Large Format photography group - From "zero to hero!"

StephenM

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Well done for getting your finger out before I'd even moved. The benefits of youth... Where did you find it? I'll admit that having thought about it overnight, I wasn't certain whether the question was optical or chemical i.e. non imaging forming light from the lens or the effect of same on the emulsion.

Of possible interest (from my GoogleDrive).
 
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Woodsy

Woodsy

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The issue is really how one thinks about "contrast". There's microcontrast which is intrinsically linked to both sharpness and chromatic aberration, colour contrast, and what people talk about as global contrast. The latter of which is, I tend to think, the most commonly conjured in ones when someone says contrast, despite it being a little simplistic.

The first of these is clearly affected by lens coatings, but is mostly the result of the optical design and how well the lens performs achromatically over the spectral (visible) range.

The second is not what I'd first think of when someone talks about contrast generally. This is to do with the spectral transmission function of the coatings, and the comparison between uncoated, single coated and multicoated. On a global (over the whole image) scale, this will affect the perceived 'contrast', as certain tones may suffer more of less loss (difference in the transmission percentage) than others, thereby affecting the contrast as compared with a different coating or no coating at all, and indeed the perceived relative brightness between that tone/colour and another. So this is really more the global contrast that people think of outside of contrast that you might alter with general levels in post. The issue is that there are multiple effects which are all amalgamated into the word contrast, some effects are independent, others are linked to one another, so it's really not a trivial thing to explain in a forum post.

Doing what I do, I tend to work from a more fundamental effect upwards. As such, when someone says contrast, I immediately simplify to monochromatic light and go from there. Say for example that you illuminate a piece of perfectly white paper with only a monochromatic light but such that there is a range of illuminating intensities with step-like boundaries (like a test exposure on a new dark room enlarger) - the brightness would be different on one part of the paper than another. If you photograph this, you could say there is range of contrast between the brightness regions. As the light is monochromatic, the lens coating, or lack thereof, will not alter the perceived contrast between the brightness regions when compared to the opposite lens coating case. In other words, if the lens offers 4% loss at that colour or 0.4% loss, and if the overall exposure is the same, the contrast will not change. This is physically impossible without a non-linear process. But, the sharpness at the boundaries between the brightness regions may still change.

Now, as soon as multiple colours are used, as in a real scene, now the global contrast may vary depending on the coatings. But it must be understood that this is a property of the spectral profile of the coatings and hence how different colours are transmitted more or less than compared with another coating or no coating at all. AND, (and this is where my issue mainly comes from) this does not imply that contrast will be stronger. It may well be that, given two colours in a scene, their relative brightness difference may be reduced if the coatings work that way. Therefore the notion of adding coatings always improves contrast is nonsense A) because of the above, and B) because "improve" is a relative term.
 

StephenM

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My simplistic take on the contrast issue runs like this.

1. Non image forming light that strikes the film isn't focused and will increase the illumination over the entire field.

2. This will be uniform.

3. Along the straight line portion of the characteristic curve, density is proportional to amount of light.

4. The increased light from internal reflections will proportionally affect the darker areas more than the lighter ones.

5. The net effect is lighter dark tones with less effect on the high values.

Hence lower contrast.
 
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Presumably "veiling flare" also reduces overall contrast? I would guess that lens design and issues like shiny interior surfaces of the barrel would have a big effect, I'm not sure how much coatings would affect it.

However, maybe if there's more reflection from an uncoated lens, there would be cases where some of that reflected light might end up back in the image. But at 4% (1/25), it's already 4 and half stops down on the main light so I don't see that having much effect... unless there's the sun in the image, perhaps.
 

StephenM

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You might care to read the information I gave a link to in post 1921 above. I found it very interesting to read, and I think you might too.
 

StephenM

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Just as a related aside, some years ago I tested the effect of flare on spot meter readings. I used a black plastic seed tray placed against a white UPVC window frame and took a reading from a distance where the spot fell completely in the black area, but the outer region took in the frame. A second reading from close in, where the black completely filled the viewfinder resulted in a very different reading. I can look up the number of stops (yes, stops, not a half stop) if you're interested. It rather made me think that if you can approach closely enough to fill the frame as it were with a normal reflected meter, it would be just as good (if not better) than using a spot meter from a distance.
 
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Woodsy

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Apologies if this breaks the rules, please delete if so.

If you want a recessed copal 0 lens board, and have a copal 0 non-recessed board going spare and want to swap (No cash either way), please see the classifieds! I need to get sorted by Thursday if possible :)
 

Andysnap

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First evening in Cornwall and already 4 shots taken......all of St Michaels Mount..... Some lovely light and great clouds.
 

Andysnap

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StephenM

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It's far more likely that I've stuffed all four of them
Consistency is a valuable attribute for a large format photographer. I'm sure that they'll all be fine - anyone who can cope with an Ensign folder and red window winding will have no trouble with a simple large format camera.
 
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Woodsy

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4 shots?! Pah! I’ve taken 3 rolls of 120! Looking forward to catching up :)
 

Asha

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anyone who can cope with an Ensign folder and red window winding will have no trouble with a simple large format camera.
Yes I can vouch for that having had numerous red windowed folders.
In comparison LF is a piece of cake!
Consistency is a valuable attribute for a large format photographer.
Good job as my end results are consistently crap!:sulk:
 

Andysnap

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You're not the only one... I shall be staying very firmly on the mainland. :)
 

StephenM

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It seems like an easier, more direct, route based on the old principle of putting an object of known size in the frame, measuring it on the ground glass, and then using the very simple exposure increase based on magnification that has no squares or square roots in sight.

Well found.

Edit to add:

The simple equation is F = (M + 1) f

where
F = effective aperture
M = magnification
f = marked aperture

In the simple case of 1:1 where everyone knows off the top of their heads that you need a 2 stop increase in exposure, this gives F = 2f, or in other words the lens might say f/8 but it's letting in the light as though it were f/16 - 2 stops less.
 
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Asha

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It seems like an easier, more direct, route based on the old principle of putting an object of known size in the frame, measuring it on the ground glass, and then using the very simple exposure increase based on magnification that has no squares or square roots in sight.

Well found.

I can understand anything so long as it's like me……..SIMPLE! :LOL:

( @RaglanSurf , be careful what you say young man!!!:bat::LOL:)
 
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