Understanding Image Resolution

Understanding Resolution and DPI

I don't know how many times I have had this conversation only to see eyes glaze over with that deer-in-the-headlights look.

It really is not that complicated.

There are three things to consider.

1. Cameras


2. Monitors


3. Printers
   

Cameras are marketed by the number of pixels on their sensors . A pixel is the smallest element of an image.

Digital cameras range from 1 or 2 Mega Pixels up to 40 Mega Pixels or even larger.

A typical digital camera would be 8 or 10 Mega Pixels. For this discussion we will use the Canon EOS Rebel Xti 10 Mega Pixel Camera.

The Rebel Xti produces an image file that is 3888 pixels x 2592 pixels for a rough total of 10 million pixels.

For comparison we will use the Canon Powershot A590 8 Mega Pixel Camera that produces an image file 3264 pixels x 2,448 pixels. Multiply these two number and you get roughly 7.9 Million Pixels.

So why is having 2 million more pixels better?

Well, It's sort of like painting a wall. The more paint you have, (pixels) the more wall you can cover. Or, trying to cover a large wall with little paint will result in a splotchy paint job…

How does this apply to digital photography you ask…

Well, first we need to talk about DPI, (We should be talking about PPI or pixels per inch but for the sake of conversation and the fact that DPI is more commonly used we will stick to DPI or dots per inch even though technically we are talking about pixels). So, understand that a “pixel” is a “dot”. If you are a tech-head don't bust me out on this, trying to keep it at a tenth grade level here.

The human eye has its limits. Even the most near sighted person with his or her nose held directly up to a printed photo cannot resolve any more than 300 DPI. For this reason 300 is used as sort of a benchmark in photography.

Simply put, a human being can see the difference between 200 dots per inch and 300 dots per inch but cannot see the difference between 300 and 400… (Again, don't bust me out tech-heads… I know this is largely dependant on the content and detail of the image. We are working with generalities here today.)

So 300 DPI is our benchmark . Using the two cameras mentioned above we can now calculate the size of the image we can print using these cameras and maintaining our benchmark.

With the Rebel Xti 10 Megal Pixel camera we can print,

(3888 / 300) x (2592 / 300) or 12.9 inches x 8.6 inches

With the Powershot A590 we can print,

(3264 / 300) x (2,448 / 300) or 10.8 inches x 8.1 inches

So you see, The Rebel Xti puts more paint in your bucket…

As the printed image size increases, the 10MP camera will hold up longer.

Now, keeping things at a 10th Grade Math level you should be able to deduce that if we print duplicate photos from both of these cameras at 5”x7” we will se no difference as far as resolution is concerned. Both of these cameras have enough pixels to make decent 8X10 and smaller printed photographs.

A note about our Benchmark.

The 300dpi Benchmark is based on a person viewing a photograph at some standard distance, something in the neighborhood of 18 inches. So, for images in books and magazines 300 is a good number to shoot for.

For portraits and art that will hang on a wall or otherwise be viewed from several feet, a much smaller dpi can be used. In other words, you can print much larger for these images and still get away with it.

As mentioned above, the amount of detail in an image also plays into mix. A photo of a smooth surface takes little information to reproduce whereas a macro image of a bee or other insect will take allot of information or resolution to reproduce those details on the printed page.

But you said Monitors too…

Yes I did. You ever get an email from a friend with this image attached that is several times larger than your monitor?

Computer monitors generally display images at 72 DPI (Back off tech-heads!) You can do the above math using 72 instead of 300 to find out why that image is so big on your screen…

So, be nice to your friends and re-size or downsize that photo before you send it in an email.

Here are the top-ten screen resolutions used by visitors to weisd.com over the last year,

• 1024 X 768 (75% of the total)

1. 1280 X 1024


2. 1280 X 800


3. 1440 X 900


4. 800 X 600


5. 1680 X 1050


6. 1152 X 864


7. 1920 X 1200


8. 1300 X 2300


9. 1280 X 768

So you can now see that if you take the Rebel XTi image file and email it to you friend with a 1024x768 screen resolution without resizing it first, the image will be near 3 times larger than his or her screen.

My preference is to send images that are 800 pixels wide for horizontal or 600 pixels tall for vertical compositions.

Some programs like Microsoft's PictureViewer or Windows Explorer and others will automatically resize images so that they fit on your computer screen. This is convenient but these programs use simple and fast algorithms to do this and many times will result in an image that is degraded and just looks bad.

For best results, resize images for use in computing.

One more thing…

If you are shopping for a new printer you will see resolution specs that look something like, 4800 x 2400 dpi

Not going to go into a long discussion on this but I will say that everything said above has very little to do with this spec.

Simply said, most all printers that one would use to print a photograph are capable of putting 300 dots, (pixels) in one linear inch of paper. In keeping with the dummied-down 10th grade, (not picking on you Sophomores) explanation, one can think of printer resolution specs as how many dots a particular printer can fit inside one pixel.

This has more to do with reproducing good tones and colors than it has to do with the actual resolution we discussed above.

In Closing,

This is a very simple explanation of what can be a complicated subject. Visit any photographer's forum on the Web and you will find plenty of debate and discussion. However, I think this covers the basics…

DTV Antenna System Design

DTV Over-the-Air Antenna System Design
Patrick Ziegler Wholesale Electronics Inc.

The advent of DTV has left antenna installers in fringe areas sometimes scratching their heads.

We welcome any RF engineers or subject matter experts to share your thoughts on this subject by emailing us at info@weisd.com

In the past antenna system design was a pretty straight forward procedure. Estimate or measure the field strength of the transmitting stations, choose an appropriate antenna or antennas, calculate your losses in the down lead and choose an appropriate preamplifier to compensate.

If you needed a distribution system, you designed that and picked the appropriate distribution amplifier to maintain adequate signal strength to all receivers on the system.

DTV problem #1: Because of the nature of the DTV signal, you cannot measure the field strength of the transmitting station. So, you have to make the assumption that your favorite station is providing similar service as they did with analog TV.

Some stations have migrated away from their old frequencies. Others, who were previously transmitting DTV on temporary frequencies, have moved back to their original frequencies. Some will make these changes in June of this year.

Many low-band TV frequencies, (Chnls 2 through 6) have been abandoned for fear of issues with electrical interference.

To add to the confusion, we now have both, “Virtual Channels” and "Actual Channels, normally the channel you are use to referring to your favorite station as and what appears on you DTV receiver, and “Actual Channels”, the actual bandwidth and frequency that the station is transmitting on. Many times these two are not only different but in different bands.

You may think you are watching Chnl 5 but really be tuned to Chnl.23.

The so called, “Signal Level Meter” imbedded in most digital TV's and Digital Converter boxes is really not a “Level” meter but a measure of signal quality or readability. It is possible to have good signal level and poor signal quality. In analog we had, "Signal to Noise" ratio in the DTV world we have, "Bit Error Rate" or BER. Noise, interference and ghosting, (multipath interference) cause "bits" of data to be lost. When enough data is lost your receiver simply stops showing you the video stream.

DTV problem #2: Trouble shooting receiver problems with analog signals was very easy. Your receiver showed you the trouble. If you had a lot of noise in the picture that meant you had either no signal at the antenna or you where losing too much via the down lead or distribution system.

Multipath, skip and other atmospheric issues were evident in the picture..

With DTV, you cannot see these issues. They do not manifest themselves in the picture at all. The picture simply goes away or becomes too intermittent to watch.

People who live in out lying areas have become accustomed to watching less than perfect reception. Snowy pictures, multipath and electrical interference are things that people learned to live with. With DTV, these issues cause complete loss of the picture.

DTV Problem number 3: Digital TV's and Digital to Analog converters do not tune on the fly, as did analog TV's.

If you make changes to the antenna system or if the transmitting station has changed, you need to rescan the channels with your TV or converter box.

This is important if your antenna system includes a rotator. After turning your antenna, you will need to rescan.

So, What can we do?

From the output of the receiving antenna down, we can still go about things the old-fashion way. We can determine the highest frequency on our down lead, (using the “Real Channel”) and use this to calculate our cable loss and choose the appropriate preamplifier to compensate. We can do the same for our distribution system.

Calculating Cable Loss:

I think a good rule of thumb to use here is, (UHF –6dB/100') & (VHF –3dB/100ft). However you can look at the exact numbers for your particular cable on the manufacturer's data sheet.

For any hardware on the line add –1dB insertion loss plus whatever loss or gain that device may have at the frequency of the receive signal.

Add up all your losses and then choose a preamplifier to offset these losses.

Choosing the Antenna:

As a former Broadcast Engineer this is where things begin to depart from the old for me. It used to be easy to estimate what one could expect for field strength at the receiving end using propagation models and any free online service such as tvfool.com

Now we really have no numbers to work with. Signal strength and signal-to-noise ratio are impossible to measure with DTV today.

At this point, the best we can do using what we already know about antennas and TV reception and what we have learned about DTV reception is make an educated guess.

Metro Areas:

It is possible to have too much signal. If you are located close to a transmitting tower, you may need to attenuate these near stations. Overdriving the front end of a receiver creates noise; DTV and noise are a bad combination.

Multi Path in metro areas can also lead to issues. DTV receivers will not tolerate a strong Multi Path signal.

Rural and Deep Fringe areas:

Multi path is a problem here also as in metro areas as well as low signal. These with the addition of any other interference can result in a “Bit Error Rate” that your receiver will find unacceptable.

High-Gain Directional antennas and stacked antenna arrays can help. The more directional the antenna the less susceptible it is to ghosting (Multi Path) and the higher gain will give you more signal strength.

Vertically stacked antennas need to be ½ wavelength of the lowest receive frequency apart on the mast and the interconnect cables need to be exactly the same length. This allows the signal from both antennas to add together in the combiner.

Two vertically stacked antennas will give you near +3dB of gain over a single antenna.

Transmitting Stations in Multiple Directions:

In many rural areas you may find yourself between transmitting stations. You might have stations located in two or three different directions.

In this event, you may need to install two or three different antennas orientated in different directions.

Or, you might want to install a rotator to rotate a single antenna. The trouble here is, every time you rotate your antenna, you will need to re scan the channels with your TV or converter box.

A multiple antenna system might be a better alternative.

However, multiple antennas on the same down lead can lead to multi-path or ghosting issues.

To minimize this, antenna spacing is critical. Space antennas no less than 1/2 wavelength apart using the lowest frequency on the system. It might also be necessary to install traps or filters between antennas and combiners to attenuate multi-path signals.

Band-Pass filters built for NTSC or Analog Television may be problematic when used with DTV. Band-Stop filters might work better until the industry brings DTV filters to market.

In my opinion, many viewers on the fringes have lost Free-Over-The-Air television. As said above, they have watched snowy or otherwise marginal pictures for years and grown use to it. Now, on the fringes, they have lost reception.

Some can get it back with a new re-designed antenna system and others we will just not be able to help.

The FCC has always been an advocate for free over the air TV largely because of the Emergency Broadcast Network now known as EAS.

It will be interesting once all the broadcasters have made the switch what will happen and or adjustments will be made to our new DTV transmission system.

Hello and Welcome to Wholesale Electronics new blog. This is our first post and we are looking forward to hearing from you...

Wholesale Electronics is located in Mitchell, SD and has been in business here for over 30 years. We launched www.weisd.com eight years ago and have since found customers all over the world.

We are very excited to now have a forum for our visitors and customers to share their thoughts and ideas.