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

machine vision lens

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

Click hereA 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.

f-number
Diagram of decreasing apertures, that is, increasing f-numbers, in one-stop increments; each aperture has half the light-gathering area of the previous one.

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

s-stop 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.

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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

 

 

 

Allied Vision’s high-resolution Prosilica GT now with 12, 16 and 25MP ON Semi PYTHON sensors

Allied Vision GT camera

Allied Vision Cameras

Allied Vision extends its Prosilica GT Large Format camera platform with three high-resolution ON Semi PYTHON image sensors (Python 12K, Python 16K & Python 25K)

Combined with robust thermal housing designed to operate in extended temperature ranges and fluctuating lighting conditions, the new Prosilica GT Large Format cameras are ideal for high-definition imaging applications with demanding requirements of robustness and design-in flexibility. They provide a great option for high-quality imaging for anyone looking to switch over from existing CCD cameras to CMOS cameras with similar resolutions and optical formats.

Features include:

  • Extended temperature range (-20 deg. C to + 50 deg. C)
  • NIR sensitivity up to 1100nm
  • Modular concept providing various lens mounts (M58, M42, and EF-mount)
  • Fixed Pattern noise correction,  Defect Pixel Correction, and Trigger over Ethernet.

Prosilica GT5120, Prosilica GT4096, and Prosilica GT4090 at a glance

AVT GT specifications

See full specification for the series below:  

Prosilica GT5120,  5120 x 5120, 25MP

Prosilica GT4096, 4096 x 4096, 16MP
This is a great replacement for the KAI-16000 CCD sensors!

Prosilica GT4090, 4096 x 3092, 12MP

Now that you have a great high resolution camera, what lens works best?Kowa F-mount lens

High resolution cameras with the ON Semi Python sensors are great, but only as good as the lenses you use!  It is imperative to have the sensor sizes matched with the lens formats.  Additionally, we need to make sure the lens resolution is adequate for the image sensor pixel size.

To aid in the lens selection, we have the following recommendation on lens series.  Recommendations as follows

Prosilica GT5120,  5120 x 5120, 25MP:  This camera has a  32.58mm diagonal image sensor, so a F-mount lens is required.  Two options are Kowa and Schneider
KOWA – F mount series
Schneider – F mount series

Prosilica GT4096, 4096 x 4096, 16MP:  This camera has a 26.06mm diagonal image sensor, also requiring a F-mount lens.
 KOWA – F mount series
Schneider – F mount series

Prosilica GT4090, 4096 x 3092, 12MP :  This camera has a 23.09mm diagonal image sensor which falls into a 4/3″ format.
KOWA 4/3″ lens series

As a note, as the pixel size is 4.5um, this is not as demanding on the lens resolution and will only require 111 line pairs/mm (lp/mm) making these adequate selections. 

Need more help in understanding lens terminology and format sizes?  Here are some related links

1stVision has experienced engineers to help you develop and further understand the best camera and lens combination for your application.

Contact us to talk to an expert!

Considerations in selecting a lens for a large format camera sensor

For most cameras in the automated imaging marketplace, the sensor sizes are 1” and less. (1″  sensors have 16mm image circle/diameters)

Note the size in inches and the image circle size in millimeters do NOT match!  1” does not convert to 16mm!.  But that is a topic for another blog post!)

Industrial cameras with C/CS mount sensors normally range from ¼” to 4/3” ( up to 22mm image circle, which is the largest a C mount is by definition).

However, there are many cameras that have sensors larger than this, with many being a full 35mm sensor.  These sensors typically require larger F or EF mounts as seen in the picture above.

Click here for F-mount lenses from Kowa

Click here for F-mount lenses from Schneider

The question becomes, how do you find a suitable lens for these cameras?

First, let address the Issues with F mount lenses

The mount most commonly used by camera vendors for sensors larger than C mount are F mounts.  F mounts are most popular mount for SLR cameras, designed by Nikon about 1960. https://en.wikipedia.org/wiki/Nikon_F-mount

F mount lenses for commercial and consumer/prosumer cameras are excellent optically and priced well but there are several issues you need to consider with these lenses for automated imaging applications.

  1. First, F mount lenses are bayonet, not screw mount. That means they are not as secure as C mount, and therefore, much more prone to vibration, or even coming lose.
  2. F mount lenses for the consumer/prosumer market are designed to take fantastic color pictures. To do this, the lens coatings are optimized for color. This can affect mono images.
  3. Consumers want their cameras to be light, so the F mount lenses are made with as much plastic as possible. This is not great for industrial applications
  4. These lenses are designed for automatic focus and exposure and have controls for that in the lens itself.
  5. Finally, the last issue is that many of these lenses are designed for the automated controls with prosumer cameras, autofocus and autoexposure.

If you can find an all metal manual F mount lens from 20 to 30 years ago, it would be great for an industrial application!  If not, we suggest you turn to one of the companies that make F mount lenses specifically for this market. The major vendors are Schneider, Edmund Optics, and Kowa.  All have a series of F mount lenses that are much more rugged than a prosumer F mount lens, and also don’t have the extra controls for automatic focus/iris.  They have lock screws as well to hold their settings.  The major limitation to these lenses are that they cost more than prosumer lenses, and are in just a small range of focal lengths.

Contact us for further specifications and pricing

EF Lenses

Since many of the new large format lenses can be controlled electronically via the camera, the automated imaging industry has created several products to use these lenses. The solutions from Birger Engineering, www.birger.com are straightforward to use. They make a mount that attaches to the  camera and allows for the use of EF mount lenses.  Attached to their mount are connections to drive the lenses, via serial commands with their SW.  Several camera manufacturers have built cameras with EF mounts as part of the camera.  This is an especially nice solution since there are no wires hanging out for control, and the lens is controlled directly from the camera’s SDK.

This provides a high level overview of the considerations in selecting a lens for large format camera sensors.  1st Vision has extensive knowledge with lenses and cameras and will help you in the selection process.

Please contact us and discuss your application with our sales engineer! 

 

Related Blog posts:

Canon EF mount integrated into Allied Visions GT1930L

Imaging Basics – Calculating lens focal lengths

Not all lenses are created equal!

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

Improper selection of the correct quality and type of machine vision lens can make your camera selection useless!

In many cases, we see customers attempting to choose a low quality lens for a camera that really needs quality optics to resolve the pixel sizes. Conversely, we also see customers trying to over specify a lens, where you simply need a lens that’s “good enough”


1st Vision has a white paper to help in the lens selection and provide valuable knowledge on various topics to make the best decisions. 

 

In this comprehensive white paper, you’ll learn the following

  1. Background basics on lenses and what they really do!
  2. The science in some basic formulas to calculate MAG, focal length and angular FOV
  3. What are extension rings used for?
  4. How are lenses rated?  MTF explained
  5. The myth behind “Megapixel” lens ratings
  6. How many pixels do I really need?
  7. What is the ” f# ” of a lens
  8. What does the “lens format” mean?
  9. Comparison of lens types from security to Factory Automation (FA)
  10. How to get some practical advice!

Download the White Paper

1st Vision has many other resources in our educational blogs.  Please find related links to lenses below.  

We are happy to discuss your imaging application in details  Just contact us!

Links: