What are the f-numbers on machine vision lenses? f-stop explained!

Why does 1stVision focus (no pun intended) so much on machine vision lenses. As the old saying goes, if you have garbage in, you get garbage out.

The lens is the input to the machine vision system. A low quality lens means that you have already degraded the image coming into the sensor. For instance, let’s say you chose a camera with 5um pixels, which equates to a lens being able to resolve 100 lp/mm. If your lens’ Modular Transform Function (MTF) is only 50 lp/mm, you should have chosen a camera with 10um pixel size, because the lens can’t do any better than that. As a note, don’t infer that a camera with 10um pixels is worse than a camera with 5umpixels from this example, as that is not true. Learn more on MTF here

A machine vision lens gathers light and then focuses it. When we talk about focus, we are talking about the MTF, but when we discuss light gathering properties, we need to discuss the lens f-number.

FUJI -f-stop — FUJI lens showing f-stops

f-number
The f-number is defined as the ratio of the focal length by the aperture width (diameter of the entrance pupil). So a 50mm focal length lens with a f-number of 2 has a 25mm entrance pupil. The lower the f-number, the more light will be allowed into the system, however this equates to more expensive lens as you need more glass to make a wider entrance pupil.

f-stop
Many camera lenses have an adjustable iris that opens and closes at the front of the lens to limit the amount of light coming in. When open all the way, the f-stop is the f-number. From there, each f-stop from wide open halves the amount of light, which corresponds to reducing the size of the aperture by 1/sqrt(2) or about 0.707 and in turn halving the area.

The f-stop is represented by a sequence of these numbers below, each letting in half the light.

Sequence: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64, f/90, f/128

The sequence is obtained by approximating the geometric sequence

Characteristics of the f-stop

Most lenses are designed to be optimal in the F4-F5.6 range, in which they have the best MTF.
The higher f-number (ie f/8 ) is, or the more closed the aperture is, better the depth of field if achieved
The lower the f-number (ie f/1.4) is, or the aperature being wide open is where you get the least depth of field, but not great MTF.

In a practical application, you need to trade off exposure time, depth of field, and available machine vision lighting. These three variables are always in tension. If you need fast exposure AND depth of field this means very small amounts of light gets to the sensor. If you need high contrast images in this situation, something has to change. Either get more light, accept less depth of field, or have some image blur.

For a full listing of machine vision lenses, click here and use the filter to help in your selection.

1st Vision’s sales engineers have over 100 years of combined experience to assist in your camera selection. With a large portfolio of lenses, cables, NIC card and industrial computers, we can provide a full vision solution!

Related Blog posts

Not all Lenses are created equal! MTF comparisons

10 key things you need to know when selecting a machine vision lens

Demystifying Lens performance specifications

Information courteous of Wikipedia

December 10, 2018December 19, 2018

Get a 5MP lens for the price of a 3MP one! FUJI’s new XA-5M lenses.

Fujinon introduced their first 5MP series, the 5MP SA-1s over a decade ago. Back then they were the first really high quality lenses for under $500.

Now, they have introduced their next generation 5MP series, the new XA-5M , and they have made them smaller, better, and less expensive! Now you can get a 5MP rated lens for very close to what 2 or 3MP lens cost with the new HF-XA-5M series lenses! 1stVision has preferred pricing and stock! Contact us for a quote

The Fujinon XA-5M are designed to work at 3.45um resolution (and very good at 2.5um), so they are a great choice for the 3.45um pixels found in the Sony Pregius IMX line of sensors. See image below showing a heat map of the resolution.(Darker red is higher resolution, 2.7um, orange is 3.3, yellow is 4um, light blue is 5um, and dark blue is 6um)

Further, the new CMOS sensors have more of a ‘stacked’ architecture, meaning each pixel is tall. Without having a lens that has a small chief ray angle, which keeps the light rays as close to perpendicular as possible to the sensor, each pixel possibly shades its neighbor. The new Fujinon lenses are designed to solve that problem for these sensors. So when comparing this line to their competitors, there is less shading at the edges and more even illumination!

The results can be seen when comparing the center to edges in an application. In the example below, the text in the center and edge on the FUJI “4D High Resolution” lenses is crisp vs the competition on the right.

Finally, Fujinon has dramatically reduced the size of the lens, all the lenses are 29.5mm in diameter except the 6mm, which is 39.5mm. Now you can get a 5MP rated lens for very close to what 2 or 3MP lens cost with the new HF-XA-5M series lenses! 1stVision has preferred pricing and in stock! Contact us for a quote

Watch this video for more details on how the HF-XA-5M lenses compare to the competition

Related Videos – FUJI is now incorporating anti-shock and vibration into their lens series!

Related Posts

Not all Lenses are created equal! MTF comparisons

There is NO such thing as a “Megapixel” machine vision lens!.. Say what??

February 22, 2018February 22, 2018

There is NO such thing as a “Megapixel” machine vision lens!.. Say what??

There has been a lot written about the ratings of machine vision lenses. 1stVision had created white papers that describe this in detail. However, the lens industry continues to use the marketing term, Megapixel Machine Vision Lens.

Let’s get this out of the way right now.

There is NO such thing as a Megapixel Machine vision Lens.

But since it is me against the world, let me explain why sometimes a 12 MP lens is really the same resolution as a 5 MP quality lens.

The first thing to understand is that lenses are evaluated on their resolving power, which is a spatial resolution. For lens used in the industrial imaging marketplace, this is normally given in terms as “Line Pairs per mm” (LP/mm). The reason it is expressed this way is because to resolve a pixel of “X” um, you need to use the formula, 1 / 2X where “X” is the pixel size and 2 is the Nyquist limit. So to resolve a pixel of 5um we need a resolution of 1/ ( 5um*2) per line pair. In LP/mm, this becomes 100 LP/mm.

A graph showing a lenses performance is shown in a plot below, plotting intensity vs. LP/mm. This is called the Modulation Transfer Function (MTF). Note that as the LP/mm increases and the lens can’t resolve it as well, the intensity falls off. This measurement is variable to F stop and angle of light, so real MTF charts will indicated these parameters. This is the only real way to empirically evaluate how a lens will perform.

You can visually compare lenses, but to truly compare Brand A vs. Brand B you would have to test them under identical situations. You can’t compare Brand A’s MTF vs. Brand B’s if you don’t know what the parameters used to test them are (need the same camera, with the same lighting, with the same focus, with the same f stop, the same gain, etc. etc.). Unfortunately its very hard to get that information from most lens manufacturers.

1, 3, 5, 9, 12 Megapixel lens?

Tamron 12MP MPY lenses — Compliments of Computar

What does this mean? As an example, Sony has recently introduced a new line of image sensors which have 5MP, 9MP and 12MP sensors. Many clients have called and said, “I want to use the 12MP sensor, so please spec a lens that can do 12MP.” Unfortunately, this isn’t correct as each of these sensors uses a 3.45um pixel. They ALL need the same quality lens! Why? Because it is the size of pixel, what you have to resolve, that dictates the quality of the lens!

In the above situation, the 5MP sensor needs a 2/3” format lens, the 9MP needs a 1” lens, and 12 MP needs a 1.1” format lens. (Multiply the size of the pixel by the number of H and V pixels to get the sensor format – more on format HERE ). However, this sensor needs about 144 LP/mm of resolving power as its a 3.45um pixel size. As much as I detest the nomenclature of “5MP lens” etc., I do appreciate what Fuji does; as they will state, “…. This series of high-resolution lenses deliver 3.45um pixel pitch (equivalent to 5MP) on a 2/3″ sensor”. Now this make more sense!

In turn, if you see a lens stated as a “Megapixel Machine vision” lens, question this! It really needs to be stated in terms of its capability to resolve the pixel size in LP/mm!

1stVision has a staff of machine vision veterans who are happy to explain this in more detail and help you specify the best lens for your application! Contact 1st Vision!

Additional References:
For a comprehensive understanding on “How to Choose a Lens”, download our whitepaper HERE.

Blog post: Demystifying Lens performance specifications

Blog post: Learn about FUJI’s HF-XA-5M (5 Megapixel) lens series which resolves 3.45um pixel pitch sensors! Perfect for cameras with Sony Pregius image sensors.

Use the 1st Vision lens selector allowing you to filter by focal length, format and manufacturer to name a few

November 29, 2017November 30, 2017

Best in class OEM machine vision lenses, low cost, high performance with an easy to use lens selector

1st Vision‘s factory automation OEM machine vision lenses are designed using glass from one of the top Japanese lens manufacturers. These lenses are not branded and allow our clients to have excellent performance at a better price than the ‘named’ brand manufacturers, getting the most bang for your buck!

In comparing specifications, our OEM lenses have better resolution in many cases over brand name lenses, but at ~ 30% less cost in some cases.

1st Vision lens comparison — Conventional lens (left) vs 1stVision OEM lens (right)

In resolution chart comparisons, the 1st Vision OEM lens on the right outperforms the leading lens in its class. All lenses have locking screws for focus and iris. High temperature options are available providing ranges from – 30 C to +70 C.

Details specifications and pricing can be found HERE

Its now easier than every to find a desired lens it the new 1st Vision lens selector. Sort by focal length, optical format and lens quality to name a few sort attributes. Try it now HERE!

Need help in your lens selection? Our sales engineers all have over 20 years of vision experience and can help you identify the best lens for your application.

If you like our lens selector, check out the new camera selector! Watch our short video highlighting helpful new features and selectors on our website

We also have several blog posts providing additional education on lens selection including handy focal length calculators. Click on the resources below.

Resources:
Calculating Lens Focal length
Video: Using the 1st Vision Lens calculator
Demystifying lens performance – MTF
Calculating your resolution for machine vision applications