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Can scanning digital backs do this...


brit

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<p>But first a little background to help you understand the question because no doubt I will not be too clear in my confusion :)</p>

<p>All my LF is done outside but in a greenhouse/glasshouse in the UK. The weather is often changeable leading to changes in light intensity and colour. With film I can just wait for the right light and *click* - the film is exposed for a few seconds at most. But how about digital? I read the cheapest backs are scanning backs and take a several minutes to complete. So here comes the question...</p>

<p>Can a scanning back do this....while it is scanning the whole frame, can it also be reading off <em>a grey card and a white card</em>* and apply the variations it sees with those during the scanning cycle to the information it is capturing at that time, so that differences in incident light intensity and colour are corrected for the final complete image?</p>

<p>I am just wondering how feasible it is to rely on a digital scanning back under changeable light intensity and colour because different parts of the scan could have been captured under different light eg a white cloud day but half stop light increase for 30 secs half way through a 6 minute scan (I realise there'd be no control for contrast).</p>

<p><em>*grey card and a white card</em>* = or any equivalent system</p>

<p>Also could I even use one to my laptop? http://uk.computers.toshiba-europe.com/innovation/jsp/SUPPORTSECTION/discontinuedProductPage.do?service=UK&toshibaShop=false&com.broadvision.session.new=Yes&PRODUCT_ID=1058205</p>

<p>EDIT. btw I now have a Sinar C</p>

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<p>Scanning backs are really only suitable for studio work with stationary objects with hot lights. That's perfectly suitable for their target market, which is studio work on product photography. For moving subjects or changeable light, they are not really useful.<br>

Your application really is limited to medium format digital backs. There's nothing larger for sensors that don't scan.</p>

<p> </p>

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<p>I saw an article a long time ago about a guy who used a 4x5 scanning back to do landscapes--but he was doing black and white work only. He did use his laptop and the images--the one I remember was a waterfall in Yosemite--were interesting in their own way.</p>

<p>In your case, I don't know that there is a good solution to color balance if the light changes during exposure (reshoot when they are more consistent would seem to be a solution). Just a caveat about scanning backs is that sometimes pixels do die and this will cause a colored line across the entire image where the dead sensor resides. It is not an uncommon occurrence apparently from those I know who have used them extensively.</p>

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<p>Brian:</p>

<p>No, a scanning back can't do that. Indeed, they provide no auto exposure at all. (although many have an amazing view camera auto focus feature) Using one is just like using a pro scanner, you preview the image, set your curves and balance, scan and save the resulting file on your computer.</p>

<p>During the days long ago when I myself used scanning backs, I would often take them out in the sunshine or set up still lifes under bright windowlight, but when the weather changed, I'd have to start again.</p>

<p>Finally, the newer models by Better Light (for example) use a regular USB connection to hook up to your computer. There's no problem at all. Older ones will use SCSI interfaces and will be a modest challenge to hook up.</p>

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<p>With the proviso that I've never used any of this technology... Here's how I'd do it as a software engineer:<br />

<br />

1) Point a web cam/other camera with fixed exposure at a white card, and start recording video.<br />

2) Do the scan, noting the start and end time.</br >

3) Get an average white level out of the images in the video stream, and use it to generate an image that varies in grey scale (or white balance!) over one dimension of an image the size of the final scan, and is constant in the other dimension.<br />

4) Demultiply the output of the scan by the resulting image.<br />

5) Swear a lot and spend a week fiddling with it.<br />

<br />

It's technically possible, and cheap, but probably less convenient than just waiting for the weather to hold still for a few minutes. I've done something not dissimilar to even out the illumination in a video sequence shot with a high speed camera under flickering lighting. Good luck if you try it...</p>

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<p>Andrew:</p>

<p>Your idea is amazing, but it would be far easier to just make another scan.</p>

<p>Actually and on second thought, I suspect that Brian's biggest problem out there in the greenhouses will be power - perhaps a long extension cord will do the job, but a battery and inverter might be called for.</p>

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<p>Ok folks. Thanks for the replies.</p>

<p>Looks like a medium format back of a non-scanning nature would come as a second consideration then. I read a bit about the Mamiya ZD but still looking to get probable prices on that. I only recently started thinking along the digital large format lines because I was on the cusp of getting into strobes for small format and was checking with myself I was chosing the best way forward. If anyone has a suggestion for the lowest price digital setup I could look for my ears are open! :) The ZD is a 22 MP back so you know whereabouts I'd be happy to be. Then again I'm not sure if my lens - Sironar 150mm 5.6 - would be good enough for that small a sensor. </p>

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<p>Brian Y: Yes, I don't deny that it's overkill, I was just saying it was possible. :-) (A webcam is overkill as well - all you actually need is an LDR and some kind of ADC - but they're so cheap these days I'd go for convenience.) Just using some continuous lighting (500W workshop lights are cheap and powerful) would also solve it.<br />

<br />

Brian T: You're thinking a scanning back for 22MP? My impression - as a small format shooter - was that scanning backs (at least, commercial ones) are pretty expensive beasts, but a good way to get 100+MP. If you only want 22MP, wouldn't a Canon 5D2 be vastly easier? (Northlight-images has a section on mounting one to a LF back if you want movements - if you just want a shallow DoF then there's the 50mm f/1.2.) The quality of medium format may be slightly better in good light; something like a Pentax 645 is again likely to be more convenient and give a quality boost. That said, maybe ZDs are cheap these days, although the 5D2 is reasonably likely to get an update within the next few months and therefore get cheap, if you can wait... Just my $.02.</p>

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<p>Hi Andrew. No, I was thinking of a scanning back at first but then because there was no easy (Yes I read your post! :) ) way to auto adjust ambient light changes during a scan, I picked up on what John Shriver said about medium format backs. It struck me I could still use my LF camera and get good IQ without extended exposure times and maybe for the same price as a top of the range 35mm SLR. I had a search (but didn't really know what I was looking for) and happened on a photographers website (pebble pebble something) who really rated the ZD - thats where the 22Mp comes from. I think I can quote him that the difference between the 22Mp ZD and Canon 1D Mark ...eeek...3? Was <em>night and day</em>. It was a Canon full sized 22MP DSLR he was comparing either Mark 3 or 4. If it was not that it was the 5D Mark 2. Though I must admit it was a ZD MF camera he was using and not just the back on ANOther body.</p>

<p>In the mean time, looks like I should get a darkroom set up and shoot some film. Haven't got the space to dev but I need somewhere to load/unload sheets.</p>

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<p>Hi Brian. I'm prepared to believe that a ZD has the resolution edge, at least because I believe it lacks a low-pass filter over the bayer sensor, but I doubt the difference is all that massive. I acknowledge that they should be cheap (as medium format digital backs go). It depends whether you're really after very high resolution images - if you were considering using the medium format back with movements on your large format camera and stitching the results, you'll have to do some kind of exposure balancing anyway. I'm inclined to say that a nice big sheet of Velvia and a drum scan might be much less effort all round, though. Good luck!</p>
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