Image circle too large?

[mingliaozi]

Hello guys and girls,

I am just getting started in large format photography, I bought and assembled my own "Bulldog" LFC and recently I bought a Fujion 180mm/5.6f lens of ebay (Image circle was 280phi according to seller). Could someone tell me, how many milimeters that is?

And most importantly, it seems that I have to move the lens very far away from the camera, before I get a sharp image (at infinity). This point is almost the outer limit of my forward extension. Now, is this a normal phenomenon? Is there a formula that, given a focal size and image circle, tells you which distances from the lens to the focusing screen make an object which is x meters away from the camera look sharp?

Thank you very much
Kind regards!

[Jon Shiu]

Focal length of 180mm indicates approximate distance from center of the lens to image plane at infinity focus. For closer objects, the distance would increase. For example at 1:1 close-up, distance between lens and ground glass would be about twice, ie 360mm.

Jon

[Noah A]

The lens focal length is what determines how far from the film your lens will be, not the image circle. As Jon said, the bellows extension when focused at infinity is very close to the lens focal length in MM. I don't know how much extension your camera offers, but you should have around 180mm of extension when focused at infinity.

If the distance is much longer than that, do you have a front and rear cell, one in front of the shutter and one behind the shutter and lens board? Some lenses will effectively give a longer focal length without the rear lens cell.

The image circle does not affect how much extension you need. The image circle is the size of the image that the lens projects onto the film plane. A larger image circle will allow you to use more camera or lens movements (rise and fall, etc.).

Not to confuse the issue, but lens extension does affect image circle. As you focus closer, the lens will be further from the film plane. This means the lens will project a larger circle onto the film plane. That's why image circle specifications are usually quoted for infinity focus.

[Argentum]

didn't bulldog make 4x5 and a 10x8 diy build cameras? Which one have you got and what are the specs for it. The spec should tell you the minimum lens extension and the maximum lens extension.

The minimum lens extension will be close to shortest focal length lens that you can use on that camera. Any shorter than that and you wouldn't be able to focus at infinity.
The maximum lens extension is just that. i.e. as far as you can extend the lens for close up focus.

Just as an example, If your minimum lens extension was 150mm and your maximum lens extension was 300mm then the shortest lens you could use would be a 150mm lens. At infinity focus it would be all the way back and for a close up doing a 1:1 image it would be extended all the way to 300mm. (you could use a shorter focal length lens but only if its image circle is big enough and you didn't need to focus at infinity).

The smallest image circle to cover 4x5 at infinity is approx 160mm and for 8x10 its 325mm

Easiest if you tell us what the minimum and maximum lens extension is on your camera and whether its 4x5 or 8x10 and then suggestions on which lens is good for it will be forthcoming... And also what the capabilities of you 180 lens will be on your camera.

Also tell us what it is tyou are trying to photograph and close or far away it is. i.e. is it a landscape, a portrait of someone fairly near or a still life of something very close etc.

[EdWorkman]

Yes there is lots of information about image circles, flange to film plane distance at infinity and the amount of movement of the lens required to focus at a given film to subject distance.
A hearty welcome to LF photography
And
I appears you have a lot of homework to do to get into the basics
There are tutorials on this site and references.
Go to a recent post by Dan Fromm to get a reference to a site that contains a ton of information about Fuji lenses, and many other lens sites.
There are at least hundreds of books/pamphlets that can be downloaded for free from say google books and Internet Archive.
A lot of them will be 100 years old, or so, due to copyright laws, but the basic principles of optics haven't changed.
There are plenty of economical lenses in the 150 mm focal length- normal for 4x5.
Get one and practice focusing-the shorter focal length should be an easier fit for your camera's extensability.
Make yourself aware of "depth of Focus, then you can discover how it changes with aperture and focal length
If part of your question was to ask how to convert inches to millimeters, one inch is 25.4 mm Thus a 'normal' lens for 4x5 is about 6 inches [do the arithmetic and you will see it's a little over].
If "normal lens" is not a familiar term, please start your homework.
Half the fun is the learning journey
regards
ED

[Bob Salomon]

The lens focal length is what determines how far from the film your lens will be, not the image circle. As Jon said, the bellows extension when focused at infinity is very close to the lens focal length in MM. I don't know how much extension your camera offers, but you should have around 180mm of extension when focused at infinity.

If the distance is much longer than that, do you have a front and rear cell, one in front of the shutter and one behind the shutter and lens board? Some lenses will effectively give a longer focal length without the rear lens cell.

The image circle does not affect how much extension you need. The image circle is the size of the image that the lens projects onto the film plane. A larger image circle will allow you to use more camera or lens movements (rise and fall, etc.).

Not to confuse the issue, but lens extension does affect image circle. As you focus closer, the lens will be further from the film plane. This means the lens will project a larger circle onto the film plane. That's why image circle specifications are usually quoted for infinity focus.

Unless it is a Tele design!

[koh303]

Hello guys and girls

That very polite, but misguided, sadly there are no females active on this forum.

Now - back to technobabble.

[Len Middleton]

Not to confuse the issue, but lens extension does affect image circle. As you focus closer, the lens will be further from the film plane. This means the lens will project a larger circle onto the film plane. That's why image circle specifications are usually quoted for infinity focus.

One way to visualize what Noah is mentioning is think of the light coming from the rear of the lens to the ground glass / film plane as a cone.

The usable cone is shortest when focused at infinity, and the resulting circular base of the cone (image circle) is he smallest. I say usable cone, as you cannot form an image if the lens is placed closer to the ground glass than the infinity focus point. Focusing closer moves the lens further away from the ground glass and thus makes the image circle larger. Hence the reason typically the image circle is quoted at infinity.

The exception to that situation is where for a particular use (i.e. process lenses) that may be quoted at 1:1.

Welcome to the community, but suggest you look at the LF Home Page (see tab at top of page) and read some of the articles there.

[mingliaozi]

Thanks to everyone. Yes, I do realize that I have lots of reading to do. However, as a mathematician I sometimes find some of the articles a bit handwavy and lengthy for the information they provide. So I was asking for a quick formula, and thanks a lot. So, I have a bulldog 4x5 DIY camera , my minimum lens extension is around 100mm and maximimum about 180mm ish. This doesn't seem to be very good, basically only allowing 90mm lenses, am I right?
And also, which formula relates f stops to depth of focus? For pinhole cameras, everything is in focus. So, by stopping down heavily, I could account for the limitied lens extension ?!

Thanks to everyone!
:)

[StoneNYC]

That very polite, but misguided, sadly there are no females active on this forum.

Now - back to technobabble.

Speak for yourself! :-p


Thanks to everyone. Yes, I do realize that I have lots of reading to do. However, as a mathematician I sometimes find some of the articles a bit handwavy and lengthy for the information they provide. So I was asking for a quick formula, and thanks a lot. So, I have a bulldog 4x5 DIY camera , my minimum lens extension is around 100mm and maximimum about 180mm ish. This doesn't seem to be very good, basically only allowing 90mm lenses, am I right?
And also, which formula relates f stops to depth of focus? For pinhole cameras, everything is in focus. So, by stopping down heavily, I could account for the limitied lens extension ?!

Thanks to everyone!
:)

As someone else mentioned, things change with "telephoto" lenses, but for normal LF lenses, the rule is.

Lens FL=bellows extension.

90mm lens = ABOUT 90mm of bellows extension.

150mm lens = ABOUT 150mm of bellows extension.

Etc.

Although I don't often care for actual calculations, as a more "artist" type, just view the ground glass and estimate based on experience how much depth I want... But here is the "Hyperfocal Distance" calculation, that is, the distance to which you can focus and stop down to have mostly everything in focus.

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

I hope this helps you.

That camera does sound very limiting. Look into the "Intrepid Camera" company, they make very cheap 4x5's that are not fancy but have a lot more options than yours seems to.

If you want to get a fancy one, I recommend a Chamonix 45N-2.

Hope this helps.

[Jac@stafford.net]

Thanks to everyone. Yes, I do realize that I have lots of reading to do. However, as a mathematician I sometimes find some of the articles a bit handwavy and lengthy for the information they provide. So I was asking for a quick formula, and thanks a lot. So, I have a bulldog 4x5 DIY camera , my minimum lens extension is around 100mm and maximimum about 180mm ish. This doesn't seem to be very good, basically only allowing 90mm lenses, am I right?
And also, which formula relates f stops to depth of focus? For pinhole cameras, everything is in focus. So, by stopping down heavily, I could account for the limitied lens extension ?!

Please visit here (http://www.largeformatphotography.info/) for formulas and a lot more information. No sense in duplicating it here. You will find that depth of focus is different from depth of field. Good stuff. Enjoy the read.

Regarding using a lens longer than bellows draw, one can use a lens board extension (which can sometimes be a recessed lens board mounted backwards). You can then use your 180mm and longer. Surf the big auction site, or google for examples.

[mingliaozi]

Thanks for the quick reply and the formula. Unfortunately, the Intrepid Camera seems to have been sold out, but I am considering making my own, apart from the bellows, it doesn't seem too difficult :)

[Jon Shiu]

Thanks to everyone. Yes, I do realize that I have lots of reading to do. However, as a mathematician I sometimes find some of the articles a bit handwavy and lengthy for the information they provide. So I was asking for a quick formula, and thanks a lot. So, I have a bulldog 4x5 DIY camera , my minimum lens extension is around 100mm and maximimum about 180mm ish. This doesn't seem to be very good, basically only allowing 90mm lenses, am I right?
And also, which formula relates f stops to depth of focus? For pinhole cameras, everything is in focus. So, by stopping down heavily, I could account for the limitied lens extension ?!

Thanks to everyone!
:)

No, not good to use a 90mm lens with a camera with > 100 minimum extension, because it likely won't focus to infinity. For that camera it would probably be better to get a 120mm lens.

[Winger]

That very polite, but misguided, sadly there are no females active on this forum.

Now - back to technobabble.

Excuse me? I really hate to interrupt the thread to answer this, but I'm quite female, thank you. I do try to be active on this forum. Sorry if nobody notices.

[Argentum]

Thanks to everyone. Yes, I do realize that I have lots of reading to do. However, as a mathematician I sometimes find some of the articles a bit handwavy and lengthy for the information they provide. So I was asking for a quick formula, and thanks a lot. So, I have a bulldog 4x5 DIY camera , my minimum lens extension is around 100mm and maximimum about 180mm ish. This doesn't seem to be very good, basically only allowing 90mm lenses, am I right?
And also, which formula relates f stops to depth of focus? For pinhole cameras, everything is in focus. So, by stopping down heavily, I could account for the limitied lens extension ?!

Thanks to everyone!
:)

If minimum extension is only 100mm then maximum lens focal length would be approx 100mm unless you can fit a recessed lens panel. It may be that the camera was designed for use with a 90mm lens, I don't know but if max extension is only approx 180mm then a 180mm focal length lens will only be able to focus at infinity or very close to it which is not much use.
Those cameras are very basic. Don't get it wet. MDF expands when it gets wet and it will become a 10x8 before you know it ;)

This software may be a bit advanced for you at this stage but as a mathematician its probably right up your street.

http://www.winlens.de/index.php?id=70

you can play around with numbers. Just remember to put object(the subject) distances and magnification preceded with a negative sign.

You can use it to calculate all distances, depth of field, depth of focus, circles of confusion, airy discs, what difference apertures will make to DoField and DoFocus, lens extension, distances near and far, etc etc.

Also available for andorid at https://play.google.com/store/apps/details?id=net.opticalsoftware.calclensthin

[Jim Jones]

. . . my minimum lens extension is around 100mm and maximimum about 180mm ish. . . .
As Jon Shiu says in post #13, a lens about 120mm or a little more should be good on your camera. Press camera lenses with a focal length of 127 or 135mm might work, and are less expensive than many of the lenses intended for view cameras. Look up the back focus specification on any similar lens before investing. To focus closer with the lens you have, buy or improvise a top hat lens board to extend the lens further.

[Alan Gales]

Excuse me? I really hate to interrupt the thread to answer this, but I'm quite female, thank you. I do try to be active on this forum. Sorry if nobody notices.

I see you on here and APUG, Bethe. You don't say as much as some of us but what you do say is both helpful and interesting.

[mingliaozi]

Hey Jim Jones, thanks a lot! If I want to aquire a press camera lens, what terms should I use on ebay? And what do you mean by back focus specification? :)

[Bob Salomon]

Hey Jim Jones, thanks a lot! If I want to aquire a press camera lens, what terms should I use on ebay? And what do you mean by back focus specification? :)

What do you mean by "a press camera lens"? Any lens that can be mounted on a press camera could qualify.

You don't care about back focus. To determine if a lens will work at infinity on your camera you need to know the flange focal length of the lens in question.

[Argentum]

I agree with what most are saying in that a 90 to 120mm lens is what you need for that camera. If you can work out what the shortest lens that will focus at infinity is for that camera, then that would be best for you. The manufacturer should be able to tell you if a 90mm Angulon will focus at infinity.

It would make sense if it does but is not guranteed so check with manufacturer what is shortest focal length lens the camera was designed to work with. If in doubt then a 100-120mm lens will probably be OK. A 90mm lens might be marginal. All depends on exact lens and exact minimum extension you really have.
180mm lens definitely sounds like it is too long for that camera to be useful.

[Theodoros]

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.

[Bob Salomon]

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.

[Theodoros]

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....

[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.

[Theodoros]

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...

[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'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.

[Theodoros]

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?

[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?

[Theodoros]

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.

[Argentum]

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.

[Jac@stafford.net]

You guys! Water, pipeline as analogies? Come on.

CoC is not inherent in the medium, lens or anything else. It is what a person chooses to use as a metric to determine how much fidelity is determined by degrees of enlargement.

[Theodoros]

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.

Yes it will be the same... the supply depends on the pipe diameter and the time the pipe is active... The surface size doesn't change either as to alter reception of supply per area... only the distance of the surface from the pipe's mount changes... if you collect the liquid is of the same amount... and since the reception surface is of constant size in both cases, you'll have the same amount of liquid on it if you evenly distribute it on the surface (which is the lens design job)... It's the distance from the entrance pupil that will alter the DOF though... It's then back in physics (from engineering) distances change, so magnitudes change in proportion too... Think of vignetting, the more the image circle the less of it... isn't it?

[Theodoros]

You guys! Water, pipeline as analogies? Come on.

CoC is not inherent in the medium, lens or anything else. It is what a person chooses to use as a metric to determine how much fidelity is determined by degrees of enlargement.

Only if you use an enlarger... which then reverses the process of capturing and thus reverses the image circle projected from the capturing process (the larger IC is projected as smaller and the smaller IC as larger for the same size print)... It's not the same if you print from an image sensor (or a per area scanned film).... Then the mounting distance difference is the only truth you are left with...

EDIT: ...and the lens resolution of course...

[Argentum]

so you're saying the image circle is the same for both lenses but the DoField is different?

[Theodoros]

so you're saying the image circle is the same for both lenses but the DoField is different?

It's not what I'm saying, but it can be also true if the position of the entrance differs... Don't expect dramatic differences though... it is a proportional change... but it is there.

[Jac@stafford.net]

Only if you use an enlarger... which then reverses the process of capturing and thus reverses the image circle projected from the capturing process [...]

Oi! Your mental gymnastics could drive a man to drink! Thank you for that. Carry on. I'll shut down and have a bourbon.

[Argentum]

I still want to know about the difference between your Fuji GX680 210/5.6 which if I'm not mistaken is designed with a larger image circle to cope with anticipated tilts and shifts which your Zeiss Contax 210/4 isn't. So my question still stands, has your fuji lens had its aperture modified to maintain expsoure across different lenses or is its aperture a true ratio of the focal length.

[Theodoros]

I still want to know about the difference between your Fuji GX680 210/5.6 which if I'm not mistaken is designed with a larger image circle to cope with anticipated tilts and shifts which your Zeiss Contax 210/4 isn't. So my question still stands, has your fuji lens had its aperture modified to maintain expsoure across different lenses or is its aperture a true ratio of the focal length.

I don't understand the question... my Fuji 210 never had any modification... why should it? Distribution of light is (supposed to be) even across image areas no matter the size of them... for equal image areas you get the same amount of light distributed despite the lens image circle, the rest is (supposed to be) lost or absorbed! (although it "passes" through the lens) So why modify it? ...The entrance pupil of the lens is positioned at a significantly larger distance from the image area and this causes DOF to be shallower...

[Argentum]

I'm telling you that if your lens is spreading the image across a wider area it affects the exposure. The inverse square law states this. It is a physical law which you can't ignore. The lens designer can do one of two things. They can ignore it which means if you do a side by side test, your fuji lens will require a longer exposure than your contax lens or they can tweak the lens aperture so that although it actually says F8 its actually giving somewhere between 5.6 and f8. And that would result in your fuji lens and your contax lens giving the same exposure at f8 even though your fuji lens is actaully using a slightly wider aperture. I want to know which is the case for your fuji lens. If its the latter then you are not comparing f8 with f8 when you are talking about different DoField. You can easily test this by framing and focusing same area on your digital back with both lenses and seeing if required exposures are the same of if the fuji requires a longer exposure or not. If it requires longer exposure then apertures are probably very close. But if it requires same expsoure then i am saying its aperture will actually be wider than f8. The physics says so. Do the test.
A lens of any focal length has a fixed field of view so one lens can not capture more light than another of the same focal length at the same aperture (excepting glass absorption but we'll ignore that for now).

[Theodoros]

I'm telling you that if your lens is spreading the image across a wider area it affects the exposure. The inverse square law states this. It is a physical law which you can't ignore. The lens designer can do one of two things. They can ignore it which means if you do a side by test your fuji lens will require a longer exposure than your contax lens or they can tweak the lens aperture so that although it actually says F8 its actually giving somewhere between 5.6 and f8. And that would result in your fuji lens and your contax lens giving the same exposure at f8 even though your fuji lens is actaully using a slightly wider aperture. I want to know which is the case for your fuji lens. If its the latter then you are not comparing f8 with f8 when you are talking about different DoField. You will can easily test this by framing and focusing same area on your digital back with both lenses and seeing if required exposures are the same of if the fuji requires a longer exposure or not. If it requires longer exposure then apertures are probably very close. But if it requires same expsoure then i am saying it aperture will actually be wider than f8. The physics says so. Do the test.

Aaaah... I now see what you are getting at... Well no... it doesn't work like this, you see when focal length is constant and aperture the same, your pipe's mouth is the same but the liquid entering has more pressure as to supply more liquid for the larger image circle... (sucks more, spits more :)) It's only that the rest than what concerns the image area is lost... (thrown to garbage)... What you are left with is with a pipe that supplies the same pressure (in proportion) for only the image area but is positioned further apart....

[Argentum]

well no becasue if image circle of lens A is 164mm(approx your fuji) and image circle of lens B is 108mm(approx your contax) and they are both same focal length at same aperture, then to get same image area on your film or digital back, then lens A will need to be further away than Lens B so for the same image captured you can't make a direct comparison about DoField which you are trying to do.
And you can not magically make lens A capture more light than lens B at same aperture so methinks your claim that DoField is different is probably becasue your subject distance is different for same image with the two lenses which obviously means doField will be different.

[Theodoros]

well no becasue if image circle of lens A is 164mm(approx your fuji) and image circle of lens B is 108mm(approx your contax) and they are both same focal length at same aperture, then to get same image area on your film or digital back, then lens A will need to be further away than Lens B so for the same image captured you can't make a direct comparison about DoField which you are trying to do.
And you can not magically make lens A capture more light than lens B at same aperture so methinks your claim that DoField is different is probably becasue your subject distance is different for same image with the two lenses which obviously means doField will be different.

Actually things are never so complicated... If you mount a (say 37x49) MFDB on the Fuji and then on the Contax, and focus them the same for the same scene, then the Fuji lens "sees" a wider angle than Contax (the sucks more -spits more case) and what is not recorded on the image area is lost... What you are left with is with two lenses, both recording the same AOV but see a different one and that supply the same through the same pipe "hole" (meaning the aperture) but the "hole" is in different distance from the image area... There is really no "magic" into it... Its simple physics... Theory presumes that lenses are symmetrical, they "see" the same AOV and are positioned in the same position... If one of the three doesn't apply, then theory doesn't apply... In this case ALL THREE don't apply...

[Dan Fromm]

I don't understand the question... my Fuji 210 never had any modification... why should it? Distribution of light is (supposed to be) even across image areas no matter the size of them... for equal image areas you get the same amount of light distributed despite the lens image circle, the rest is (supposed to be) lost or absorbed! (although it "passes" through the lens) So why modify it? ...The entrance pupil of the lens is positioned at a significantly larger distance from the image area and this causes DOF to be shallower...

cos^4, also known as optical vignetting, anyone?

[Jac@stafford.net]

I nominate a new forum category for absurdity and to name it Argentum vs. Theodoros, a pair or insignificant particles orbiting one another in a universe made of words rather than substances practical in the world of LF.

Done?
.

[Dan Fromm]

You do sound like a "drunk"... or at least have the behavior of one... Never the less people forget that theory makes assumptions... and among the assumptions that it makes, is that lenses are symmetrical (and hence their entrance pupil is always at the mounting distance)... or ignores that (some) people drink too much! ...do you have a theory if the lens is one of retrofocus design though? ...Think of it after you are back and sober...

Dunning-Kruger, anyone?

[Jon Shiu]

Well, it doesn't make any sense for them to design a lightweight compact camera that can't use a 90mm lens. I looked on B&H and they say the lens can be 50-125mm, so it appears around 90mm lens would be good. So, I guess more research is in order. Perhaps there are different versions.

[Theodoros]

I think that if two lenses are mounted in different distance from the image area, then the proportion of image area to lens and lens to subject change (and thus DOF does) is far easier for one to understand... :)

[Argentum]

back on track

[Jac@stafford.net]

cos^4, also known as optical vignetting, anyone?

I thought you would never ask. :) I do happen to have a 3" Biogon that FAPP does defeat the very most of optical vignetting over 4x5", but to accomplish such it cheats by having a rear element larger than 4x5". Oh, on 8x10"? Don't ruin my day.

Call upon our theo/argen pair to put it through the mixmaster.
.

[Jac@stafford.net]

Dunning-Kruger, anyone?

Well that Trumps everything.
.

[Theodoros]

I thought you would never ask. :) I do happen to have a 3" Biogon that FAPP does defeat the very most of optical vignetting, but to accomplish such it cheats by having a rear element larger than 4x5". Oh, on 8x10"? Don't ruin my day.

Call upon our theo/argen pair to put it through the mixmaster.
.

He isn't asking Jac... he is entering a further level in the conversation... I think I'll rest with my last post: "I think that if two lenses are mounted in different distance from the image area, then the proportion of image area to lens and lens to subject change (and thus DOF does) is far easier for one to understand... :) " :eek::cool:

[Dan Fromm]

I thought you would never ask. :) I do happen to have a 3" Biogon that FAPP does defeat the very most of optical vignetting over 4x5", but to accomplish such it cheats by having a rear element larger than 4x5". Oh, on 8x10"? Don't ruin my day.

Call upon our theo/argen pair to put it through the mixmaster.
.

Interesting. My little 38/4.5 Biogons (remember, I've sold 18, still have 2) have rear elements that fit through a Century Graphic's lens throat. Yours must be a Pacific Optical version that wasn't made to the original prescription.

[Dan Fromm]

In the words of the Lord Protector, "I beseech you, in the bowels of Christ, think it possible that you may be mistaken."

[Argentum]

correct me if I'm wrong. Field of view (what is in front of the lens) is largely dictated by focal length. Angle of coverage(what is behind the lens) is largely dictated by lens design. The two are not necessarily similar angles.

[mingliaozi]

What if I use like a huge extension board? Like 50mm for example, or even 70?

[B.S.Kumar]

I'm sorry your relatively simple question got too technical, even for the most experienced amongst us :)

To get back on track, when you focus most lenses at infinity, the distance from the lensboard to the film plane will be approximately the focal length. A 90mm lens will be ~90mm away from the film plane, a 180mm lens ~180mm away, and so on. Some lenses are designed to focus at closer or longer distances than their focal lengths. This is common with lenses for rigid body cameras like 35mm or medium format, where the distance between the lens mount and the film or digital sensor is fixed.

You say your camera has a maximum extension of ~180mm. So it should focus at least to infinity. If your camera will not let you do this, or let you focus closer, use an extension board. We do this all the time when we want to use for example, a 360mm lens on a camera which has only a 350mm bellows, or when we want to focus closer than the bellows will allow.

Kumar

[Jac@stafford.net]

Interesting. My little 38/4.5 Biogons (remember, I've sold 18, still have 2) have rear elements that fit through a Century Graphic's lens throat. Yours must be a Pacific Optical version that wasn't made to the original prescription.

Twenty 38/4.5 Biogons!

Indeed, I was referring to the Pacific Optical 3". I don't need more than a couple. I just don't have that many doors in my little house. I would use bricks as door stops but the POs were easier to find here.

Picture of one mounted onto a 4x5 Sinar Alpa. (http://www.digoliardi.net/standw.jpg) And a picture of it with the camera bellows removed (http://www.digoliardi.net/super-wide-biogon-1/new_lens_wo_bellows.jpg). Close up of rear element (http://www.digoliardi.net/super-wide-biogon-1/close1.jpg)when focused to infinity.

Which brings us back to the subject. This is a 3" lens. Now, how does its physical dimensions fit the description of focal length given by B.S. Kumar? Remember, Kumar, this is a 4x5 camera.

[Dan Fromm]

I seen my opportunity and I took it.

[B.S.Kumar]

I would think that a 38mm Biogon, or a 3" Pacific Optical are some of the lenses not included in most people's definition of "most lenses" :) I must confess to never having seen either!

Kumar

[Argentum]

Well, it doesn't make any sense for them to design a lightweight compact camera that can't use a 90mm lens. I looked on B&H and they say the lens can be 50-125mm, so it appears around 90mm lens would be good. So, I guess more research is in order. Perhaps there are different versions.

I've seen somewhere else where it says 50-200mm so in answer to OP, yes a 90mm should be good for that camera.

[Bob Salomon]

correct me if I'm wrong. Field of view (what is in front of the lens) is largely dictated by focal length. Angle of coverage(what is behind the lens) is largely dictated by lens design. The two are not necessarily similar angles.

Depth of field is in front of the lens and is dependent on focal length, aperture, desired CoC.
Depth of focus is behind the lens and is dependent on focal length.
Angle of coverage is the cone of light thrown by the lens towards the film. It is dependent on the optical design of the lens and the focal length and aperture and magnification ratio.
Angle of view is dependent on the format size of the film and the focal length of the lens.

[Dan Fromm]

I would think that a 38mm Biogon, or a 3" Pacific Optical are some of the lenses not included in most people's definition of "most lenses" :) I must confess to never having seen either!

KumarKumar, the 3" PO is very closely related to the 75/4.5 Biogon as sold for Linhofs etc. The 38/4.5 Biogon was most commonly fitted to the Hasselblad Superwide and successors. I'm surprised you haven't seen one.

[Theodoros]

correct me if I'm wrong. Field of view (what is in front of the lens) is largely dictated by focal length. Angle of coverage(what is behind the lens) is largely dictated by lens design. The two are not necessarily similar angles.

There are two ways of looking at field of view... One is the FOV that the lens projects, the other is the size of image area you use, only the second is focal length dependent.... Angle of coverage is the same as the first. What is in front of the lens is the whole angle that a lens can see, the angle of coverage is the same and is both in front or behind the lens... The lens doesn't know what size image area one uses, nor it "cares".

[Theodoros]

Depth of field is in front of the lens and is dependent on focal length, aperture, desired CoC.


...and on mounting distance. There is no way (according to physics) that two lenses of the same focal length, projecting on the same size image area, stopped down to the same aperture and used for the same size print, to saw the same DOF if they are mounted in different distances from the image area...

[Oren Grad]

I'm going to close this thread.

It was started by a LF beginner asking a straightforward question about flange focal distance, flavored with a bit of confusion about image circle that could also have been cleared up with a simple response. Instead, the poor OP has been drenched with a convoluted mess of irrelevant stuff that can't be anything but confusing.

To mingliaozi: welcome to the Forum! Please do have a look at the various learning resources as recommended but also feel free to post additional questions here as you continue your LF explorations.

Everyone else: no, we're not outlawing all forms of thread drift. But before posting, please take a moment and ask yourself whether what you intend to say really is appropriate and helpful in context. If you want to argue about third-order subtleties of applied optics, then please define a suitable topic and launch a thread for that purpose rather than hijacking a beginner's thread on an unrelated, very basic topic.