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Author Topic: Real Center Filters vs. Digital Center Filters: Which is Better?  (Read 890 times)
JoeKitchen
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« on: February 12, 2013, 10:03:18 AM »
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What do we think?  I know many people have given up on the real things, but is that because the digitals are better or are we just lazy and finding shortcuts?
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Doug Peterson
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« Reply #1 on: February 12, 2013, 10:18:20 AM »
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What do we think?  I know many people have given up on the real things, but is that because the digitals are better or are we just lazy and finding shortcuts?

"Digital Center Filter" is a marketing term. It just means the software is brightening up (or "pushing") parts of the image after capture. Like any "push" it increases noise in the shadows. In this regard it matters where you are starting from. If starting with an IQ180@ISO35 and a properly exposed image you'll increase the noise from "essentially none" to "very very little". If starting with a P25@ISO800 and an underexposed image you'll increase noise from "a good bit" to "a ton".

The Schneider/Rodenstock options for this can only be applied to a TIFF or JPG. If you're going to do digital correction it's much better to do this at the raw file stage for both workflow and quality reasons. This is what Capture One's LCC tool is for. It can be applied quickly, in batch, and draws off the quality of the original raw data.

The Schneider and Rodenstock optical center filters are of very high quality. Like any addition of glass there is an increase in propensity to flare and sometimes a minor loss of sharpness when using a very high res back and an absolutely ideal aperture like f/8. However I, and most of our clients, are glad to accept that minor loss of sharpness in exchange for a raw file which requires minimal, if any, light fall off correction. I want to emphasize that the loss of sharpness is considered very minor by most of our clients even in a subgenre which places a very high emphasis on image quality.

Obviously some lenses benefit from it more than others, and the amount you plan to shift the lens has a very large impact on whether you need to care about this issue at all. A Schneider 47XL shot straight on (no movement) at f/11 will not show much fall off so it won't matter much if you correct it optically or digitally. But a 47XL shifted 20mm will show a lot of fall off and if you correct it entirely using LCC you will definitely notice a difference between correcting optically and digitally.

In general with the Schneider wides I recommend using a center filter unless you are handholding and can't afford the loss of light the center filter mandates, or are shooting towards the limit of the long exposure capability of your back. With the Rodenstock wides it's a trickier question.
« Last Edit: February 12, 2013, 10:23:55 AM by Doug Peterson » Logged

DOUG PETERSON (dep@digitaltransitions.com), Digital Transitions
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theguywitha645d
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« Reply #2 on: February 12, 2013, 11:34:07 AM »
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A real center filter is better as it actually evens out the exposure giving more signal rather than just trying to pull it out from the noise in the data.
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Doug Peterson
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« Reply #3 on: February 12, 2013, 12:22:01 PM »
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A real center filter is better as it actually evens out the exposure giving more signal rather than just trying to pull it out from the noise in the data.

I assume you agree however that there are exceptions to this rule, like when handholding in a situation where a CF would lower the shutter speed below handhold-ability.

Also, we don't always want to correct the light fall off. Especially for street photography the vignette (IMO) is often very nice as-captured without a CF. While sometimes there is enough light to allow you to expose evenly and then add vignette, this is not always possible, nor can you see it's effect in the field (which lowers your ability to compose and react to the image with the vignette while in situ).

There is never a 100% right or wrong answer to a question like this. Though, as my post suggests, in general I agree with you: an optical filter approach is nearly always better.
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DOUG PETERSON (dep@digitaltransitions.com), Digital Transitions
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pixjohn
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« Reply #4 on: February 12, 2013, 02:29:05 PM »
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Real filter 100% better, but I use the digital lens correction in C1 7.

I just loose to much light with strobe. If I am using available light I use the filter. With the digital correction i see lots of noise zoomed in but in prit most clients will never see it.
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Paul2660
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« Reply #5 on: February 12, 2013, 02:53:27 PM »
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For sure, on the Schneider 35mm and 43mm, and the Rodenstock 28mm, the physical CF makes a huge difference.  The only really issue I have is the fact that most of the CF's correct by 2.5 stops and thus slow the exposure down considerably.   They even out the image on center and make a big contribution on a shift.  By use of just the LCC, I find that even at iso 50 you are asking the software to do a bit do much work and noise really starts to become an issue.

To me the answer is create a lens that works like a Mamiya wide, in that it sits out farther (due to mirror box) and thus has no vignetting issues.  Since most of these "digital" Schneider and Rodenstock lenses are meant to be used with a Digital back, I don't understand why this can't be designed into a lens. 

Paul Caldwell
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Paul Caldwell
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Doug Peterson
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« Reply #6 on: February 12, 2013, 03:35:31 PM »
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This article I wrote on wide angle lens design might be partially illuminating:
http://www.digitaltransitions.com/blog/dt-blog/lens-design

The Phase One 28D and other lenses designed around a mirror box have much less vignetting but the tradeoff is it is far harder to make them sharp at wide apertures, harder to make them evenly sharp across the frame, harder to control chromatic aberration, and harder to control distortion. All else equal it also makes the lens larger and heavier the further you place it from the normal symmetrical-design distance. Given that light-falloff is easily corrected with an optical center filter I would rather have a bit of light fall off than compromise on any of these other attributes. Wouldn't you?

Also when making comparisons of design bear in mind the Rodenstock 28HR and 32HR have a decently large image circle which allows shift/tilt/rise/fall while the Phase One 28D does not. It's also sharp to the edge of that larger-than-645 image circle while the 28D is not perfect in the corners when using a back like the IQ180 which is full frame and high resolution.

Also notably the 32HR has much less fall off than the 35XL for instance. Largely because it is designed as a mild retrofocus placed further from the sensor (but not as far as if it was designed around a mirror box).
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DOUG PETERSON (dep@digitaltransitions.com), Digital Transitions
Dealer for Phase One, Mamiya Leaf, Arca-Swiss, Cambo, Profoto
Office: 877.367.8537
Cell: 740.707.2183
Phase One IQ250 FAQ
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