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Using iPhone 4 as Light Meter to Calibrate a Digital Camera

Posted: 26 Jul 2010, 10:21
by univurshul
IMPORTANT: The comparison tests in this topic do not use photos taken from the iPhone, but they use the iPhone as the light meter (or exposure meter) to calibrate digital cameras based on given settings. All the images are taken from a Canon A590IS.

I've been experimenting with the iPhone 4 and it's camera. I'm routinely reminded of how intuitively it makes adjustments and captures images 'on-the-fly' in various situations. This isn't unique to solely the iPhone, but seeing how I have this development of a smartphone scanner-build in the works, I thought I'd run some cloning tests up against the Canons I already predominately use for scanning.

Does the iPhone have an algorithm that's worth cloning? The goal is better images, sharper photos, on the Canons. There's continual discussion about 'the best' setting for each person's camera setup for their respective mechanisms, lighting, etc. So, if the iPhone has a way to auto adjust very well in a multitude of environments, what--if anything--can the Canons learn from the iPhone?

I decided to try this clone-test and approximate these capture-settings on my scanner with 2x Canon A590IS. I snapped images of a book page on my scanner rig that use the same platen, same lights, etc. as the Canons normally do. This is what the metadata produced from a capture (no image pictured, only data) taken from the iPhone 4 on my scanner hardware:
iphone.jpg
iphone.jpg (74.26 KiB) Viewed 8328 times
I then mimicked these settings the closest I possibly could on the Canons

The photo below is my standard settings for scanning with a Canon A590IS, captured with the Canon:
ISO: 100
Shutter: 1/80
f/stop: F8
Standard Canon Calibration.jpg
(372.98 KiB) Downloaded 827 times
Cloning within proximity to what the iPhone 4 assessed on the same setup captured from the Canon:
ISO: 80
Shutter: 1/640
f/stop: 4
Canon cloning iPhone 4 Calilbration.jpg
(341.91 KiB) Downloaded 827 times
This a test-in-progress. I noticed I have 'exposure settings' limitations on my Canon A590IS. I'd like to try this on a HQ camera digi camera.

Otherwise I like where it's going. The clone settings appear to have less noise, less blown-out color halos and richer images for post processing. Although I have no post-processing tests posted yet, the results so far look pretty good to further the testing.

I'll update with more images other media when I have time. I have a feeling text-body books could have universal ideal cloned 'sweet-spot', and art-books, images, etc., could each have a unique new calibration.

Light Meter App for iPhone

Posted: 28 Jul 2010, 02:34
by univurshul
This app below is essentially a light meter and performs my above longer process in an easy short-hand form:

http://www.apptism.com/apps/lightmeter-2

Although you don't need Light Meter app to read the metadata of the iPhone's images it auto-snaps, this app will give you instant readings (as opposed to offloading the images into a photo-editing application that reads metadata). Having an app like this shaves at least 2-3 minutes every time you need a single exposure reading.

Notice the Lux readings. Maybe someone can tell me the usefulness of this feature, as I'm beginning to test multiple types of lighting and light strengths for scanning.

Re: Using iPhone 4 as Light Meter to Calibrate a Digital Cam

Posted: 30 Jul 2010, 01:54
by univurshul
Here are processed text images through Scan Tailor; one is a calibration performed by 'eye' using the Canon A590IS, and the second image is the Canon A590 calibrated to the iPhone's Light Meter app (as close as possible).
Canon Calibration by eye.jpg
Canon Calibration by eye.jpg (248.9 KiB) Viewed 8274 times
iPhone Calibration.jpg
iPhone Calibration.jpg (242.55 KiB) Viewed 8274 times
Results: The image calibrated according to the iPhone Light Meter app produced smoother, better-defined text characters. (notice the letter "k" in the loupe between the 2 images) Both are close, however after looking at the images in greater detail, the iPhone has the upper hand in metering exposures vs. human eye. More importantly, the calibration 'unknowns' are corralled by performing this technique. I can't tell you how many times I've looked through my viewfinder thinking I needed to make a change to my Canon's settings; most of the time these settings were incorrect, although they were tweaked because I thought I saw what "looked" correct.


I have to assume that color/artistic/detailed illustration media will benefit from this too. More testing to come...

Re: Using iPhone 4 as Light Meter to Calibrate a Digital Cam

Posted: 30 Jul 2010, 09:24
by daniel_reetz
I like the approach here -- iPhone specificity is not so great, but the rest is.

At SIGGRAPH this week, I spoke with many people about camera technology. One interesting fact that came up was that the color meter/WB sensor in small digital cameras has gotten so good that it's basically lab-grade. Presumably the same is true of the metering.

I'm not sure how each specific camera calculates the "right" settings (meaning, sometimes it's an extra sensor, sometimes it's through the imaging sensor), but at this point it's probably a good idea for us to start consolidating techniques into an article.

Guess that's my job. ;)

Re: Using iPhone 4 as Light Meter to Calibrate a Digital Cam

Posted: 30 Jul 2010, 15:08
by univurshul
daniel_reetz wrote:I like the approach here -- iPhone specificity is not so great, but the rest is. ...At SIGGRAPH this week, I spoke with many people about camera technology. One interesting fact that came up was that the color meter/WB sensor in small digital cameras has gotten so good that it's basically lab-grade. Presumably the same is true of the metering...I'm not sure how each specific camera calculates the "right" settings (meaning, sometimes it's an extra sensor, sometimes it's through the imaging sensor), but at this point it's probably a good idea for us to start consolidating techniques into an article.
Yeah, being camera noob myself, this method--if anything--is a friendly approach to engage the user with a creative way to enhance one's own self-teaching about taking 2D images with good results right off the bat. It throws you right into 'camera-speak', and kind of lays out the variables. I've learned that flat bed scanning is a system of absolutes, but with cameras, the subtleties are perplexing enough to desire answers in a way that are affirming on other devices. It's a great option to make use of devices like the smartphones, because it's a form-factor of testing that allows thousands of users to implement immediately and run with it.

Why not just use the Canon's Auto exposure adjustment?--Because the firmware is a biotch, and I can't seem to disable the flash in auto mode. Plus, Canon A590IS requires the user to offload the images like above just to see the image metadata.

Re: Using iPhone 4 as Light Meter to Calibrate a Digital Cam

Posted: 03 Aug 2010, 10:55
by Misty
univurshul wrote:Why not just use the Canon's Auto exposure adjustment?--Because the firmware is a biotch, and I can't seem to disable the flash in auto mode. Plus, Canon A590IS requires the user to offload the images like above just to see the image metadata.
Auto-exposure per page is probably a bad idea anyway. Different pages will read as being lighter or darker than each other and cause the camera to use different settings for them. It's better to use one setting across all pages to preserve original brightness variance.

Re: Using iPhone 4 as Light Meter to Calibrate a Digital Cam

Posted: 03 Aug 2010, 11:18
by univurshul
Misty wrote:[
Auto-exposure per page is probably a bad idea anyway. Different pages will read as being lighter or darker than each other and cause the camera to use different settings for them. It's better to use one setting across all pages to preserve original brightness variance.

It's per book, not per page. I don't have that kind of time anyway. Using a light meter (iPhone) to calibrate the camera has actually improved my images and processing.

Re: Using iPhone 4 as Light Meter to Calibrate a Digital Cam

Posted: 03 Aug 2010, 11:19
by Misty
Ahh, misread you. Thought you meant using the camera's auto mode so it calibrated settings each shot.

Re: Using iPhone 4 as Light Meter to Calibrate a Digital Cam

Posted: 03 Aug 2010, 11:25
by univurshul
Yeah, the issue with the Canon A590IS is that it doesn't have a way to act as a light meter, send you back metadata about the image, etc. If it engaged the user a bit more, maybe using a separate device to aid in calibrating wouldn't be necessary.

Re: Using iPhone 4 as Light Meter to Calibrate a Digital Cam

Posted: 03 Aug 2010, 11:45
by daniel_reetz
univurshul wrote:Yeah, the issue with the Canon A590IS is that it doesn't have a way to act as a light meter, send you back metadata about the image, etc. If it engaged the user a bit more, maybe using a separate device to aid in calibrating wouldn't be necessary.
Umm, that's not correct. It does have a meter -- that's the exposure meter on the right of the camera display on the rear. And it reports the same values in EXIF metadata, accessible in any capable image editor. Check it out!