making a photography darkroom

[whisky-dave]

I don;t insist, that;s what other have called it.

for video work.

yep shows where the film or sensor plane is.

mORE THAN YOU IT SEEMS.

[NY]

Yes, you want to make sure you still get a reasonable amount of signal in relation to the noise. It's like overexposing parts of a picture that's got very contrasty light: once one or more of the channels reaches maximum you get featureless colour (typically a blue sky becomes a horrible cyan, or parts of the face become fake-tan orange). Any sensor (digital or film) has its minimum and maximum values, and one of the few problems with digital is that the maximum can be a bit of brick wall rather than a gradual roll-off, which is why, as for slide film, I expose for the highlights and correct in post-processing.

Sounds intriguing. It's sad that a package like Photoshop or PaintShop Pro could achieve this with a few button presses whereas you really have to work at it with film and feel as if you've really achieved something then.

I experimented with printing a slide onto B&W paper (which gives a negative) and then contact printing this onto another piece of B&W paper (wait till it's dry otherwise the emulsions stick together - been there!). Results can be quite good for some subjects - you have to allow for the fact that the "negative" will be very non-panchromatic so red/orange shades in the slide render as white in the neg and hence black on the final positive. Printing from colour negs yields horribly muddy results (and very long exposures) because the orange base of the neg behaves as a safelight! The paper can only see the reddish tones in the original picture (which come out as blue on the neg), so it's like taking a B&W photo in red light.

[Andy Burns]

My dSLR also has one

Or indeed, the plane of the sensor ...

[whisky-dave]

Nom I mentioned them before anyone elee .

Few record to vinyl, but vinyl records are still made and sold.

You don't know much about this do you.

but not explain them you can gove 5min talks on quantumm mechanics too.

That is not the point though is it.

[Martin Brown]

And the colour temperature of the continuum light source. All bets are off with film in narrowband or emission line based illumination.

It is mainly of historical interest CCDs get it right. It so happens that panchromatic and colour emulsions have a safelight wavelength sensitivity gap that exactly matches the bright green OIII nebula line in many astronomical nebulae. This meant that until about the mid 1970's when a special emulsion sensitive to this line was created all the Palomar deep sky slides showed nebulae to be red, pink and powder blue with no hint of green, yellow or turquoise.

It always seemed a bit odd that something that visually looked dirty oily grey green photographed as mostly pink and blue.

An interesting quirk of colour film is that to get flesh tones exactly right in different parts of the world different makers bias their colour films slightly differently (and so do digital cameras). The residual errors are hidden along the line of purples which makes a few rare plant flowers with just the wrong peak wavelength reflected look very strange indeed when using film. Notocactus Ubelmannianus (purple form) is one such plant that photographs badly on most colour films.

Digital images can easily have their white balance and flesh tones tweaked afterwards if necessary and the residual colour errors are also considerably less than film on decent kit.

Choosing the speed can still be relevant in digital if you want to deliberately create a motion blur or freeze fast action. You are always trading signal to noise for shorter exposures (courser grain in film and more intrusive thermal noise/less resolution on a digital camera).

One thing a one shot colour camera struggles with is monochromatic images at certain wavelengths. Some sensors really don't like red H-alpha and leaks in the other filtered channels gives a weird effect.

You could do some of these things by scanning slides or negatives back when film was the only high resolution game in town.

You could in the old days push process film or bake it in a dry N2/H2 gas mix for a day or two before use to trade shelf life for sensitivity. You didn't have to develop it exactly by the book.

In extremis uranium intensifier could sometimes rescue faint under exposed silver based images to printable negatives.

The main advantage of digital especially in hard to repeat situations is that you have instant feedback and know almost immediately whether or not you have a decent quality record of the event.

We used to carry a Polaroid instant camera around with negative capable film stock as an insurance policy when taking important images.

Polaroid was doomed the moment that Mpixel digicams became affordable.

[NY]

I'd never heard of depth of focus but a quick google has educated me on that. Strictly speaking the "London Underground" sign marks the position of the film, rather than the range of positions of the film where the lens would produce an acceptably sharp image, which I'm sure isn't a constant and varies according to focal length of lens.

I was misled by the way you phrased it into thinking that they were alongside each other, either on the lens or the camera, rather than being two completely different things which happen to share the same acronym.

Yes, but the difference isn't a constant for all lenses so you couldn't mark it by a different film plane Underground mark on the camera. Instead you'd measure from the film plane to the subject and then set your lens to the measured distance using the IR mark on the lens rather than the visible mark. Or if you were focussing using the viewfinder, you'd focus using optical light, read the distance against the visible mark and re-focus slightly to set that same distance against the IR mark.

I never said it was a compact camera. I should have modified what I said to "digital *SLR* cameras have the same mark" to clarify that. Mind you, I have seen a compact camera with a focal plane mark - I've no idea what make/model it was, but I remember noticing it at the time. And some video camcorders have the mark as well (as had some Super 8 film cameras).

How standard is it for the tripod mounting thread to be aligned with the focal plane and the centre of the sensor/film so the camera always rotates about the sensor/film? Looking at the three DSLRs that I can lay my hands on right now, the tripod bush looks to be in about the right place (*) but on my two compact digital cameras it's a fair way off to one side; I'm not sure whether the Instamatic-type film camera that I used to have even had a tripod mount.

(*) It's about the right distance front-to-back to line up with the focal plane mark and it's roughly aligned left-to-right with the middle of the lens mount.

[whisky-dave]

I think early cameras such as plate it was importan to make sure the plate was in teh right place eraly cameras weren't acurratly made.

I don;t think it does. if yuo have any camera with interchangable lens all of tehm have to be in focus at the same point and that is where the film or sensor is. In the old days yuo could take the film pack out and replace it. If teh fiml/sensor is in teh wrong place the picture will be out of focus. Those with difital camera don;t consioder this and probley have never even throught about it.

Well I've yet to see a use for this in the digital photography world. I've yet to see a compact with this mark, well a compact that doesn;t have interchangable lenses.

it's mostly a function of focal lenth.

I know which is why I say it has nothing to do with the lens, which is why it's on the camera, and why they put IR marks on lenes mostly telephoto rather than WA.

Yep that's how I did it. I never used the DoFocus mark for IR. I used it a few times for macro when calculatign guide No.s but I found trial an error worked best.

It does seem that those that have experince of film know far more about these marks than those brought upo on digital cameras.

I'd say never but I've never seen one.

Can;t see the3 point of doing that the tripod mount should be so the camera balancies better on the tripod and not stressing anyhting. It's why you have tripod mounts on telephoto lenses and not normally on WA ones.

irrelivent to film or sensors the tripod mount is about balance or it's most convient to put it.

[whisky-dave]

It is a DoF mark that's what it's called that's what it is.

[dennis]

Well go and read it again because you have it wrong.

That has nothing to do with either DoF.

That depends on the lens, The focal length has to be corrected for all the colours and you can correct for IR too but its usually only done for specialist uses. A mirror lens will bring all the colours including IR to the same focus.

Well you got that bit right.

[dennis]

Who? Have you corrected them yet?

Irrelevant, they are the same.

So how come you keep getting stuff wrong?

[dennis]

I know why it happens, do you want to explain it or me?

One is simple the other is even simpler.

No the point is that you can teach photography easier and cover more using digital rather than film. You can't teach darkroom techniques using digital. You still maintain there are enough differences between film and digital to make it difficult to use digital but refuse to say what they are and keep getting the basics wrong and claim its not you that's wrong. Not very promising for your pupils is it.

[whisky-dave]

then prove you're not talking bollocks oh you can;t.

ld-and-depth-of-focus

DEPTH OF FIELD - The range of object distance within which objects are in satisfactory sharp focus, the limits being the establishment of a circle of confusion of greatest acceptable size. DEPTH OF FOCUS - The range through which the image plane (the emulsion of the film) can be moved backward and forward with respect to the camera l ens such as defined under the depth of field and circle of confusion. This term is often confused with depth of field and vice versa.

In common English, Depth of Field is what the photographer is interested in ; it is what is in acceptable focus in front of the lens. Depth of Focus is what only a technician is interested in; it is what is in focus behind the rear lens element which the film or image sensor "sees."

Yes it has.

not it doesn't IR always focus at a differnt point it's to do with the way light refracts through a glass lens. Of course I realise you know NOTHING about such things.

What do you mean by specialst use's you've no idea have you. Have you ever used IR flim ? Didn;t think so.

Mirror lenes aren't the standard lens for most cameras.

I got it all right.

[whisky-dave]

Anyone that knows about what photography is. Those that think smart phones invented photography need educating.

you're the one that needs correcting.

they are NOT the same.

DEPTH OF FIELD - The range of object distance within which objects are in satisfactory sharp focus, the limits being the establishment of a circle of confusion of greatest acceptable size. DEPTH OF FOCUS - The range through which the image plane (the emulsion of the film) can be moved backward and forward with respect to the camera l ens such as defined under the depth of field and circle of confusion. This term is often confused with depth of field and vice versa.

In common English, Depth of Field is what the photographer is interested in ; it is what is in acceptable focus in front of the lens. Depth of Focus is what only a technician is interested in; it is what is in focus behind the rear lens element which the film or image sensor "sees."

I don't.

[whisky-dave]

you, don't make me laugh you haven't a clue.

it wasn't me that said the above "they aren;t..... either.

and there's few as simple as yuo .

You canm but it deosn't work like that.

ou know nothing about teaching do you.

You could but it wouldn;lt be any better than using a flight similar to get someone their pilots licence.

I've told you what they were, because you have NEVER been in a teaching enviroment you will NEVER understand.

[NY]

I doubt whether anyone with a camera with a fixed and defined focal plane (eg the polished plate across which the film runs, with the 36x24 mm or

120-sized aperture in it) will have thought much about it. After all, the position of the focal plane is no more adjustable for most film cameras than for a digital camera.

Where the focal plane mark comes into its own is if you are setting focus of your lens by tape measure rather than by adjusting the focus ring until the correct part of the image is in focus on the focussing screen (or letting the auto-focus do its job).

How certain are you that "depth of focus mark" is the correct term for the "underground symbol" mark? In camera manuals it's described as "focal plane". Depth of focus (like depth of field) refers to a *range* of distances - either side of the focal plane (in the case of depth of focus) or either side of the subject (in the case of depth of field):

It is probably technically incorrect to refer to the "underground mark" as "the depth of focus mark" (because this is a range rather than an absolute distance) but nevertheless it may be that it is common parlance. If so, fair enough, although all the references I've seen to the mark on cameras have called it the focal plane mark.

Since the rigidity of the camera body keeps the lens perpendicular to the sensor/film and the correct distance away, depth of focus isn't really an issue for cameras unless you use bellows between lens and film. Position of focal plane is another matter and you'd use it for specialised focussing as I described earlier.

I doubt whether all lenses focus IR the same distance closer/further than visible light. My gut feeling is that it may vary depending on the quality of the lens (as well, almost certainly, on the focal length). Here's why. A simple lens made of a single piece of glass focuses different colours of light at different distances, which results in chromatic aberration. To counteract this, photographic and telescope lenses have elements made of a combination of two pieces of glass of different refractive index to minimise the difference. In general, the more you pay for a lens, the less chromatic aberration you'll get - ie the smaller will be the depth of focus between the red and violet ends of the spectrum. Extending this further, a good lens will correct over a wider range, extending to some part of the IR spectrum. It may not do it perfectly, but the degree to which it does governs how far apart in depth the lens will focus visible and IR. For this reason, and the fact that as you say a longer lens probably has a greater offset between IR and visible focal planes, I'd expect it to make sense only to mark the two focus points on the lens focussing scale and not on the camera.

Effectively the underground mark is saying "this is where the sensor/film is". A lens's focussing scale is marked such that a visible light image will be focussed at the plane, by virtue of the camera having a fixed and precisely controlled spacing between mount and film. For IR, a different lens-dependent offset is needed to counteract the fact that when the lens is correctly focussed for visible light, IR will be focussed at a different plane *whose position depends on the lens* both in terms of focal length and degree of chromatic aberration correction.

The only time when the precise position of the tripod mount is critical (as far as I am aware) is when taking multiple overlapping photos eg for a panorama. I have seen special brackets with knurled knobs to move the rotation point accurately to the position of the sensor if the camera's own tripod bush isn't in the right place, though I'm not sure how you calibrate it. It is probably important for movie cameras where the geometry has to remain correct when panning during filming so a subject moving on a circular path centred on the rotation point will remain in focus.

I agree that apart from this case, it makes sense to put the tripod point on the lens (if it's a heavy lens) for better balance.

[whisky-dave]

I did in the mid 70s, I tried using it once to help me calculate the magnification of a macro I was doing, as I was using bellows this meant the usual film plane to rear objective was quite difernt from what you usually encounter. I;m betting Dennis will now claim I was trying to keep a afire alight because I was using bellows.

It idsn;t really meant to be that mark is the FIXED location for measuring purposes.

werent many auto-focus cameras back in the mid 70s of course. As I've said I always thought of it as an aid in macro work. I was also tiold it was useful when doing copyiong using a copying stand, don;t see that sort of thing very often now. If I need to copy documents I now use my ipad. Have yet to find any DoF mark/indicator on that ;-)

only 99.48567892%

Yep as I said.

and it doesn;t move depending on teh lens either.

which is the depth of focus, as that is where the point of focus of visible light will be at infinity IIRC.

Yes but what's specailed abouyt this focusing are you saying that you can't use the lens for focussing.

well yes a poor quality lens will give fringing of colour that's easily seen.

so three is a term depth of focus YES> it is the term used when all visible light focuses on that point when teh light is parellel i.e focused at infinity, or a gants bollck away from infinity as you can never reach infinity can you. :)

Mayeb but I'm not sure that will be a better quality lens.

yes and with large instumnents it might be easier moving teh focal plane than focusing think telescopes.

The vast majority of camera have that and I still haven;t seen teh undergropund mark on canerqas with fixed lenses so that give's me further evidence.

so you could change the focal plane to rear objective distance as yuo do with telescopes and microscopes but rarley with photographic lenses .

No don't agree there.

would yuo really mount this lens on yuor camera and use your camera tripod socket.

There's a reason why long lenese come with tripod sockets even my M3 to EOS converter as a tripod socket.

each to ther own. ;-)

what do you mean by isn;t in teh right place why would a camera maker not put the tripod mount not oin the right place ?

there's special gimble and things for that.

I used to use a rifle grip arrangement.

[NY]

Actually that page describes the point as the focal plane or film plane mark and doesn't use the term "depth of focus".

I agree that it would be horrendously out of balance. It's one hell of a lens. I wouldn't like to hand-hold something 2.8 kg in weight and as long as that - no smutty comments :-) And with a 2x converter - with a 5200 mm lens you could probably almost have seen Neil Armstrong doing his "great leap" ;-) If 50 mm is regarded as 1:1 magnification then this thing is over 200x magnification. Camera shake, thermal currents over long distances and optical quality might be a problem.

I did say "precise" and I meant it as opposed to approximate. For balance you mount a heavy lens as close to the centre of gravity of the lens+camera unit, but the exact position isn't too critical.

Where it becomes critical, so I am told, is when taking several photographs to join together. And then you'd mount the camera to rotate about its sensor/film point. The two different uses wouldn't really come into conflict as you are unlikely to use a 2600 mm lens to take separate images of a panorama!

As I said earlier it seems from a very quick sample of cameras I can lay my hands on that DSLRs (and almost certainly film SLRs) do put the tripod mount at that point (Nikon D90, Canon 10D), but compact cameras don't always (Canon SX260). But that's a very limited sample. I can't find my older G9 compact to see where its mount is.

[whisky-dave]

dam I had a long list of those :-)

I used a 400mm with a 2X and a 3X converter

I make it 104X with a FF sensor 166X with my EOS M3. I'm thinking of buying one, but also considerign I:d be better off with a telescope. I got some reasonable moon pics with my 70-300mm

yes that's why it's in a light colour too.

dependiong on the lens of course. I doubt many people even pros choose a camera based on where the tripod mount is.

True and with modern software I don't think it's that importent. I've done basic panos of 3 pics or so hand held.

pdreview might have the info too. I have a canon G10

[dennis]

Early cameras had adjustments so you put your bit of frosted glass in and adjusted it and then replaced the frosted glass with a plate and took the picture.

Do you know what a technical camera is? You can still do that with one of them and they aren't that old and are very precise and accurate.

The depth of focus is the otherend of the depth of field equation, you change one and you change the other, you change the depth of field when you change the focal length so you also change the depth of focus. You don't change the focal plane as that is nothing to do with DoF.

The same as in the film world.

Why do you keep bringing in compacts are you only going to teach using film compacts?

Its mostly a function of design and which glass is used in which element. Mirror lenses are usually telephoto but bring all the colours to the same focus.

that's because depth of field and depth of focus are different for different lenses so you can't put it on the camera unless you put the depth of field on the info screen as a guide like my compact does. It knows what the zoom and focus is set to and can workout the depth of field.

What's it got to do with guide numbers?

Only those who had SLRs as virtually all compact film cameras didn't have them.

You don't want it to rotate about the film plane anyway.

There are good and bad places to put the mount, they are seldom put in the best place, just the convenient place.

[dennis]

The place that you need to pivot around isn't the film plane. There is a node inside all lenses where rotating about that point doesn't change the perspective as the image moves across which it will do if you rotate the camera around the film plane.