# Thread: Image circle too large?

1. ## Re: Image circle too large?

Originally Posted by StoneNYC

Hyperfocal Distance
Setting focus at the Hyperfocal Distance gives maximum depth of field from H/2 to infinity.

H = (L x L) / (f x d)

Where:
H = Hyperfocal Distance (in millimeters)
L = lens focal length (ie, 35mm, 105mm)
f = lens aperture f-stop
d = diameter of circle of least confusion (in millimeters)
for 35mm format d = 0.03
for 6x6cm format d = 0.06
for 4x5in format d = 0.15
Hi,

One has to take in mind that the above is true only if the lens image circle is "just enough" as to cover the above mentioned image areas... The circle on confusion value (d) increases further with the image circle size... Therefore, there cases where one may try two different 180mm lenses on a (say) 4x5 image area and find that DOF of one is narrower that the other... The larger image circle lens will always have the narrower DOF.

2. ## Re: Image circle too large?

Originally Posted by Theodoros
Hi,

One has to take in mind that the above is true only if the lens image circle is "just enough" as to cover the above mentioned image areas... The circle on confusion value (d) increases further with the image circle size... Therefore, there cases where one may try two different 180mm lenses on a (say) 4x5 image area and find that DOF of one is narrower that the other... The larger image circle lens will always have the narrower DOF.
Depth of field of two different 180mm lenses will be identical if you use the same CoC for both of them.
The size of the CoC that you decide to use will vary only by the amount of magnification that you want to use for the,print and the distance that you will view the print from.

3. ## Re: Image circle too large?

Originally Posted by Bob Salomon
Depth of field of two different 180mm lenses will be identical if you use the same CoC for both of them.
The size of the CoC that you decide to use will vary only by the amount of magnification that you want to use for the,print and the distance that you will view the print from.
That would happen only if you print the image using an enlarger and crop into the area for the same AOV... If you digitize the neg at same magnification per area digitized (as >99.9999999% of people are doing) or if you shoot on a digital image sensor and print at the same dpi, the larger image circle lens will have narrower DOF....

4. ## Re: Image circle too large?

Teddy, might you be confusing blur circle, also called circle of confusion, an arbitrary number chosen by the photographer when calculating depth of field given magnification and relative aperture, with circle of least confusion, an attribute of a lens?

As Bob implicitly pointed out, neither has anything else to do with the circle a lens covers.

5. ## Re: Image circle too large?

Originally Posted by Dan Fromm
Teddy, might you be confusing blur circle, also called circle of confusion, an arbitrary number chosen by the photographer when calculating depth of field given magnification and relative aperture, with circle of least confusion, an attribute of a lens?

As Bob implicitly pointed out, neither has anything else to do with the circle a lens covers.
Think about it this way Dan... How can two lenses of the same focal length but their entrance pupil mounted at different distance from the image area have the same DOF? ...can they? ...Isn't the larger image circle lens always mounted further away? (provided the same type of camera and format of camera). Physics assumes constants, Engineering is how things work... Otherwise my Fuji GX680 210/5.6 would have the same DOF as my Contax 210/4 if both where stopped down to f8 and the same MFDB was used on them (hardly the case). Or my Nikkor 85/2 AIS would have (slightly) narrower DOF than my Contax 80/2 when mounted on my Nikons... But it's the opposite...

6. ## Re: Image circle too large?

just to compound the argument, would both your 210 lenses actually use the same aperture for say f8. I mean, if they are both to give the same exposure at f8 would the wider circle lens require a wider aperture (entrance pupil (or is it the exit pupil, I can never remember)). In other words is f8 tied to exposure or to ratio of focal length. Do the lens manufacturers modify it to maintain consistant exposure across lenses or not.
Or to put it another way, is f8 on both lenses really f8 as we would normally understand it. i.e. the ratio of aperture to focal length?

I'm thinking that if you spread the image over a wider area then the inverse square law sayas it will be less bright and therefore would require more exposure. So how is that handled? Byu adjusting aperture to be something different than the ratio of aperture to focal length? If that is happening then you are not comparing f8 with f8 in which case you would get different dof for each lens. If it is not happening then you would required different exposure time with each lens.

7. ## Re: Image circle too large?

Originally Posted by Argentum
just to compound the argument, would both your 210 lenses actually use the same aperture for say f8. I mean, if they are both to give the same exposure at f8 would the wider circle lens require a wider aperture (entrance pupil (or is it the exit pupil, I can never remember)). In other words is f8 tied to exposure or to ratio of focal length. Do the lens manufacturers modify it to maintain consistant exposure across lenses or not.
Or to put it another way, is f8 on both lenses really f8 as we would normally understand it. i.e. the ratio of aperture to focal length?
I think your contribution really helps the argument here... You do have two pipes with a "hole" of the same diameter which both "throw" some liquid on a surface... forget about gravity, the two pipes though, are of different dinstance from the surface... will the mark on the surface be of the same radius when the liquid hits it? Yet the amount of liquid (the exposure) will be the same... Simple ain't it?

8. ## Re: Image circle too large?

I don't think it will. If the flow rate is constant it will require longer for the same volume of water to hit any single point the further the pipe is away since the water will be spread more thinly. i.e. a longer exposure time would be required.
So it comes back to my question, is aperture setting on a lens modified to link to eposure or is it maintained as a ratio of focal length?

9. ## Re: Image circle too large?

Originally Posted by Argentum
I don't think it will. If the flow rate is constant it will require longer for the same volume of water to hit any single point the further the pipe is away since the water will be spread more thinly. i.e. a longer exposure time would be required.
So it comes back to my question, is aperture setting on a lens modified to link to eposure or is it maintained as a ratio of focal length?
No it won't... the rate depends on the pipe diameter, which is the same in both cases, if you turn off the supply (terminate the exposure) at the same time, the supply will be the same.

10. ## Re: Image circle too large?

If they are the same and one is spread over a greater area then the exposures will be different. Are they? Simple to test with a digital back.

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