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  #16  
Old 7th August 2019
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Re: F Stop.............!!

Quote:
Originally Posted by royd63uk View Post
Hi
So having just purchased an Olympus om-d e-m1 mkii, with thoughts of changing to MFT, I am now wondering if it was the right move.
I have a Canon EOS R and do not seem to have this issue with DOF, the smaller the aperture the better dof and detail, as far as I can see.
Am I confused here?
Roy
Diffraction affects all lenses when they are stopped down too far. With FF you can stop down more before the effects are apparent.

But remember you don't need to stop down as much on (Micro) Four Thirds to achieve the desired depth of field.

It's simply a matter of scale.

Also, in the old days there was often a necessity to stop down in order to achieve optimum lens sharpness, but modern lenses are now very sharp wide open.

Ian
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  #17  
Old 7th August 2019
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Re: F Stop.............!!

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But remember you don't need to stop down as much on (Micro) Four Thirds to achieve the desired depth of field.
Thats was one of the questions i was going to ask.

So one of my favourite f stops on full frame was F8, so would 5.6 on MFT work the same?
Roy
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  #18  
Old 7th August 2019
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Re: F Stop.............!!

The only thing that works the same as f/8 on "full frame" is f/8 on full frame The whole thing is a combination of focal length, sensor size and aperture, they all work together. As a very rough guide, I think the idea is that if you want to match a 100mm lens at f/8 on FF, you can use a 50mm lens at f/4 on MFT. This will give roughly the same field of view and depth of field. But even that is approximation, the best bet is to use it as a starting point and see how it works in your circumstances.

John

Last edited by Bikie John; 7th August 2019 at 04:24 PM. Reason: Add formatting
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  #19  
Old 7th August 2019
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Re: F Stop.............!!

Quote:
Originally Posted by royd63uk View Post
Thats was one of the questions i was going to ask.

So one of my favourite f stops on full frame was F8, so would 5.6 on MFT work the same?
Roy
You basically halve the number so f/4 is equivalent to f/8.

A very handy benefit is that you get greater brightness for the same DOF and so the benefit of faster shutter speed and/or lower ISO options.

What I mean is that the brightness at f/8 is the same regardless of sensor size/format (given the same subject).

Ian
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  #20  
Old 11th August 2019
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Naughty Nigel Naughty Nigel is offline
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Re: F Stop.............!!

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Originally Posted by pdk42 View Post
Diffraction can be numerically estimated using the Rayleigh criterion for the size of the Airy disk. For closely-spaced objects this approximates to:

x = 2.44 L N

where:

x = size of Airy disk diameter
L = wavelength of light
N = f-stop

Plugging this in for green light (in the middle) and f7.1, we get:

x = 0.009mm

This is still much smaller than the usual circle of confusion used for DoF calculations (0.015mm). In other words, at f7.1, diffraction is not a significant factor.

We don't get an Airy disk larger than the circle of confusion until about f11.

In other words, up to f11 you're fine in practical terms, although if you nit-pick, keep it at f5.6 and below.
That's useful information Paul, thank you.

Does the above apply equally to all focal lengths? And does diffraction effect film in the same way that it effects digital sensors? (Obviously film provides a bigger 'sensor' in most cases.)
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  #21  
Old 12th August 2019
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Re: F Stop.............!!

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Originally Posted by Naughty Nigel View Post
That's useful information Paul, thank you.

Does the above apply equally to all focal lengths? And does diffraction effect film in the same way that it effects digital sensors? (Obviously film provides a bigger 'sensor' in most cases.)
Film is different because there are no photosites on the surface of film emulsion.

I can't if there is a comparable effect but it's certainly not the same.

Ian
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  #22  
Old 12th August 2019
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Re: F Stop.............!!

Quote:
Originally Posted by Ian View Post
Film is different because there are no photosites on the surface of film emulsion.

I can't if there is a comparable effect but it's certainly not the same.

Ian
I may be wrong, but a film is less likely to suffer from diffraction as the emulsion grains have an element of randomness to them, so the diffraction patterns that can be seen are less likely to occur. A sensor is very regular and precise in its construction so diffraction patterns are more likely.

Or something?
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  #23  
Old 12th August 2019
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Re: F Stop.............!!

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Originally Posted by Walti View Post
I may be wrong, but a film is less likely to suffer from diffraction as the emulsion grains have an element of randomness to them, so the diffraction patterns that can be seen are less likely to occur. A sensor is very regular and precise in its construction so diffraction patterns are more likely.

Or something?
I'm sure that's a factor but the primary one is the size of the photosites (basically holes in the surface of the sensor) on the sensor surface in relation to the airy discs.

Ian
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  #24  
Old 12th August 2019
Harold Gough Harold Gough is offline
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Re: F Stop.............!!

Quote:
Originally Posted by MJ224 View Post
I think I am a bit still in the old school where the F stop controlled the DoF as much as anything....
I suspect that a lot of current lenses were designed to give optimum performance at the wider apertures.

In film days, lenses were designed to be best at around f8 or f11. My Kiron 105 has the f16 marked in blue and the handbook says to use it for macro.

My default aperture for macro at all magnifications is f11 but I may use f8 for a shallow subject. At the higher magnifications I remove diffraction digitally.

Harold
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