Can Photoshop kill the need for centre filters for WA lenses?

It was proposed sometime ago in this Forum that Photoshop filters

could be used to compensate for exposure variations throughout the

original image, simulating the effect of a centre filter. Has anyone

tried it and could anyone provide specifics as to how that is done?

The gradient filters I see in Photoshop 7 do not seem to me to lend

themselves easily to achieve similar results as centre filters, but

perhaps someone in this Forum knows how that can be done and will

share the secret. That would be much appreciated, thanks.

I don't know about Photoshop, but I do use a program called Picture Window Pro which has a center filter simulation built into it. You specify the negative size and the focal length and it attempts to make an adjustment. This can then be tweaked manually. I works fairly well as long as you don't crop before trying to use it. Do a search on Digital Light and Color to find the website from which the program can be downloaded. There is a trial version available for free. It is a powerful program designed with the photographer, not the graphic artist, in mind. Someone else asked about perspective control, for instance. Picture Window Pro has a good procedure for doing this.

I'm using PS 5, but can't imagine it's too different in PS 7. Select the radial gradient tool and set it for "color dodge" and draw the gradient line from the part of the image that corresponds to the lens axis out to the corner of the frame. Adjust the opacity and redo until it looks right.

It is better to do the correction "in camera". How much better depends on severe the light falloff is, and the exposure lattitude of the film (or digital sensor). If the light falloff is too much, the information might not be there on the film to be corrected, e.g., there might not be any detail in a shadow in the corners of the film. In some cases a post-exposure digital correction will work well, in others not so well.

 

As one example, by no means the worse case, with an 80 mm Super-Symmar XL focused at infinity and centered on 4x5 film, the light falloff in the corners is 1.8 stops. (Which is something to think about the next time one reads a photo writer pontificating on the need for 1/3 stop accuracy in determining the exposure.) Detail might not be recorded in a shadow in the corner, esp. with a slide film. Without the information on the film, no post-exposure correction method can bring the information back. With negative film, 1.8 stops light loss is more tolerable, especially if you increase the exposure to give the corners the correct exposure and to overexposure the center -- negative film has far more lattitude for over exposure than under exposure. On the other hand, movements such as a substantial front rise will make the light falloff worse.

 

Digital "center filtering" is a useful technique, but saying that it will "kill the need for centre filters" in all cases is going too far.

The answer is absolutely not. This question presumes that large-format photographers are all willing to print digitally, or that large-format photographers would intentionally produce technically inferior negatives/chromes with the intention of "fixing" them digitally, both of which are highly unlikely to happen on a large scale.

 

Photographers have long been able to dodge and burn images when enlarging and, theoretically, correct for exposure differences throughout the negative just like Photoshop filters, yet this has not obviated the use of center filters.

 

I agree with Michael Briggs that the center filter is used to correct for light falloff in the corners of the original negative/transparency. If that light falloff isn't compensated for by exposure increase and the filter, detail may be lost on the original and it can never be recovered.

Correcting for light fall-off works to some extent - I use it often to correct images where a CF was not available to use.

 

One important fact to keep in mind when working with color images is that effect on color saturation is highly dependent on the color space used - I have had good results un the Lab space, whereas digital dodging and burning in the Adobe 1998 space leads to higher saturation in darkened areas and lower saturation in brightened areas.

 

Photoshop can never rescue underexposed corners well, though. So if you use a 47XL on 4x5" then Photoshop cannot really compensate for a lack of CF.

While it is true that you can't get back information that never was recorded on film, and indeed it is better to get it right on film by using a center filter, if you know you are going to attempt to correct digitally, you can expose for it.

 

Just as you need to expose for the shadows to get all the information you need on neg film, for instance, if the shadows fall at the edge of the image circle, you may need to take that into account, and consider that you need an extra 1.5 stops (or however much is appropriate for your lens) to get the information, bearing in mind that you don't want to blow out the highlights, which may fall in the brightest part of the image circle. Of course with transparency film, you might have to split the difference or adjust the composition and placement of the lens axis (using rise/fall and shift) to get the hotspot where you want it, so that it doesn't detract from the composition.

It can be done but I'm with the rest about doing it in camera. It is awhole lot

more realistic. I've always done the things in camera, in camera masking,

filtering ect. and it has always been a better product. I have worked with PS

since version 2 and feel that there are alot of things you can do but in camera

is still the best way to do things.

It is also much faster to do it with a filter on the lens. And while the initial cost

for the filter is high, it is a one time investment as opposed to the time it takes

to do an equally high quality photoshop correction on many images, each

with diffeeent needs. If you are just thinking about doing one image every now

and then that is one thing, but if you are doing it a lot it would quickly become

a pain. Even if you are not a professional time is valuable because you could

be doing something else with it, like spending it with family, friends or maybe

even making photographs. As you get into digital darkroom work you will

discover that there is big difference between being able to simply automate

the purely mechanical steps (like this one) and the creative work.

as David says, incorporate it into your methodology. this isn't rocket-science, and is as easy as any dodge/burn operation. it isn't perfect, but you don't have to lose a stop or two with a centre-filter, don't have to deal with potential glare and other optical contributions, and as the inverse of Ellis's comments, you can take the monies you saved by not buying centre-filters for your lenses and taking the family and dog to Hawaii or Disneyworld.

 

within certain qualifications, we don't have to deal with absolutes here. photography should be about explorations. adjust your exposure to deal with the light gradient, if possible, and go off exploring.

I don't think anyone mentioned the part about changes in color rendition that result from various parts of the film getting more or less exposure. This is a bigger and killing problem than evening out the tone. You really need to buy the filter. In Photoshop, select the area thats too light or too dark and feather the selection a LOT (hundreds of pixels) then use curves to adjust the tone. You can use this to correct small differences in tone with varying degrees of success. Or you can make a layer via copy, adjust its tone to match your desires and erase the part that you don't need. But you'll still have a color problem. If you want to do consistent first-class work, buy the filter.

Henry, experiment with LAB colour-space and changes to the luminosity channel only. by the way, not everyone photographs in colour.

How about some of you guys posting some examples, some with and without the center filters, and then showing what photoshop can do on the one without a center filter so we can compare photoshop to the picture taken with the center filter?

It's only going to improve and become simpler in PS - with the sudden boom in pro digital cameras, everyone and their dog is bringing out suites of filters and they are getting better and better. I recently came across one with a one click action to correct falloff in corners - didn't do too bad a job, and I'm sure there will be plenty more where that came from

Hi Daniel,

Converting to LAB color space does not change whats on the film or what you have in our scan. LAB is a good place to do color corrections but it does not solve this particular problem. Imagine a solid color wall running all the way across the frame, blue perhaps. If the edge of the frame gets a full stop less light than the center of the frame it gonna be -different colors of blue- not just lighter or darker. Unfortunatley I experienced this first hand when using a rented super wide for a job and being supplied the wrong center filter. The wrong filter made a halo like ring which fortunately I caught on Polaroid before I went to film. I shot without the center filter and got the lesson from which I comment. I was able to mostly fix it in Photoshop. I've been scanning film and doing digital work since 91 or 92 and have handled thousands and thousands and thousands of files, this "no center filter" fix was NOT easy and its still not really perfect, but it got me by. I know more about Photoshop than I do Large Format cameras but not using the correct center filter on really wide lenses is asking for a lot of trouble! If its a 90mm you may not need one but I promise you if you're shooting color and you're using really wide lenses you're gonna want the filter and want it bad. Of course you're right about shooting B&W but even then its better to use the center filter. If you want a realistic reproduction of the thing being photographed then buy and use the filter. (I ended up buying a proper center filter for the rented lens because it was a month long job and I needed it - I KNEW I needed it.) I still say buy the filter. Why make life harder than it is?

Boy, I can just see it now. The magazine editor gets in a submission of 30 4x5 chromes & as he lays them out on the light table half get stuck from the scotch tape used to put the digitally printed neutral density filters in place. A couple of helpers cut their thumbs from the ones stapled in place after the scotch tape ran out. Then as he holds a chrome up to the window to look closely a staple falls into his eye & after surgery there are complications & he dies.

 

All because someone was too damn cheap to buy a graduated neutral density filter.

 

And I didn't even go into the international ramifications of what happens if the editor is Iraqui & Geo W thinks this may be some kind of new 'weapon of mass destruction'.

 

Worse than a paper cut I bet.

Thanks y'all for a comprehensive treatment of the subject. You have indeed been encyclopedic and I have learned much from your postings.