focal length comparison between 35mm, 120, and 4x5.

[Jim Galli]

Clark, to answer at least one of your questions, a 127mm on 6X7 will see about the same as the 210 0n 4X5. If you put a rollfilm back on the 4X5 and you are using the 210, bellows factor doesn't start until you go well past 210. It's still a 210, but your rollfilm will "see" approximately what a medium format with a 210 telephoto would be seeing. It is correct that you can't really compare the 3:2 ratio of 35mm with the 4:5 of 4X5, but we do it anyway. For my brain, I choose to consider the width in that ratio, not the height. since 35mm is 1.5" wide, you could fit 3 in the 5" width of 4X5 and have 1/2 inch left over. I use 3.33 to make that conversion for what it's worth, making a 165mm closer to a 50mm on 35. There's no set rule because it's technically an improper comparison. As far as metering the scene, either you're confused, or I am. If I had 3 cameras set up, a 35mm with a 63mm lens, a 120 with 127mm lens, and a 4X5 with 210mm lens, all focused at infinity, and all had the same film, ie. velvia, I would take one reading for XX ASA and set all 3 cameras the same. Hope this helps some.

[Walter Glover]

Clark,

With regard to your query about focal length comparisons with varying formats I suggest use of the following equation in a spreadsheet such as Excel:

Angle Of View =ATAN(Image Length / 2 / Focal length) X 180 / PI() X 2

which becomes:

=ATAN(B1/2/A1)*180/PI()*2

when

Column A = Focal length

Column B = Image Length

Another useful application of this equation :

Focal Length =A1/(1/(B1/C1)+1)

where:

A1 = Subject Distance B1 = Image Width C1 = Subject width

In circumstances where space might be limited this equation will give the focal length required to fit a given subject into a particular film size. Importantly the same units of measurement must be used for each element (ie: inches, mm.) To convert a focal length in inches to mm simply multiply by 25.4.

To determine bellows factors use this equation on a seperate worksheet:

Bellows factor =(Measured Bellows Extension X Measured Bellows Extension) / (Focal length X Focal Length)

or

=(A1*A1)/(B1*B1) where A1 is Bellows Extension and B1 is Focal Length

A further modification of the second equation that may come in handy is:

=1+(A2/((A1*B1/(A1-B1))-B1))

This will give the magnification.

I have these formulae in templates on a spreadsheet in my Palm Pilot and am able to make the necessary calaculations instantly wherever I am.

Try it ... it might make light of a number of confusing issues.

WG

[Clark King]

Thank you all for the info. I appreciate it. I'm a little new at all this and needed to clarify a few particulars. One thing about this forum is that you can learn a great deal of information, and I appreciate any and all responses.

Thanks

CK