Opening moves in Photoshop

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NNSW
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Joined: Oct 8 2000

I wonder if I'm doing the best thing when I open up a new scanned image in Photoshop (for general purposes, or no particular purpose, just to save for the future)

I do a first crop. Then Levels - mainly the middle slider. Then Unsharp Mask. And then Save, either as a TIFF or good quality JPEG.
I would want the possibility of printing up to A4 size. I'm not in business; I just want something half decent.

peter millard
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Joined: Oct 19 2000

Hi there.

Sounds like you're doing fine. When you use levels, you're effectively altering the brightness and contrast of the image, but in a far more controlled way than simply using the Image>Adjust>brightness/contrast command.

When you scan/capture a 24-bit (8-bit per channel) image it's represented in Photoshop in 256 levels, with 0 as black and 255 as white. Calling up the levels dialogue allows you to adjust the image as a whole (or just individual channels) by either moving the sliders beneath the histogram, by entering numbers in the corresponding boxes, or by clicking on the eyedropper buttons and physically selecting an area on your image - known as setting the black and white pioints.

(BTW, you can also "neutralise" any colour casts in your image by selecting the grey (middle) eyedropper and choosing a neutral area of your picture - try it.)

By shifting the black and white points of the image closer together on the slider controls you increase the contrast of the picture. By moving the mid (grey) point one way or the other, you can effectively give more "space" to the lighter or darker tones in the image.

With your image open, choose the eyedropper tool and move the cursor over your image, and you'll get a readout of the RGB values in the "Info" palette. I try and avoid setting whites in a picture higher than 245, and blacks below 20, as they rarely print outside of those levels - but try it for yourself and see.

If you haver the HD space, archiving as uncompressed TIFF is best, or Max quality JPEG if you're sure you've finished working on it.

If you're keen to learn more, I can recommend "Adobe Photoshop 5.x for Photographers" by Martin Evening, about £25
from Amazon.

Whatever, enjoy making great pictures.

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Peter Millard
petermillard.com

Peter Millard
www.petermillard.com

tom hardwick
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Joined: Apr 8 1999

Peter: Excellent! I learn something new every day here. Can you expand on what "auto levels" does? It seems to work fine for me (and it's quick) but is it like auto exposure where manual could be better?

tom.

peter millard
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Joined: Oct 19 2000

Hi Tom.

If you open the levels dialogue you get a histogram which shows the tonal distribution of the image ranging from black(0) to white(255). There's there's often a "blank" area at either end of the scale before the image data starts as the tonal range of scanned images often needs to be expanded - not always, but often.

What auto levels does is move the black & white points in to just where the image data begins, which now become the new 0 and 255 points, thus spreading out the bits in between. This will usually work fine for a lot of pictures, but it can be fooled - very much like auto exposure, as you say. Setting the black & white points manually (like manual exposure) simply gives you more control.

There's no harm in trying auto levels (it's quick after all, and easily undone) but if it sometimes throws up a change you're not too happy with, then try tweaking the levels manually. Sometimes you'll want more adjustment than Auto Levels gives, sometimes less - try it.

One word of warning though; the black & white points are literally just that - the end of the line as far as image detail goes. If you do an Auto Level adjustment and find that it's too dark (black point moved in too far, for example) you have to undo it before you can make the changes manually.

Finally, once you're happy with the black & white points, use the Output Levels slider to reduce the overall contrast of the imaage as required.

As always, have fun making pictures.

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Peter Millard
petermillard.com

Peter Millard
www.petermillard.com

Alan Roberts at work
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Joined: May 6 1999

Another hint. Beware of the Brightness and Contrast controls in graphics software, they don't work the way that identically named controls work in video. In video, Contrast changes the gain leaving black level fixed, Brightness adds dc to the entire image . In graphics, Brightness changes the gain leaving the centre level (127) alone, Contrast does the dc shifting. (I may have got those names the wrong way around, but certainly not the gain change operations). So you should ALWAYS use the histogram window to see what you're doing.

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seaDog
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Joined: Sep 3 2000

In the layers dialogue box drag the background down to the new layer button, you will then get a new duplicate layer above your existing one. Do with it what you will (levels/curves/colour/mask) etc you are not damaging the real picture. If you don't like it drag the layer to the layer bin and you are back to your original picture and have lost nothing. If you like what you've done then flatten the image.

The safest way to work in Photoshop rather than relying on the history.

Beacuse you are using layers you get to use opacity etc between the layers and the world as they say is your oyster.

Hope this helps

NNSW
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Joined: Oct 8 2000

Thanks for the very helpful advice.

Any thoughts on the effective use of Unsharp Mask? (Sometimes I wonder if it's best to not use this at all.)

tom hardwick
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Joined: Apr 8 1999

I love the unsharp mask, finding its level of control and preview most excellent. Always worth dialing in a bit of UM isn't it guys?

tom.

peter millard
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Joined: Oct 19 2000

Absolutely. What Unsharp Mask does is create the impression of sharpness by selectively increasing the contrast of pixels as defined by the settings in the dialogue box. Here's a quick explanation of what each control does in Photoshop;

Amount - how much you want to sharpen. The softer the original the higher the amount.
Radius - the area affected by the sharpening. If the radius is not high enough the scan will look soft, if the radius is too high you get a 'halo affect' around edges.
Threshold - defines at what point USM begins to take effect - what we want to do is set the limit so that we only sharpen the image and not the film grain. A limit of 0 will sharpen everything. The grainier the film the higher the limit.

Here's how it works; Photoshop inspects the colour of each pair of adjacent pixels. If the colour difference is less than the threshold then nothing happens. Otherwise the contrast of the pixels within the Radius value is changed by the Amount value.

Juggle the three controls and preview the scan until you get the effect you want. One word or caution; it's best to try not to apply USM more than once on any image, so try to make it the last thing you do before printing.
Have fun.

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Peter Millard
petermillard.com

Peter Millard
www.petermillard.com

Alan Roberts at work
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Joined: May 6 1999

Peter, your description of the Unsharp Mask is an exact description of Aperture Correction in TV and film. It is a useful technique for gently tweaking images, but can easily wreck them if overdone.

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peter millard
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Joined: Oct 19 2000

Hi Alan.

I never realised there was so much common between the media types; amazing what you can learn on these boards, isn't it? Great stuff!

Best,

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Peter Millard
petermillard.com

Peter Millard
www.petermillard.com

Alan Roberts at work
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Joined: May 6 1999

Yup, neither of us can really teach the other much, except about specific hardware properties.

As a side issue, has anyone any real information on the temporal aperture of a film (movie) camera? To explain:

A video camera's temporal aperture is rectangular, it opens and closes instantaneously. At each readout of the ccd, all the charge is dumped, wiping it clean, so the next integration period starts with a bang, sharp edges to the temporal aperture.

A movie camera has a moving shutter to blank the light while the film moves to place the next frame in the gate. What effect does the moving shutter, out of focus behind the lens, have on the shape of the temporal aperture? Is it near rectangular, trapezoidal, even triangular?

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tom hardwick
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Joined: Apr 8 1999

First to Peter: You say "Photoshop inspects the colour of each pair of adjacent pixels...." What? It does this in what appears to be no time at all. As I click the mouse, so is it done. Do larger image files take noticeably longer? Amazing.

To Alan: Movie cameras have a focal plane shutter that "wipes" the image open or shut with a soft edged mask, in effect distorting the picture in the same way as cameras of old distorted moving car wheels into ellipses.

You of course know this. The exposure is even across the frame because the wipe in's unevenness is counteracted by the wipe out's unevenness. Higher shutter speeds don't affect this (the speed of the shutter curtains remain constant), but higher shutter speeds brought about by using slow mtion do, as the speed of the shutter blades is increased. How this affects the t a I've no idea.

tom.

Alan Roberts at work
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Joined: May 6 1999

Yes, Tom, it's the degree of unsharpness that I'm interested in.

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Alan Roberts at work
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Actually, this reminds me of an incident from my school days (must be about 58 or 59). The physics master had brought in hes brand new SLR and wanted to show us how high tech it was. He showed us the shutter blinds moving, then put the lens back on a set it to 1/1000 sec exposure, pressed the button and there was a loud, slowish, "kerlunk" as it all happened. He the said "It's amazing to think that all that happened in a 1/1000 of a second". I hadn't the heart to tell him that it actually took about 1/30 second, it was only the gap between the opening blind and the chasing, closing, blind that lasted 1/1000 second.

Ho hum, and we was a really nice guy, very bright, damn good teacher as well.

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peter millard
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Joined: Oct 19 2000

Hi Tom.

Yep, that's how it works. Bigger files can take ages (relatively speaking...) depending on the speed of the computer - which is why applying USM to a large image is used as a "real world" speed test by many computer mags. with a graphics-orientated readership.

I shot some stuff the other day which was then res'd up to about 70Mb, and applying USM took a good couple of minutes to each file - I know, it's time I got a new computer, but it's just that this one's working, it's well calibrated, the colour's right, the camera works well on it etc..etc... Basically, if it ain't broke...

All best,

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Peter Millard
petermillard.com

Peter Millard
www.petermillard.com

tom hardwick
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Joined: Apr 8 1999

Focal plane shutters often used to show the effects on screen of the uneven acceleration and deceleration of these blinds, didn't they Alan? I remember Olympus with their half frame Pen F SLR that got around this by using what was effectively a movie camera's focal plane shutter, a disk that spun up to speed well before the gate was reached, so giving much more even exposure.

If your teacher was demonstrating his SLR's top speed in 1959 he was very optimistic to think that the marked 1000th was within +- 30% of that anyway. Great flapping curtains that travelled the long dimension of the frame were notoriously unreliable as far as speeds and even-ness of exposure were concerned.

tom.

Alan Roberts at work
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Joined: May 6 1999

100% with you Tom. The motion of the shutter in still cameras gives as much pause for thought as it does in film cameras (and in some specialised HDTV video cameras that I know of and use). That's why I'm trying to gather information.

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