645/135 equivalence

cluttered · Nov 7, 2015

I'm helping someone choose a lens for their Pentax 645nII. I'm more familiar with 6x7 or 4x5 lens equivalents, so I'd like to double-check the 645 equivalents for them.

They're considering buying a 75mm FA lens. I would have thought that it is equivalent to a 45mm lens in 135 (35mm) terms. ie, pretty much a 'normal' lens, which is what they want. Doing the maths (working out the equivalent diagonals), I'm fairly sure this is right. However that particular lens at the B&H website (http://www.bhphotovideo.com/c/produ...6121_75mm_f_2_8_smc_fa_645.html#Specification) is allegedly a 59mm equivalent, which is a fairly substantial difference. And the Pentax web site agrees with this.

So who's right?

MattKing · Nov 7, 2015

The difference arises because of the difference in aspect ratios.

If you compare the short edge of the two formats you will get a markedly different result than if you compare the diagonal, which will be different again then if you compare the longer edge of the two formats.

The 59mm equivalent probably arises from the comparison between the diagonals. I tend to prefer a comparison between the short edges, but others would prefer a comparison between the long edges.

cluttered · Nov 7, 2015

MattKing said:
The difference arises because of the difference in aspect ratios.

If you compare the short edge of the two formats you will get a markedly different result than if you compare the diagonal, which will be different again then if you compare the longer edge of the two formats.

The 59mm equivalent probably arises from the comparison between the diagonals. I tend to prefer a comparison between the short edges, but others would prefer a comparison between the long edges.

I can't see how they get 59mm regardless of approach; diagonals gives the numbers I mentioned before, and using just the horizontal or vertical also don't get anywhere near 59mm. It's definitely possible I'm missing something obvious however.

narsuitus · Nov 7, 2015

A 45mm lens on a 35mm camera has a 42.6 horizontal angle-of-view and a 51 degree diagonal angle-of-view.

A 59mm lens on a 35mm camera has a 33.9 horizontal angle-of-view and a 40.3 degree diagonal angle-of-view.

A 75mm lens on a 645 camera has a 40.9 degree horizontal angle-of-view and a 49.8 degree diagonal angle-of-view.

Therefore, the 75mm lens on a 645 camera has an angle-of-view closer to a 45mm lens on a 35mm camera than a 59mm lens on a 35mm camera.

Jeff Bradford · Nov 7, 2015

The store could be calculating per the sensor size of the 645D or 645Z which are not as large as the exposure in the 645n film camera.

cluttered · Nov 7, 2015

Jeff Bradford said:
The store could be calculating per the sensor size of the 645D or 645Z which are not as large as the exposure in the 645n film camera.

That makes sense, thanks for the info!

narsuitus · Nov 7, 2015

The Pentax 645 film camera produces an image that is close to 6 x 4.5 cm.

The Pentax 645D produces a digital image that is 4.4 x 3.3 cm.

How can a camera that produces such a small image be confused with a 645 medium format film camera?

MattKing · Nov 7, 2015

cluttered said:
I can't see how they get 59mm regardless of approach; diagonals gives the numbers I mentioned before, and using just the horizontal or vertical also don't get anywhere near 59mm. It's definitely possible I'm missing something obvious however.

The B & H link you referred to quotes a single angle of view of 40.5 degrees. Which, if you use the calculations referenced on this Wikipedia link, appears to correspond to a diagonal angle of view, on a full frame 35mm, that is approximately the same as a 60mm lens: https://en.wikipedia.org/wiki/Angle_of_view

Alan Gales · Nov 8, 2015

When you change aspect ratios it screws everything up. I don't care what the math says, I just know what my eye sees and others may see differently.

A 75mm to 80mm is considered a "normal" focal length on 645 or 6x6. If your friend likes a 45mm lens on a 35mm camera a 75mm on a Pentax 645 is as close as he is going to come. The next widest lens for Pentax 645 is a 55mm from what I remember.

cluttered · Nov 8, 2015

Alan Gales said:
When you change aspect ratios it screws everything up. I don't care what the math says, I just know what my eye sees and others may see differently.

A 75mm to 80mm is considered a "normal" focal length on 645 or 6x6. If your friend likes a 45mm lens on a 35mm camera a 75mm on a Pentax 645 is as close as he is going to come. The next widest lens for Pentax 645 is a 55mm from what I remember.

Agreed.

My friend already has the 35mm Pentax 645 lens which is quite wide (about 22mm equivalent in 135), and she now wants to add a "normal" lens or maybe a little wider than "normal", so the 75mm is what I've recommended to her. I was confused when I saw B&H and Pentax describe it as 59mm-equivalent, but now that I realise that they are describing it in terms of the new Pentax "645" digital sensor which is smaller than a 645 film frame, it all makes sense.

wiltw · Nov 8, 2015

MattKing said:
The difference arises because of the difference in aspect ratios.

If you compare the short edge of the two formats you will get a markedly different result than if you compare the diagonal, which will be different again then if you compare the longer edge of the two formats.

The 59mm equivalent probably arises from the comparison between the diagonals. I tend to prefer a comparison between the short edges, but others would prefer a comparison between the long edges.

I am the short edge equivalence photographer myself. Vertical AOV...

24mm on 135 is height of frame, 24mm lens sees 53 degree AOV vertically.
42.5-43mm on 645 is height of frame, 43mm lens sees 53 degree AOV vertically.

...then the horizontal AOV is defined by the aspect ratio of the frame

adelorenzo · Nov 9, 2015

Don't overlook the 45-85mm FA zoom lens, it's a beauty. I have the 35mm as well (and a 135mm) but I use the zoom 90% of the time.

cluttered · Nov 9, 2015

adelorenzo said:
Don't overlook the 45-85mm FA zoom lens, it's a beauty. I have the 35mm as well (and a 135mm) but I use the zoom 90% of the time.

Thanks for the tip, it sounds like a good option. I'll put it to my friend.

Alan Gales · Nov 10, 2015

I owned the 35mm manual focus lens, 75mm FA, 120mm FA Macro, and 150mm FA lens. All were excellent performers.

gzhuang · Nov 10, 2015

mklw1954 · Nov 13, 2015

Pentax states on page 28 that 35mm equivalent is approximately 0.6 x focal length of 645 lens:

http://www.pentax-manuals.com/manuals/medformat/a lenses_s.pdf

pentaxuser · Nov 13, 2015

I think I had seen the equivalence ratio as being 0.62 but 0.02 makes very little difference. Anybody considering the 45-85 zoom for the first time needs to be aware of the extra bulk from the zoom lens

Not a reason for not getting it- just an aspect to take into account

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