File size

[Rekusu]

What ho one and all,

I normally shoot for stock and my MO is shoot RAW, open with ACR, fiddle around and save as the largest pixel offering in 16 bit tif. Then I have plenty of leeway for PS editing.

However, I have been doing a bit of pro bono work for my local parish council; pretty much reportage of local events, so lots of images have been created. If I go my normal processing route, it will take forever and at the end of the day, they only want jpg, not tif files.

In the past, I have found that using my normal MO, I can get an A4, 8 bit tif, save it as a jpg, and it is around (for example, 10mb.) Given that I know the pixel measurements, if I open the RAW reportage images in ACR, correct and set the defaults to the same pixel measurement, then hit the Save button, the saved file, although more or less A4 overall, is around half the mb size (or less) than a similar saved from an 8 bit tif.

So my question is, what is the difference between converting from an A4 tif and saving as a jpg, or saving straight from ACR. Both files are overall, A4, but the mb count is significantly different. What data is being lost? What difference does that 'lost' data make to image quality?

Toodle pip and tanks

Rex

 

[thebestcpu]

Hi Rejusu

Since you weren't specific, I believe when you save from Photoshop to JPEG you get a file size that is twice as big as saving the image (with similar pixels dimensions) from ACR to JPEG

There are a number of things that can impact file size yet the largest is the quality level settings.

Did you use the exact same quality level settings in ACR and Photoshop and were you sure to not constrain the file size in the ACR settings (which is one of the options).

It is possible that the options have changed with different versions of ACR and PS yet I believe the most likley culprit is quality setting being different between the two images.

As far as what you have lost, for the pro bono work that you are doing, if you cannot see the difference then likely it does not matter. Technically, the JPEG standard has many options that can be set by the software "quality" level yet in general, the image is broken down between luminosity and chroma channels and the chroma (color) content is compressed to a higher level than the luminosity channel as the human eye cannot detect compressed color as easily as it can detect compressed luminosity. Also, the highest levels of compression occur in the darkest areas and the brightest areas where it is even less noticed.

Hope that helps

John Wheeler

 

[Rekusu]

Thanks for your reply. Now that you mention it, there probably is a difference in the file saving quality. When saved from ACR, I have the quality setting on Max, but the numerical box displays 10. From PS, it is possible to go up to 12. That probably accounts for it. Why would Adobe not keep the settings range the same?

I'm not too overly concerned about loss of quality. Most of the images will be deleted, a couple may make the quarterly newsletter but at no more than around postcard size, and the remainder will probably drop into the parish archive where they would only ever (if at all!) be viewed on the web. These were really shot for the newsletter so once the editor has made their selection, that is it.

Toodle pip

 

[thebestcpu]

Your welcome Rekusu. In ACR it is also possible to go to 12. You just have to type in the number in the box instead of just using the dropdown. I bet the file sizes would match very closely if you typed in the same number used in PS.

I think the mystery is solved.

John Wheeler

 

[Rekusu]

Didn't think of doing that. Figured the options avaiable where the only options. However, I would say that I am using CS3 with whatever the default ACR is. I am happy with CD3, it does what I want but am unable to upgrade ACR without upgrading Ps as well. There are certainly more useful things in newer version of ACR.