LumoLabs: Understanding Image Sharpness

We have prepared our first White Paper. It shall serve as a base to better understand our methodologies to measure image resolution and more importantly, it is meant to help understand what factors can prevent an image from turning out tac-sharp.


LumoLabs website

We changed the layout of our site too.

LumoLabs is now at www.falklumo.com/lumolabs and hosts a repository of articles.

Therefore, we will use the blog to announce new articles or important updates to followers and interested parties. And to enable their discussion.

The actual articles are not posted as a blog article as its format was deemed unsuitable. But you'll find links to both the online article and a printable PDF version. If possible, we always recommend to download and read the PDF version. The PDF version does update more frequently too ;)


Understanding Image Sharpness


(Sample chart form the article)
Hint: The article image URLs actually open as larger images as they appear embedded in the article.

This article is a recommended read for anybody loving to dig into technology and who isn't afraid of a bit of math.

It's abstract and table of contents is:

Abstract
This White Paper is one in a series of articles discussing various aspects in obtaining sharp photographs such as obtaining sharp focus, avoiding shake and motion blur, possible lens resolution etc. This paper tries to provide a common basis for a quantitative discussion of these aspects.

Table of Content
1. Measures
1.1. Modular Transfer Function
1.2. Blur
1.2.1. The hard pixel
1.2.2. The perfect pixel
1.2.3. The real pixel, sharp and soft
1.3. More realistic resolution measures
1.4. Combining blur
2. Sources of blur
2.1. Defocus
2.1.1. Ability of deconvolution operators to reduce defocus blur
2.2. Bayer matrix and anti aliasing
2.3. Diffraction
2.4. Lens aberrations
2.4.1. Defocus, Spherical aberration, Coma, Astigmatism
2.5. Shake
2.5.1. Measuring shake
2.5.2. Expected shake
2.5.3. Empirical results
2.5.4. Tripod classification
2.6. Motion blur
2.7. Noise
2.8. Atmospheric perturbations
2.9. Precision and calibration
3. Practical considerations and examples

Please, proceed here:

• Online article (same as click onto blog title)
• PDF print version

 

Apple iPhone 4 camera specs

On 2010, June 7 His Steveness has introduced the Apple iPhone 4 which features a new camera module. Two actually but I am going to talk about the main camera (on its rear side) only. And only about its still photography capabilities (it records 720p 30fps HD video as well).

It is typically hard to figure out the exact camera specification for cell phones and to compare them with a P&S camera. E.g., the HTC EVO 4G is said to have an 8 megapixel camera but little else is known. Images shot with the EVO all lack the significant EXIF information!

However, I think I managed to extract the full iPhone 4 camera specs now.

First, the pixels are 1.75 µm (according to Job's keynote) and we may assume that this is the pixel pitch. Sample images are 2592x1936 pixels (5.0 MP, 4.02:3). This makes a 4.54 x 3.39 mm^2 chip (5.67 mm diagonal) which corresponds to a 35mm film camera crop factor of 7.64.

In a P&S, this would be called a 1/3.2" sensor and P&S sensors range between 1/3.8" and 1/1.5" (crop factors between 8.6 and 3.9). So, it is an imaging sensor at the lower end of the P&S range, but it is in the P&S range and with a reasonable number of pixels and a back-illuminated sensor like the Sony Exmor R sensor. Here is a sample image from Apple's web site:


(click on the image for the sample in original size; hosted on apple.com)

Gizmodo.de has published the full EXIF data for exactly this sample photo (it is magically absent from the web sample though ...):

Aperture: 2.97
Color space: Uncalibrated
Date and time: 2010/05/07 15:14
Exposure: Auto exposure
Exposure time: 1/887
Flash: Flash did not fire, auto mode
FNumber: 2.8
Focal length: 3.85
ISO speed rating: 80
Metering mode: Spot
Sharpness: Hard
White balance: Auto

So, the lens is a 3.85 mm f/2.8 lens.

This corresponds to 35mm film camera equivalent lens of 29.4mm f/21.4.

I should note that even an aperture this small is not restricted by diffraction yet (limit at f/2.8 is at 54% of pixel width). This is a good value. But one really shouldn't stop down beyond f/4 and it seems the iPhone 4 has an f/3 fixed aperture lens anyway.


Summary

iPhone 4 main camera:

• 5.0 Mpixels (2592 x 1936)
• 1/3.2" back-illuminated CMOS sensor
• 4:3 aspect ratio
• 35 mm film camera crop factor: 7.64
• Low ISO 80 (or better)
• 3.85 mm lens focal length
• f/2.8 lens aperture
• Autofocus: tap to focus

Equivalent 35mm film camera and lens:

• 30 mm f/22

So, don't throw away your FourThird, APS-C or full frame SLR ;)

Besides the deep depth of field, an f/21.4 aperture at ISO 80 produces as much noise as an f/8 lens at ISO 570 (for a given shutter speed). Where 35mm film camera f/8 and APS-C f/5.6 produce the same level of noise.


Conclusion

The iPhone 4 seems to have a camera which should deliver an image quality on par with the lower segment of P&S cameras. It's field of view (no zoom!) corresponds to a 30mm lens (on a 35mm film camera) and it captures as much light (produces as much noise) as a system camera with a lens aperture between f/11 (FourThird) and f/22 (35mm film).