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Viewing Angle – how to compare?

DBF

Member
I have looked at the Data-Sheet of the Rodenstock HR Digaron-S 28mm.

The Viewing Angle is given with 101º.

For the Hasselblad HCD28 there are three Values given: diag./horiz./vert. 95°/83°/66°
Are these for a given (Size of) Sensor? If so, which one?

How can the Values of both Lenses be compared?
 
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Jamaicanpi

New member
If you are using both lenses with the same digital sensor/back, the image produced by each would cover the same field of view, as they are both 28mm lenses (there are some caveats, including numerical rounding, floating elements, distortion, etc).

Rodenstock provides the diagonal viewing angle based on the circular image produced by the lens (focused at infinity). They use the image cast by the lens, in part, because they expect their lens to be used with many different sensor sizes/shapes and also, importantly, shifted. Few, if any sensors, are large enough to capture a 101º viewing angle with the Rodenstock 28mm in one capture; However, it could be done by stitching multiple shifted images. If you used the lens unshifted, the sensor would be sitting within a large circle, and there would be a projection of an image all around the sensor that wasn't being recorded. Essentially, you would be cropping the image down to the sensor size.

The HCD lens is designed to be used with an H-series back, so Hasselblad provides the viewing angles based on the H-series sensor size. On the datasheet, they say it is designed for sensor sizes of 37 mm x 49mm (link). So, the HCD describes it's viewing angle based on an ~61.4mm image circle, while the Rodenstock is based on a 70mm image circle (link).

The HCD lens probably casts a slightly larger circle than the sensor, but it is designed around that ~61.4mm. How do you define how big of an image circle is cast by a lens? Arbitrarily... It's up to the manufacturer to decide based on their own criteria how large of a sensor/circular_image the lens is designed for.

I suppose one takeway is: if you want a wide field of view (and you aren't planning on shifting the back), shorter focal length determines which lens will produce a wider field of view.

I hope that helps...

Details
Actual focal length: Hasselblad is good about stating the actual focal length in their datasheets, the HCD28 is actually 28.9mm (datasheet). I didn't see it in Rodenstock's data sheet (link), but I think the Rodenstock is very similar at 28.8mm (based on a quick estimation using their 70mm image circle and 101 degree viewing angle).

The calculation is (note there is plenty of rounding in data sheets that can matter here):

Diagonal_Field_of_View = 2 * ARCTANGENT (.5 * Diagonal_Image_Size(or Diagonal_Sensor_Size) / Focal_length )

Don't forget to keep the units the same (generally mm), and to compute the arctangent (inverse tangent) using degrees.
 

DBF

Member
Thank You for Your Explanation.
I am looking for a Camera for "Walking around" should be as light as possible.
The HCD28 is my favorite Lens for Landscape Photos. So I looked at the Rodenstock (Data-Sheet) and found that Lens.
I want to use the my H5D60 Back (54x40 mm Sensor) and something like the Alpa TC
 

Jamaicanpi

New member
Conveniently, the sensor shapes are similar (aspect ratio of 1.32 vs 1.35 for the H5D60). When the shapes are substantially different, it's worth considering whether you want an equivalent horizontal/vertical/diagonal field of view. I am going to ignore that since there aren't infinite lens options available at all focal lengths.

Since the H5D60 is 10-12% larger along any one axis, an equivalent field of view would have a focal length that was ~11% longer: ~32mm (28.9 * 1.11).
Unfortunately for your wallet, Rodenstock has you covered :) But it may also be worth looking at Rodenstock's 35mm. I never hear much about it, but it's MTF is among Rodenstock's best. It's a narrower field of view by about 10% in any one axis, but it's a bit lighter and cheaper. Hasselblad also runs a HC35, which should cover the H5D60 too (based on the MTF chart, link).
Alternatively, you can capture a 10% wider field of view by using Rodenstock's 28mm, and again save money and weight.

Hopefully, a dealer or other members of the forum can help you refine your choice.

But the above math(s) is how you can look at the problem of equivalent fields of view. Good luck with your kit, and don't forget to share photos!
 
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spb

Active member
The title grabbed my attention! "Viewing Angel – how to compare?" - as I have never seen an angel ............
 

spb

Active member
No problem it made me laugh and grabbed my attention - I took it as a little typo that we all make....... nought wrong with your English.
 

DBF

Member
[U]Jamaicanpi:[/U] I found a used "Apo-Sironar digital 4,5/35 mm" on EBAY.
I looked at the MTF-Charts and for me these look very good.
Is that the Lens you ment?
I think, I will construct the "Camera-Body" myself. This is "only" a Thing, that keeps the Lens and the Back in the correct Distance together - of Course "tight" against ambient Light.
It will have a "Handgrip" with the Battery for the Back inside. I have CNC-Machine - so it should be possible.
 

Jamaicanpi

New member
I found a used "Apo-Sironar digital 4,5/35 mm" on EBAY.
I looked at the MTF-Charts and for me these look very good.
Is that the Lens you ment?
I was referring to a newer model of that lens, the Digaron-S f4, but I am not qualified to give buying advice for medium format; I shoot mostly scientific and 35mm cameras with tilt-shift lenses. You may want to create a new post asking about people's experiences in this 28mm-35mm range, and list the price range that you want to pay. There are many older lens options that might be satisfactory. There may be a great 32mm Schneider, etc. that could even match your HCD 28mm

I think, I will construct the "Camera-Body" myself. This is "only" a Thing, that keeps the Lens and the Back in the correct Distance together - of Course "tight" against ambient Light.
It will have a "Handgrip" with the Battery for the Back inside. I have CNC-Machine - so it should be possible.
That sounds very doable. As you say it's just an interface to hold the lens the correct distance away. However, you'll either need to add a focus ring or always shoot at infinity. I have pondered similar projects, but always come back to simpler solutions, evolving my setup: adding shift lenses, etc. There's a lot of knowledge on this board, and it may be worth a new post asking opinions and experience. Audii-Dudii has modified a handful of cameras, as have other members. Best of luck!
 
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