View Full Version : Lens for Full-Scale Schlieren System!

Schneider Ron
5-Jul-2013, 11:28
Im working about a Full-Scale Schlieren System and I want to take pictures about the schlieren effect!
For this, I have a FASTCAM SA3 brand PHOTRON (digital camera) and I have to select and buy a large format lens!
whats the criterion for to select a Lens?
Let me explain the application!
1. The lens proyects the image on a Frosted glass or screen or photographic film (This is the image plane).
2. The FASTCAM takes pictures to this image plane.
I have two lens options:
a) APO SYMMAR 480mm/8.4 and b) APO DIGITAR 210 mm/f5.6, whats the best option!

5-Jul-2013, 11:42
There's insufficient information to assist in my opinion.

Knife edge schlieren, color schlieren or synthetic schlieren ?

What is the image plane size ?

Do you have a diagram of your set-up ?

Bob Salomon
5-Jul-2013, 12:38
You have two lens options or you already own these two lenses? There should be better lens options.

Since both are corrected for applications other then flat field copy work neither sounds very practical. Also both would need a lot of extension for closeup work but the 480 would need several feet of extension unless you are quite a few feet or yards from the image plane.

Schneider Ron
8-Jul-2013, 08:05
Hellow Amadeus!
Full-Scale Schlieren System is integrated for three sub-systems: 1) cam, lens and ilumination, 2) test section and 3) source grid!.
The cam, lens and ilumination configuration is integrated for a FASTCAM (digital cam), a LARGE FORMAT LENS and an ilumination system.
The Full-Scale Schlieren System operation is described bellow:
1) the lens and ilumination are located in the same plane. after the system is alignated, the lights are turned on. The lights travel to the section test and cross it. After, the lights iluminate the "source grid" (done with retroreflective material) and come back to the lamps and the lens region (obviously, the lights cross the test section again!).
In this moment, the light crosses the lens and the image is proyected on a on a Frosted glass or screen or photographic film (This is the image plane). After, the FASTCAM takes pictures to this image plane.

In cam-lens section I have to employ a cam-lens configuration and I have to select a lens of this two options: a) APO SYMMAR 480mm/8.4 and b) APO DIGITAR 210 mm/f5.6. Why those options? APO SYMMAR 480 mm/f8.4 was employed in a Full-Scale Schlieren System done in the Gas Dynamic Laboratory in the Penn State University.This lens was selected for its chromatic fidelity, resolution, and resulting sclieren sensivity98385. I want to use this lens because is a tested lens in Schlieren photography. But I, this is the issue: APO SYMMAR 480mm/8.4 is a Large Format Film lens and I have a Digital Camera. Schneider dealer told me that APO SYMMAR 480mm/8.4 and FASTCAM SA3 is a nonconvenient configuration beacause I have a digital CAM and APO SYMMAR 480 mm/8.4 is a Large Format Film Lens. He recomended me APO DIGITAR 210 mm/f5.6 for this issue becasue the APO SYMMAR 480 mm/8.4 get a "soft image".

For the system, I only have defined the test section. This has a section of 2m x 2m and the average distance between the "source grid" and lens region is 10m.!

I send an image of the Full-Scale Schlieren System of the Penn State University!


8-Jul-2013, 19:30

What is the distance between the 2m x 2m test section and the lens ? This would allow you to accurately calculate the angle of view required.

The lens should cover ideally the same angle of view. Since you know the size of the image sensor in the FASTCAM SA3 (nice high speed camera ;-) with a square sensor) ... you can now calculate the optimal focal length of the lens required.

The 480mm lens is probably inconvenient from a coverage perspective with an 18-20mm sensor (assumption of my part ... ) even the 210 mm is mostl likely too long but one and other really depends on the distance lens-to-test section as this will determine the angle of view needed.

The distance of the 4m x 5m source grid to the lens is somewhat important as at a distance of 10m with a diagonal of ~ 6.4m you'll need an angle of view of ~35 degrees. Assuming you want to cover the source grid and equally assuming your sensor has a diagonal of ~25mm, you'll need a lens with a focal distance of roughly 78mm (or shorter) to approximately cover the source grid.

Since the test section is positioned between camera and source grid, expect the focal length needed to be longer than the previously calculated 78mm but I believe also shorter than 210mm. (2m x 3m test section diagonal is 3.6m and at a distance of 10m would require an angle of view of 21 degrees or a focal length requirement of roughly 135mm.

So, assuming a 25mm sensor diagonal, you'll end up with a focal length of 78 to 135mm.

Schneider Ron
9-Jul-2013, 08:13
Hello again Amadeus!
Thank you for your advices!
I did the calculation and my angle of view is near of 22! This angle is convenient for lens with a focal lenght of 70mm-135mm!
I did the calculation employing the data describe bellow:
The distance between lens and source grid is 11.88m and the distance between the test section and lens is 5.94m (half of 11.88m).
moreover, the sensor dimensions are 20.48mm x 20.48mm (diagonal of 28.96mm).
Considering this, How can I calculate the optimal focal lenght of the lens requerid?
How affect the sensor dimensions on the optimal focal lenght?

10-Jul-2013, 17:06
Hi Ron,

Since your test section is 2m x 3m and 5.94m away from the lens, the angle of view for the diagonal of the test section is 2 arctan (1/2 diagonal test section / distance lens-to-testsection) or roughly 34 degrees

You have to satisfy this equation also at the other side of the lens if you want to capture this test section fully without any cropping. You'll need a lens of a FL of roughly 47mm (or shorter) to yield an angle of view of 34 degrees if your diagonal of the sensor is 29 mm

For a more or less detailed explanation abour angle of view and camera field of view that will help you calculate your requirements in case they change, you can check http://en.wikipedia.org/wiki/Angle_of_view

Hope this helps,