you are referring to.common options are 645, 6x6 & 6x7 but I also have cameras that take 6x9 shots and there are wider options.

MF is also a dodgy abbreviation to use, as it also applies to manual focus.
 
stevelmx5 said:
“A very simplistic diagram…”

Maybe you could summarise the 45 page very technical document into an explanation to help the OP with their question instead of just dropping a link?
Click to expand...
The document was meant for you. The OP wasn't asking about DOF.
 
Monopix said:
The document was meant for you. The OP wasn't asking about DOF.
Click to expand...
Barney said:
Do i need to make any adjustments for MF exposure or is the relativity of the F stop purely a compression/DOF consideration?
Click to expand...

Thanks for you effort to educate me, but the OP was asking about the general comparison between 135/medium format with regards to exposure/compression/DOF/aperture, hence my generalised basic comparison of how our eyes/brain perceive a difference.
 
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A big thankyou to all who have tried to help me understand whats going on here. I will have to read the thread again later and attempt to digest the information provided.
Every little bit helps and many apologies if my post is causing consternation.

As I outlined, my first post was likely from a point of confusion.
 
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