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how to compute medium format lens FL in 35mm terms?
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PostPosted: Mon Aug 15, 2011 9:30 pm    Post subject: how to compute medium format lens FL in 35mm terms? Reply with quote

in reading about medium format lenses, their focal lengths are often described in 35mm terms to make their field of view more generally understandable. for example, an 80mm lens for a 6x6 cam is often referred to as 'like a 50mm lens' used on a 35mm cam. this same lens on a 6x9 cam would be 'something like a35mm lens' on your 35mm film cam.

my question is does anyone know the formula for computing the 35mm FOV equivalent for lenses used on medium format cams? with so many different size MF cams--6x6, 6x7, 6x8, 6x8 and even 6x12 and 6x17--i think it would be helpful to know how to interpret different lenses used on these systems in the 35mm terms with which we are most familiar.

any light anyone can shed on this would be appreciated!
tony


PostPosted: Mon Aug 15, 2011 9:40 pm    Post subject: Reply with quote

I don't have a scientific formula, only a practical one for 6x6
Divide by 1.6 the MF focal lenght to obtain the SF equivalent.
Example:
80mm : 1.6 = 50mm
45mm : 1.6 = 28,1 mm

hope this helps


PostPosted: Mon Aug 15, 2011 10:44 pm    Post subject: Reply with quote

Crop factor is simply the ratio of the diagonal lengths of two formats.

Remember Pythagoras' Theorem, for a rectangle the diagonal is the square root of the sum of the squared lengths of two adjacent sides.

=============================================

For APS-C of 15.7mm x 23.7mm, is 15.7^2 + 23.7^2 = 808 and 808^0.5 = 28.4mm diagonal

For FF/35mm of 24mm x 36mm, is 24^2 + 36^2 = 1872 and 1872^0.5 = 43.3mm diagonal

So, for APS-C compared to FF/35mm is 43.3 / 28.4 = 1.53 crop factor.

=============================================

For 6x6 of 60mm x 60mm, is 60^2 +60^2 = 7200 and 7200^0.5 = 84.8mm diagonal.

So, for FF/35mm compared to 6x6 is 84.8 / 43.3 = 1.96 crop factor.


PostPosted: Tue Aug 16, 2011 5:58 am    Post subject: Reply with quote

For square formats, just multiply one side by 1.41 Smile


PostPosted: Tue Aug 16, 2011 2:08 pm    Post subject: Reply with quote

thanks to all, but it seems we have a pretty big discrepancy even with one format, 6x6. orio says the crop factor is 1.6, V says its 1.9 and ludoo says its 1.4!

because i am not comfortable with math generally, i have a very healthy respect for it and i do actually (to my surprise) understand the mathematical underpinnings of V's equation. orio and ludoo, how do you each feel about your crop factors in light of V's math equations? it basically changes orio's 80>50 to 80>43mm. it would obviously have a bigger impact on ludoo's method of determining equivalence.


PostPosted: Tue Aug 16, 2011 2:11 pm    Post subject: Reply with quote

rbelyell wrote:
thanks to all, but it seems we have a pretty big discrepancy even with one format, 6x6. orio says the crop factor is 1.6, V says its 1.9 and ludoo says its 1.4!


Nope, I just said that instead of computing the diagonal using powers and square roots, for square formats the diagonal is simply side*1.41. Then apply the same rules to get to the ratio compared to 35mm. The diagonal for 6x6 is 60mm * 1.41 = 84.6mm.


PostPosted: Tue Aug 16, 2011 4:58 pm    Post subject: Reply with quote

Like I said, I have no scientific foundation for my calculation.
I have always read that 80mm equals 50mm on small format, 45mm equals 28mm, 35mm equals 21mm, and 120mm equals 75mm.
The division factor to obtain these figures is 1.6, so that's how I figure out the corresponding focal lenghts.
Of course, I have no way to tell if the 1.6 division factor gives a precise value or only an approximation.


PostPosted: Tue Aug 16, 2011 5:12 pm    Post subject: Reply with quote

Wikipedia: Crop Factor


PostPosted: Tue Aug 16, 2011 6:05 pm    Post subject: Reply with quote

The math above, while the method is sound, uses figures that are not quite accurate, rather estimations of film size. Using this Film Size Chart for more accurate measurements, here are the most accurate crop factors related to 35mm film size:



The column FL shows the FL lens for that format with angle of view same as 50mm lens on 35mm camera.

Interesting the crop factor for 645 is 1.62, same as Orio gives. Wink


PostPosted: Tue Aug 16, 2011 7:50 pm    Post subject: Reply with quote

ludoo: like i said, math is not my thing! Embarassed frankly, i was amazed i comprehended V's equations at all.

orio: thank you for your help as there is no substitute for a great photographers real world experience.

V: that chart is super! ive made a copy and if it helps anyone, using these calculations the crop factor on 6x9 is approximately 2.5, and for 6x12 is approxiately 3.

this has been immensely helpful, thank you to all!
tony


PostPosted: Tue Aug 16, 2011 8:43 pm    Post subject: Reply with quote

I have no argument with the maths, but could someone explain why the length of the diagonal is significant? How do you get from a 43mm diagonal to the "normal" 50mm FL on 135?

When I look through any 135 viewfinder with a 50mm lens mounted, the image is larger than I see with the other eye. I'd need a lens of about 43mm to make them appear similar, which is remarkably similar to the 135 diagonal. Why is that?


PostPosted: Tue Aug 16, 2011 10:19 pm    Post subject: Reply with quote

peterqd wrote:
I have no argument with the maths, but could someone explain why the length of the diagonal is significant? How do you get from a 43mm diagonal to the "normal" 50mm FL on 135?

When I look through any 135 viewfinder with a 50mm lens mounted, the image is larger than I see with the other eye. I'd need a lens of about 43mm to make them appear similar, which is remarkably similar to the 135 diagonal. Why is that?


In spreadsheet, 50mm lens was chosen for 35mm/FF, only for illustrative purpose, to compare the 'equivalent' focal lengths of other formats giving the same angle of view. It could have been any focal length lens, the crop factors remain the same regardless of lens chosen to compare.

'Normal' is different. (no there's an oxymoron Laughing) Meaning 'normal' has nothing to do with crop factors, but only with focal length equal to diagonal measurement of format.

Eye angle of view is 180 degrees, same as ultra-wide of around 16mm (on 35mm). The foveal region (best focus) angle of view is around the same as 135mm lens on (35mm).

'Normal' lens angle of view retains relative perspective of objects at different distances. A wider lens increases the apparent distances. A longer lens reduces the apparent distances.


PostPosted: Tue Aug 16, 2011 11:12 pm    Post subject: Reply with quote

visualopsins wrote:
'Normal' lens angle of view retains relative perspective of objects at different distances. A wider lens increases the apparent distances. A longer lens reduces the apparent distances.

I once thought the same but was corrected by Rick Oleson, and a few years ago I did a little test. It's an optical illusion - the apparent distances between near and far actually appear exactly the same whether with a telephoto or a wide angle.
http://forum.mflenses.com/perspective-and-focal-lengths-t12219,highlight,%2Bperspective.html

I still don't see what the diagonal length of the frame has to do with the focal length of a "normal" lens, or "standard" if you prefer. Perhaps it's just a coincidence?


PostPosted: Tue Aug 23, 2011 4:53 am    Post subject: Reply with quote

peterqd wrote:
visualopsins wrote:
'Normal' lens angle of view retains relative perspective of objects at different distances. A wider lens increases the apparent distances. A longer lens reduces the apparent distances.

I once thought the same but was corrected by Rick Oleson, and a few years ago I did a little test. It's an optical illusion - the apparent distances between near and far actually appear exactly the same whether with a telephoto or a wide angle.
http://forum.mflenses.com/perspective-and-focal-lengths-t12219,highlight,%2Bperspective.html

I still don't see what the diagonal length of the frame has to do with the focal length of a "normal" lens, or "standard" if you prefer. Perhaps it's just a coincidence?


There is so much confusion surrounding the term 'perspective'. Yes some of it is 'optical illusion', but that's not what I refer to. Check Rauschenbach's Theory of Perspective and Photography for explanations of all types.



By definition, a 'normal' lens is focal length equal to the length of diagonal of the film/sensor. On apug, Viewing distance and perspective -- what's the basis? Roger Hicks gives the basis, "The logic is that the angle subtended by each element in the picture should be the same as the angle subtended at the camera by the same element in the original scene. "


PostPosted: Thu Mar 17, 2016 4:09 pm    Post subject: Reply with quote

Orio wrote:
Like I said, I have no scientific foundation for my calculation.
I have always read that 80mm equals 50mm on small format, 45mm equals 28mm, 35mm equals 21mm, and 120mm equals 75mm.
The division factor to obtain these figures is 1.6, so that's how I figure out the corresponding focal lenghts.
Of course, I have no way to tell if the 1.6 division factor gives a precise value or only an approximation.


Only Canon APS-C is Cropfactor x1.6 Orio, Pentax, Nikon & Sony is x1.5, the Sensors are a little bigger.
(but differ also in very neglible points)


PostPosted: Sat Mar 19, 2016 1:13 am    Post subject: Reply with quote

Embarassed

Now I see I misunderstood what Peter was talking about -- he is correct, if the two lenses are used from the same position the perspective does not change. However, I was talking about using lenses from different positions, which does change the perspective, when used to obtain images with the same magnification...

I should also clarify why comparison of diagonals is used for determining crop factors -- the diagonal is the maximum the lens image circle must cover, for each format, no matter if one is square and one rectangular...

Lastly, something should be said about what is a "Normal" lens for any format -- again, this is the length of the diagonal, with a caveat.

Gerald wrote:
CBokeh wrote:
I was always under the impression that "normal" was 43mm on 35mm format but 50mm was "close enough."


The conventional definition of a normal lens is that one whose focal length is equal to the diagonal of the format. This gives a focal length of 43.3mm for 35mm and 79.2mm format for 6x6 format.

I prefer another definition: a normal lens is the one whose angle of view matches the angle of view of the person viewing a print of a picture taken with that lens. This definition can be extended to pictures viewed on a screen of a computer, TV or cinema.

I will try to explain with a simple example. Suppose an image captured by a FF sensor is enlarged by a factor M equal to, say, 10x to produce a print with dimensions 24 x 30 cm. If the print is to be viewed at a distance of 50cm, the normal lens is the one with a focal length of 50cm / M = 50 mm. However, if the print is viewed at a distance of 40 cm, the normal lens has focal length of 40 mm. And if the viewer is a young person that prefers to see the print at 20 cm, the normal lens is the 20 mm!

According to the second definition, the focal length of a normal lens is always given by:

FL = D / M

where:
D = distance of visualization
M = magnification of the displayed image relative to the sensor size


PostPosted: Sat Mar 19, 2016 2:19 pm    Post subject: Reply with quote

I don't feel the need to convert FL to the fourth decimal point. A factor of 2 is close enough for me.

50mm on a 35 is 100mm for my 6x7.