1. Re: Epson V800 Calibration Area

Originally Posted by IanBarber
Is there a method you can suggest to measure the value of the white or is an understanding of the formula required for this
Lots of simple photo tools, even a text editor if you use an image with consistent density, e.g. clear film, Photoshop probably as well but I never use it, if I measured your graph at 92%. Then in photoshop if you took the RGB value of and I am guessing this is monochrome image, and then expressed it as a percentage it should either be 92% or 83% depending on if you told photoshop that the file was not gamma encoded. 83% is 92% ^ 2.2

Ultimately if the real value is 92% then you want to all your calculations with 92%, all behind the scenes calculations need to be done in a linear colourspace, otherwise they don't work... How you display that to a user is something different entirely. Our perceptual model is not linear (mathematically) at all that is why we have things like the L in photoshop in the first place.

If you post the file that is probably the easiest :-)

2. Re: Epson V800 Calibration Area

Ted,
I have found this a very interesting discussion. I understand nearly all of what you have explained, just a little confused with the latter about measuring the white. I use Photoshop mainly.

My initial thought pattern was that if the RAW output gave me an L value of 97 after the file was assigned a gray gamma of 2.2 then this would have represented 97% brightness. I just need to work more on this to get a grasp of what you was explaining.

3. Re: Epson V800 Calibration Area

Originally Posted by IanBarber
Ted,
I have found this a very interesting discussion. I understand nearly all of what you have explained, just a little confused with the latter about measuring the white. I use Photoshop mainly.

My initial thought pattern was that if the RAW output gave me an L value of 97 after the file was assigned a gray gamma of 2.2 then this would have represented 97% brightness. I just need to work more on this to get a grasp of what you was explaining.
the L is very complicated formula that attempts to model human perception and includes colour in that percetpion, but the input is the real values that can measures with instrumentation. In the case WE ARE discussing is to get the real values used in the INPUT. Plus you never know if photoshop does it correctly... Basically cut to the chase and don't confuse the issue.

I have attached a tiff file that is 100x100 pixel it contains all pixels that have a value of 92% of 2^16 i.e. 92% of the biggest number you can store. If you open it in Photoshop hopefully there is something that shows it 92%,or the number 60292, or 235 if you using an 8bit scale. I put the number 60292 directly. If your doing digital image processing you need to use a real linear scale, i.e. 1,2,3,4,5,6,8,9,10 and you want to use the real numbers that get from the sensor, or you use some calibrated number and you are aware its calibrated.

Its a complex topic, which I tried to simplify...

Actually I can't seem to add a tiff file so I will attach a jpeg with the same data. (actually 92.16%)

4. Re: Epson V800 Calibration Area

Thank Ted.

In Photoshop using the 8bit RGB sampler, it does indeed show 235

5. Re: Epson V800 Calibration Area

Originally Posted by IanBarber
Thank Ted.

In Photoshop using the 8bit RGB sampler, it does indeed show 235

Ok so take that value divide by 255 and you have the raw value, without gamma applied its 92.16%.

If we imagine that value represents the light hitting the sensor in a give time by the hardware, and you put a piece of film in between the light and sensor that lets 18% of the light through. It would look to the human eye about half of the light was removed, but the sensor would record 18% of 92% or 18% of 235 which is 42. The sensor is not perfectly linear but it not bad through its sweet spot.

42 is 16.4% yet most people would describe this perceptually as around 44% give or take. For image processing your are usually interested in the former value. Real people are more interested in perceptual scale that makes some sense, and there is a disconnect. In analogue photography we use a real values but we use logarithms so we can use ratios to deal with the real linear measurements underneath.

I.e. a char curve has the real values that are measured by a linear sensor, plotted as log values.

6. Re: Epson V800 Calibration Area

Do you have any RGB exposures in vuescan you have recorded for say FP4 FomaPan or HP5, just curios to see how they relate to what Im testing at the moment or would that be a false comparrison

7. Re: Epson V800 Calibration Area

Originally Posted by IanBarber
Do you have any RGB exposures in vuescan you have recorded for say FP4 FomaPan or HP5, just curios to see how they relate to what Im testing at the moment or would that be a false comparrison
I will try and find some test shots of grey scales that I am sure I created with HP5. I do my testing and software dev with color film, and real photography in black and white. If your shooting in B&W a lot of things are simplified. Often as a starting point you want to try and mimic the resultant tonal scale of grade 2 paper when used with normal b&w film.

I think color perfect offers that, I have done something similar.

For B&W and 16bit you can always recreate anything with a curve, that is NOT the case in colour.

8. Re: Epson V800 Calibration Area

Before I retire for the night, something just occurred to me.

Because the film edge is very narrow, when placing the rectangle around it in VueScan to establish the exposure for nearly white, under the crop Tab in Vuescan is a Buffer % which ignores inside the crop box when calculating the exposure. I am wondering whether this needs turning down from the default value of 8%

9. Re: Epson V800 Calibration Area

Originally Posted by IanBarber
Before I retire for the night, something just occurred to me.

Because the film edge is very narrow, when placing the rectangle around it in VueScan to establish the exposure for nearly white, under the crop Tab in Vuescan is a Buffer % which ignores inside the crop box when calculating the exposure. I am wondering whether this needs turning down from the default value of 8%
Possibly, I just used the film leader, and for 4x5 I just had many spare sheets that I forgot to press the shutter, before I developed the film... A long as you get around 80-90% that is fine. Its the consistency that I am mainly after.

10. Re: Epson V800 Calibration Area

Ignore this comment, user error

Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•