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Author Topic: Focal Length question  (Read 2576 times)
x-ray
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« on: September 24, 2005, 07:49:30 AM »
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Hi All

Just wondering if anyone can answer this question for me? Have checked with the manufactures web sites and not been able to find the data I need. I am cosidering the purchase of a digital back for use with a Hasselblad 503cw system. I want a good selection of lenses, and at this time do not feel like changing to a 645 platform. I like the way my 503 handles and feel that it has a future. I understand from my reading that the focal lengths and angle of view match up with a 645 platform more easily due to the 4x5 cm sze of the CCD.

This is where my question comes in. The CCD on the digital back measures 4x5cm. So what is the factor the focal length of the lens is multiplied by on a Hasselblad V kit? I have had numbers from 1.2 to 1.49 thrown at me. What is the angle of view I can expect from the lens.

I ask these questions because at present I use a pentax 67II kit. Having used this commercially for a number of years I know what lens to pick up for a type of shot I am trying to produce. So 45mm for wide angle for example. The only problem is that after many years loyal service the Pentax is going to have to be retired as it can not except a digital back. But what I want to do is Purchase a system of lenses for the 503cw that I can use in a similar way. But as I can not at present find any info on what the effects opticaly will be capturing to the smaller CCD size I am a little stuck. I know the wide angle will an issue. I intend to use a flex body here. It will also allow me a little movement from time to time if its needed on a job.

I thought this would be easy as Hasselblad made the A16 120 Roll Film Holder for 6x4.5cm. But do you think I can find any info on how the lenses perform with this? Any help would be greatly appreciated. Thanks.

X-Ray
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Anon E. Mouse
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« Reply #1 on: September 24, 2005, 09:06:44 AM »
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Usually, the crop conversion is a ratio of the diagonals of the two formats. If I did my math right, it is a crop factor of about 1.3. however, you are going from a square to a rectangle and I find the psychology of how we percieve the two areas is different. I would suggest dropping a 4x5 mask in your Hassleblad viewfinder and looking through it.

BTW, is 4x5 the sensor size or the area of the sensor used to capture the image? There is a small difference both for the crop calculation and mask size. But the mask should give you a feel for the new format.
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williamrohr
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« Reply #2 on: September 24, 2005, 10:48:05 AM »
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This is a chart of calculated focal length equivalents, based on angle of view, which as Anon notes is the generally accepted way to answer your question. The top row is the 35mm focal length, 2nd row the calculated angle of view across the diagonal. The following rows are the medium format focal lengths necessary to achieve the same angle of view for a particular format. The Aptus numbers can be substituted for any other digital back which uses the same size imaging chip. As you can see the smaller imaging surface of the digital backs leads to the common problem of a lack of available lenses to achieve "wide angle" images. That, combined with the issue of chromatic aberation is why I still use film for medium format "wide angle". One unique digital medium format "wide angle" solution is the Alpa with their 24mmXL or 35mmXL lenses. You give up all automation but the camera construction is beautiful and the results spectacular. Bill
 
35mm 20 24 35 50 85 105
AOV 94.5 84.1 63.5 46.8 28.6 23.3

Aptus 17 24.7 29.7 43.3 61.9 105.2 129.9
Aptus 22 27.7 33.3 48.5 69.3 117.8 145.5
6X4.5 31.9 38.3 55.9 79.8 135.7 167.6
6X6 36.6 43.9 64.0 91.5 155.5 192.0
6X7 41.4 49.7 72.5 103.5 176.0 217.4
6X9 49.0 58.8 85.7 122.4 208.1 257.0
6X17 82.7 99.2 144.7 206.7 351.4 434.0
 
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Jonathan Wienke
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« Reply #3 on: September 24, 2005, 11:13:22 AM »
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The correct answer is "it depends", but here's an easy an easy way to calculate it:

If the print aspect ratio is wider than 5:4 (like 3:2 or 2:1), then the crop factor is 7/5 or 1.4.

If the print aspect ratio is squarer than 7:6 (like 1:1), then the crop factor is 6/4 or 1.5.

If the print aspect ratio is between 7:6 and 5:4, it will be somewhere between 1.4 and 1.5.
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x-ray
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« Reply #4 on: September 25, 2005, 03:58:28 AM »
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Hello Gents

Thank you for your thoughts on this one. They have been a great help. Just a quick comment for Bill, re the very useful format table he posted. Does he know of one which covers any greatef focal length than 105mm?
Any way all your info has been a great help on this one.

Thanks again.

X-ray
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howard smith
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« Reply #5 on: September 25, 2005, 02:05:45 PM »
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I understand that the customary angle of view is quoted along the diagonal.  I don't know any body that uses the diagonal dimension as a primary measure for composing images.  Why not give an angle of view on the horizontal dimension?  Or both the horizontal and vertical?
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Anon E. Mouse
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« Reply #6 on: September 25, 2005, 06:12:55 PM »
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Quote
I understand that the customary angle of view is quoted along the diagonal. I don't know any body that uses the diagonal dimension as a primary measure for composing images. Why not give an angle of view on the horizontal dimension? Or both the horizontal and vertical?
Some manufactures do (I know Horseman does this for its SW series cameras), but for the most part the diagonal is the easy single figure. It works fairly well until the difference in aspect ratios becomes large. I am sure horizontal and vertical angle of view would be more common if there were more requests for it.
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williamrohr
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« Reply #7 on: September 27, 2005, 08:54:04 AM »
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Actually, I simply calculated the table above from the simple physics of the optics. I'd be happy to do it for height and width or for any other focal length if anyone wants (I just didn't want to clog the forum with long charts ... and the diagonal gives the relavent information anyway). The major point for me is that because of the reduced sensor size in most formats (except for Canon), the actual availability of lenses with a wide angle of view is very limited. Add to that the CA issue with retro-focus designs ... and "wide angle" landscapes become a challenge in digital (unless one resorts to stitching, etc.) Bill
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howard smith
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« Reply #8 on: September 27, 2005, 09:35:29 AM »
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"The major point for me is that because of the reduced sensor size in most formats (except for Canon), the actual availability of lenses with a wide angle of view is very limited."

Exactly.  I think that is the main reason for quoting angle of view on the diagonal is so good for lens makers and deceptive for buyers.  The buyer isn't getting what he expects unless pretty savy.  Like TVs or computer monitors that eevn include the part you can't see.  The diagonal just gives a bit higher number.
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