# Another question - How to convert medium format lens to equivalencyof a 50mm normal lens (35mm camer

Discussion in 'Digital Photography' started by aniramca, May 31, 2009.

1. ### aniramcaGuest

I need help on this one (also), as I seem to have a block in my brain
to do the math and follow the logic. Perhaps someone can explain this
in a more practical and easy to understand way?.
I just recently used my old Mamiya M645 lenses on my digital camera
(APS-C) using a lens adapter. I recall the following:
1. 50mm normal lens in a full frame (35mm camera) will be equivalent
to approx. 31-33mm APS-C lens (crop factor of 1.5 or 1.6).
2. I heard a long time ago that for a 6x4.5cm medium format camera,
the normal lens will be around 90mm. Normal means as a standard 50mm
lens in the 35mm camera.

So, if I have 35mm, 90mm and 210mm M645 lenses, what are the focal
lengths represented by these lens in the digital APS-C cameras
(equivalency of the stardard 35mm camera)? When I tried them with my
digital camera, the 35mm was a little too short for being called a
wide angle, the 90mm lens was great, useful and powerful, but the
210mm did not appear to have the effect of a long telephoto lens. It
could just be in my mind, as I perhaps expected the 35mm and 210mm to
be a wide angle and a telephoto lens, respectively.

aniramca, May 31, 2009

2. ### aniramcaGuest

I am trying to answer my own question here. Could someone confirm if
this is correct?
90mm M645 = 50mm normal 35mm = 33mm APS-C Crop factor 90/33= 2.7
Therefore 35mm M645 will be equivalent to 13mm, and 210mm M645 will be
equivalent to 78mm
The 35mm lens did not look like it has a wide angle view than, say, my
Nikkor 18-55mm lens at the 18mm zoom, though?

aniramca, May 31, 2009

3. ### NickoGuest

Using the lens adapter almost certainly throws everything off but in
any case, continue

I recall the following:
56mm

144mm

336mm

designed to cover. That is just an image circle size issue. 210mm is
210mm regardless of the manufacturer, and the only difference is what
size sensor or film that a given lens will cover--the size of its
image circle.

Focal length is absolute.

But again, whatever adapter you are using is probably going to
complicate things beyond multiplication by the crop factor.

I hope that probably didn't help.

But you are probably a troll, and I am a pencil-necked geek, so I aint
complaining.

Nicko, May 31, 2009
4. ### NickoGuest

Yes.

Why did you steal my sausages?

Nicko, May 31, 2009
5. ### Doug JewellGuest

Not quite. A 90mm lens is a 90mm lens regardless of what
camera you put it on. The only thing that changes is how
much of the image that the lens casts that you are looking at.

So if you stick the 90mm lens on a 645 camera, because the
film is quite large, you are looking at a large area of the
image. When you put the 90mm lens on a 35mm film camera you
see a smaller amount of the image, and on an APS-C camera
you are looking at a smaller portion again. Because a
smaller amount of the image fills the sensor/film frame, you
have a smaller field of view.

The concept of "35mm equivalent" etc comes because to get
the same field of view you need a lens with a different
focal length. So when you put your 90mm lens on your 645
camera you get a certain field of view. If you put the 90mm
on a 35mm film camera you will have a smaller field of view.
If you want the same field of view that you had with the
645, you will need to mount a 50mm lens. If you want the
same field of view on an APS-C camera, you would mount a
33mm lens. So 33mm (APS-C), 50mm (35mm) and 90mm (645) can
be said to be "equivalent" because on their respective
formats they all give basically the same image.
That's right - when you put the 35mm lens on the Nikon it is
still a 35mm lens. If you dial your 18-55 lens to the 35mm
position, it will give exactly the same image that your
35mm(645) lens gives when mounted to the Nikon.

Your 13mm figure is correct though, you are just applying it
the wrong way. When you put the 35mm lens on the 645 camera,
it will give a very wide angle view - the same as you would
get if you put a 13mm lens on your Nikon. You will only get
that wide angle of view with in on the 645 though - on the
Nikon it will be the same as any other 35mm lens on the Nikon.

Doug Jewell, May 31, 2009
6. ### WoodyGuest

What needs explaining here is what at 'standard lens is - and
that is simple.

It is the lens which has a focal length equivalent to the longest
measurement of the format in use, i.e. the diagonal. It this
follows that a 'standard' lens for 35mm will, by Pythagorus, be
about 42mm, and for 645 will be 75mm.

AP-C cell sizes vary, but assuming 23x15mm this would make a
'standard' lens about 27mm. The ratio of 35mm to AP-C is thus
about 42/27 or 1.5 which is what is generally accepted as 'about'
the magnification factor. Due to fanufacturing processes and the
way the brain 'sees' a picture, 50mm (with a magnification factor
or 50/42 or 1.2) is usually accepted as 'standard' for 35mm.

By the same token a 645 'standard' lens on an AP-C camera will be
75/27 or about 2.8, which in 35mm lens terms would be the

HTH

Woody, May 31, 2009
7. ### aniramcaGuest

The lens adapter do not have an optical element on it. The Nikon
camera's register distance is 46.6mm. The Mamiya M645 register
distance is 63.3m. So, to use a Mamiya lens on a Nikon, you do not
need an optical element, and infinity focus is not a problem here.
Regards

aniramca, May 31, 2009
8. ### aniramcaGuest

Thanks Doug, but I am still a little bit fuzzy as shown below

I agree

If you dial your 18-55 lens to the 35mm
You dial 18-55 lens to the 35mm position meaning 18-55 APS-C lens?
Then this would be around 50mm normal 35mm camera, and it is not the
same as 35mm(M645)
I am getting fuzzy here again.
I agree

You will only get
I thought the 35mm(M645) would be equivalent to 13mm(APS-C)?

aniramca, May 31, 2009
9. ### Rob MorleyGuest

44.4mm 70.6mm

Just in case anyone thought Pythagoras was playing up.

Rob Morley, May 31, 2009
10. ### WoodyGuest

Sorry - fogot the spel-chequer!

Woody, May 31, 2009
11. ### WoodyGuest

Let's split the difference shall we? (36^2+24^2)^0.5 = 43.2.
(60^2+45^2)^0.5=75 as I said. (This using Windows Calculator.)

Woody, May 31, 2009
12. ### Rob MorleyGuest

It seems I succumbed to one of the fundamental laws of Usenet.
The neg size of 645 is more like 56Ã—41.5mm, because 120 film is based
on 2 1/4", not 60mm.
Next time I'll double check before posting. Or just try to abstain
from nitpicking.

Rob Morley, May 31, 2009
13. ### Matt IonGuest

This is an excellent and succinct explanation of "crop factor".

Matt Ion, May 31, 2009
14. ### Doug JewellGuest

Ok, One camera, your Nikon APS-C. Mount the 35mm 645 lens
with adaptor to it. It will give a certain field of view.
Now take it off and put your 18-55 zoom lens on the Nikon.
Dial it up to the 35mm position. The field of view will be
exactly the same. Both lenses are 35mm, so both will give
the same view, when fitted to the same camera.
If you put the 35mm lens on the 645 camera, it will give a
much wider angle view than it does on the APS-C camera,
because the 645 camera is looking at more of the image that
the lens produces.
Yes - the 35mm lens WHEN FITTED TO THE M645 CAMERA will be
roughly equivalent to a 13mm lens FITTED TO THE APS-C CAMERA.
When the 35mm lens is fitted to the APS-C camera it is
exactly the same as any other 35mm lens fitted to the APS-C
camera. The 35mm stays fixed, so on the SAME CAMERA the view
is fixed. It doesn't matter whether the lens was designed
for 645, 135, APS-C, 8x10, it is still a 35mm lens. You
could have 4 35mm lenses designed for each of the above
formats, but when fitted to the APS-C camera every one of
them will give the same image.

Doug Jewell, May 31, 2009