67 Lens with 645 back: equivalent focal length confusion?

[windhorse]

Hi, I can find out how to calculate the 35mm equivalent of 6x7 lenses and 645 lenses, but my brain turns to soap suds when I try to calculate what the 35mm focal length equivalent of a 110mm 6x7 lens on a camera with a 645 back would be?

How far off would be my guess that in this case I would just have to calculate "as if" the 110mm 6x7 lens was a 645 lens ie something like 35mm equivalent of 110mm 645 lens = about 70mm?

 

[WilmarcoImaging]

Absolute precision is not needed. Precise calculations on angle of view vs the diagonal, the long edge and the short edge of the film are unnecessary.

Think of 645 as being very close to 6x6. Also look at “kit” lenses that were offered for 645 systems. A 75mm lens was considered “normal”. IMO 75mm is long for 645 but that’s just me.

90mm is generally considered normal for 6x7. Mamiya also positioned the 127 as a normal-ish 6x7 lens. So 110 on 6x7 is close to normal. A rough equivalent on 645 could be in the 70mm-80mm range.

I caution against thinking of a 50mm lens on 35mm film as normal. While this was a convention in the past, many people agree that human perception is very similar to a 35mm lens rendered on 35mm film. This is my view.

It’s important to know what you want to do with this 110mm equivalent lens. Portraits? Landscapes? Architecture? Each image type has different conventions and approaches. Of course these don’t need to be adhered to but if you are striving to replicate a certain style, the conventions at least need to be in mind.

To answer your question, 70mm is roughly normal focal length on 645. Kit lenses were close to this length.

 

[E. von Hoegh]

Hi, I can find out how to calculate the 35mm equivalent of 6x7 lenses and 645 lenses, but my brain turns to soap suds when I try to calculate what the 35mm focal length equivalent of a 110mm 6x7 lens on a camera with a 645 back would be?

How far off would be my guess that in this case I would just have to calculate "as if" the 110mm 6x7 lens was a 645 lens ie something like 35mm equivalent of 110mm 645 lens = about 70mm?

This is a bit like trying to learn the metric system by calculating every equivalence, it doesn't work. If your camera is an SLR or TLR, you have the equivalence right in front of you on the groundglass.
If you must, Pyhthagoras will help you calculate the format diagonal, but this will work exactly only for the 35mm's 3:2 aspect ratio i.e 35mm to 6x9cm (and almost on 6x4.5) and it disregards the difference in reproduction ratio. It's better and much faster to just become familiar with whichever lenses you are using on whatever format. For instance, I use formats 35mm, 6x4.5, 6x6, 6x7, 4x5, 5x7, 8x10, occasionally 16mm submini; if I noodled through the focal length equivalencies I'd never have time to make pictures.

 

[Dan Fromm]

OP, what does the lens' native mount have to do with anything? All 110 mm lenses that cover 6x4.5 produce the same image at the same magnification.

If you believe that the format's diagonal is the right dimension to use when thinking about equivalent focal lengths for different formats, comparing 35 mm still with 645 is easy. 645's diagonal is 70 mm, 24x36's is 43 mm. 70/43 = 1.628. 13/8 is close enough,

 

[cramej]

110 is still 110 regardless of 6x7 or 645. Calculate the equivalent of 110 on 645 and you've got it. There's no magic just because it's a "6x7 camera".

 

[Sirius Glass]

Myself when I compare 6x6 with 35mm I use 80m as the equivalent of 50mm and then use ratios.

My 500mm lens is 500/80 = x/50 ==> x = 500*50/80 = 312.5mm for a 35mm lens.

 

[E. von Hoegh]

A bad pitfall is thinking in "equivalent focal lengths" because there aren't. A 300mm lens is a 300mm lens regardless of format. However, 300mm is "normal" on 8x10, and "telephoto" on 35. Just like 50mm is "wide" on 6x7 and "normal" on 35, but it's always a 50mm lens.
Think instead "angle of view" and don't waste time and effort with 'equivalencies'

 

[MattKing]

110 is still 110 regardless of 6x7 or 645. Calculate the equivalent of 110 on 645 and you've got it. There's no magic just because it's a "6x7 camera".

+1
The only extra consideration to add is due to the fact that when you are using a 6x7 body, you need to modify the viewing system in order to be sure to get everything you expect into your photos.
I use a viewfinder mask in my RB67 when I have the 6x4.5 backs on the camera. Others either mark the focusing screen or use a gridded version.

 

[benveniste]

Because of the differences in aspect ratio, the "crop factor" for 645 can be anything between about 0.57 and 0.64. That means that a 300mm lens on a 645 is roughly equivalent to a 170-195mm lens on a 35mm SLR. It doesn't matter if the 300mm lens was designed to cover a 645 image circle, a 6x6 image circle or something larger.

In fact, I use a 300mm from the Pentax 67 system on my Pentax 645 (via an adapter).

 

[wiltw]

Hi, I can find out how to calculate the 35mm equivalent of 6x7 lenses and 645 lenses, but my brain turns to soap suds when I try to calculate what the 35mm focal length equivalent of a 110mm 6x7 lens on a camera with a 645 back would be?
How far off would be my guess that in this case I would just have to calculate "as if" the 110mm 6x7 lens was a 645 lens ie something like 35mm equivalent of 110mm 645 lens = about 70mm?

The easiest and most accurate way to think of it is to IGNORE 'diagonal and IGNORE 'normal' as the basis of comparison. To use The Bronica GS-1 (6x7) and ETRS (645) as comparison examples...
The GS-1 frame size is 55.6 x 69mm and has a 100mm 'normal' and the ETRS is 42.5 x 55.1mm, and 75mm 'normal'

• GS-1 'normal' is 1.798 * frame height, ETRS 'normal' is 1.764 * frame height ... they have DIFFERENT VERTICAL Angle of View!!!
• 'Ultrawide' equivalent to 24mm on 135 format (1 * frameheight) is 56mm for GS-1, 43mm for ETRS will all have identical vertical angle of view
• 'Head and shoulders' portraiture equivalent to 100mm on 135 (4.17 * frame height) is 232mm for GS-1 and 177mm on ETRS will all have identical vertical angle of view

and the short dimension of the frame will capture identical shot with all three cameras at the same tripod position. They will have the same ANGLE of View, and also the same FIELD of view when comparing the short direction of the frame capture!

So back to your question about 110mm on GS-1, would have 85mm on ETRS and 48mm on 135 to capture the same amount of subject along the short dimension of the frame (and varying amounts along the long dimension and different diagonal captures due to the different aspect ratios of the three frames!)

 

[jwd722]

For 645 use a conversion rate of 1.6, e.g. 50mm x 1.6 = 80mm
For 6x7 use 1.8, 50 x 1.8 = 90

 

[Tobes71]

There's very little point getting too hung up about it as most medium format systems don't make lenses in 5mm increments. They'll have a normal, short teles, moderate wides etc. but they won't necessarily conform exactly to 35mm lens standards or even correspond exactly in 67 to 645. The prevalence of slightly long standard lenses on many MF systems is a case in point. Couple that with the fact that you'll be buying secondhand so it might be a case of what you can find rather than what's totally ideal.

 

[tomfrh]

110mm 6x7 lens simply projects a bigger image circle to cover the whole negative. The actual image is the same (it just extends further). So yes, treat the 110mm 6x7 lens as a 110mm 645 lens.

Same as a 50mm 35mm format lens mounted on an APS camera is the same as a 50mm APS lens on the APS camera.